An equivalent circuit with the physical structure of depletion region considered for determination the parasitic capacitances of MESFETs and HEMTs

An improved equivalent circuit model for GaAs MESFETs and HEMTs under zero drain bias and pinch-off is proposed. The presented method utilizes the physical structure of the depletion region under the gate in channel. Under the bias of pinch off at V/sub ds/=0V, the depletion region where internal fringing capacitances exist looks like a fiat box (hexahedron). Using the basic equation of parallel plate capacitors, the parasitic capacitances of MESFET and HEMT with the physical dimension of depletion region considered are calculated from the measured S-parameters under the pinched-off coldFET measurement.