Although gate modulation (GM) spectroscopy performed by combining optical measurements with gate bias application in a field effect transistor (FET) is one of the most straightforward techniques for electronic state evaluation of operating FETs, the origin of the spectral response to the gate bias application has not been fully elucidated. We here address the origin of the GM signals from variety of GM spectroscopic techniques performed for pentacene-based organic FETs. The GM spectrum under the negative gate bias in the condition that the source and drain electrodes are short-circuited exhibits a complicated lineshape with many unresolved peaks. We demonstrate that the GM signal under FET driving condition with drain bias applied is dominated by second-derivative (SD) Stark signals. From the comparison of the first and second harmonic response GM signals, the SD-Stark signal is found to be given by local electric fields from hole carriers injected from the electrodes, rather than given by an external electric field from the gate-bias. The SD signal intensity is enhanced when a high degree of molecular orientation exists around the carriers, thus the SD signal can be used to investigate molecular orientation around gate-induced carriers.

尽管在场效应晶体管（FET）中通过将光学测量与栅极偏置应用相结合而执行的栅极调制（GM）光谱是用于操作FET的电子状态评估的最直接技术之一，但对栅极偏置应用的光谱响应的起源尚未完全阐明。我们在这里解决了基于并五苯有机FET的各种GM光谱技术产生的GM信号的起源。在源极和漏极短路的情况下，在负栅极偏置下的GM谱图显示出复杂的线形，其中有许多未解析的峰。我们证明，在FET驱动条件下施加漏极偏置的GM信号主要由二阶导数（SD）Stark信号控制。通过对第一和第二谐波响应GM信号的比较，发现SD-Stark信号由来自电极注入的空穴载流子的局部电场给出，而不是由栅极偏置的外部电场给出。当载流子周围存在高度的分子取向时，SD信号强度会增强，因此SD信号可用于研究栅极诱导的载流子周围的分子取向。