Photoeffect on diffused P-N junctions with integral field gradients

A detailed analysis of a diffused junction photodiode is presented in which the illumination, monochromatic or broad-band, is applied to the diffused face. The electric field produced by the impurity distribution, assumed exponential, assists the transport and collection of minority carriers created by photons absorbed in the graded region. The theoretical study covers both the steady-state and the transient response, and takes into account the effect of surface recombination velocity. The presence of the built-in field increases the photocurrent and reduces the dark current compared with homogeneous base diodes. For p-n silicon photodiodes with 5-micron base widths and acceptor concentrations of say, 2 × 10¹⁸atoms/cm³at the surface, photosensitivities of approaching 0.01 ampere per lumen may be achieved. The transient-response analysis considers the extrinsic delay imposed by the time constant of the junction capacitance and the load resistance, and also the inherent delay caused by the transit time of the minority carriers. With moderate or high load resistances, the extrinsic delay is much larger than the transit-time delay. However, for comparable graded- and homogeneous-base photodiodes, the capacitances of graded junctions are lower, and therefore the transient response is improved on this account. The graded junctions also are shown to have greatly reduced transit-time delays because of the built-in field effect.