Design and optimization of high-power, high-frequency FET's using pseudomorphic layers.

Abstract

Strain modifies the bandstructure of a semiconductor such as GaAs in such a way that the threshold energy for impact ionization is increased in the strained layer. This effect and its application to submicron gate FET's are studied theoretically and experimentally. In the theoretical study the effects of strain on a semiconductor's bandstructure are studied. The effect of the bandstructure changes on impact ionization threshold energies is then shown. These calculated threshold energies are used in a two dimensional solution to the moments of the Boltzmann transport equation which includes impact ionization. This model is used to calculate the breakdown voltage in FET's and study the effects of the strained layers on these breakdown voltages. The model shows that breakdown in AlGaAs/GaAs HEMT's is dominated by impact ionization generation in the AlGaAs region. The most effective use of the thin, high threshold pseudomorphic material is in place of the AlGaAs layer under the gate. In the In$\sb{0.52}$Al$\sb{0.48}$As/In$\sb{0.53}$Ga$\sb{0.47}$As system the model shows that device breakdown is dominated by generation in the In$\sb{0.53}$Ga$\sb{0.47}$As channel. The best use of the pseudo-morphic layer in this case is in place of the channel. The experimental study includes the development of a submicron gate FET fabrication sequence. This fabrication study emphasizes the optimization of ohmic contacts to provide low access resistance and still be compatible with the rest of the fabrication process. After the FET fabrication sequence is developed, it is used to fabricate strained layer devices in both the GaAs system and the InGaAs on InP system. The power and frequency performance of these devices is compared to the power and frequency performance of lattice matched samples of similar design. These studies show that the pseudomorphic layers can improve the breakdown voltage of GaAs FET's by 65% with a corresponding improvement in the rf power output of 62%. The breakdown voltage of InAlAs/InGaAs FET's has been improved by 47% with the strained layers. The strained layer devices show no degradation in frequency performance compared to similar lattice matched devices.