On the Drain-to-Source Capacitance of Microwave FETs in Triode Region

This paper explains the apparent negative drain-to-source capacitance observed in microwave field effect transistors operating in the triode region. Its physical origin is explored using TCAD simulations, where the terminal capacitances are directly obtained from the electron channel charge density applying the Ward-Dutton partition. Then, a theoretical derivation for the apparent negative drain-to-source capacitance is presented by modeling the channel with a lossy transmission line composed of several distributed RC elements. The correct modeling of the channel is fundamental to predict the dynamics when the device operates in these regions, such as RF switches or resistive drain mixers.