van Dalen, Rob; (1998) Organisation of p-type two dimensional semi-conductor structures for FET applications. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The phenomenally high electron mobility that can be achieved in GaAs-based heterostructures has led to the development of n-type Heterostructure Field Effect Transistors (HFETs) with enhanced power handling capabilities and high-frequency performance. Performance of p-channel HFETs is substantially lower, essentially because of the lower hole mobility as compared to that of electrons. The latter has almost completely prevented their use, despite the intrinsic advantages of complementary circuits. An investigation to enhance the performance of p-type FETs for power applications by means of selective doping and wavefunction engineering was performed, both theoretically as well as experimentally. A self-consistent multiband 'k-dot-p' effective mass model was set up to provide detailed bandstructure information, this model was then used in conjunction with an RPA scattering model to provide estimates for the hole mobility. The experimental mobilities obtained for a set of 60Å channel-delta-doped InGaAs-AlGaAs QWs show an improvement in the mobility by a factor 2.5 when moving the impurity plane from the centre of the QW toward its interface, in qualitative agreement with the theoretical predictions. This verifies the capabilities of such a model to optimise p-type performance, as well as illustrates the possible gain in device performance by careful design of the structure. A related investigation in this thesis concerns the elementary question of how to connect a wavefunction across an interface between two different materials, which has long been a major cause of controversy in effective mass theory. Insight in this problem was not provided until the derivation of an 'exact' envelope function theory, and it was shown that the resulting boundary conditions for [001] growth differ considerably from the conventional ones. In this thesis, the extension to arbitrary growth directions is provided in the form of a set of boundary condition rules that is to replace the traditional symmetrisation procedure.

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