The Early Voltage and Current Gain of InGaAs Double Heterojunction Bipolar Transistor

Abstract

Early effect is the variation in the width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage, named after its discoverer James M. Early. An increase in the collector-base voltage causes a greater reverse bias across the collector-base junction which increases the collectorbase depletion region width, and decreasing the channel width of the base, which is defined as “Early Effect”. The early effect has been studied extensively in conventional bipolar junction transistors (BJTs). In the present work closed form analytical model were derived for Early Voltage (VA) and common emitter current gain (β) for uniform and exponential base doping profiles with arbitrary Indium (In) profiles heterojunction bipolar transistor (HBT). Field depended mobility (DnInGaAs), band gap narrowing (BGN) (due to both heavy doping and presence of Indium content in the base) and electron velocity saturation effects (vs) were considered in this model. The effects of base doping profiles and Indium profiles on VA and β are observed in this work. Here it is observed that if Indium mole fraction at emitter and (yE) is 0.01 and it increases at collector and (yC) from.0.01 to 0.5 it increases VA exponentially. It can be found from this analysis that yC increases due to BGN effect, effective intrinsic carrier concentration (nieInGaAs) increases towards base-collector junction which minimizes “Early Effect” and increases VA. For a particular yC and yE , VA found highest for uniform base doping profiles. The results shows that VA is proportional to base doping concentration. Also keeping yC at 0.5 and if yE vary from 0.01 to 0.5 it reduces VA. It was also observed that by keeping yE at 0.01 if yC can be increased from 0.01 to 0.5 it reduces collector current density (JCO). It can also be observed that vS has significant impact on both JCO and DnInGaAs for uniform and exponential base doping profiles. The results obtained by using this analytical model compared with the results available in the previous literature and found in good agreement.