Piezoelectric pressure sensors and pyroelectric infrared detectors based on ZnO thin film have been integrated with GaAs metal-semiconductor field effect transistor (MESFET) amplifiers. Surface micromachining techniques have been applied in a GaAs MESFET process to form both microsensors and electronic circuits. The on-chip integration of microsensors such as pressure sensors and infrared detectors with GaAs integrated circuits is attractive because of the higher operating temperature up to 200 oC for GaAs devices compared to 125 oC for silicon devices and radiation hardness for infrared imaging applications. The microsensors incorporate a 1${mu}$m-thick sputtered ZnO capacitor supported by a 2${mu}$m-thick aluminum membrane formed on a semi-insulating GaAs substrate. The piezoelectric pressure sensor of an area 80${ imes}$80 ${mu}$m2 designed for use as a miniature microphone exhibits 2.99${mu}$V/${mu}$ bar sensitivity at 400Hz. The voltage responsivity and the detectivity of a single infrared detector of an area 80${ imes}$80 $mu extrm{m}$2 is 700 V/W and 6${ imes}$108cm$.$ Hz/W at 10Hz respectively, and the time constant of the sensor with the amplifying circuit is 53 ms. Circuits using 4${mu}$m-gate GaAs MESFETs are fabricated in planar, direct ion-implanted process. The measured transconductance of a 4${mu}$m-gate GaAs MESFET is 25.6 mS/mm and 12.4 mS/mm at 27 oC and 200oC, respectively. A differential amplifier whose voltage gain in 33.7 dB using 4${mu}$m gate GaAs MESFETs is fabricated for high selectivity to the physical variable being sensed.