This work has been concerned with the characterization of deep levels in GaAs MESFET's fabricated by selective ion-implantation into undoped and Cr-doped LEC semi-insulating substrates. The deep level related substrate bias effect and backgating effect in these devices have also been investigated. For the characterization of electron traps in the ion-implanted layers, MESFET channel current DLTS and channel resistance transient techniques have been developed and used. Utilizing these techniques a method for determining trap concentrations has been developed. For the characterization of shallow defect levels near the n('+)/substrate interface, thermally stimulated current measurement technique has been found very useful. Deeper levels in this region have all been characterized using the channel resistance transient technique combined with substrate bias pulsing. Several electron traps present in the implanted layers of MESFET's have been detected and characterized. Among these EL2 is the dominant deep level in undoped and lightly Cr-doped substrates. There are two electron traps (at 0.22 eV and 0.85 eV) that exist only in undoped material, and with the addition of Cr they are suppressed and a new level at 0.64 eV emerges instead. A deep level at 0.53 eV is seen in all samples based on both undoped and Cr-doped materials. The substrate bias and backgating effects in MESFET's have been critically examined to determine their relationship with the deep levels. Reduction of MESFET channel current due to a negative substrate bias (the substrate bias effect) is very small in undoped devices and is quite significant in Cr-doped devices and this arises due to ionization of Cr and other hole traps. The role of electron traps in this effect has been analytically examined and it is shown that they cannot completely compensate the negative charges caused by the ionization of hole traps. In contrast to the substrate bias effect, a negative bias applied to an adjacent n('+) region (the backgating effect) is quite effective in reducing the MESFET channel current in both undoped and Cr-doped devices. This current reduction is effective only beyond a threshold backgate voltage. Variable energy infra-red excitation has been used to establish that the backgating effect is not caused by the ionization of hole traps. The influence of the surface conditions on conduction in n('+)/S.I. GaAs/n('+) structure, and in the related backgating effect, has been examined utilizing different test structures. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.
