Electronic properties of quantum wells for field effect transistor applications

Several alternative techniques for improving the transport properties of delta-doped quantum well structures have been investigated. The most successful have used edge delta-doping and compositional grading to produce significantly enhanced electron transport characteristics in AlxGa1-xAs/GaAs, AlxGa1-xAs/InyGa1-yAs, and Al xIn1-xAs/InyGa1-yAs quantum well structures. This compositional grading has been achieved using the digital alloying growth technique---which has been confirmed as a viable method of producing good quality, delta-doped material. Improvements of up to 100% in mobility (to 2630cm2/Vs) and 67% in electron saturation drift velocity (to 1.7 x 107 cm/s) have been achieved using these techniques. The dependence of electron saturation drift velocity (vsat) on mobility (mu) has been determined as vsat alpha mu0.8+/-0.3. In addition, we present our investigations of the free-carrier loss observed in a series of highly Si delta-doped AlxGa1-xAs/GaAs quantum well structures. We interpret the results of Hall measurements and self-consistent Poisson Schrodinger modelling in terms of a model of DX centre formation which includes Coulomb interactions, a result of the charged nature of the DX state. Our interpretation implies a strongly growth dependent DX centre energy---which explains the range in published values for the GaAs DX centre energy. The data allows investigation of the DX centre distribution between 214 and 249 meV above the F minima, and confirms a considerably broadened DX centre density of states ≥35 meV).