Improved current gain at high collector current densities for SiGe heterostructure transistors

Abstract

A two-dimensional simulation study is presented to analyze the current gain performance of an NPNSi/SiGe/SiGe graded heterojunction bipolar transistor (SiGe GHBT) having a uniform 20 atomic per cent (at%) Ge profile in its base and a linear tapering of Ge at% in collector with zero at% Ge at collector ohmic contact. A contemporary NPN Si/SiGe/Si double heterojunction bipolar transistor (SiGe DHBT) having a uniform 20 at% Ge profile in the base region and zero Ge at% in collector is also simulated for comparison. The analysis predicts that a valence band offset for holes at base-collector junction gives rise to the formation of dynamic retarding potential barrier for minority carrier electrons at the base-collector heterojunction of the DHBT structure, which increases with the collector current density. However, the formation of such a potential barrier is smoothed out in an SiGe GHBT structure with base-collector homojunction. The GHBT structure shows an improved current gain and a better current-gain fall-off at high collector current density in comparison with the conventional SiGe DHBT, thus providing the option of operation at higher current densities.