Theoretical Study of a Potential Ultraviolet Avalanching Detector Based on Impact Ionization Out of Confined Quantum States

We present a detailed analysis of a possible new ultraviolet photodetector based on impact ionization out of confined quantum states using a GaN—AlxGa1_ XN multiple quantum well array. The GaN-AlGaN materials system is continuously gradeable in composition and has a large conduction band-edge discontinuity, which makes it an attractive candidate for asymmetric confined quantum state photomultipliers. The impact-excitation rate is determined for various device geometries and doping concentrations. As the carrier concentration increases in a quantum confined structure, the excitation probability increases. The ionization rate increase is due in part to the increase in the number of carriers within the high-energy subbands of the well with the resulting reduction of the carrier ionization threshold energy. The presence of significant carriers in energy levels near the top of the well, however acts to increase the thermionic dark current and therefore reduce performance of the device. Hence, an interesting tradeoff in the design of the structure exists; a large carrier concentration in the well is favorable in terms of device gain but at the potential risk of increased dark current. The calculated total impact-ionization rate, thermionic, and tunneling dark currents are presented for various asymmetric multiple-quantum-well arrays. It is predicted that an appreciable ionization rate, ~1010 s-1, can be realized in a 200/400 A [well/(well and barrier)] width device with a free carrier concentration of 5.0 x 1019 cm-3 within the well region.