Abstract
This thesis describes experimental studies of perpendicular transport in multi-layer III-V semiconductor structures. Impact ionization, charge trapping at heterointerfaces and resonant tunnelling in multiple quantum well photodiodes were studied. Long-wavelength high-speed multiple quantum well avalanche photodiodes were studied. Ga0.47In0.53As/InP structures grown by trichloride, and metal-organic, vapour phase epitaxy exhibited avalanche multiplication at room temperature. The pulse response at high fields was not limited by carrier pile-up at the heterointerfaces. High quality Al0.48In0.52As/Ga0.47In0.53As photodiodes, grown by molecular beam epitaxy, exhibited multiplication of ≈ 70, microwave gain of ≈12 and an intrinsic response time of ≈100 ps at room temperature and high electric fields, indicating no carrier pile-up despite the very abrupt heterointerfaces. In multiple quantum well photodiodes which exhibited large reverse-bias leakage currents, a new carrier multiplication effect was observed. This was attributed to impact ionization across the heterojunction discontinuity of carriers "stored" in the quantum wells. Experimental evidence for this effect is presented for several material systems. Single carrier-type multiplication was demonstrated in a multilayer photodiode with compositionally-graded interfaces. Sequential resonant tunnelling and resonant Zener tunnelling were studied in high quality Al0.48In0.52As/Ga0.47In0.53As structures with low background doping. Carrier transport in undepleted quantum wells was studied in Ga0.47In0.53As/InF structures grown by gas source molecular beam epitaxy.