Modulation Efficiency Limited High Frequency Performance of the MODFET

Epilayer design for the modulation doped field-effect transistor (MODFET) is frequently performed using the depletion approximation [1,2]. The work of authors such as GRAY and LUNDSTROM [3], STERN and DAS SARMA [4], and PONSE et al. [5] have provided tools by which electron distributions and capacitances can be calculated without this assumption. Their results demonstrate that for two-dimensional electron gas (2DEG) sheet densities significantly below the saturation value, free and bound electrons are present in the electron supplying layer. Because these electrons must be modulated with the 2DEG electrons, the gate capacitance is increased while the transcon- ductance is decreased [6]. The unity current gain frequency, fT, is thus decreased.