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 m 60 ( ) 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 7 10 cm 3 while the subcollector etch-stop is doped either above 6 10 cm 3 (current gain/sheet resistance ratio, = 0 435 at = 1mA) or below 3.5 10 cm 3 ( = 0 426 0 438 at = 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 of InGaP/GaAs HBTs can improveas much as 69% with the optimized doping concentration in subcollector and subcollector etch-stop.