Ge/GaAs Hetero-structured n-p-n Transistor

The presented n-p-n Heterostructure Bipolar Transistor (HBT) is comprised of Ge/GaAs heterojunctions consisting of 0.15μm thick emitter and 0.65μm collector junctions. High diffusivity of carriers in GaAs base was the major motivation of current design. Grading of the base has been avoided in this structure, which is otherwise common in heterojunction bipolar transistors, in order to keep the electron diffusivity as high as possible. The electrons were injected into the 0.25μm thick p-type GaAs base with not very high doping(10
¹⁷
cm
⁻³
). The designed HBT enables a cut-off frequency on the order of 130GHz. The Ge/GaAs heterojunctions presented in this paper have proved to work better than comparable HBTs having GaAs bases and emitter/collector junctions made, for example, of AlGaAs/GaAs or other III–V compound semiconductors. The difference in lattice constants between Ge and GaAs is less than 2%. Therefore, there is no need of transition layers between Ge emitter and GaAs base. A significant difference in the energy gap of these two materials presents new scope for improving the performance of the emitter. With the complete structure being modeled and simulated using TCAD (Technology Computer Aided Design) on Silvaco, the collector/emitter offset voltage of the device has been limited to a reasonable value of 63 millivolts by dint of low energy band gap value associated with Ge emitter. The efficiency of the emitter in the HBT is 86%. The use of Germanium in the emitter and collector regions presents new opportunities for integration of this vertical device structure into silicon substrate.