Nuclear Instruments and Methods in Physics Research A 434 (1999) 67}70 Monte Carlo simulator of the charge signal induced by external radiation on semi-insulating GaAs detectors Adriano Cola*, Lino Reggiani, Lorenzo Vasanelli Istituto per lo Studio di Nuovi Materiali per l'Elettronica, CNR, Via Arnesano 73100, Lecce, Italy Unita+ INFM, Dipartimento di Scienza dei Materiali, Universita+ di Lecce, Via Arnesano 73100, Lecce, Italy Abstract We present a Monte Carlo simulator of the charge signal induced by external radiation on a semi-insulating GaAs detector. The relative contribution to the charge signal of fast and slow time components as well as the time constant of the slow component are studied as a function of the applied voltage. The collected charge e$ciency is evaluated in terms of relevant time scales. Calculations compare well with experimental results.  1999 Elsevier Science B.V. All rights reserved. Keywords: GaAs detectors; Monte Carlo; Trapping; Semiconductors; Simulators; Charge collection 1. Introduction The main problem for semi-insulating GaAs to be a competitive material for radiation detectors is the low value of the charge collection e$ciency (CCE) associated with an electron transport dom- inated by trapping centres. As a consequence, to optimize the CCE a reliable modelling of the de- tector is a main issue to be addressed. This paper aims to present relevant advances in this task based on an original Monte Carlo simulator of the charge signal induced by external radiation on semi-insu- lating GaAs detectors. In this way, the relative contributions to the charge signal of fast (nano- second scale) and slow (microsecond scale) time * Corresponding author. Tel.: 39-0-832-320-243; fax: 39-0- 832-325-299. E-mail address: adriano.cola@ime.le.cnr.it (A. Cola) components are investigated and compared with experimental results. 2. Theoretical model The model is based on the knowledge of the dark characteristics of the detectors, which has a Schottky Barrier Diode (SBD) structure. For a one-dimensional geometry and speci"c material parameters (trap levels, carrier-mobility, etc.), the local electric "eld and the transport coe$cients (carrier concentration, drift velocity, mean energy, etc.) at the given bias point are obtained from a drift di!usion calculation [1]. From the above `darka condition, we consider the generation of an excess electric charge (electrons and/or holes) at a given point inside the SBD and simulate its full dynamics (i.e. motion and trapping/de-trapping processes). We assume that the excess charge does not modify the local electric "eld, which is commonly 0168-9002/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 4 3 4 - 9 II. CHARACTERIZATION/MODELLING