A COMPARATIVE-STUDY OF STRAIN RELAXATION EFFECTS ON THE PERFORMANCE OF INGAAS QUANTUM-WELL-BASED HETEROJUNCTION PHOTOTRANSISTORS

The performance of a GaAs based heterojunction phototransistors (HPT's ) using an n-p-i-n configuration, where the absorption is provided by InGaAs quantum webs (QW's) have been studied. Structures with differin g numbers of QW were investigated, This allowed the tradeoff between t he benefits of increased light absorption and the drawbacks of increas ed lattice relaxation, caused by the mis-match between InGaAs and the GaAs substrate, to be examined. All the HPT's investigated showed resp onsivities (A/W) far larger than unity, as well as large wavelength to lerance, for example 44 A/W +/-15% from 950-970 nm, for 10 mu W incide nt optical. Electrical common-emitter current gains, of up to 3000 wer e measured for our HPT's and then confirmed by subsequent HBT measurem ents. Small relaxation levels (<10%) had no significant detrimental ef fects, allowing a large improvement in HPT performance. More heavily r elaxed HPT's showed a degradation in both the inherent photodetector a nd transistor action, though this was not catastrophic in nature. A si mple simulation of the results is carried out, suggesting that the dis locations adversely effect the carrier transport across the collector region, and also reduce the minority carrier diffusion length in the b ase.