Integration of sensors and GaAs electronic circuits on a single chip

Surface micromachining techniques have been applied in a GaAs MESFET (Metal Semiconductor Field Effect Transistor) process to form both piezoelectric pressure sensor and infrared radiation detector. These sensors are based on piezoelectricity and pyroelectricity of ZnO thin film. The microsensor incorporates a 1 μm-thick sputtered ZnO capacitor supported by a 2 μm-thick aluminum membrane, which is 1 μm elevated above the semi-insulating GaAs substrate. The piezoelectric pressure sensor of an area of 80 x 80 μm² exhibits 2.99 μV/μbar sensitivity at 400 Hz and agrees well with the value predicted by a model based on the small deflection theory of thin plates. The voltage response of a single infrared detector of an area of 80 x 80 μm² is 6.2 mV at 10 Hz and the time constant is 53 ms. A differential amplifier whose voltage gain is 33.7 dB using 4 μm gate GaAs MESFETs is fabricated for high selectivity to the physical variable being sensed. GaAs MESFETs fabricated in a planar, direct ion-implanted process feature lightly doped shallow channel layers and the measured transconductance is 25.6 mS/mm at room temperature.