THEORETICAL-STUDY OF THE QUASI-STATIC CAPACITANCE OF METAL-INSULATOR-SEMICONDUCTOR STRUCTURES IN AMORPHOUS-SEMICONDUCTORS

We present the theoretical analysis,of the quasistatic capacitance of metal-insulator-semiconductor structures for the case of amorphous sem iconductors. The contribution of the bulk of the semiconductor is emph asized. We show that the semiconductor bulk capacitance is simply give n by the ratio, taken at the insulator/semiconductor interface, of the space-charge density to the electric held. From the explicit expressi ons of these quantities as a function of the surface potential, a nume rical calculation of the capacitance versus bias curves is performed. This is used to discuss the ability of the capacitance to reproduce th e underlying structures of the density of states (DOS) in the gap. We derive also approximate analytical expressions of this capacitance in the case of exponentially distributed band-tail states. Moreover, we s how that it is possible to reconstruct the DOS of the amorphous semico nductor from the bias dependence of the semiconductor capacitance usin g simple approximate analytical expressions. In particular, the square of the bulk semiconductor capacitance can lead in most cases to a rea sonable DOS reconstruction. Using the capacitance versus bias curves d erived from the numerical simulation, the accuracy of the reconstructi on is then checked on DOS examples consisting of two exponential band tails and a Gaussian deep defect density, which can be representative of typical amorphous semiconductors such as hydrogenated amorphous sil icon (alpha-Si:H). We emphasize the influence on the deep gap stares r econstruction of the bulk Fermi-level position, whether it is located in a DOS minimum or not. We also discuss the influence of the characte ristic temperature in the case of an exponential band tail which shoul d be met in the accumulation bias regime. As this was done in the crys talline case to develop the metal-oxide-semiconductor technology, the method proposed can be used as a characterization tool to investigate metastability phenomena and to optimize technological processes relate d to amorphous semiconductor field-effect devices. (C) 1995 American I nstitute of Physics.