Abstract
In this study, we investigated the effect of gate length and surface area of the ZnO ultraviolet (UV) absorbing structure on the transient characteristics of AlGaN/GaN HEMT based UV photodetectors. The gate-areas (2, 6 and 18 μm lengths with same width of 100 μm) of AlGaN/GaN HEMTs are covered with ZnO thin film and nanorods (NR) separately. The fabricated devices show enhancement in response speed with the reduction of gate length or the increase of ZnO surface area. The best response speed of ∼10 ms response time and ∼190 ms recovery time is measured from the NR-gated device with a gate length of 2 μm. A model for the oxygen desorption kinetics is proposed for the first time which theoretically shows that the response speed is dependent on two key parameters; light absorbing surface area and gate length. From our model analysis, it is shown that predicted response time is a strong function of these two device parameters, and the calculations show a good agreement with the experimental measurements.
Highlights
INTRODUCTIONBased on the theoretical estimations, we examined the effect of surface area of ZnO floating gate on the conduction mechanism of AlGaN/GaN HEMTs. Basically, HEMT-based sensors include a two-dimensional electron gas (2DEG) channel region of high electron carrier concentration induced by piezoelectric polarization of the strained AlGaN layer and spontaneous polarization of GaN.[23] Positive counter charges on the HEMT surface layer are induced by the 2DEG at the AlGaN/GaN interface
In this work ZnO nanorods/AlGaN/GaN HEMT structure is used as an active UV sensing device
The physics and device parameters influencing on the response speed such as desorption kinetics of oxygen at the surface of ZnO and device variables related to it have not been fully studied in depth
Summary
Based on the theoretical estimations, we examined the effect of surface area of ZnO floating gate on the conduction mechanism of AlGaN/GaN HEMTs. Basically, HEMT-based sensors include a two-dimensional electron gas (2DEG) channel region of high electron carrier concentration induced by piezoelectric polarization of the strained AlGaN layer and spontaneous polarization of GaN.[23] Positive counter charges on the HEMT surface layer are induced by the 2DEG at the AlGaN/GaN interface. Any slight changes in the ambient conditions can affect the surface charge of the HEMT, and it can change the electron concentration in the 2DEG channel at the AlGaN/GaN interface. The charging and discharging mechanism due to adsorption and desorption of ambient oxygen molecules affects the carrier concentration in 2-DEG channel. To experimentally validate our theoretical model, we fabricated HEMT-based active UV sensors as described in the followings by varying gate lengths from 2 to 18 μm and measured their photoresponse transient characteristics
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