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A high-speed monolithic sample-and-hold (S/H) implemented with a GaAs/AlGaAs heterojunction bipolar transistor (HBT) IC technology is presented. This circuit is believed to be the first GaAs HBT S/H demonstrated. The S/H fabrication employs molecular-beam epitaxy and circuit integration with 3 µm emitter HBT, integrated Schottky diode, thin-film resistor, MIM capacitor, and double-level metal interconnect. S/H speed performance includes track bandwidth (-3 dB) of 1.8 GHz and sampled-mode analog bandwidth of 400 MHz at a sample rate of 800 Ms/s. The low frequency distortion (3rd harmonic) was at least -50 dBc for an input amplitude of 250 mV peak-to-peak. Preliminary test results indicate that the S/H has the potential for the accuracy required in 8-bit data conversion systems up to 400 Ms/s. Further development in processing is expected to improve both sampling rate and accuracy.