Frequency response of the surface state admittance in weakly inverted thin SiO2–Si MOS capacitors

Abstract

The frequency response of surface states in the weak inversion region has been studied using MOS capacitors with very thin oxide on p‐type silicon. To account for the conductance and capacitance data, an exact formula is derived for the Shockley‐Read‐Hall (SRH) centers having interaction not only with the majority carrier band but also with the minority carrier band located at the surface of the semiconductor. The energy distribution of surface states and the random spatial variation of the surface potential are taken into account. Accurate data are obtained on devices fabricated on p‐type silicon substrates with a very thin oxide (about 35 Å) in the energy range of 0.32 to 0.73 e V at room temperature. Good agreement between the experimental and the theoretical conductance‐ and capacitance‐frequency characteristics is obtained. The time constant for the SRH transitions of holes in the weak inversion region is evaluated by the exact formula, and shown to be exponentially dependent on the surface potential precisely as in the depletion region.