Effect of Ge Concentration on Static and Microwave Performances in Ge x Si 1-x Heterojunction Bipolar Transistors under High-Level Injection

We report the theoretical investigation of the effect of Ge concentration on both static and high-frequency performances in polycrystalline silicon emitter Ge x Si 1- x heterojunction bipolar transistors (HBTs). The calculation is carried out up to 0.95 V, before the onset of the Kirk effect [C. T. Kirk: IEEE Trans. Electron Devices ED-9 (1962) 164]. From this model, we find that the current gain, transconductance, cutoff frequency, and maximum oscillation frequency increase with increasing Ge concentration. In addition, the base transit time and the switching delay time in the emitter-coupled logic circuit slightly decrease as Ge content increases. Furthermore, it shows that the high-level injection effect becomes more pronounced in the Ge x Si 1- x HBT as Ge concentration increases. Compared with the conventional low-level injection model, the improved model provides much more accurate expressions for electrical characteristics of Ge x Si 1- x HBTs operating under high-level injection.