HBTs and HEMTs (High Electron Mobility Transistors) are among the most advanced semiconductor devices. They both benefit from the use of heterojunctions formed by different materials. Among other different material parameters, the most important is the bandgap difference. This is in contrast to conventional homojunction devices, where junctions are utilized by p-type or n-type doping in the same material. The HBTs make use of wide bandgap emitter and narrow bandgap base. In an npn-transistor this favors the electron injection from the emitter to the base, and restricts hole injection from the base to the emitter. This advantage can be maintained even if the base is highly doped to get a low base resistance and the emitter is lightly doped. Microwave, millimeter-wave, and high-speed digital HBT ICs are used for microwave power and low power wireless communications applications between 0.9 GHz and 100 GHz.