Principal Research Results

Investigation of Forward Voltage Degradation of Silicon Carbide (SiC)
pn Diode and Fabrication of High Voltage SiC pn DiodeBackground
Since silicon carbide (SiC) power devices have a potential of high-voltage and low-loss compared to silicon power devices,
their practical use is desired for more low-loss and miniaturized power-electronics apparatuses. On SiC pn diodes applied basic structure of semiconductor devices, degradation of a forward voltage (an on-state loss) is caused by sending a current. This issue is themost serious problem for practical use of SiC pn diodes. And high voltage devices require thick epitaxial layer, it is very important toinvestigate forward voltage degradation of thick layers. And we also need to investigate the degradation suppression technique. (Thiswork is collaborated with The Kansai Electric Power Co., Inc..)

Objectives
To investigate the forward voltage degradation on SiC pn structure and fabricate high voltage SiC pn diode.

Principal Results
We fabricated SiC pn diode using SiC substrate with SiC layer made by our developed vertical hot-wall CVD reactor. Andwe investigated the degradation characteristics on n-type layer thickness, crystal polarities and off directions. And finally, we tried tofabricate high voltage SiC pn diode using those results (Figure 1).
1： Investigation of degradation characteristic for thick layer
It became clear quantitatively that thicker n-type layer lets to more increase degradation (Figure 2). This result shows thatthe degradation is serious problem to high voltage SiC pn diodes, which need thick layer.
2： Investigation of degradation suppression technique
We investigated on the degradation of crystal polarity ＊ 1 and off direction ＊ 2 SiC pn diode used C-face <11-20> toward offsubstrate is the least degradation of other substrates (Figure 3).
3：Fabrication of high voltage SiC pn diodeWe succeeded in fabricating 4.55kV SiC pn diode using thick and low-doped n-type layer (~60 μm, ~6 × 1013cm-3) on<11-20> toward off C-face substrate with JTE (Junction Termination Expansion)＊ 3, which expands localized electric field on pnjunction termination (Figure 4). This result is the highest performance in C-face SiC devices.

Future DevelopmentsOn the basis of these results, we’re going to develop low degradation, high reliability, high voltage (over 5kV) and low loss
SiC pn diode.
Main Researcher: Toshiyuki Miyanagi,
Research Scientist, Material Functions and Mechanism Creation Sector, Material Science Research Laboratory
Reference
T. Miyanagi et al, 2003, “Development of process technologies for high voltage SiC pn diode fabrication-Investigation of reduced technique for forward voltage degradation and fabrication of high voltage SiC pn diode- “, CRIEPI REPORT
W03033 (in Japanese)＊１：SiC has crystal polarities called Si-face and C-face, which show different chemical characteristics. In general, Si-face is using to SiC device
fabrication, because thick and low-doping layer to need high voltage devices is difficult to make on C-face substrate.
＊２：SiC substrate with tilt off-cut toward specific direction is used to make same crystal structure as substrate. In general, <11-20> toward off
SiC substrate is used to fabricate of SiC devices.
＊３：A p-type layer like a doughnut was fabricated into n-type layer surface using ion implantation technique. By this technique, the blockingvoltage become near theoretical value, because the electric field expands widely.

20Ti/Al anode
SiO2JTE

p-typelayer

SiO2n-type layer

JTE

forward voltage degradation(V)

B. Creation of integrated energy service

100
Si-face，<11-20> toward off

101
0.1

[current stress test]
current density : 100A/cm2
test time: 60min0.01

n-type substrate

0.001
0Ni cathodeFig.1 SiC pn diode

50
100
150
n-type layer thickness (μm)

200

Fig.2 degradation characteristics of ntype layer thickness (size 2.6mmφ)

A n-type layer thickness defines forward
voltage and blocking voltage.

(0001) Si-face<1-100> direction<11-20> direction

SiC crystal polarityand off direction

100100leak current (mA/cm2)

forward voltage degradation (V)(000-1) C-face101

type A
type B

0.1

0.01

C-face，<11-20> toward off

80
60
4020

BV=4.55kV
0Si-face
C-faceSi-faceC-face11.25<11-20><11-20>
toward off 2.5 <1-100><1-100>3.75
toward off0.0

1.0

2.0

3.0

4.0

5.0

Blocking voltage (kV)Fig.3 degradation characteristics of polarityand off directions (thickness 40μm, size
2.6mmφ, type A and type B are madeFig.4 blocking voltage of C-face SiC pn
diode (thickness 60μm, size 0.26mmφ)by different products.)

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