The covalent functionalization of GaN and AlN surfaces with organosilanes is demonstrated. Both octadecyltrimethoxysilane and aminopropyltriethoxysilane form self-assembled monolayers on hydroxylated GaN and AlN surfaces, confirmed by x-ray photoelectron spectroscopy and atomic force microscopy. The monolayer thickness on GaN was determined to 2.5± 0.2 nm by x-ray reflectivity. Temperature-programmed desorption measurements reveal a desorption enthalpy of 240 kJ/ mol. The application of these devices for electronic detection of specific biomolecular processes is a promising approach for novel biosensors based on molecular recognition, such as specific antibody detection or label-free detection of deoxyribonucleic acid ͑DNA͒ hybridization. For this purpose, the covalent attachment of specific molecules with controlled structural order and composition on group III-nitride devices is a basic requirement.Covalent coupling of biomolecules to oxidized and hydrogen-terminated silicon surfaces has been investigated in numerous works during recent decades. 3,4 Covalent grafting of self-assembled monolayers ͑SAMs͒ of organosilanes on oxidized silicon surfaces 5-7 is widely used as the first step of surface functionalization with biomolecules for electronic detection of enzyme activity and DNA hybridization with Si-based field effect transistors. 8,9 In contrast, the surface chemistry on AlGaN alloys has not yet been studied in great detail. Pioneering work has been carried out by Bermudez, 10,11 who has analyzed the adsorption of different organic molecules, such as anilines or octanethiols on GaN surfaces, from the gas phase. In a recent publication Kang et al. 12 have reported the electrical detection of immobilized proteins on AlGaN / GaN transistors, modified with aminopropyltriethoxysilane ͑APTES͒ molecules in a liquid phase reaction. However, the covalent immobilization of molecules was not proven.In this letter, we report the covalent attachment of SAMs of octadecyltrimethoxysilane ͑ODTMS͒ and APTES on hydroxylated GaN and AlN surfaces. Complementary analysis by x-ray photoelectron spectroscopy ͑XPS͒, atomic force microscopy ͑AFM͒, and temperature-programmed desorption mass spectrometry proves covalent coupling of the SAMs to the surface. Immobilization of single-stranded 20-mer oligonucleotides on APTES-modified nitride surfaces and hybridization is demonstrated by fluorescence microscopy.GaN and AlN layers with a thickness of 2 m were grown by metalorganic chemical vapor deposition ͑MOCVD͒ on c-plane sapphire substrates. AFM over a scan area of 4 m 2 revealed a root-mean-square surface roughness of 0.2 nm for GaN and 0.3 nm for AlN, respectively.The presence of hydroxyl groups on the hydrophilic surface is an essential requirement for the silanization process. 5-7 Therefore, the substrates were immersed in H 2 SO 4 :H 2 O 2 ͑3:1͒ solution for 20 min, rinsed with deionized water ͑18 M⍀ cm, Millipore͒ and dried under nitrogen flux prior to silanization. In addition to the activation of surface hydroxyl groups, this...

An AlGaN / GaN electrolyte gate field-effect transistor array for the detection of electrical cell signals has been realized. The low-frequency noise power spectral density of these devices exhibits a 1/ f characteristic with a dimensionless Hooge parameter of 5 ϫ 10 −3 . The equivalent gate-input noise under operation conditions has a peak-to-peak amplitude of 15 V, one order of magnitude smaller than for common silicon-based devices used for extracellular recordings. Extracellular action potentials from a confluent layer of rat heart muscle cells cultivated directly on the nonmetallized gate surface were recorded with a signal amplitude of 75 V and a signal-to-noise ratio of 5:1.

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.

Enzyme-modified field-effect transistors (EnFETs) were prepared by immobilization of penicillinase on AlGaN∕GaN solution gate field-effect transistors. The influence of the immobilization process on enzyme functionality was analyzed by comparing covalent immobilization and physisorption. Covalent immobilization by Schiff base formation on GaN surfaces modified with an aminopropyltriethoxysilane monolayer exhibits high reproducibility with respect to the enzyme/substrate affinity. Reductive amination of the Schiff base bonds to secondary amines significantly increases the stability of the enzyme layer. Electronic characterization of the EnFET response to penicillin G indicates that covalent immobilization leads to the formation of an enzyme (sub)monolayer.