COLLECTOR EMITTER OFFSET VOLTAGE IN SINGLE-BASE AND DOUBLE-BASE INGAAS(P)/INP HETEROJUNCTION BIPOLAR-TRANSISTORS

The effects of base current on the collector-emitter offset voltage for InGaAsP/InP heterojunction bipolar transistors (HBTs) have been examined both experimentally and theoretically. The junction area ratio of emitter-base and collector-base obviously affects the value of offset voltage. We demonstrate that decreasing base current increases the offset voltage. A double-base InGaAsP(p)/InGaAs(p+)/InP HBT (DBHBT) and a traditional single-base InGaAsP/InP HBT (SBHBT) have been fabricated and characterized. We examined theoretically the electron diffusion current of the DBHBT and found that the turn-on voltage of the EB junction for a DBHBT is low, and the offset voltage of a DBHBT is low. The DBHBT with low-high doping and wide-narrow bandgap design shows a small offset voltage that is not due to a heterojunction at the collector. The experimental results show that low offset voltage (DELTA-V(CE) = 63 mV) and high current gain (beta(ac) = 140) of the DBHBT are excellent compared to those (DELTA-V(CE) = 130 mV and beta(ac) = 100) of the SBHBT. The offset voltage of the SBHBT is increased with decreasing base current. However, the offset voltage of a DBHBT is insensitive to the base current value. The effective electron diffusion lengths in the bases of the DBHBT and SBHBT are about 3.35 and 3.83-mu-m respectively.