Junction Barrier Schottky Rectifiers in Silicon Carbide

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Junction Barrier Schottky Rectifiers in Silicon Carbide

Fanny Dahlquist

KTH, Royal Institute of Technology

Department of Microelectronics and Information Technology

Stockholm, 2002

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Junction Barrier Schottky Rectifiers in Silicon Carbide

A dissertation submitted to Kungliga Tekniska Högskolan, Stockholm, Sweden, in partial fulfillment of the requirements for the degree of Teknisk Doktor.

 2002 Fanny Dahlquist

KTH (Kungliga Tekniska Högskolan) Royal Institute of Technology

Department of Microelectronics and Information Technology Electrum 229,

SE-164 40, Kista SWEDEN

ISRN KTH/EKT/FR-2002/4-SE ISSN 1650-8599

TRITA - EKT

Forskningsrapport 2002:4

Printed in 250 copies by Kista Snabbtryck AB, Kista 2002

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ISRN KTH/EKT/FR-2002/4-SE, KTH, Royal Institute of Technology, Department of Microelectronics and Information Technology, Stockholm, 2002

ABSTRACT

Silicon carbide (SiC) is a semiconductor material that may enable the transition of traditional silicon (Si) power electronics into smart power. SiC material properties allow devices with higher voltage rating and higher operating temperatures compared to Si, which translates into smaller and less expensive components. Switches and rectifiers are key components in power electronics and the Junction Barrier Schottky (JBS) and Schottky rectifier in SiC are candidates to replace Si PiN diodes in the 300- 3300 V blocking voltage range.

The JBS rectifier combines a Schottky and PiN diode structure making use of the advantages of both types. The forward voltage drop was investigated and analytic equations formulated, considering the Schottky barrier height, the drift region and the geometrical layout. A p

⁺

grid structure was implemented and a design procedure to minimize the drift region resistance for any blocking voltage was derived.

JBS diodes and reference Schottky diodes were fabricated on several 4H (and 6H) SiC wafers with epitaxial (epi) designs for 600-3300 V blocking voltages. The increase in forward voltage for the JBS diode compared to the Schottky diode due to the p

⁺

grid resistance is compensated by the fact that higher blocking voltages are reached. For example, JBS diodes were shown to withstand 1500 V blocking voltage where Schottky diodes only yielded 1100 V on the same epi layer. The reason is that JBS diodes can withstand 20% higher junction electric field compared to Schottky devices. This favorable scaling applies to all the investiga ted voltages. Blocking voltage up to 3300 V was reached for JBS diodes with less than 2.1 V forward drop for 2A current rating.

Furthermore, the JBS diodes show higher blocking yield than the Schottky diodes, especially on those wafers where poor Schottky contact properties were measured.

This is explained by the different blocking mechanisms (p

⁺

n junction versus Schottky junction) and shows that the JBS design is less sensitive to imperfections and crystal defects in state-of-the-art SiC material.

Keywords: silicon carbide, JBS rectifier, Junction Barrier Schottky (JBS), Schottky

rectifier, MPS rectifier, power rectifier, punch-through design, power loss, high

blocking voltage

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Table of Contents... i

Appended papers... ii

Related papers not included in the thesis ... iii

Summary and author’s contribution to the appended papers ... iv

Acknowledgements... vi

1. Introduction ... 1

2. Background ... 3

2.1 Silicon carbide properties... 3

2.2 Device fabrication... 4

3. Power rectifiers ... 6

3.1 Power diode concepts ... 6

3.2 Important parameters for power rectifiers ... 9

4. Forward and reverse characteristics of Schottky and JBS diodes ...16

4.1 Forward conduction characteristics ...16

4.2 Reverse blocking characteristics ...19

4.3 Summary of leakage current mechanisms...23

4.4 Other variants on JBS structures ...25

5. Device design for 600-3300 V diodes...27

5.1 Minimized drift resistance by punch-through epitaxial design...27

5.2 Ideal and state-of-the-art parameters and forward voltage calculations ...33

6. Fabrication process...43

6.1 Critical steps in JBS (and Schottky) diode process ...43

6.2 Experimental ...48

7. Results and discussion ...51

7.1 Papers I-V...51

7.2 Electrical characterization and parameter extraction...53

7.3 Paper VI and Paper VII...55

7.4 Transient measurements ...59

8. Conclusions...60

9. References ...62

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ii

Appended papers

I. Junction Barrier Schottky diodes in 6H SiC

C.-M. Zetterling, F. Dahlquist, N. Lundberg, and M. Östling, Solid-State Electronics, 42, 1757 (1998)

II. Junction Barrier Schottky Diodes in 4H-SiC and 6H-SiC F. Dahlquist, C.-M. Zetterling, M. Östling, and K. Rottner, Materials Science Forum, 264-268, 1061 (1998)

III. A 2.8 kV, 2 V forward drop JBS diode with low leakage F. Dahlquist, J.-O. Svedberg, C.-M. Zetterling, M. Östling, B. Breitholtz, and H. Lendenmann,

Materials Science Forum, 338-342, 1179 (2000)

IV. A High Performance JBS Rectifier - Design Considerations F. Dahlquist, H. Lendenmann, and M. Östling,

Materials Science Forum, 353-356, 683 (2001)

V. Long Term Operation of 4.5 kV PiN and 2.5 kV JBS Diodes H. Lendenmann, F. Dahlquist, N. Johansson, R. Söderholm, P. A. Nilsson, J. P. Bergman, and P. Skytt,

Materials Science Forum, 353-356, 727 (2001)

VI. A JBS diode with controlled forward temperature coefficient and surge current capability

F. Dahlq uist, H. Lendenmann, and M. Östling, Materials Science Forum, 389-393, 1129 (2002)

VII. Junction Barrier Schottky (JBS) and Schottky diodes in silicon carbide for the 600-3300 V blocking voltage range

F. Dahlquist, H. Lendenmann, and M. Östling,

Submitted to IEEE Transactions on Electron Devices (May 2002)

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Related papers not included in the thesis

VIII. Demonstration of Lateral Boron Diffusion in 4H-SiC Using the JBS Device as Test Structure

F. Dahlquist, H. Lendenmann, M. S. Janson, and B. G. Svensson,

Presented at the International Workshop on Ultra low- loss Power Device Technology, UPD2000, and printed in Journal of Future Electron Devices, 11, 2, (2000)

IX. Performance and Reliability of High Power SiC diodes

H. Lendenmann, F. Dahlquist, N. Johansson, J. P. Bergman, H. Bleichner, and C. Ovrén,

Presented at the International Workshop on Ultra low- loss Power Device Technology, UPD2000, and printed in Journal of Future Electron Devices, 11, 2, (2000)

X. 4.5 KV 4H-SiC diodes with ideal forward characteristic

H. Lendenmann, A. Mukhitdinov, F. Dahlquist, H. Bleichner, M. Irwin, R. Söderholm, and P. Skytt,

Proceedings of the International Symposium of Power Semiconductors, 31 (2001)

XI. High Power SiC diodes: Characteristics, Reliability, and relation to material defects

H. Lendenmann, F. Dahlquist, J. P. Bergman, H. Bleichner, and C. Hallin,

Materials Science Forum, 389-393, 1259 (2002)

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iv

Summary and author’s contribution to the appended papers

The author’s contribution to the work in Papers I-VII is as follows:

Paper I: Junction Barrier Schottky diodes in 6H-SiC

In Paper I JBS diodes in silicon carbide were reported for the first time. In this work the goal was to develop a process to verify the first design of JBS devices in SiC and electrically characterize them in comparison with Schottky and PiN diodes. The author participated in the process development, processed the devices and did most of the analysis and electrical characterization as well as contributed to the manuscript.

Paper II: Junction Barrier Schottky Diodes in 4H-SiC and 6H-SiC

In Paper II, the author fabricated JBS devices in both 4H and 6H-SiC. The goal was to demonstrate the usefulness of 4H- material for power devices and improve the results from Paper I. The author did further process development compared to Paper I, which included improvement in implantation profile and Schottky contact formation. The author did all processing, electrical characterization, most of the analysis part and wrote the manuscript.

Paper III: A 2.8 kV, 2 V forward drop JBS diode with low leakage

In Paper III the author did a complete new experiment design based on the results in the previous experiments. The goal was to fabricate 3 kV JBS diodes with as low forward voltage drop as possible. The author did a comprehensive experiment design in both layout and process. The author did part of the processing, all electrical measurements of the finished devices, the analysis and writing of the manuscript.

Paper IV: A High Performance JBS Rectifier - Design Considerations

In Paper IV the author performed a more extensive analysis of the design variations in

Paper III and wrote the manuscript.

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Paper V: Long Term Operation of 4.5 kV PiN and 2.5 kV JBS Diodes

In Paper V the JBS diodes in Paper III and Paper IV were presented together with results on separately processed PiN diodes. The author contributed in the analysis and writing of the manuscript.

Paper VI: A JBS diode with controlled forward temperature coefficient and surge current capability

In Paper VI the temperature dependence and capability to handle high current densities in a JBS diode was studied. The author did the experiment design, the analysis part and writing of the manuscript.

Paper VII: Junction Barrier Schottky (JBS) diodes in silicon carbide for the 600- 3300 V blocking voltage range

For Paper VII JBS and Schottky diodes were processed with an improved design based

on the previous results and the goal was to demonstrate the advantages by using a JBS

diode concept for different blocking vo ltages. The author designed the experiments,

developed the process, did all electrical characterization, analysis and wrote the

manuscript.

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vi

Acknowledgements

In March 1997, I started as PhD student at the Electronics department and eight months later, in November, I decided to accept the opportunity to continue with an industrial PhD working for ABB. During five intensive years I have now combined practical research work in an industrial environment with the academic side of research.

This thesis is a result of a daily work that has been carried out with both short term and long term goals and perspectives. But what really makes this thesis possible is that I have gone into a deep analysis of the normal work from time to time. The difference between being an engineer and a researcher became obvious to me during these times of diving deeply into my research field.

Now, I am bringing my thesis to an end and it is a collected analysis of my work. It has been decisive to discuss my research with my supervisors and colleagues.

First of all I want to thank Professor Mikael Östling, for creating a PhD position and for being such inspiring, encouraging and professional supervisor! I also want to express my gratitude to Dr. Heinz Lendenmann, Dr. Christer Ovrén and Ove Albertsson at ABB for all the support. Thanks goes also to colleagues at the EKT and FTE departments.

Special thanks to Dr. Carl-Mikael Zetterling and Dr. Erik Danielsson for reading my thesis manuscript. I would also like to thank my colleagues at ABB for a great working atmosphere.

I am in the happy situation to be surrounded by a caring family. My parents Gudrun and Sven-Gunnar, my brother Mårten and his Johanna, my brother Olof and his Hanna. My grandparents Birgit and Börje Åstrand, and Inga-Lisa and Arne Dahlquist. Thank you for your support in all kind of ways!

Thanks also to my friends Jenny, Lotta and Denny, you have meant a lot to me during this time. You are great friends, always there when I need you.

Finally, to Jens Nordquist in Göteborg, thank you for your unconditional support.

Fanny Dahlquist

May 2002

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1. Introduction

The role of efficient power electronics and power devices become more important in the modern society since we consume more and more electricity. One issue is saving energy to suppress increase in CO

2

gas, another is that the information society puts stronger requirements on reliable and stable electrical energy supply. Power electronics control or modify the flow of electrical energy between sources and their loads in many applications. One example where more efficient power electronics is necessary is for future energy sources such as wind power, solar cells and fuel cells. The transmission of electricity from these sources needs to be more efficient to realize the m into economically competitive alternatives to conventional energy sources.

Silicon carbide is considered as the semiconductor material that will enable the transition of traditional silicon power electronics into smart power. Silicon carbide has material properties that allow devices with higher voltage rating and higher operating temperatures compared to traditional silicon, which translates into smaller and less expensive components. Reduced energy loss, more efficient use of the power grid, increased controllability and better switching properties are all attributes to devices made of silicon carbide.

In power electronic systems, such as high voltage DC transmission (HVDC), control electronics, power supplies and motor drives, switches and rectifiers are key components. This thesis is about high voltage rectifiers in silicon carbide intended to replace the silicon rectifiers utilized today. Unipolar rectifiers in silicon carbide (SiC), Junction Barrier Schottky (JBS) or Schottky diodes, are candidates to replace silicon (Si) bipolar PiN diodes in the 300-3300 V blocking voltage range. The first SiC Schottky diodes for 300 V or 600 V are now commercially available [1-4]. The Junction Barrier Schottky (JBS) rectifier is a device, which combines a PiN diode and a Schottky diode making use of the advantages of both types [5,6].

In this thesis, the JBS diode concept is designed and verified experimentally for 4H and

6H silicon carbide, and compared to Schottky and PiN rectifiers. Chapter 2 gives a

background to silicon carbide and why its material properties give outstanding device

performance for power devices compared to other semiconductor materials. Chapter 3

presents the important parameters for power rectifiers and in Chapter 4 an analytic

model for the total forward drop over a Schottky and JBS diode is discussed and

compared. In Chapter 5 parameters affecting the trade-off between forward voltage and

blocking voltage are identified and summarized. In Chapter 6 the most critical steps in

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2 the processing of JBS diodes are identified and described and the fabrication process is

presented. Electrical characterization and discussion of the results in the appended

papers are found in Chapter 7. Finally, this thesis work is concluded.

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2. Background

2.1 Silicon carbide properties

The high electric breakdown field strength, high thermal conductivity, low intrinsic carrier concentration and the high saturated drift velocity are important properties that give silicon carbide high potential in the field of high-power devices. Power losses are substantially reduced since devices with both higher blocking voltage, lower on- resistance and higher operating temperatures than comparable silicon (Si) devices can be manufactured. It is the wide bandgap energy (

≈

3 eV), which translates into high electric breakdown field strength, about ten times higher than in Si. Rectifiers and switches can then be designed with ten times thinner drift layer, resulting in one to two decades of performance improvement. SiC has for more than 50 years received attention as a material for high power devices but until early 90’s no wafer bulk material of device quality was available. During the last ten years rapid development in material quality has resulted in intensive research in the areas of high-power, high-temperature and high- frequency devices.

SiC consists of equal parts of silicon and carbon atoms and exists in more than 300 crystal structures, called polytypes. 4H SiC (Figure 1) and 6H SiC are the polytypes showing best physical and electrical properties for device fabrication. 6H- material is mainly used for high frequency devices while 4H- material is used for high power devices due to the higher electron mobility.

Figure 1 The 4H SiC crystal structure where each plane contains one carbon atom

layer and one silicon atom layer. (Photograph is taken from a crystal structure model.)

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4 Table 1 Comparison of electrical properties for the traditional power semiconductor materials Si and GaAs and the wide band gap semiconductors SiC, GaN and diamond.

Property

at T=300K Si GaAs 6H-SiC 4H-SiC GaN Diamond

E

g [eV]

1.1 1.4 3.0 3.3 3.4 5.5

E

c

[MV/cm]

0.29

*

0.3 2.5 2.4

**

3.3 20

µn

[cm²/Vs]

1350

*

8500 400 880

**

1000 2200

µp

[cm²/Vs

] 490

*

400 80 120 30 1800

ε

11.8 12.8 10 10 8.9 5.7

λ ^[W/cmK]

1.5 0.5

^3.0-3.8*** 3.0-3.8***

1.3 20

n

i

[cm^-3]

1.5e10 2.0e6 - (low) 5e-8 - (low) - (low)

νsat [10⁷ cm/s]

1 1 2 2 2.5 1.5

* for Nd=1⋅10¹⁴ cm^-3, ** for Nd=6⋅10¹⁵ cm^-3 (≈2500 V), parallel to c-axis, *** at E>2⋅10⁵ V/cm

In Table 1 the electrical properties are compared for the semiconductor materials that are of interest for high power. Si and gallium arsenide (GaAs) are the traditional materials. Gallium nitride (GaN) and diamond are, like SiC, wide bandgap materials also considered as future power semiconductor materials. Diamond, which is the material with the highest inherent potential for high-power devices, is behind SiC in high-quality bulk material development and for example no n-type dopant has yet been found, which makes device development a difficult.

2.2 Device fabrication

State-of-the-art 4H-SiC material still contains a variety of defects affecting the device

properties. The crystal defects that still are present in the substrate material, makes

processing of working devices in SiC a challenge. Table 2 summarizes the most

common reported defects in 4H-SiC and their effect on device characteristics. Due to

the high binding energy SiC also has high chemical stability and extreme mechanical

hardness. This makes process technology more complicated compared to Si although

many Si processes can be used with some modifications.

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Table 2 Most common defects in 4H-SiC, typical density and effects on power device characteristics. (The table is taken from Paper V.)

Defect type Typical density Effect on device

Micropipes 1-30 cm

^-2

reduced blocking, <50 - 70% E

c

Carrots 0.1-10 cm

^-2

E

c

, leakage current, ideality factor Major pits 1-100 cm

^-2

E

c

, leakage current

Screw dislocations 10

³

cm

^-2

reduced blocking, < 80% E

c

Edge dislocations 10

^4-

10

⁵

cm

^-2

not known

Low angle grain boundaries 10

^2-

10

³

cm

^-2

lifetime reduc tion Threading dislocations a few cm

^-2

not known

Stacking faults 10

^0-

10

²

cm

^-2

lifetime reduction

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6 3. Power rectifiers

Switches and rectifiers are key components in power electronic systems, which cover a wide range of applications, from power transmission to control electronics and power supplies. The total power handling ranges from 40W in control electronics, to several MW in power transmission.

Table 3 Example of common applications that utilize power rectifiers.

Application Diode blocking voltage

Diode current Switching frequency High Voltage DC

transmission (HVDC)

5000-25000V 100-3000A 50Hz - few kHz

Traction and

industrial drives 1700-6500V 500-1500A 50 - 1kHz Power supplies and

motor drives 300-1200V 3-100A 2kHz - 250kHz

Control electronics 40-300V 1-10A 50kHz -

several 100kHz

3.1 Power diode concepts

When realizing SiC power devices that will operate in applications with lower power losses compared to Si is it important to design and fabricate devices that really make use of the better electrical properties of SiC. The improved device performance should result in higher blocking voltages for the same total power losses as for Si devices and higher possible operating temperatures (above 125 °C). Then the benefit is fewer components and less cooling equipment. The natural approach is to start with power diode structures well known in Si and GaAs technology and then adapt and modify design and process to the SiC material. For power diodes there are three main device structure concepts:

1) Schottky diode

Unipolar diode that offers extremely high switching speed, but suffers from high

leakage current. A unipolar diode means that the current conduction is governed only by

majority carriers (electrons).

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2) PiN diode

Bipolar diode that offers low leakage current but shows reverse recovery current charge during switching as a consequence of minority (holes) and majority (electrons) carriers both are involved in the current conduction.

3) Junction Barrier Schottky (JBS) diode

Unipolar diode, which combines Schottky- like on-state and switching characteristics with PiN - like blocking characteristics.

3.1.1 The Junction Barrier Schottky (JBS) diode

A pn junction in SiC has a large forward voltage drop (about 3 V) because of the wide bandgap energy. For low and medium voltage applications, 300-4500 V, the forward drop becomes a significant part of the static losses in SiC PiN diodes. On the other hand, using a Schottky diode as rectifier where the forward voltage drop (1-1.5 V) is proportional to the Schottky barrier height may result in excessive reverse leakage current, thus limiting the desired blocking voltage.

The JBS device was first demonstrated in silicon [5,6] and is a Schottky structure with a p

⁺

n junction grid integrated into its drift region. Schematic cross sections of Schottky and PiN diode structures in comparison with a JBS structure are shown in Figure 2. In forward conduction mode the current flows unipolar through the multiple conductive channels under the Schottky contact with a voltage drop determined by the metal- semiconductor Schottky barrier height. In reverse blocking mode the p

⁺

n junctions become reverse biased and the depletion layers spread into the channel and pinch off the Schottky barrier. After pinch-off a potential barrier is formed which limits the electric field at the Schottky contact while the drift region supports further increase in voltage.

The spacing between the p

⁺

regions should be designed so that pinch-off is reached

before the electric field at the Schottky contact increases to the point where excessive

leakage currents occur due to tunneling currents. Lowering of the leakage current

without too much increase in on-resistance can be obtained for the JBS if an optimized

spacing is used in the p

⁺

grid design.

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8 Figure 2 Schematic diode structures of Schottky, PiN and JBS diodes.

In Si, the difference in barrier voltages in a PiN diode and Schottky diode are small thus giving similar forward voltages (about 0.8 V). Hence the reduced leakage current in the JBS diode is not justifying the increase in on-resistance [7]. The JBS structure in Si is mainly used to lower the recovery transient losses. By operating the diode at a forward voltage where the p

⁺

regions are injecting but at the same time having current conduction through the Schottky contact the reverse recovery current is lowered with only a little sacrifice in forward voltage and leakage currents. When the JBS diode is operated in this mode it is usually referred to as the Merged Pinch Schottky (MPS) rectifier.

SiC JBS or Schottky diodes could replace Si diodes with much lower reverse recovery charge during turn-off of the rectifier while still exhibiting low conduction losses.

Cathode Anode P+

Ohmic contact

N+ substrate N- epi N+ substrate

N- epi

Schottky metal

Anode

Cathode

Schottky PiN

Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode

JBS

Ohmic contact

N- epi

Schottky metal

Anode

Cathode

Schottky PiN

Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode

JBS

Ohmic contact

N+ substrate N- epi

P+

Ohmic contact

N- epi

Schottky metal

Anode

Cathode N+ substrate

N- epi

Schottky metal

Schottky PiN

Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode

JBS

Schottky metal/

Ohmic contact

N+ substrate N- epi

P+

JBS

Ohmic contact

N+ substrate N- epi

Ohmic contact

N- epi

Schottky metal

Anode

N- epi

Schottky metal

Anode

Cathode

Schottky PiN

Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode

JBS

Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode

N+ substrate N- epi

Cathode P+

Anode

JBS

Ohmic contact

N+ substrate N- epi

Ohmic contact

N- epi

Schottky metal

Anode

N- epi

Schottky metal

Anode

Cathode

Schottky PiN

Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode

JBS

Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode

N+ substrate N- epi

Cathode P+

Anode

JBS

Ohmic contact

N+ substrate N- epi

P+

Ohmic contact

N- epi

Schottky metal

Anode

N- epi

Schottky metal

Schottky PiN

Schottky metal/

Ohmic contact

N+ substrate N- epi

Cathode P+

Ohmic contact

N+ substrate N- epi

Cathode P+

Anode

N+ substrate N- epi

Cathode P+

Anode

JBS

Schottky metal/

Ohmic contact

N+ substrate N- epi

P+

JBS

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J (A/cm²)

V

_F

≈2.8V V

_F

≈φ

B

≈1V V

_F

≈φ

B

≈1V

R

_on

=f(V

_B

,J,τ) R

_on

=R

_drift

= t

^epi

/(qµ

ⁿ

N

^d

)

R

_on

=R

_drift,JBS

+ R

_grid

SiC PiN SiC Schottky SiC JBS

( ≈ 0.8V for Si)

J (A/cm²)

V

_F

≈2.8V V

_F

≈φ

B

≈1V V

_F

≈φ

B

≈1V

R

_on

=f(V

_B

,J,τ) R

_on

=R

_drift

= t

^epi

/(qµ

ⁿ

N

^d

)

R

_on

=R

_drift,JBS

+ R

_grid

SiC PiN SiC Schottky SiC JBS

( ≈ 0.8V for Si)

3.2 Important parameters for power rectifiers

The most important parameters when quantifying a power rectifier are blocking voltage (V

B

), on-resistance (R

on

), and forward voltage drop (V

F

). How these parameters change with temperature have to be considered. For rectifiers the static on-state losses can be expressed in the forward voltage drop over the diode (V

F

) and the on-resistance (Ron) in the drift region, which accommodates the specified blocking voltage. In Figure 3 these parameters are compared for a SiC PiN, Schottky and JBS diode. The barrier voltage over the diode is lower for a Schottky and JBS diode compared to the PiN diode since it is determined by the metal-semiconduc tor barrier height (

ΦB

) instead of a p

⁺

n junction barrier. On the other hand the on-resistance is lower for the PiN diode since the forward current is conductivity modulated. The PiN diode on-resistance is a function of blocking voltage, current density and carrier lifetime in the base. In the Schottky and JBS diode the conduction is a unipolar electron current giving a linear current dependence with forward voltage drop (see Figure 4).

Figure 3 Contributions to the total on-state losses for a PiN diode, Schottky diode, and JBS diode respectively. The JBS diode has an additional resistive part from the p

⁺

n junction grid compared to the Schottky diode.

For the total on-state losses contact resistances and substrate resistance must also be

accommodated for. Silicon carbide wafers (substrates) are normally 300 µ m thick with

10

¹⁸

cm

^-3

nitrogen (N) doping. Then the substrate resistance is 0.1-0.3 m

Ωcm²

resulting

in voltage drops of 10-30 mV at 100 A/cm

²

. Reproducible contact resistances to n-type

SiC using nickel as contact metal are in the 10

^-5Ωcm²

to 10

^-4Ωcm²

range, which results

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10 in voltage drops of around 10 mV at 100 A/cm

²

[8]. Then a good estimation is that the substrate plus contact resistance contribution to the total forward voltage drop is maximum 100 mV.

In Figure 4 the typical current- voltage characteristics is shown for a Schottky, PiN and JBS diode. In comparison with a SiC PiN diode, the Schottky or JBS diode are attractive only as long as the unipolar on-resistance gives a lower voltage drop than that of the PiN diode. The “cross-over” point depends on blocking voltage, but also on operating current density and operating temperature. This is under the assumption that the transient losses are the same for the Schottky, JBS and PiN diode.

Figure 4 (Left) Schematic comparison of forward characteristics for a PiN, Schottky and JBS rectifier in SiC. (Right) Si PiN diode with strong dependence on forward voltage with switching frequency.

3.2.1 Unipolar drift region resistance

The unipolar drift region resistance (diode on-resistance if contact and substrate resistances are neglected) is determined by the epi layer thickness (t

epi

), doping concentration (N

d

) and electron mobility (µ

n

) according to Equation 1 [9]:

d n ,

µ N q

R^on^sp= t^epi

[

Ω

cm

²

] (1)

where q is the electron charge. In an ideal no n punch-through structure the depletion width W is equal to the epi layer thickness at voltage breakdown (see Figure 5). Then the reverse bias voltage V

B

is given by:

JF (A/cm²)

Ron,Sch=R_drift

≈Φ^B

Ron,JBS=R_drift,JBS+ R_grid

VF (V)

JBS JBS

Schottky Schottky

R_on,PiN= f(V_B,J,τ)

≈2.8V

PiN PiN

JF (A/cm²)

≈

0.8V

^{VF (V)}

Increasing frequency

Si PiN Si PiN

JF (A/cm²)

≈

0.8V

^{VF (V)}

Increasing frequency

Si PiN

(21)

n- n+

Schottky metal

Ec

Electric field

tepi= W

Distance

s B d

W V qN

ε 2

= 2

[V] (2)

Figure 5 Electric field distribution for a metal-semiconductor junction where the depletion width is equal to epi thickness at breakdown voltage. (Equivalent for PiN and JBS diodes where the Schottky metal is replaced by a p

⁺

layer.)

The depletion width W at breakdown voltage can be expressed in terms of the critical electric field at the junction and the doping:

Nd

q

W = ε^sE^c

[cm] (3)

or

Ec

2V_B

W =

[cm] (4)

The critical electric field E

c

has a doping dependence according to equation 5, which is experimentally determined by Konstantinov et al. [10]:



 



− 

= ⋅

10 16 6

log 10 4 1 1

10 49 . 2

d

c N

E

[V/cm] (5)

Combining (3) and (4) gives the maximum blocking voltage for a given drift region

doping:

(22)

12

d c s

B qN

V E 2 ε 2

=

[V] (6)

The drift region resistance in Equation 1 can now be rewritten by Equations 3 and 6 to an expression in terms of the designed blocking voltage and critical electric field, usually called the specific on-resistance R

on,sp

(when t

epi

=W):

3 c n s

2

,

E

4 µ ε

B sp

on

V

R

=

[Ωcm

²

] (7)

Equation 7 is often used as a figure of merit for unipolar power devices since it gives the differential on-resistance for a designed blocking voltage. The on-resistance increases quadratically with blocking voltage and is the reason why unipolar devices have non-attractive on-state losses for higher voltages compared to bipolar devices. The electron mobility µ

n

has a doping dependence that also has to be taken into account when calculating the on-resistance [11]:

61 . 0

1017

94 . 1 1

947



 



 + ⋅

=

d

n N

µ

[cm

²

/Vs] (8)

In Figure 6 Equation 7 is plotted for Si and 4H-SiC showing the usefulness of unipolar

devices in SiC compared to Si. For Si a critical electric field of 0.25 MV/cm is used and

for calculating on-resistance the doping and electron mobility are assumed to be

constants. For SiC a critical electric field of 2.0 MV/cm is used (dashed line) which

corresponds to a constant doping of 1e15 cm

^-3

. The solid line is the optimized (lowest)

on-resistance where the doping dependence in the critical electric field (equation 5) is

taken into account. Equation 5 together with Equation 6 are used to maximize the

doping concentration for each blocking voltage.

(23)

Figure 6 Comparison of specific on-resistance as function of blocking voltage for Si and 4H-SiC. For Si a critical electric field of 0.25 MV/cm is used (ideal value, above current state-of-the-art for high voltage Si power components). For SiC a critical field of 2.0 MV/cm is used (dashed line). The optimized on-resistance (solid line) is when the maximum doping is used for each blocking voltage. T= 30 °C.

Temperature dependence

In unipolar devices the on-resistance increases with temperature due to a decrease in mobility with increasing temperature,

µn∼

T

^-2.15

[11], see Equation 9. In comparison with bipolar PiN diodes this dependence is a disadvantage with respect to losses since PiN diodes show a negative temperature coefficient. But for paralleling of devices a positive temperature coefficient is advantageous for obtaining uniform current distribution.

However, in a trade-off comparison between unipolar (Schottky or JBS diodes) and PiN

diodes is it important to take the different temperature dependencies into account, i.e.,

make the power loss comparison at the temperature at which the diodes will be operated

in the application.

(24)

14

15 . 2 61

. 0 17

300 10

94 . 1 1

947

⁻



 



⋅



 



 + ⋅

=

T

N

_d

µ

n

[cm

²

/Vs] (9)

Influence of electric field

The maximum electric field reached at the junction is also an important parameter affecting on-state losses, R

on,sp

∼1/E

c3

, as was shown in Equation 7. It has been reported that it is realistic to reach 80% of the critical electric field strength because of imperfections in the epi material, doping uniformity etc, [12]. However, if the device could be designed for 100% critical field, the on-resistance (drift region resistance) would be lowered to 51% of the 80%-field resistance according to Equation 10:

51 . ) 0 8 . 0 (

3 3

% 80 ,

% 100

, = =

c c on

on

E E R

R

(10)

From Figure 7 the gain by reaching the theoretical critical electric field is clear. The on- resistance at 125

°

C and theoretical field is almost the same as for 30

°

C and 80%-field.

In order to reach the corresponding breakdown voltage to the theoretical field strength, a

proper junctio n termination such as floating field rings, JTE etc [9] is also needed to

extend the surface field over a sufficiently wide distance.

(25)

Figure 7 Specific on-resistance versus blocking voltage for T=30 °C and T=125 °C for

80% and 100% reached critical electric fields E

c

. The 100%Ec resistance at 125 °C is

close to the 80%Ec resistance at 30 °C.

(26)

16 4. Forward and reverse characteristics of Schottky and JBS diodes

In this chapter an analytic model for the total forward drop over a Schottky and JBS diode is discussed and compared. The reverse leakage current mechanisms are also discussed and the most important parameters affecting the trade-off between forward voltage and blocking voltages are identified and summarized.

4.1 Forward conduction characteristics

4.1.1 Forward voltage drop in a Schottky diode

Forward conduction characteristics in experimental SiC Schottky diodes (n-type) [13, 14] have agreed well with the thermionic emission theory, which is also the dominating current transport mechanism in Si Schottky diodes [15]. The forward voltage drop is a function of temperature, Schottky barrier height and drift region resistance. Then the forward voltage drop V

F

at a defined current density J

F

can be written as [15]:

F on

F

R J

T A

J q

V

^F

kT

+ +



 



=

η



ηφ

^Β

*

2

ln * , (11)

where k is Boltzmann’s constant, q is the electron charge, T is the temperature, η is the ideality factor,

φB

is the Schottky barrier height and J

_F

is the forward current density at V

_F

. A** is the Richardson’s constant, theoretically calculated to be 146 A/cm

²

K

²

for SiC [13]. R

_on

is the drift region resistance and already presented in Equation 1.

4.1.2 Forward voltage drop in a JBS diode

During forward conduction in the JBS diode the current flows unipolar between the anode and cathode in channels between the p

⁺

n junctions. Consequently, in normal operation (100 A/cm

²

) the Schottky current dominates and the forward current analysis can be based on thermionic emission theory for Schottky junctions.

For a JBS diode the relationship between the forward voltage drop and current density is

equal to that of a Schottky diode (Equation 11), except that the expression has to be

(27)

modified to allow for the area taken up by the p

⁺

regions in the structure, see Figure 8.

The current density across the Schottky barrier J

FS

will be modified to [9]:

F

FS

J

A J A

Schottky total

=

(12)

where J

F

is the total current density over the metal contact. For a striped (linear) p+ grid design Equation 12 can be written in terms of the grid spacing s and grid width w:

F

FS J

d s

w J s

−2

= +

(13)

The area relation between total contact area and Schottky area is:

Schottky

total A

d s

w A s

−2

= +

(14)

where w is the width of the p+ regions and s is the spacing in between, i.e., the Schottky area region. d is the junction depletion width from the p+ regions, according to Equation 15, due to the built- in voltage V

bi

and has to be considered for forward voltages up to ≈2.8V (V

bi

at RT). The voltage drop V

ch

in Equation 15 is the potential at the bottom of the channel (p

⁺

grid junction depth). If the Schottky barrier adds 1V to V

ch

and the grid resistance adds 0.05V (R

grid

is typically 0.5 m

Ωcm²

), then V

ch

equals 1.05 V. A typical doping N

d

=3e15 cm

^-3

then gives a depletion width of d=0.8 µ m.

) N (

2

d

ch bi

s V V

d = qε −

(15)

(28)

18 w s

n- Anode

Cathode

p+

Schottky metal

R

^drift

,

^JBS

R

grid

Φ

^B

Current spreading due to p⁺grid

n+

w s

^p+

d d

w s

n- Anode

Cathode

p+

Schottky metal

R

^drift

,

^JBS

R

grid

Φ

^B

n+

w s

n- Anode

Cathode

p+

Schottky metal

R

^drift

,

^JBS

R

grid

Φ

^B

n+

w s

^p+

d d

w s

^p+

d d

Figure 8 (Left) Part of JBS grid showing the main contributions to the total forward voltage drop over the diode. (Right) Upper part of JBS grid showing depletion regions and current spreading due to the p+ regions.

If a 45 degree current spreading is assumed below the channels the JBS drift resistance R

drift,JBS

can be written as in Equation 16 (with homogenous current conduction assumed). The electron mobility parallel to c-axis is 20% higher than the mobility perpendicular to c-axis [11]. A 45° current spreading, by assuming isotropic mobility is, however, considered a sufficiently good estimation for the analytical calculations.

d n j epi JBS drift

N q

w x R t

µ

) 2 / (

, = − −

(16)

where t

_epi

is the total epi thickness and x

_j

is the p+ grid depth. Resistive contribution R

_grid

from the channels and current spreading is given by Equation 17:



 





−

 +



 



 +

 +

 

= +

d s

w s d s

w s N q

w R xj

D n

grid

ln 2

2 2

/

µ (17)

The total JBS on-resistance is the sum of R

grid

and R

drift

,

JBS

:

JBS drift grid JBS

on

R R

R

^, = + ^,

(18)

Now the forward voltage drop of a JBS diode can be written by modifying Equation 11

with Equations 13 and 18:

(29)

F JBS on

F

n R J

T A

J d s

w s q

V

^F^JBS

kT

,

*

2

* ) 2 (

) ln (

, + +



−

=

η

+

φ

^Β

(19)

Equation 19 can be used to calculate the forward voltage drop for a JBS diode at a defined current density.

4.2 Reverse blocking characteristics

4.2.1 Leakage current mechanisms in a Schottky diode

The basic reverse current leakage mechanism in Schottky rectifiers is thermionic emission, which depends on the Schottky barrier height, temperature and applied bias.

Thermionic emission means that electrons are thermally excited over the Schottky barrier. The relationship between the thermionic emission reverse leakage current density and Schottky barrier height is [15]:

( ¹ )

*

² ⁽ ^/ ⁾ ⁽ ^/ ⁾ −

= ⁻ ^B ^kT ^qV ^nkT

R

A T e e

J

^φ

(20)

The thermionic reverse leakage current is also affected by image- force barrier height lowering, which means that the effective Schottky barrier height is decreased by an amount that depends on the electric field:

s B

qE φ πε

=

4

∆

(21)

where ∆φ

B

is the image- force barrier height lowering and E is the electric field at the metal-semiconductor interface. To account for the barrier lowering in the leakage current, Equation 20 can for large negative voltages V be rewritten to:

) / ( ) / (

* 2

* ^B ^kT ^B ^kT

R A T e e

J = ⁻^φ ^∆^φ

(22)

The strong dependence of leakage current on barrier height, temperature and electric

field is the reason why Si Schottky diodes are not practically used above 150 V. In

Table 4 typical Schottky barrier heights to n-type Si, GaAs and SiC are shown. In

(30)

20 silicon relatively low barrier heights are formed; consequently there is a substantial increase in leakage current with increasing temperature.

Table 4 Comparison of energy bandgap, critical electric field (relevant values) and typical n-type Schottky barrier heights in different semiconductors [9,14-16].

A second leakage mechanism that also has to be taken into account is caused by generation in the depletion region. Corresponding leakage current J

G

can be written:

r i

G qnW

J = 2τ

(23)

where n

i

is the intrinsic carrier concentration, W is the depletion width and τ

r

is the carrier lifetime within the depletion region.

4.2.2 Leakage current in a SiC Schottky diode

The Schottky barrier height to SiC is usually about two times higher (Table 4) than for Si and the leakage currents are relatively low also at elevated temperatures. However, the leakage current has been found experimentally to be larger than predicted by thermionic emission theory. The increase is also larger with increasing field than what can be explained by image- force barrier lowering. It has been shown that the larger electric fields used in SiC substantially increase thermionic field emission and field emission [17], which are negligible leakage mechanisms in silicon. Both thermionic field emission and field emission are tunneling mechanisms that depend on barrier height, and thermionic field emission also on temperature. The consequence is high leakage currents at electric field values lower than the theoretical electric breakdown field strength, especially for higher temperatures. The reported dependence between tunneling current density J

_tunnel

and electric field and Schottky barrier height is expressed according to Equation 24 [17]:

E

g

(eV) E

^c

(MV/cm) φ

_B

(eV)

1.12 0.25 0.5-0.7

1.43 0.3 0.8

Si GaAs 4H SiC 6H SiC at 300 K

3.26 2.2 0.8-1.7

3.03

2.4 0.6-1.5

(31)

3 )

* 2 8

2 ( ^m ³^/²/ ^hqE

tunnel E e

J ∝ ⁻^π ^φ

(24)

where m is the electron effective mass, and h is Planck’s constant. Since the electric* field E at the Schottky contact increases with the square of the applied voltage V according to Equation 25 (from Equation 6) the tunneling leakage current is directly proportional to the applied reverse voltage.

s d

V E qN

ε

=

2 (25)

Leakage current caused by generation in the depletion region, see Equation 23, is low in SiC since the intrinsic carrier concentration is very small (n

i

=5.27e-8 cm

^-3

at RT in SiC compared to n

i

=1.5e10 cm

^-3

in Si at RT).

In conclusion, SiC Schottky diodes show low thermionic leakage currents because of higher Schottky barrier heights while the high electric fields enhance the tunneling leakage current, which limits the blocking voltage. Schottky diodes with blocking capability up to 4.9 kV have been reported [18,19]. However, a high Schottky barrier is used (

∼1.5 eV) and the drift region doping is extremely low (<10¹⁴

cm

^-3

) which allow the electric field to be designed to only about 60% of the theoretical value.

Consequently, the high blocking voltage is demonstrated but the forward voltage drop is much too high for normal device application (>6V at 100A/cm

²

). Since the reverse characteristics depends strongly on temperature the maximum blocking voltage is also defined by the operating temperature. Schottky diodes with reasonable forward characteristics and blocking voltages up to 2000 V at elevated temperatures have been reported [20] and also shown in this thesis in Paper III.

4.2.3 Leakage current in a SiC JBS diode

The important feature of the JBS diode is that the depletion regions from the p

⁺

n junctions pinch off the channel and the electric field is reduced at the metal-SiC junction. The electric field E at the Schottky contact depends on the channel pinch-off voltage V

_p

and the doping N

_d

:

) 2 (

bi s p

d

V V

E

=

qN

+

ε (26)

(32)

22 where V

bi

is the junction built- in voltage. The pinch-off voltage is determined by the Schottky spacing s between the p

⁺

regions since that gives the voltage at which channel pinch-off occurs:

bi s

P qNd s V

V = ²−

8ε

(27)

In Figure 9 the pinch-off voltage V

P

is plotted versus Schottky spacing s (channel width).

Figure 9 Calculated pinch-off voltage showing the quadratic dependence on Schottky spacing s.

How much the electric field is reduced depends not only on the Schottky spacing but

also on the doping concentration in the channel, the doping profile shape and depth of

the p

⁺

regions.

(33)

4.3 Summary of leakage current mechanisms

The reverse leakage current mechanisms that should be considered in a JBS or Scho ttky diode are summarized below.

Thermionic emission leakage current density

From Equation 20 (as the exponential term in brackets becomes negligible for high reverse voltages):

Js e

T A T f

J

^R =

( φ

_B

, )

=

* *

² ⁽⁻^φ^B^/^kT⁾ =

(28)

For Schottky barrier heights higher than 1.0 eV and temperatures below 125 °C this contribution gives negligible current densities.

Schottky barrier lowering

From Equation 21:

kT R Js e B

J = ⋅ ⁻^∆^φ ^/

(29)

In Figure 10 the leakage current contributions from Equations 28 and 29 are plotted

versus typical Schottky barriers in SiC.

(34)

24 Figure 10 Calculated Schottky leakage current density from thermionic emission theory (solid lines) and with Schottky barrier lowering (dotted lines) at electric fields of 0.5 MV/cm and 2.0 MV/cm.

Generation in depletion region

Neglected in SiC due to the low intrinsic carrier concentration as mentioned in 4.2.2.

Tunneling leakage current density

From Equation 24:

3 )

* 2 8

2 (

/

) ,

(

_B ^m ³^/² ^hqE

tunnel

e

B

E E

f

J

=

φ

∝ ⁻ ^π ^φ

(30)

The tunneling leakage current is a strong function of the electric field at the Schottky contact and consequently also of the doping concentration in the drift layer (Equation 5).

By using a JBS structure both the Schottky barrier lowering and tunneling current contributions to the leakage current are decreased due to the electric field reduction at the Schottky contact. As seen in Figure 10 the leakage current from the Schottky barrier lowering is not severe, the leakage currents are still very low even at 125 °C, at least for barrier heights higher than 1 eV. Thus the electric field dependent tunneling current is the leakage current mechanism that should be suppressed by use of the JBS grid.

0,8 0,9 1 1,1 1,2 1,3 1,4

Schottky barrier height (eV)

1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e-10 1e-09 1e-08 1e-07 1e-06 1e-05 1e-04 1e-03 1e-02 1e-01 1e+00

with barrier lowering at 2.0 MV/cm with barrier lowering at 0.5 MV/cm Jr from Equation 28

Leakage current density (A/cm2)

30 °C 125 °C

0,8 0,9 1 1,1 1,2 1,3 1,4

Schottky barrier height (eV)

1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e-10 1e-09 1e-08 1e-07 1e-06 1e-05 1e-04 1e-03 1e-02 1e-01 1e+00

with barrier lowering at 2.0 MV/cm with barrier lowering at 0.5 MV/cm Jr from Equation 28

Leakage current density (A/cm2)

30 °C 125 °C

(35)

4.4 Other variants on JBS structures

In order to improve the forward voltage versus leakage current trade-off other variants on the same theme as the JBS structure have been suggested and experimentally verified in the literature.

4.4.1 Dual Metal Trench (DMT) diode

In the dual metal trench (DMT) diode (Figure 11) the p

⁺

n junctio ns in the JBS structure are replaced by trenches. At the bottom of each trench a relatively high Schottky barrier metal are formed. At the top of the mesa a lower barrier Schottky metal is deposited for current conduction during forward bias. The DMT forward characteristics is then dominated by the lower barrier regions and reverse characteristics is dominated by the higher barrier regions giving lower leakage currents than for a Schottky diode with only the lower barrier. Reported is a combination of titanium (0.84 eV) and nickel (1.51 eV) [21,22]. This diode concept is a pure Schottky barrier structure and therefore it will be sensitive to high electric fields (giving tunneling currents) as described in section 4.2.2.

The fabrication of this diode is often claimed to be simple since no p-type ion implantation is required. On the other hand, a stable process for forming a uniform and reproducible Schottky contact with low leakage currents on dry etched surfaces has to be developed on both vertical and lateral trench walls.