Enhancing GaN/Al_xGa_1-xN-Based Heterojunction Phototransistors: The Role of Graded Base Structures in Performance...

This research explores the architecture and efficacy of GaN/Al_xGa_1-xN-based heterojunction phototransistors (HPTs) engineered with both a compositionally graded and a doping-graded base. Employing theoretical analysis along with empirical fabrication techniques, HPTs configured with an aluminum compositionally graded base were observed to exhibit a substantial enhancement in current gain. Specifically, theoretical models predicted a 12-fold increase, while experimental evaluations revealed an even more pronounced improvement of approximately 27.9 times compared to conventional GaN base structures. Similarly, HPTs incorporating a doping-graded base demonstrated significant gains, with theoretical predictions indicating a doubling of current gain and experimental assessments showing a 6.1-fold increase. The doping-graded base implements a strategic modulation of hole concentration, ranging from 3.8 × 10¹⁶ cm^-3 at the base-emitter interface to 3.8 × 10¹⁷ cm^-3 at the base-collector junction. This controlled gradation markedly contributes to the observed enhancements in current gain. The principal mechanism driving these improvements is identified as the increased electron drift within the base, propelled by the intrinsic electric field inherent to both the compositionally and doping-graded structures. These results highlight the potential of such graded base designs in enhancing the performance of GaN/Al_xGa_1-xN HPTs and provide crucial insights for the advancement of future phototransistor technologies.