AlGaAs/InGaAs quantum doped-channel field-effect transistors with a mesa gate and an airbridge gate were fabricated and compared. An additional processing technique was employed in fabricating the required airbridge with multiple piers. Experimental results reveal that the airbridge technology does not degrade device yield even if the materials were not selectively removed. Both dc and rf performance are enhanced when the airbridge technique is used in device fabrication. We obtained maximum extrinsic transconductance, available current density and breakdown voltage to be 240 mS mm −1 , 350 mA mm −1 and 8.5 V for a 1 × 100 µm 2 airbridge-gate device. A mesa-type device exhibits smaller values than the airbridge-gate one. Furthermore, the measured unit-current-gain frequencies were 19 and 14 GHz for airbridge-and mesa-gate devices, respectively.

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