Optimal channel grading in p-type Si/SiGe metal oxide semiconductor field effect transistors (MOSFETs)

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Abstract
Quantum numerical modelling is employed to show that the cutoff frequency of p-channel Si/SiGe metal oxide semiconductor field effect transistors can be optimized by fully grading the Si1−xGe1−x channel from 0% Ge (at the substrate end) to 40% Ge (at the gate side). A comparison is made with previously reported, partially graded channel devices. The valence band potential drop owing to Ge grading is shown to provide sufficient isolation between the free carriers in the channel and the acceptor atoms in the δ-doped layer underneath. A simple formalism is used to calculate saturation region cutoff frequency from one-dimensional simulations and to show how free carrier mobility profiles can be obtained from low-frequency small-signal measurements.