Low-offset NpN InP-GaAsSb-InP double heterojunction bipolar transistors with abrupt interfaces and ballistically launched collector electrons

We have developed MOCVD-grown, C-doped, abrupt heterojunction InP-GaAsSb-InP DHBTs which feature a very small V/sub CE/ offset voltage <0.1 V and a low turn-on voltage (J/sub c/=1 A/cm/sup 2/) at V/sub BE/=0.3 V, by taking advantage of the staggered band line-up at InP-GaAsSb interfaces: the GaAsSb conduction band sits above the InP conduction band, eliminating any possibility of current blocking at the B-C junction; instead, the GaAsSb base injects electrons ballistically (with ΔE/sub C/=0.1-0.3 eV) into the widegap InP collector when they travel with a high saturated drift velocity and low rates of impact ionization. One of the fundamental advantages of InP-GaAsSb-InP DHBTs is that they can be implemented without compositional grading and with nominally abrupt interfaces: the turn-on and offset voltages are therefore determined by the band line-ups, doping, and junction areas. They are not altered by the effectiveness of compositional grading or pulse doping schemes that are intended to eliminate conduction-band spikes and barriers. Other important advantages of MOCVD-grown GaAsSb bases are that they can be C-doped to 10/sup 20/ cm/sup -3/ without H-passivation effects, and that the low p-type Schottky barrier heights on antimonide compounds lead to low resistance base ohmic contacts, with measured base contact resistances lower than 10/sup -7/ Ωcm/sup -2/. Our DHBT prototypes display a base current ideality factor of n/sub B/=1.1 and a current gain of 15. The fabrication of InP-GaAsSb-InP DHBTs is presented, with particular attention to the growth details and their impact on device performance.