Factors limiting the current gain in high-voltage 4H-SiC npn-BJTs

Abstract

The dependence of the base current gain β on the collector current I C has been measured in high-voltage 4H-SiC bipolar junction transistors at collector current densities j C from 20 to 700 A/cm 2. With increasing collector current, the β value grows, reaching a maximum β max=23 at j C max of about 250 A/cm 2 and then decreasing sharply with further increase in current. The peculiarities of this dependence are analyzed in terms of a model taking into account the carrier recombination in the emitter space charge region (SCR), the surface recombination, and the emitter current crowding effect. It is shown that at relatively small j C the recombination in SCR plays a key role in limiting the β magnitudes. The decrease in β at high j C is mainly due to the surface recombination enhanced by the emitter current crowding effect. The effects of surface recombination and recombination in the SCR reduce the maximum current gain almost threefold: from 65 to 23.