The Simulation of a -Si:H Junction Capacitance Measurements
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THE SIMULATION OF a-Si:H JUNCTION CAPACITANCE MEASUREMENTS FINLEY R. SHAPIRO AND ARIJIT DAS Department of Electrical and Computer Engineering, Drexel University, Philadelphia, PA 19104 USA. ABSTRACT Junction capacitance measurements have been used by many researchers to study the density of states in the mobility gap of hydrogenated amorphous silicon. However, the data analysis methods used for these studies are based on approximate analytic models which may not always be appropriate. In order to understand better the experimental method and the models, we have performed simulations using a numerical simulator which can calculate the complete time-dependent response of an amorphous semiconductor device. The current in a device is simulated as a function of time when a small sinusoidal voltage applied in addition to a DC bias voltage. The out-of-phase and in-phase components of the sinusoidal part of the current are used to calculate the capacitance and series resistance, just as they measured in an experiment. The results of simulated experiments at different temperatures are shown. INTRODUCTION Reverse bias capacitance measurements are widely used to characterize all types of pn junctions, Schottky junctions, and MOS junctions. Most commonly, the capacitance is measured by superimposing a small high frequency sinusoidal voltage on top of a larger DC bias voltage, and using the sinusoidal part of the current in the device to extract the incremental capacitance. Capacitance can measured as a function of temperature and reverse voltage, and the results can be analyzed to determine the doping level and the interface and bulk defect state densities. The time dependent characteristics of interface and bulk states are studied by measuring the capacitance as a function of frequency or the transient capacitance after an excitation. Steady state, frequency dependent, and transient capacitance measurements have been used by many authors to study defect levels in hydrogenated amorphous silicon (for example [1-7]). However, the measurements from these experiments are difficult to analyze, and for this reason they have not always been well accepted in the field. One cause of this difficulty is that the assumptions which are routinely employed to analyze capacitance data from crystalline semiconductors are invalid, or questionably valid, in hydrogenated amorphous silicon (a-Si:H). In crystalline semiconductors, it is assumed that the discrete bulk defect levels are separated by energy ranges with no defect levels, the applied reverse voltage is large compared to the depth of the defect levels, the reverse current in the junction is negligible, the doping concentration is large compared to the concentration of defect levels, the carriers which are emitted from localized states in the depletion region quickly leave the region without being recaptured, and the conductivity of the semiconductor outside the depletion region is quite high. An understanding of experimental data is much more difficult to achieve if some or all of these
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