Carbon doped InP/GaAsSb HBTs via MOCVD

Abstract

Heterojunction bipolar transistors (HBTs) need heavily doped p-type regions. However, at high concentrations (>10/sup 19/ cm/sup -3/), most p-type dopants diffuse into other regions of the device, ruining performance and preventing stable, reliable operation. The discovery that carbon as a p-type dopant does not significantly diffuse has lead to reliable operation of GaAs-based HBTs. For the first time, carbon-doped double heterojunction InP/GaAsSb/InP HBTs have been fabricated (emitter area of approximately 70/spl times/70 /spl mu/m/sup 2/). The base doping was P=4/spl times/10/sup 19/ cm/sup -3/. Base thickness was varied from 350 /spl Aring/ to 1500 /spl Aring/, giving sheet resistances of 850 to 200 /spl Omega//sq. The devices had DC current gains ranging from 5 to 45 that scaled sheet resistance. While these gains are low, they are comparable to the best InP/GaInAs HBTs fabricated, where the gain is limited due to Auger recombination in highly doped bases. BV/sub ceo/ is on the order of 6 volts. The typical turn-on voltage for both emitter-base and base-collector junctions is approximately 0.4 V, even with the emitter-base junction grown nominally abrupt: i.e., no undoped setbacks or intentionally graded layers were used for these structures. The authors report on the growth, fabrication, and properties of the MOCVD-grown carbon-doped InP/GaAsSb HBTs.