Abstract
The microwave performance potential of Si/SiGe pseudomorphic MODFETs are studied, in comparison to state of the art InGaAs pseudomorphic HEMTs. Both devices have equivalent structures corresponding to a physical HEMT used for calibration. We use an RF analysis technique based on transient Monte Carlo simulations to estimate the intrinsic noise figures, the RF figures of merit fT and fmax, and the effect of contact and gate resistances. Both devices exhibit velocity overshoot below the gate region. It is shown that the difference in noise figures and fT values can be mainly attributed to differences in device channel velocity, fmax exhibits a strong dependence on device contact resistance, eroding some of the performance advantage of the pseudomorphic HEMT.
Highlights
Recent theoretical and experimental studies show that low field mobility and velocity overshoot are enhanced in Si layers grown pseudomorphically on relaxed SiGe substrates
The average velocity in the channel of both devices is compared in Figure 2 for a d.c bias of VD 1.5 V and V -0.25 Vo In the pseudomorphic HEMT
This corresponds to the region of maximum transconductance, while in the pseudomorphic Modulation doped field effect transistors (MODFETs) it is the region of maximum transconductance achieved while constraining parallel conduction in the -doped region to less than 10% of total conduction
Summary
Recent theoretical and experimental studies show that low field mobility and velocity overshoot are enhanced in Si layers grown pseudomorphically on relaxed SiGe substrates. Induced strain breaks the six-fold degeneracy of the Si conduction band, resulting in an .improved band offset for the two conduction valleys whose transverse effective mass is in the plane of the heterojunction. This increases the in plane effective mobility and reduces intervalley scattering in the Si layer [1]. Modulation doped field effect transistors (MODFETs) based on this material system have been demonstrated, and show significant potential for RF applications [2,3]. Comparison of strained Si channel devices with well established members of the III-V family will provide greater insight into their potential performance and limitations
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