Reverse current-voltage characteristic of almost ideal silicon p-n junctions

Abstract

The forward current-voltage characteristic of silicon p-n junctions obtained by means of suitable gettering processes is well described by the Shockley diffusion theory. Their reverse characteristic, however, according to the preparation procedure and final annealing temperature, may deviate from both the Shockley model and the generation-recombination (GR) theory of Sah, Noyce, and Shockley owing to an ohmic contribution proportional to the reverse-bias voltage. Such an ohmic current has previously been ascribed to pure-generation centers of the depletion layer. The invoked generation mechanism is a field-assisted thermal emission (FATE) of electron-hole pairs from donor-acceptor twins. Here, we show that both GR and ohmic currents may be ascribed to a single type of defect, namely, four-state traps (FST) which may be empty, or filled by an electron or a hole, or both, and to different Frankel–Pole effects for electrons and holes in their emission process from such traps. The model requires determining the kinetic equations of the FSTs, solving them in the depletion layer for steady state and reverse bias, and computing the Frankel–Pole effect both of a single ion and an ion dipole. In agreement with the experimental results, the new approach also shows (i) that, apparently, the electric field acts and, through the FATE mechanism, induces the ohmic current only when it is greater than a critical field, (ii) that the constant contribution of the reverse current may be much lower than the Shockley saturation current, (iii) that the activation energy is equal for both the GR and the ohmic current, and (iv) that FSTs generate no extra charge in the depletion layer. In more general terms, the model accounts well for all the experimental results of the new ideal and almost ideal silicon p-n junction obtained by means of new gettering processes.