Enhanced Sensitivity Pt/AlGaN/GaN Heterostructure NO₂ Sensor Using a Two-Step Gate Recess Technique

Based on our proposed precision two-step gate recess technique, a suspended gate-recessed Pt/AlGaN/GaN heterostructure gas sensor integrated with a micro-heater is fabricated and characterized. The controllable two-step gate recess etching method, which includes O₂ plasma oxidation of nitride and wet etching, improves gas sensing performance. The sensitivity and current change of the AlGaN/GaN heterostructure to 1-200 ppm NO₂/air are increased up to about 20 and 12 times compared to conventional gate device, respectively. The response time is also reduced to only about 25 % of value for conventional device. The sensor has a suspended circular membrane structure and an integrated micro-hotplate for adjusting the optimum working temperature. The sensitivity (response time) increases from 0.75 % (1250 s) to 3.5 % (75 s) toward 40 ppm NO₂/air when temperature increase from 60°C to 300°C. The repeatability and cross-sensitivity of the sensor are also demonstrated. These results support the practicability of a high accuracy and fast response gas sensor based on the suspended gate recessed AlGaN/GaN heterostructure with an integrated micro-heater.

I. Introduction

Recently, there have been growing concerns about environment pollution, such as photochemical smog and acid rain, was caused by the growing nitrogen dioxide (NO₂) emission mainly from combustion of automotive exhaust [1], [2], industrial processes [3]. Especially, the monitoring of exhaust gas of the automotive vehicles was required for the feedback control of the catalytic reduction system (SCR) in order to reduce NO₂ emission. Accordingly, the NO₂ sensor installed in the automobile exhaust gas after-treatment-system have to work in the harsh environment of high temperature. The harsh environment, low power, continuous NO₂ monitoring sensors with excellent sensing performance was desired.