technology GaN transistors
Figure 5. Optical microscopy image of a GaN-based cantilever
Testing of these sensors in an environment capable of producing pressures up to 60 bar revealed that the drain-source voltage decreases in a linear fashion with increased voltage, diminishing by 0.02 percent per bar. This agrees with finite element models developed in the MORGaN project, which reveal that any deviations from linearity result from piezoelectric relaxation and imperfect clamping. When the sensor is placed in a bespoke package, it can operate at up to 80 bars and at 400 °C.
dogleg’ growth window with this lithographic process that enables ELOG growth along optimum crystallographic directions.
Simulations to optimise the design of the drumskin sensor have been performed by the University of Bath and the University Joseph Fourier. These universities have developed mechanical models for the sensor, which has six sensing elements (see Figure 4). Modelling enables fine-tuning of the thickness of the sapphire substrate for a particular pressure range.
Another contributor to this effort is the IEE Slovak Academy of Sciences, which has developed a high- temperature compatible fabrication process for this sensor. Conductive metal oxides are formed on the gate interface through thermal oxidation of evaporated and patterned thin nickel and iridium interfacial layers. Conductance is increased with this novel gate metallization process, and the device’s impressive transport characteristics are maintained after device annealing at 800 °C.
AlGaN/GaN cantilevers have also been fabricated on silicon substrates, thanks to efforts at MicroGaN. In this process cantilevers are defined, before dry etching selectively removes the silicon substrate from underneath. The chip design includes a temperature sensor and two cantilevers in a Wheatstone bridge configuration. The sensor operates by measuring the deflection of one cantilever – the other one serves as a reference in order to compensate for temperature effects (see Figure 5). Measurements have been made on this sensor at temperatures up to 300 °C and a range of deflections.
Acid or base?
Another aspect of the MORGaN project is the development by the Technical University of Ulm of ion- sensitive FETs that feature diamond electrodes on AlInN/GaN HEMTs. These chemical sensors can be built by either monolithic integration on one chip or by hybrid integration. With both designs the nano-crystalline diamond electrode is exposed to the electrolyte, and when hybrid integration is used this electrode is also connected to an external AlInN/GaN HEMT. This heterostructure must have resistance to the nano- crystalline diamond thermal deposition budget, which is a temperature of 800 °C for many hours.
Figure 6.(a) A packaged InAlN/diamond ion-sensitive FET (b) Steady-state values of the drain current of InAlN/nano-crystalline diamond electrochemical sensor as a function of the pH value
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www.compoundsemiconductor.net April / May 2012
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