| Summary: | 碩士 === 國立成功大學 === 光電科學與工程研究所 === 97 === In this study, AlGaN/GaN hetero-junction photo-transistors (HPTs) in detecting ultraviolet spectra have been fabricated and characterized. One of the HPTs features as low-temperature grown GaN (LT-GaN) insertion layer between based and emitter layers to reduce dark current. The internal gain and response time are also studied for the HPTs with and without the LT-GaN layer.
The lack of internal gain in conventional p-i-n PDs often limits its applications in detecting low incident photon levels. The principle of neutral charges and band bending of band discontinuity in HPTs enhances internal gain and photo response. In principle, the band discontinuity at Base/Emitter hetero-junction leads to accumulation of photo-excited holes originating from the Base/Collector homo-junction. Thus electrons from external circuit must enter the device to neutralize the hole accumulation while the excess electrons inject from Emitter. The free electrons and holes accumulate at two sides of Emitter to change the continuity of band diagram for Base/Emitter junction and then attain high internal gain.
In addition, the conventional PD response time is mainly limited by the carrier drift velocity and RC time constant. To improve the performance regarding to aforementioned phenomena, we inserted a LT-GaN film between Base and Emitter. However, owing to the poor material quality of LT-GaN layer, photo-generated carriers would be trapped by native defects to limit the improvement.
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