Analysis of the emitter-down configuration of double-heterojunction bipolar transistor

A two-dimensional numerical study of the parasitic effects on current gain β and cutoff frequency f_T of the emitter-down configuration of AlGaAs-GaAs double-heterojunction bipolar transistors (DHBTs) is reported. The studied structures include: those with and without the extrinsic emitter regions; a structure with oxygen implantation in the extrinsic emitter region; and two structures with P ⁺ dopant implantation of 1000 and 300 Å into the extrinsic emitter region. Analysis of the structures with and without the extrinsic emitter region shows that parasitic effects associated with the extrinsic emitter seriously degrade β and f_T , and lead to extremely asymmetric DHBT operation. The three structures with the extrinsic emitter region modified by the implantations exhibit improved β and f_T at low bias. However, the effectivenesses of the implant approaches in minimizing the parasitics are quite different