Carrier Deconfinement Limited Velocity In Pseudomorphic AlGaAsiin GaAs Modulation-doped Field Effect Transistors (MODFET's)

This paper describes the first experimental evidence which suggests that carrier deconfinement, rather than the low 2DEG sheet density, limits the carrier velocity in pseudomorphic Al/sub x/Ga/Sub 1x/As/In/sub 15/Ga/sub 85/ as MODFET's for 0.1/spl les/ x /spl les/ 0.45. We use C-V at 300K and 77K to characterize charge control and dc-and-rf measurements to evaluate device performance. The highest 2DEG densities are obtained for 0.20 /spl les/ x /spl les/ 0.35 while best device performance for 0.10 /spl les/ x /spl les/ 0.30. The maximum effective velocity v/sub eff/ as deduced from S-parameter measuriments, is independent of sheet density but exhibits a dependence on Al mole fraction similar to that of mobility in bulk AlGaAs [1]. An effective velocity of /spl tilde/ 1.5 x 10/sup 7/ cm/s is estimated for 0.10 /spl les/ x /spl les/ 0.30, 1.3 x 10/sup 7/ cm/s for x 0.35, and 0.9 x 10/sup 7/ cm/s for x = 0.45. Our experimental data suggests for the first time that i) the maximum carrier velocity is not limited by low sheet densities and ii) the transport properties ouside the InGaAs channel have a significant impact on device performance due to the lack of carrier confinement.