AlGaN/GaN-based diodes and gateless HEMTs for gas and chemical sensing

The characteristics of Pt/GaN Schottky diodes and Sc/sub 2/O/sub 3//AlGaN/GaN metal-oxide semiconductor (MOS) diodes as hydrogen and ethylene gas sensors and of gateless AlGaN/GaN high-electron mobility transistors (HEMTs) as polar liquid sensors are reported. At 25/spl deg/C, a change in forward current of /spl sim/6 mA at a bias of 2 V was obtained in the MOS diodes in response to a change in ambient from pure N/sub 2/ to 10% H/sub 2// 90% N/sub 2/. This is approximately double the change in forward current obtained in Pt/GaN Schottky diodes measured under the same conditions. The mechanism appears to be formation of a dipole layer at the oxide/GaN interface that screens some of the piezo-induced channel charge. The MOS-diode response time is limited by the mass transport of gas into the test chamber and not by the diffusion of atomic hydrogen through the metal/oxide stack, even at 25/spl deg/C. Gateless AlGaN/GaN HEMT structures exhibit large changes in source-drain current upon exposing the gate region to various polar liquids, including block co-polymer solutions. The polar nature of some of these polymer chains lead to a change of surface charges in gate region on the HEMT, producing a change in surface potential at the semiconductor/liquid interface. The nitride sensors appear to be promising for a wide range of chemicals, combustion gases and liquids.

I. Introduction

There is an emerging interest in the development of wide bandgap semiconductor chemical, radiation, and combustion gas sensors because of their Fig. 1. Schematic of Pt/GaN Schottky diode for gas sensing. ability to operate under harsh environmental conditions and be integrated with on-chip wireless telemetry systems [1]–[12]. In addition to homeland security, these detectors would have dual-use in automobiles and aircraft, fire detectors, fuel leak detection in spacecraft, exhaust diagnosis and emissions from industrial processes. GaN is capable of operating at much higher temperatures than more conventional semiconductors such as Si. Semiconductor-based gas sensors have been developed for a number of years using different designs including capacitance, field-effect transistors and Schottky diodes. These sensors have been shown to be highly sensitive to several gases including hydrogen, carbon monoxide, and hydrocarbons. One additional attractive attribute of GaN is the fact that gas sensors based on this material could be integrated with high-temperature electronic devices on the same chip. There have already been initial reports of rad-hard combustion gas detectors with extremely fast time response and capable of operating at high temperatures, eliminating bulky and expensive cooling systems [4], [7]–[9], [11], [12].