Temperature Dependence of Inversion-Layer Frequency Response in Silicon

Theoretical calculations of metal-oxide semiconductor (MOS) capacitance show a total capacitance approaching oxide capacitance in strong accumulation and strong inversion. 1 Experimentally, it has been found that response time of the inversion layer can be very long."' The response time can be drastically shortened, however, by lateral ac current flow in an extended inversion layer. 2,3 The lateral current flow mode requires equilibrium surface inversion beyond the metal contact. This condition is usually found in p-type silicon because of the preponderance of positive surface charge in thermally oxidized silicon. Channel cutoff frequencies are then typically in the MHz range. In n-type silicon, charge in the inversion layer can communicate with the bulk under steady-state conditions only by means of generation-recombination processes.* Inversion-layer cutoff frequencies in n-type silicon are normally below 100 Hz, sometimes below 1 IIz. These low frequencies make it difficult to measure cutoff frequencies and to determine the mechanism of generation of minority carriers. 513