Besides silicon carbide, group-III nitrides are also suitable large-band-gap semiconductor materials for high-temperature gas sensor devices. Exposing GaN-based Schottky diodes with catalytically active platinum electrodes to hydrogen, we observed a decrease of the rectifying characteristics which we attribute to a decrease in Schottky barrier height. Current–voltage and elastic recoil detection measurements were used to investigate the H-sensing behavior of such devices. Our results indicate an interfacial effect as the origin of the sensor response to hydrogen.

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In the present article recent results concerning sensor applications of AlGaN layers and AlGaN/GaN heterostructures are summarized. The piezoresistive effect in piezoelectric AlGaN layers is investigated and the dependence of the piezoresistive gauge factor on the Al content is attributed to the influence of strain induced piezoelectric fields. An enhancement of this effect is observed in AlGaN/GaN heterostructures, resulting in high longitudinal gauge factors.The response of gas sensitive Pt:GaN Schottky diodes to hydrogen and hydrogen containing gases is analyzed up to temperatures of 600°C and employed to realize gas sensitive field effect transistors which are demonstrated to operate up to 400°C. In addition, ion sensitive field effect transistors (ISFETs) have also been fabricated on the basis of AlGaN/GaN heterostructures. The GaN surface shows a high pH sensitivity which is attributed to the presence of a thin native metal oxide layer on the surface.

We review the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices. GaN films grown on sapphire by MOCVD or HVPE usually exhibit Ga-face polarity. N-face polarity is obtained either on the backside of such layers after removal from the substrate, or by turning the crystal polarity in MBE growth via a thin AlN buffer layer. In addition to rather obvious differences in their structural and morphological features, Ga-and N-face samples differ also in their electronic properties. Thus, different Schottky barrier heights are observed for both polarities, the position and detailed properties of spontaneously formed twodimensional electron gases vary with polarity, and the adsorption of gases and ions also show an influence of the two different surfaces. A particular interesting possibility is the growth of lateral polarity heterostructures with predetermined macroscopic domains of different polarity separated by inversion domain boundaries. These structures make use of the crystal polarity as a new degree of freedom for the investigation of electronic properties of III-nitrides and for novel devices. Introduction The fact that the III-nitrides InN, GaN, and AlN are essentially ionic solids with a strong charge transfer between the very electronegative nitrogen atoms and the less electronegative metal atoms (In, Ga, or Al) has important consequences for many physical properties of this material system.The thermodynamically stable wurtzite structure of III-nitrides has a polar axis parallel to the c-direction of the crystal lattice. Deviations of the real atomic charge distributions from the point charge model of the ideal wurtzite lattice give rise to a macroscopic spontaneous polarization P of the III-nitrides, which can reach values up to 0.1 C/m 2 . Electrostatically, such macroscopic lattice polarizations are equivalent to twodimensional fixed lattice charge densities s ¼ P with values between 10 13 and 10 14 e/cm 2 located at the two surfaces of a sample (about 1-10% of a monolayer) [1,2]. A direct consequence of this large macroscopic polarization and the corresponding fixed surface charges is the appearence of large internal electric fields,