Carrier type and density dependence of impact ionization characteristics in WSe₂

The impact ionization process offers advantages in achieving low-power and high-speed switching in transistors and also provides high internal gain for photodetectors. We investigate the density dependence of both electron- and hole-initiated impact ionization in WSe₂. We observe a record-low critical electric field for impact ionization and a large multiplication factor in WSe₂ when the impact ionization is initiated by holes, particularly near a charge-neutral point. As the carrier density increases, the impact ionization decreases, which is attributed to the increase in Fermi energy and the weakening of the carrier-carrier interaction by increasing the screening effect. To understand the role of screening effects on the impact ionization, we examine the carrier scatterings of several scattering sources and the rate of change of the channel current with respect to the drain source voltage, ΔI_DS/ΔV_DS. We obtain the optimal conditions for impact ionization and apply these findings to fabricate avalanche photodetectors (APDs). This study proposes that the performance of low-power transistors or APDs, utilizing impact ionization, can be enhanced under optimized conditions by examining and controlling specific external parameters.