Turn-on Process in High Voltage 4H-SiC Thyristors
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Physico-TechnicalInstitute., 194021 St.Petersburg, Russia
**Cree Research Inc., 4600 SiliconDrive, Durham, NC 27703, USA
ABSTRACT The turn-on process and the steady state current-voltage characteristics have been investigated in 4H-SiC thyristors with a forward breakover voltage close to 700 V and pulse switch current density of 16 000 A/cm 2. The rise current constant, Zr, has been found to decrease monotonically with temperature: r, n 10.5 nsec at T = 300 K and -r, 1.2 nsec at T =
500 K at high bias U0 > 300 V. The value of r, s, 1.2 nsec is the lowest observed value for SiC thyristors. At very high current density, j > 5 10 3 A/cm 2, the residual voltage drop for 4H-SiC thyristors is lower than for identically rated Si thyristors. The voltage drop Udo decreases monotonically with temperature. INTRODUCTION SiC thyristors are expected to show great performance advantage as compared to Si and GaAs thyristors. The reason for that is a higher breakdown electric field in SiC than in Si and GaAs. This allows the design of SiC thyristors with thinner and higher doped base layers than in identically rated Si and GaAs thyristors. Such devices are expected to show fast switching and low residual drop at high current densities. A large bandgap of SiC is also expected to result in much higher operating temperature than in Si and GaAs thyristors.
The first 6H- and 4H-SiC thyristors have been produced [1, 2] at the beginning of the 90ies. However, until very recently, nobody has succeeded in realizing the advantages of SiC thyristors. The most promising SiC thyristor structure to date has been an n-p-n-p 4H-SiC device, the design of which allows the use of low-resistivity n-type substrate [3]. For such devices with a forward breakover voltage close to 400 V, the values of r,. P 1.9 nsec at T = 500 K and Udo = 10 V atj = 25 000 A/cm2 have been recently demonstrated in Ref.[4]. In this work the turn-on process and the forward current-voltage characteristics at high current density have been investigated in high-voltage (700 V) 4H-SiC n-p-n-p thyristors. The results obtained have been compared with the data for 4H-SiC thyristors with a lower forward breakover voltage (400 V). We have found that the turn-on characteristics and current-voltage characteristics for these two types of 4H-SiC thyristors are qualitatively different. EXPERIMENTAL DETAILS The n-p-n-p 4H-SiC thyristors with a thickness of the p-voltage blocking layer (p-base) of 11 pm were able to block 700 V at both forward and reverse bias. The design of the thyristors is basically the same as reported previously [7]. The n-base had a thickness W,= 0.65 Am. The operating area of the devices, S, was about 6.10-3 cm2. The reverse bias and forward bias I-V 211
Mat. Res. Soc. Symp. Proc. Vol. 512 © 1998 Materials Research Society
characteristics at low current density for these devices were described inRef[5]. The n-p-n-p 4H-SiC thyristors with a thickness of the p-voltage blocking layer Wp = 4.5 gm had a relatively low forward breakover voltage Ub - 400 V. The design
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