Comprehensive study of InGaP/Al _xGa _1-xAs/GaAs heterojunction bipolar transistors with different doping concentrations of Al _xGa _1-xAs graded layers

The performances of InGaP/Al _xGa _1-xAs/GaAs heterojunction bipolar transistors (HBTs) with different doping concentrations of Al _xGa _1-xAs graded layers are theoretically studied. The use of the Al _xGa _1-xAs graded layer plays a key role in affecting the direct current and radio frequency performances of the studied HBTs. It is found that the studied devices with suitable doping concentrations of Al _xGa _1-xAs graded layers exhibit lower offset voltages, saturation voltages, and base and collector current ideality factors. Furthermore, due to the use of proper doping concentrations of Al _xGa _1-xAs graded layers, the studied devices show high values of the unity current gain cut-off frequency (f _T) and maximum oscillation frequency (f _max). It is known that, from the theoretical analysis, the appropriate doping concentration of the Al _xGa _1-xAs graded layer is 1 × 10 ¹⁶ to 1 × 10 ¹⁸ cm ^-3. Consequently, this work is promising for device engineers to design high-performance HBT structures.