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We report on a versatile processing technology for the fabrication of micro-electromechanical systems in gallium nitride (GaN). This technology, which is an extension of photo-electrochemical etching, allows for the controlled and rapid undercutting of p-GaN epilayers. The control is achieved through the use of opaque metal masks to prevent etching in designated areas, while the high lateral etch rates are achieved by biasing the sample relative to the solution. For GaN microchannel structures processed in this way, undercutting rates in excess of 30 μm/min have been attained. We propose two mechanisms to account for these high etch rates.

We report on the use of metal-insulator-semiconductor ͑MIS͒ diodes, formed on n-GaN with SiO 2 , for capacitive strain sensing. These diodes, when subjected to static strain, were found to exhibit a steady-state change in capacitance. As a result, they can be used to detect strain with frequencies all the way down to dc. We formulate a model to explain the action of piezoelectricity in the diode and obtain excellent agreement with measurements. The model is then used to develop design criteria which optimize the sensitivity of the diode to detect strain. The sensitivity of the devices tested here rivals that of the best silicon piezoresistive sensors, but could attain nearly tenfold improvement with only minor design changes. Finally, we consider the effects of interface states on sensor performance and demonstrate how static strain sensing in GaN MIS diodes is enabled by the high quality of the oxide interface.

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We report on the electromechanical response of Schottky diodes on n-GaN as a function of the strain frequency and the applied dc bias. These measurements reveal excellent strain detection sensitivity for frequencies above ∼10 Hz. The observed amplitude and phase of the electromechanical output can be largely explained using a simple model of piezoelectric charge generation on either side of the depletion layer. In addition, we report on the noise spectral density from these diodes under the same conditions and infer a signal to noise ratio for strain detection.