The Temperature Dependent Breakdown Voltage For 4H- and 6H-SiC Diodes
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The Temperature Dependent Breakdown Voltage For 4H- and 6H-SiC Diodes
Y. S. Lee, M. K. Han and Y. I. Choi* School of Electrical Eng., Seoul Nat’l Univ., Shinlim-Dong Kwanak-Ku, Seoul 151-742, Korea Tel : +82-2-880-7254, Fax : +82-2-873-9953, E-mail : [email protected] *Dep. of Molecular Science and Technology, Ajou Univ., Wonchun-Dong, Suwon 442-749, Korea, E-mail : [email protected]
ABSTRACT The breakdown voltages of 6H- and 4H-SiC rectifiers as function of temperature were modeled analytically in both non-reachthrough diode and reachthrough diode. The breakdown voltage was derived by the ionization integral employing accurate hole impact ionization coefficient. The breakdown voltage of SiC rectifiers was increased with increasing temperature and the positive temperature coefficient of breakdown voltage indicates that SiC rectifiers are suitable for high temperature applications. The breakdown voltages of both 6H- and 4H-SiC diodes were increased by M(T)-1/4 in NRDs and M(T)-1/8 in RDs.
THE BREAKDOWN VOLTAGE SiC rectifiers have attracted a considerable attention for high temperature applications due to its good thermal conductivity and wide bad-gap properties. However, the breakdown voltage of SiC rectifiers has not been modeled accurately. That over breakdown voltage as function of temperature has been scarcely reported. The breakdown voltage of the power semiconductor device is one of the most important design parameters. When SiC rectifier is reverse biased, multiplication factor becomes large due to the generation of electron-hole pairs (EHP) in depletion region and it approaches to the infinity () at the critical electric field, so that avalanche breakdown at the junction occurs. It has been reported that the breakdown voltage of SiC rectifiers is about one order larger than that of Si rectifiers due to the high electric field strength1. At the critical field of the avalanche breakdown, the impact ionization integral becomes one.
∫
W
0
α p exp[ ∫ (α n −α p )dx ]dx = 1 x
0
where α i = Ai exp(− bi / E ( x) )
mi
i = p, n (1)
where W is the drift length and αp ,αn are hole and electron impact ionization coefficient respectively. It should be noted that the effective impact ionization integral cannot be applied for 4H- and 6HSiC rectifiers because the hole impact ionization coefficient is much higher than the electron impact ionization coefficient in the broad region of electric field3. Therefore, αn is set as δ αp in which δ is considerably small constant such as 0.01. The accurate hole impact ionization coefficient with increasing temperature was extracted experimentally in the form of αi in (1) in the previously reported literature2 Due to the exponential term including electric field, it is rather difficult to perform the impact ionization integral. So that it is necessary to separate the temperature term and electric field term. In here we employed reformed αp and seventh order approximation of αi as
T6.4.1
α p ( x, T ) = M (T ) ⋅ E ( x )
7
( 2)
Temperature dependence of αp is includ
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