Investigations of the role of various transport channels have been carried out for single GaAs/AIAs and double barrier AIAs/GaAs/AIAs structures. Curremt-voltage measurements have been performed in both forward and reverse biases at pressures up to 11 kbar. Measurements at room and liquuid niterogen temperatures were carried out at various pressures while measurements at temperatures were carried out at various pressures while measurements at temperatures between 8 K and 77 K were performed at ambient pressure in a closed cycle cryostat. Variations im amplitude and position of current maxima and minima as well as regions of negative. differential resistemce with applied pressures help determine the dominant tunnelling channels. In the single barrier structure an observed increase in current with hydrostatic pressure can be attributed to tunnelling involving the X-point minima. The voltage at the peak shifts linearly downwards in voltage with applied pressure. A comparison with self-consistent theoretical calculations indicates that the peak arises due to quasi-resonances between - point confined 2D subbands in the accumulation layer and unoccupied longitudinal X-point confined subbands in the barrier. In the double barrier structure asymmetrical doping of the electrodes helps us identify the origin of resonances in terms of the X-point conduction-band minima. X-X and -X resonances have been observed at high pressure. At ambient pressure and temperature the X-minima appear to play an important role. An investigation of surface state pinning at metal/semiconducator interfaces has been based on the low temperature I-V characteristics of Schottky diodes. Pd/n+-GaAs Schottky diodes were exposed toa hydrogen plasma at temperatures between 20 to 200'C. Current-voltage measurements have been performed at temperatures down to 8 K in a closed cycle cryostat, while measurements under hydrostatic pressures up to 11 kbar were performed in a liquid pressure cell at 77 K and room temperatures. The Schottky barrier height and mean impurity concentratin of the samples were then obtained by comparison between experimental data and a self-consistent theoretical model. The Schottky barrier height has been observed to increase from 0.44 eV for unpassivated samples to 0.46 eV for sample hydrogenated at 100'C. This was accompanied by a general decrease in mean impurity concentration. Possible interpretation are discussed.