MODFET versus MESFET: the capacitance argument

In this paper a detailed charge control analysis is presented for a MESFET 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 in the solution scheme by solving the effective mass Schrodinger equation for a single band and within a self-consistent framework. In the case of the MESFET, the analysis shows that the filling of donor impurity states, which are coincident in real space with the conducting channel, gives rise to a large parasitic capacitance which severely degrades both the intrinsic transconductance and cutoff frequency performance. In the case of the MODFET, the separation of the donor impurity level in real space from the conducting channel, and in energy from the Fermi-level, results in a low parasitic capacitance over the whole operating region of the device. Thus it is shown that the MODFET appears to be more suited to millimeter-wave applications, regardless of any electron transport effects.<
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