A fully monolithically integrated L-band T/R module using InGaP/GaAs-based HBTs (heterojunction bipolar transistors) for both the transmit and receive functions is proposed. We plan to improve the efficiency by implementing a DHBT (double heterojunction bipolar transistor) InGaP HBT which is designed reduce the offset voltage. This DHBT design can offer a higher efficiency as well as increasing the safe operation region (SOA), which benefits both the low noise and high power requirements of a T/R module. The footprint of the device is expected to be reduced by 20-30% by integrating the T/R function using the same device InGaP DHBT platform. GaAs based devices are critical for the realization of reduced size and reduced power consumption MMIC (monolithic microwave integrated circuits) which provide the fundamental building blocks of T/R modules. InGaP HBT provide the lowest cost technology platform for high frequency devices in comparison to other device technologies such as Si-Ge and InP. In addition, InGaP/GaAs HBTs have been shown to be inherently radiation hard for both proton and g exposure, and are well suited for use in the space environment.