A comparative study of strain relaxation effects on the performanceof InGaAs quantum-well-based heterojunction phototransistors

The performance of a GaAs based heterojunction phototransistors (HPTs) using an n-p-i-n configuration, where the absorption is provided by InGaAs quantum wells (QWs) have been studied. Structures with differing numbers of QW were investigated. This allowed the tradeoff between the benefits of increased light absorption and the drawbacks of increased lattice relaxation, caused by the mis-match between InGaAs and the GaAs substrate, to be examined. All the HPT's investigated showed responsivities (A/W) far larger than unity, as well as large wavelength tolerance, for example 44 A/W±15% from 950-970 nm, for 10 μW incident optical. Electrical common-emitter current gains, of up to 3000 were measured for our HPT's and then confirmed by subsequent HBT measurements. Small relaxation levels (<10%) had no significant detrimental effects, allowing a large improvement in HPT performance. More heavily relaxed HPTs showed a degradation in both the inherent photodetector and transistor action, though this was not catastrophic in nature. A simple simulation of the results is carried out, suggesting that the dislocations adversely effect the carrier transport across the collector region, and also reduce the minority carrier diffusion length in the base