Abstract. 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 × 10¹⁶ to 1 × 10¹⁸ cm⁻³. Consequently, this work is promising for device engineers to design high-performance HBT structures.

Print publication: Issue 3 (March 2004)
Received 11 March 2003, in final form 5 November 2003
Published 23 December 2003