Analysis of Avalanche Multiplication in Ambipolar WSe₂ Field-Effect Transistors through Channel Length Modulation

Recently, there have been various reports of utilizing avalanche multiplication in two-dimensional (2D) materials-based devices applications such as avalanche photodetectors and transistors [1]. Previous studies have mainly focused on developing high-performance devices with unipolar semiconductor as the active material. However, fundamental analysis of the multiplication process in ambipolar materials is required to push the performances further, as well as establish novel architectures. Although ambipolar 2D materials have the advantage of simple carrier type tuning through electrostatic gating, allowing both carrier types in a single channel poses an inherent difficulty in analyzing their individual contributions to avalanche multiplication. In ambipolar WSe₂ field-effect transistors (FETs), two phenomena of ambipolar transport and avalanche multiplication can occur, and both exhibit secondary rise of output current at high lateral voltage. We distinguished between these two competing phenomena using the method of channel length modulation. In long-channel devices, minority charge carriers are accumulated near the drain side of the channel, and the critical voltage is modulated in equal amounts as the gate voltage. In contrast, short-channel devices undergo carrier multiplication at high lateral voltages, and the breakdown voltage is insensitive to change in the gate voltage. Furthermore, we extracted multiplication characteristics from both electron- and hole-induced avalanche multiplication in WSe₂ and compared the results with conventional semiconductors. Our study provides a simple and robust method to examine carrier multiplication in ambipolar materials and will foster the development of high-performance atomically thin electronic devices utilizing avalanche multiplication.<br/>[1] A. Gao et al., Nat. Nanotechnol. 14, 217 (2019).