LF Dependence of the output admittance in short-channel n- or p-power MOS transistors

This paper presents the output admittance properties of p-or n-type short-channel power MOS transistors operating beyond pinch-off, in the low-frequency (LF) range. In the Nyquist chart, it appears that the signs and amplitudes of the real and imaginary parts of the output admittance are bias dependent. These properties are accounted for by taking into account the internal feedback between the electrical mechanisms and the thermal phenomena. A theoretical expression of the admittance is proposed and compared to the experimental results. A method for determining the thermal impedance and the equivalent circuit of an MOS transistor is inferred from this analysis.