Effect of substarte parasitics on heterojunction bipolar transistors

Abstract

Though the semiconductor silicon(Si) has been used to manufacture the integrated circuits, it I for the manufacturing of high speed and high frequency circuits. In order to improve the performance of Si transistors enough to be competitive with III-V devices for RF and microwave application, while preserving the yield cost and manufacturing advantages associated with conventional Si fabrication, silicon-germanium(Si-Ge) hetero junction bipolar transistor has been introduced. SiGe HBTs can provide faster switching speeds than Si bipolar transistors mainly because of reduced base resistance and collector-to-substrate capacitance.
Analyzing the small signal model of SiGe HBT, we computed its s parameters and plotted it in smith chart to a frequency variation of 1-18GHz. Again we analyzed the circuit while taking the external parasitic effect into account and drew the S-parameters in a smith chart. So, in this frequency range we compared the performance of the device with and without taking the external substrate effect into account.
We drew the graph for maximum available power gain of common emitter SiGe HBT , with and without taking the substrate parasitic effect. We compared the graph in both the cases and verified that the maximum available power gain decreases considerably due to the external substrate effect. We also verified the substrate effect in case of common base SiGe HBT and saw that, in this case also the maximum available power gain decreases due to the external substrate effect.
By plotting graph we verified that,if we don’t consider the substrate parasitic effects then the gain of SiGe HBT in CB mode is much higher than that of in CE mode. But inclusion of parasitic degrades the superior power gain of HBT in CB mode and hence its gain reduces than that of in CE mode
Grounding the substrate is an effective way to reduce the effects of substrate parasitic on SiGe HBT used as an amplifier. We verified this, by plotting the graph of maximum available power gain of common base and common emitter SiGe HBT while grounding the substrate and comparing it with while not grounding the substrate.
Again we designed microwave oscillator using common base and common emitter SiGe HBT and saw how the oscillation frequency has been decreased due the substrate parasitic effect in both the cases.