Transient space charge limited currents in light-pulse excited silicon

Abstract

Space charge limited current (SCLC) transients have been studied by exciting over-depleted gold-on-n- type silicon surface barrier diodes. The excitation was done by short (1 nsec) light pulses from a supperradiant laser and currents due to transport of electrons through the diodes were studied. Deviations from expected pulse shapes according to the theory of Many et al. for SCLC in an insulator are caused by (a) the influence of doping atoms on the field, (b) by field-dependent mobility and (c) by lateral spreading of the discharge. Effect (a) is pronounced at low fields (voltages only slightly exceeding the depletion voltage). It causes a shortening of the cusp time with up to ⋍ 20%, a general increase in current, and a decrease in the ratio of cusp to the starting current values. Effect (b) which occurs at high fields, causes a lengthening of cusp time and changes in current magnitude. The latter is shown to give deviations from the expected quadratic dependence of SCLC on voltage. Effect (c) gives an increase in apparent space charge limited current when the light excitation is increased above the level which is sufficient for SCLC from the irradiated area. An increase in light with a factor ⋍ 7 approximately doubles the transient current.