Optimum diode geometry in a two-dimensional photovoltaic array

The quantum efficiency and the crosstalk of a photodiode in a long wavelength IR (LWIR) 2-D diode array are studied by numerically solving the 2-D diffusion equation of photocarriers in an array environ- ment. The quantum efficiency depends strongly on both the diode size and the junction depth, and can be expressed, to good accuracy, as a quadratic function of the two variables. The results, corresponding to HgCdTe n 1 -on-p backside illuminated photovoltaic (PV) diodes, are compared with published Monte Carlo results and with analytical (1-D) special cases. For a given pitch, smaller diodes have lower crosstalk, but also lower quantum efficiency. This implies an optimal diode size. The highest quantum efficiency is obtained for low junction depths (1 mm), and the optimum diode size, maximizing the quantum efficiency and minimizing the crosstalk, lies in the range of 15 to 30 mm for a pitch of 50 mm over a wide range of diffusion lengths. The quantum efficiency de- pends weakly on the pitch, for a given ratio of diode size to the pitch.