A fully integrated electromechanical resonator is described that is based on high mobility piezoelectric semiconductors for actuation and detection of nanoscale motion. We employ the two-dimensional electron gas present at an AlGaN/GaN interface and the piezoelectric properties of this heterostructure to demonstrate a resonant high-electron-mobility transistor enabling the detection of strain variation. In this device, we take advantage of the polarization field divergence originated by mechanical flexural modes for generating piezoelectric doping. This enables a modulation of carrier density which results in a large current flow and thus constitutes a motion detector with intrinsic amplification.

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The piezoelectric actuation of a micro-electro-mechanical system (MEMS) resonator based on an AlGaN/GaN heterostructure is studied under various bias conditions. Using an actuator electrode that is also a transistor gate, we correlate the mechanical behaviour to the two-dimensional electron gas (2DEG) presence. The measured amplitude of the actuated resonator is maximum at moderate negative biases and drops near the pinch-off voltage in concordance with the 2DEG becoming depleted. Below the pinch-off voltage, residual actuation is still present, which is attributed to a more complex electric field pattern supported by quantitative modelling. The results confirm that epitaxial AlGaN barriers are fully adapted to the piezoelectric actuation of MEMS.

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AlGaN/GaN HFETs up to 800°C in vacuum [3]. These devices operate up to 600°C with the use of ohmic contacts with a high specific contact resistivity (3 ϫ 10 Ϫ3 ⍀ cm 2 ). Above 600°C, an irreversible degradation of the device performance was observed [3]. In this Letter the high-temperature dc performance of AlGaN/GaN high electron mobility transistors (HEMTs) is reported. The measurement is carried out under exposure to air, which constitutes realistic but drastic conditions. DEVICE TECHNOLOGYAlGaN/GaN HEMTs are processed on epilayers grown by metal organic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. It consists of a 3-m GaN undoped layer and a 300-Å Si-doped Al X Ga 1ϪX N layer.Then, Ti/Al/Ni/Au (150 Å/2200 Å/400 Å/500 Å) metallization layers are evaporated to make ohmic contacts. These contacts are annealed under nitrogen atmosphere at 900°C for 40 s [4]. The mesa isolation is made by reactive ion etching (RIE) using 4 sccm of SiCl 4 gas, a rf power of 200 W, and a pressure of 20 mTorr, resulting in an etch rate of 180 Å/min. The gate length is defined by electron beam lithography and varies from 0.3 to 2 m. The metallization layers used for the Schottky contact are Pt/Au (100 Å/1000 Å). The gate width varies from 2 ϫ 25 to 2 ϫ 75 m, and the gate-to-drain and gate-to-source spacings are 1 m. The devices are not passivated. DEVICE MEASUREMENT AND PHYSICAL UNDERSTANDINGHigh-temperature transistor performances require a high temperature stability of the ohmic contacts. It has previously been shown that the ohmic contacts using Ti/Al/Ni/Au metallization layers are stable on GaN epilayer up to 600°C during at least 5 days of exposure to air [5]. Figure 1 shows the evolution of this resistance, versus aging time at 550°C for ohmic contacts deposited on Al 0.3 Ga 0.7 N with the same metallization schemes as described above. These contacts have a low specific contact resistivity (9.6 ϫ 10 Ϫ6 ⍀ cm 2 ) and a small contact resistance (0.46 ⍀ mm) in comparison with literature results [4]. They are stable in air at least up to 500°C for 50 h and for several hours at 600°C. It has also been observed that the sheet resistance of the AlGaN material increases after a long exposure to air at 500°C and 600°C, even when the specific contact resistivity stays constant. This phenomenon can be explained by thermal oxidation of the AlGaN cap layer due to a high Al content of 30%. The passivation of the AlGaN cap layer should circumvent this problem.After the ohmic contact technology validation for high-temperature applications, the high-temperature effect on the dc electrical behavior of AlGaN/GaN HEMTs has also been studied. The devices are heated in air from room temperature up to 700°C with a step of 50°C. dc electrical measurements are performed for each temperature step. Figure 2 shows that the AlGaN/GaN HEMTs operate up to 550°C. This result confirms that the current HEMT technology is developed enough for use in high-temperature applications. The operating temperature limit is due to the Schottky contact d...