Doping effects and compositional grading in Al(x)Ga(1-x)As/GaAs heterojunction bipolar transistors

The AlGaAs/GaAs heterojunction bipolar transistor (HBT) is now recognized as an excellent high-gain and high-speed device for microwave and digital applications. Grinberg et al. (1984) have obtained analytical expressions for the current-voltage characteristics of an HBT using the thermionic-field-diffusion model. However, the agreement between this model and the experimental results needs to be improved. A knowledge of the real shape of the energy-band diagram would provide a better insight into the various current-injection mechanisms. A study has, therefore, been conducted of the energy-band diagrams of n-Al(x)Ga(1-x)As/p-GaAs heterojunctions for varying doping densities with and without compositional grading.