MODFET VERSUS MESFET - THE CAPACITANCE ARGUMENT

In this paper a detailed charge control analysis is presented for a ME SFET and a MODFET to show how the various capacitances associated with the region under the gate contact influence device performance. Where necessary, an exact description of electron confinement is included i n the solution scheme by solving the effective mass Schrodinger equati on for a single band and within a self-consistent framework. In the ca se of the MESFET, the analysis shows that the filling of donor impurit y states, which are coincident in real space with the conducting chann el, gives rise to a large parasitic capacitance which severely degrade s both the intrinsic transconductance and cutoff frequency performance . In the case of the MODFET, the separation of the donor impurity leve l in real space from the conducting channel, and in energy from the Fe rmi-level, results in a low parasitic capacitance over the whole opera ting region of the device. Thus it is shown that the MODFET appears to be more suited to millimeter-wave applications, regardless of any ele ctron transport effects.