Capacitive effects in insulators or semiconductors. Influence of the density of states in the Gap

The ac electrical response of a dielectric material or a semi conductor is calculated. The sample is supposed to have only one type of mobile charge carriers. Using Shockley-Read statistics, generation—recombination at all the states of the gap is taken into account. Analytical expressions for parallel capacitance and conductance are given. Under certain conditions the zero frequency parallel limiting capacitance is directly proportional to the square root of the density of states at the Fermi level. The shape of the dielectric dispersion curves is examined, the parallel capacitance can present ω−1/2 zones, ω−3/2, and ω−2 frequency response. Comparison with previous space charge theories shows that the interpretation of experimental results must be made with great care, otherwise completely erroneous values of carrier density and mobility might be obtained.