Improved two-dimensional electron mobility in asymmetric barrier delta-doped GaAs/AlGaAs modulation-doped field-effect transistor structures

Abstract

We study the enhancement of electron mobility μ in barrier delta-doped GaAs/AlGaAs quantum-well-based modulation-doped field-effect transistor (MODFET) structures. We asymmetrically vary the doping concentrations Nd1 and Nd2 in the barriers on the substrate and surface sides, respectively, to obtain a nonlinear enhancement of μ as a function of the well width w through multi-subband effects. We show that an increase in doping concentration increases the surface electron density Ns, which in turn enhances μ. Interchanging Nd1 and Nd2 leads to no change in Ns but rather, an enhancement of μ as a function of w for Nd2 > Nd1 owing to asymmetric variation of subband wave functions, thereby implying a higher channel conductivity in a surface-doped structure than in an inverted doped structure. By keeping (Nd1 + Nd2) unchanged, the conductivity of a single-channel MODFET, Nd1 (Nd2) ≠ 0 and Nd2 (Nd1) = 0, can be enhanced by considering a MODFET based on an asymmetrically doped (Nd1 ≠ Nd2 ≠ 0) quantum well structure. We show that the highest Ns and μ product for these structures occurs almost before the onset of the occupation of the second subband. Our analysis of the effect of asymmetric doping profiles on channel conductivity can be utilized for the performance improvement of MODFET-like devices.