| 报告题目 | A photoconductor itself has no gain |
| 报告人 | Prof. DAN Yaping |
| 报告人单位 | University of Michigan – Shanghai Jiao Tong University Joint Institute |
| 报告时间 | 2015-09-25 |
| 报告地点 | 合肥微尺度物质科学国家实验室九楼会议室（9004） |
| 主办单位 | 合肥微尺度物质科学国家实验室、国际功能材料量子设计中心、中国科学技术大学物理系 |
| 报告介绍 | 报告摘要：
It is well known that avalanche photodiodes and bipolar phototransistors have gain. A photoconductive semiconductor material having gain is surprising, but it is clearly written in the classical semiconductor physics textbooks and widely accepted by the community for decades. The gain theory was derived in 1950s, which concluded that the gain of a photoconductor is equal to the minority carrier lifetime divided by the transit time that the carriers take to transport between the two contacts of the device. Therefore, a photoconductor itself would have a large gain if the transit time is much shorter than the minority carrier lifetime. The physical explanation for the gain is that the short transit time allows the photogenerated carriers to circulate in the circuit multiple times before recombination, equivalent to generating many times more photoexcited carriers. We call this gain theory as “recycling gain mechanism” for convenience.
Conceptually, accordingly to the theory, the gain comes from the carrier collection accumulation in time domain instead of more photogenerated carriers in physical space. It will inevitably lead to the conclusion of no gain in photoconductivity, which is in contradiction with the experimental observations. Quantitatively, there is a huge disparity between the gains predicted by the theory and those measured in experiments. Some argued that this disparity is due to the carrier trapping by surface trap states that can prolong the minority carrier lifetime, while others speculated that it is because of the built-in electric field in the device (surface static charges, for example) that separates photogenerated electron-hole pairs, resulting in longer carrier lifetimes. Up to date, this gain theory is still being widely used to explain the observed gain in photoconductors based on quantum dots, nanowires and more recently 2-dimensional materials.
In this talk, we find that this well-known recycling gain theory is highly questionable because the theory derivation is based on the assumption of zero electric field in the semiconductor, which cannot hold for a photoconductor with voltage bias. By solving the continuity equation, we conclude that a photoconductor itself has no gain, meaning that the theoretical gain will never be greater than 1 no matter how short the transit time is. The conclusion is consistent with the numerical simulation results using commercial software. The photoconductor gain observed in experiments comes from other effects, on which we will offer our perspective at the end of this talk.
报告人简介：
Dr. Yaping Dan is currently a tenure-track associate professor at the University of Michigan – Shanghai Jiao Tong University (UM-SJTU JI) in Shanghai Jiao Tong University. He received the B.S., Master’s, and PhD degree from Xi’an Jiaotong University, Tsinghua University and the University of Pennsylvania, respectively. Dr. Dan carried out his postdoctoral research at Harvard University. He joined the joint institute in 2012 and won the national “1000 young scholars” award in that fall. His research focuses on developing single atom electronics and high performance photodetectors for optical interconnect. |
