Analytical modelling of current gain and frequency characteristics under high injection levels in Si/SiGe heterojunction bipolar transistors at 77 and 300 K

This paper reports an analytical modelling of current gain and frequency ch aracteristics in Si/SiGe heterojunction bipolar transistors (HBTs) at 77 an d 300 K. Important transistor parameters, such as current gain, transconduc tance, cutoff frequency and maximum oscillation frequency are calculated as a function of Ge concentration in the base under different injection level s. The main physical mechanisms for the current and cutoff frequency rollof f at high injection levels are also analyzed. It shows that the high-level injection effect is more pronounced in the SiGe HBTs as a result of the inc reasing minority carrier concentration in the base and the Ge concentration and distribution will have a decisive influence of device performance. The results may provide a basis for the design of low temperature operation Si Ge HBTs. (C) 1999 Elsevier Science Ltd. All rights reserved.