Theory and observation of enhanced, high field hole transport in Si_1-xGe_x quantum well p-MOSFET's

We report on the observation of enhanced high field hole velocity in strained Si/Si_1-xGe_x/Si quantum wells. This effect manifests itself in the drive current capability of nanometer scale p-channel Quantum Well Metal-Oxide-Semiconductor-Field-Effect-Transistors (p-QWMOSFET's). The high-field transport of a two-dimensional hole gas confined in a Si/Si_1-xGe_x/Si quantum well is formulated and solved. The results indicate an increase in the hole saturated drift velocity in strained SiGe quantum wells with increasing Ge mole fractions up to x = 0.5. This is a consequence of the optical phonon spectrum of the strained SiGe alloy remaining Si-like (i.e., high energy) while the carrier transverse effective mass decreases with higher Ge content. To investigate the theoretical prediction of increased high-field drift velocity, p-QWMOSFET's were fabricated with Si/Si_1-xGe_x/Si quantum well heterostructures grown by Molecular Beam Epitaxy (MBE) with varying Ge mole fractions, x. The fabrication sequence maintained a low thermal budget to prevent strain relaxation in the SiGe layer and involved a mixed optical/electron beam lithography scheme to define junction-isolated transistors with a minimum drawn gate lengths of 200 nm. The measured saturated transconductance, g_msat, of the p-QWMOSFET's were 20-50% higher than that of a reference Si p-MOSFET under equivalent biasing conditions. The importance of this g_msat increase for high-speed, low-power VLSI applications is discussed.