Current gain dependence on subcollector and etch-stop doping in InGaP/GaAs HBTs

The DC current gain dependence of InGaP/GaAs heterojunction bipolar transistors (HBTs) on subcollector and etch-stop doping is examined. Samples of InGaP/GaAs HBTs having various combinations of subcollector doping and etch-stop doping are grown, and large area 60 /spl mu/m/spl times/60 (/spl mu/) HBTs are then fabricated for DC characterization. It is found that the DC current gain has a strong dependence on the doping concentration in the subcollector and the subcollector etch-stop. Maximum gain is achieved when the subcollector is doped at 6/spl sim/7/spl times/10/sup 18/ cm/sup -3/ while the subcollector etch-stop is doped either above 6/spl times/10/sup 18/ cm/sup -3/ (current gain/sheet resistance ratio, /spl beta//R/sub b/=0.435 at I/sub c/=1 mA) or below 3.5/spl times/10/sup 17/ cm/sup -3/ (/spl beta//R/sub b/=0.426/spl sim/0.438 at I/sub c/=1 mA). The data show that it is not necessary to heavily dope the subcollector etch-stop to reduce the conduction barrier and to obtain high current gain. The high current gain obtained with the low InGaP etch-stop doping concentration is attributed to the reduction of the effective energy barrier thickness due to band bending at the heterojunction between the InGaP etch-stop and the GaAs subcollector. These results show that the /spl beta//R/sub b/ of InGaP/GaAs HBTs can improve as much as 69% with the optimized doping concentration in subcollector and subcollector etch-stop.