Transient Response of Amorphous Semiconductor Devices: A Theoretical Microscopic Simulation Approach to the Physics of D
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TRANSIENT RESPONSE OF AMORPHOUS SEMICONDUCTOR DEVICES: A THEORETICAL MICROSCOPIC SIMULATION APPROACH TO THE PHYSICS OF DISORDERED SYSTEMS FINLEY R. SHAPIRO* AND YANEER BAR-YAM** *Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science and Center for Materials Science and Engineering, Cambridge, MA 02139 "**Materials Research Department, Weizmann Institute, Rehovot, Israel
ABSTRACT A general purpose simulator has been developed for transient experiments on amorphous semiconductors. Simulated time-of-flight (TOF) experiments have been used to study the effects of band tails of the form gc exp{-[(Ec - E)/kTo]"}. Values of m from 0.5 to 2.0 are compared to the pure exponential of m = 1.0. Quantitative results demonstrate that dispersive transport as observed in TOF experiments on a-Si:H requires a value of m very close to 1.0. INTRODUCTION The electrical and opto-electrical properties of amorphous semiconductors are of great interest for both the fundamental physics of disordered systems and for devices based on amorphous semiconductor technology. Disorder is known to disturb the electronic properties in suprisingly minor though significant ways in comparison to crystalline materials. Abrupt edges of crystal conduction and valence bands are replaced by tails of localized states, which extend into the gap between the bands. There are also other localized states with energies deeper in the gap. These states are poorly understood and poorly characterized as to their energy distribution, and many controversies and unresolved questions which remain. Much of our understanding of the physics of semiconductors and insulators is derived from transient experiments. This is due to the crucial role that transient characteristics play in the operation of most semiconductor devices, and also because steady-state processes are often best studied by transient responses. The interpretation of transient experiments, however, generally requires an analysis based on drastic assumptions using simple models. The problems with such analyses have been very apparent in the study of amorphous silicon hydride (a-Si:H). Many questions and controversies concerning the fundamental physics of this important semiconductor have their origin in our limited ability to interpret the data from transient experiments. In order to improve our ability to interpret the results of transient experiments, we have developed a numerical simulator for these experiments which is not dependent on many of the commonly used assumptions. The material is modeled as a normal semiconductor with conduction and valence bands and localized states with an arbitrary set of discrete levels. The densities of electrons and holes in the bands and the currents
are calculated using the semiconductor transport equations. The spatial variation of the carrier densities and the electric field is included in the calculations. The occupancies of
Mat. Res. Soc. Symp. Proc. Vol. 118. c1988 Materials Research Society
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