Monte-Carlo Simulation of Generation- Recombination Noise in Amorphous Semiconductors
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Monte-Carlo Simulation of Generation- Recombination Noise in Amorphous Semiconductors R.I. Badran, C. Main1 and S. Reynolds1, Dept of Physics, The Hashemite University, Zarqa, JORDAN 1 School of Science and Engineering, Univ of Abertay Dundee, DUNDEE, UNITED KINGDOM ABSTRACT We compare the predictions of several analytical models for conductivity fluctuations in a homogeneous semiconductor containing discrete and distributed traps, using a Monte-Carlo simulation of the relevant multi – trapping (MT) transitions. The simulation directly embodies the statistical features associated with such processes, in a simple ‘model - independent’ approach, free of approximations and assumptions. We compare the results with those of several analytical approaches. In one, the noise spectrum is assumed to reflect separately, the characteristic individual release time constants of the various trapping centers in the material. In another, the trapping time into the ensemble of electron traps is taken to be the dominant time constant, and hence, in a material such as a-Si:H, where the trapping time into tail sates is of order 1ps, this is taken to imply that this component of the conductivity noise spectrum is unobservable in practice. Our own analytical approach, incorporates coupling (albeit weak) between traps, which necessarily communicate via the extended states. Preliminary results of the simulation support our thesis, and verify that the same information is contained in the real part of the modulated photoconductivity (MPC) spectrum. A ‘full Monte’ - Carlo simulation incorporating all gap states and spatial inhomogeneities is now a priority. INTRODUCTION Noise in semiconductor devices remains an important topic for investigation, from the point of view of low-level signal applications, and also as a means of probing and understanding electronic processes occurring within semiconductors. In this paper, we restrict our discussion to conductivity fluctuations, which result from random variations in trapping and release rates in the trapping system. Much of the fundamental theory of this type of noise in semiconductors was developed in the 1960s, essentially for the case of crystalline materials with relatively few trapping levels in the gap. More recently, this theory has been applied essentially without alteration, to amorphous semiconductors, with distributed gap states, where appropriate assumptions may have to be made to simplify analysis of these more complicated systems. However, at least three plausible but qualitatively different, approaches to trapping noise in such systems, have been described in an earlier paper by the present authors [1]. In the description below, the continuous density of states function is represented by a set of closely spaced discrete levels. Independent traps [2-4] Fluctuations in trapping and release rates from each trap level modulate the free carrier density, each trap doing so independently of other traps, with its own characteristic time constant τi,, imposing thereby a characteristic Lorentz
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