GaInP grown lattice-matched to GaAs is a very attractive material system for application in heterojunction bipolar transistors (HBTs) due to its superior properties over the AlGaAs/GaAs system. In this paper, the etching characteristics of GaAs and GaInP using electron cyclotron resonance (ECR) plasma are reported. The high-density low ion energy plasma characteristic is beneficial for low-damage material processing. It was found that the GaAs and GaInP sidewall profile and surface morphology was greatly affected by the Cl-2/Ar flow rates, with near vertical sidewall profiles obtained at low Cl-2 and Ar flow rates of 10 sccm. Furthermore, the GaInP surface characteristic was very sensitive to Ar flow rate, with grass-like features observed at a flow rate of 40 sccm. Such features were absent at an Ar flow rate of 10 sccm, suggesting that high Ar flow retarded the etching by-product desorption. Increasing the DC self-bias from /20/ to /70/ V resulted in change of the GaInP sidewall profile from undercut to near-vertical characteristic along the (110) crystal plane. In the case of the GaInP/GaAs heterostructure, etching at /70/ V DC self-bias results in a smoother GaAs surface after the top GaInP layer was etched. Optimised conditions for GaInP/GaAs heterostructure. etching was established at Cl-2/Ar 10:10 sccm, /70/ V DC self-bias, 2.5 mtorr, 40 degreesC and 200 W microwave power. The relatively slow etch rate of 7.2 nm/s gives good control for mesa etching in device fabrication processes. The resulting GaAs root-mean-square (RMS) surface roughness is 4 nm.