MONTE CARLO SIMULATION OF DRIFTING CHARGE CARRIERS IN PHOTOCONDUCTIVE INTEGRATING DETECTORS

Semiconductors behaviour is often assumed and modelled under small signal conditions. One of the most common properties used to describe semiconductors is their collection efficiency (CE) the most common model being the Hecht collection efficiency model (HCE)η₀. HCE and its modified expressions for exponential absorption have been widely used in time-of-flight type transient photoconductivity experiments as well as in the assessment of the sensitivity of integrating-type radiation detectors. However, the equations apply under small signals in which the internal field remains uniform (unperturbed) while electron hole pairs (EHPs) move in the semiconductor. In this thesis I have used Monte Carlo simulation of the continuity, trapping rate and Poisson equations to calculate the collection efficiency ηᵣ (CCE). Each injected carrier is tracked as it moves in the semiconductor until it is either trapped or reaches the collection electrode. Trapped carriers do not contribute to the photocurrent but continue to contribute to the field through the Poisson equation. The instantaneous photocurrent iph(t) is calculated from the drift of the free carriers through the Shockley-Ramo theorem. iph(t) is integrated over the duration of the photocurrent to calculate the total collected charge and hence the collection efficiency hr. hr has been calculated as a function of the charge injection ratio r, the electron and hole ranges (drift mobility and lifetime products, μτ), mean photoinjection depth δ and drift mobility ratio b. The deviation of the collection efficiency hr from the uniform field case η₀, in the worst case, can be as much as 30% smaller than the small signal model prediction. However, for a wide range of electron and hole schubwegs and photoinjection ratios, the typical error remained less than 10% at full injection, the worst case. The present study provides partial justification for the wide-spread use of the uniform field collection efficiency η₀ formula in various applications, even under high injection conditions