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The InP/GaAsSb/InP "type-II" heterostructure system is of interest for high-speed devices such as photodetectors and double heterostructure bipolar transistors (DHBTs) because its band alignment enables the straightforward injection of electrons from a ~0.72 eV p-type GaAsSb layer into n-type InP without any need for interface grading. We briefly review the salient features of the InP-GaAsSb system, and consider some of the surprising device characteristics encountered in InP/GaAsSb -based devices. In several respects, the InP/GaAsSb system is shown to offer very appealing opportunities for the development of high-speed NpN DHBTs with ultrathin base and collector layers.

Absfract-In "type-11'' NpN InPiGaAsSbiInP double heterostructure bipolar transistors (DHBTs), the p + GaAsSb base conduction band edge lies dEc above the InP collector conduction band: a small ballistic energy dEc is imparted to collected electrons as they are launched into the collector, The high initial velocity should in principle reduce the collector signal delay time. In the present work, we extract the average collector electron velocity in high-speed lnPiGaAsSbIInP DHBTs, and find a peak average velocity approaching 4 x lo7 c d s across a 2000 8, InP collector. This finding provides the first evidence of the performance advantage afforded by abrupt type-I1 BIC junctions for collector transport when compared to the conventional graded 'launchers' required when a GaInAs base layer is used.

The effect of secondary impact ionization by the non-initiating carrier on the near avalanche behavior of highspeed InP collector transistors is studied. We show that secondary collector ionization by generated holes traveling back toward the base layer significantly reduces B V c~o if the hole ionization coefficient is higher than that of electrons &(E) > q,(Q]: the positive feedback due to a strong secondary ionization sharpens the breakdown characteristic by speeding up carrier multiplication, and decreases separation between the open-base collector-emitter ( B VCEO) and the open-emitter base-collector (BVcuo) breakdown voltages. The influence of secondary ionization on the BVCEO-BV~BO separation has not previously been described. Multiplication coefficient comparisons for representative InP, GaAs and Si collectors indicate that all structures can sustain low-current above B VcEo operation from a transport (non-thermal) point of view, although the different degrees of secondary ionization in various semiconductors lead to fundamental differences when InP is compared to GaAs and Si since for the latter materials W(E) < a,(@. The collector ionization integral is used to determine the maximum collector voltage before the onset of non-thermal device instabilities for TnP devices: we find that for a 2000 A collector the transistor can be operated well above B V , E~ and up to 90% of BVCSO when the base is not left open-circuited, in good agreement with measurements on InPIGaAsSblInP DHBTs.