Epitaxial technology for the monolithic integration of HBT and PHEMT

We have investigated the optimal growth conditions for integrating heterojunction bipolar transistors (HBTs) and pseudomorphic high electron mobility transistors (PHEMTs) together by metal-organic chemical vapor depositon (MOCVD). In the structure of HBT and PHEMT (BiFET), AlGaAs PHEMT is at the bottom, while InGaP HBT is on the top. The HBT and PHEMT share a heavily n-doped GaAs layer that serves as the cap of the PHEMT and the subcollector of the HBT simultaneously. A heavily n-doped InGaP layer under the HBT subcollector layer is used as an etching-stop layer for controlling the deepness of the gate recess during the PHEMT process. Through the investigation using capacitance-voltage measurement, van der Pauw measurement, high resolution secondary ion mass spectrometry, and photoluminescence and x-ray diffraction measurement, we found that the thermal cycle of InGaP HBT results in the out-diffusion of Si in the heavily doped InGaP layer and donor layer, leading to the increment in the sheet electron density and the decrement in the electron mobility, both degrading the performance of the PHEMT. After lowering the growth temperature of the HBT, Si out-diffusion is inhibited, which brings about the performance recovery of the PHEMT. Though the temperature is reduced, the DC characteristics of the HBT are still as good as those of the HBT grown at high temperature.