Effect of collector-base barrier on GaInP/GaAs double heterojunction bipolar transistor and further improvement by doping-spike

We report the effects of a collector-base barrier on GaIuP/Gansn double heterojunction bipolar transistors (DHBT's). A potential barrier exists at the collector-base heterointerface due to a small yet finite conduction band discontinuity, The voltage-dependent barrier that blocks the electrons results in a large collector current saturation voltage (knee voltage), We found, through theoretical calculation, that the potential barrier is much higher and less sensitive to the external bias when the collector doping density is low, If the conduction band discontinuity of Delta E-c = 0.2 eV is employed in the calculation, the collector-base voltage used to totally suppress the existing potential barrier varies from 2.5 V for N-C = 1 x 10(17) cm(-3) to 17 V for N-C = 2 x 10(16) cm(-3). Further improvement in eliminating the barrier was achieved using the doping spike located at the heterointerface. A zero potential barrier was obtained even if the base collector was forward biased. Experimentally, we have successfully fabricated GaInP/GaAs DHBT's with and without doping spikes at the B-C heterointerface. Large knee voltages are observed in DHBT's without a doping spike, The improved DHBT's exhibit a high current gain of 280 and a small knee voltage smaller than 1.8 V; supporting the very low potential barrier at the B-C junction.