Optimization of heterostructure design for switching pHEMT transistors

Abstract

In this work, the design of one-side-doped pHEMT heterostructures was varied to obtain better microwave switching transistor parameters. For this purpose, the doping level, the method of introducing the doping impurity and the type of spacer were varied. It was obtained that the electron density in the heterostructures increases without saturation up to a sheet donor concentration of 9 × 1013 cm−2. In the sheet donor concentration range of (5–6) × 1012 cm−2 a sharp increase in electron density in δ-doped heterostructures was observed. At an optimum concentration of the donors of 6 × 1012 cm−2 the conductivity of heterostructures with modulation doping from the supply layer is 9% higher than that of δ-doped heterostructures. The use of a spacer in the form of an AlAs/GaAs superlattice leads to a decrease in the electron density, which is more pronounced for δ-doped heterostructures. But the conductivity of heterostructures with AlAs/GaAs spacer is higher due to better mobility. Measurements of electron density and mobility, as well as the conductivity of heterostructures together with the calculation of electron concentration at different gate biases by the self-consistent solution of Schrödinger and Poisson equations showed that δ-doped heterostructures at 6 × 1012 cm−2 and AlAs/GaAs spacers are most preferable for switching transistors. These heterostructures were used to fabricate monolithic integrated circuits of single-pole double throw pHEMT-based switches with gate length and widths of 0.5 μm and 100 μm, respectively. The resulting transistors had the following parameters: gm = 400 mS/mm, saturation current density ID = 380 mA/mm, ON-state resistance 1.0 Ohm × mm, OFF-state capacitance 0.37 pF/mm. Parameters of the fabricated switches at frequency of 20 GHz are the following: insertion loss 2.2 dB, isolation 56 dB, return loss 11.7 dB, linearity P1dB 21 dBm and IIP3 - 40 dBm.