<title>Liquid-helium temperature operation of silicon-germanium heterojunction bipolar transistors</title>

Abstract

We present the first measurements of silicon-generated (SiGe) heterojunction bipolar transistors (HBT) in the liquid-helium temperature regime. We have measured the dc characteristics of SiGe HBTs from two different profile designs over the temperature range of 300 K - 4 K. The first SiGe HBT design was optimized for high-speed digital applications at room temperature (`i-p-i' SiGe HBT), and the second SiGe HBT design was specifically optimized for cryogenic operation (`emitter-cap' SiGe HBT). A silicon bipolar junction transfer (Si BJT) which has a doping profile similar to the i-p-i SiGe HBT is used as a control. The devices continue to exhibit transistor action down to the liquid-helium temperature regime, even in the presence of strong carrier freeze-out in the neutral base and collector regimes. In contrast to the Si BJT, the peak current gain of the optimized emitter-cap SiGe HBT rises monotonically from 100 to 300 K to nearly 2000 at 16 K, although parasitic base current leakage limits the useful operating range to collector currents above about 1.0 (mu) A. At very low-injection levels (less than 1.0 nA) below 77 K, we identify a non- diffusive transport mechanism in the collector current of both SiGe HBTs and the Si BJT which is unaccounted for in conventional device theory. Initial calculations suggest that this phenomenon has a bias and temperature dependence characteristic of a carrier tunneling process.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.