A process to produce very smooth and highly anisotropic through-the-wafer slot via holes using reactive ion etching in Cl2/BCl3/Ar mixtures has been developed. In this study, we have extensively investigated the GaAs etch rate and resultant etch profiles as functions of bias voltage, gas ratio, flow rate, chamber pressure, aspect ratio, and etch time. An optimum via hole etching process using a Plasma Therm 790 reactive ion etching system was determined to be 300 V bias voltage, gas flow ratio of Cl2:BCl3:Ar=4:3:10, and chamber pressure of 15 mTorr. The average etch time for 20 μm wide ×60 μm long vias on a 50 μm thick 3 in. diam wafer is 180 min using a total flow rate of 50 sccm, which corresponds to an average etch rate of 0.3 μm/min. This process has been successfully implemented on two-stage Ka-band monolithic microwave integrated circuit driver amplifiers fabricated by TRW. Under a pulsed bias with Vds=5.0 V and an input power of 15 dBm, the average power added efficiency was greater than 40% with a 13–14 dB gain between 32 and 36 GHz. A peak power added efficiency of 44% was achieved at 35 GHz. © 1999 American Vacuum Society.