Numerical Simulation and Modeling for Vacuum Channel Field Effect Transistor Using Graphene Emitter

Abstract

Recent studies have investigated the potential of utilizing semi-metallic materials, specifically Graphene, as an emission source in vacuum devices [1]. Due to its distinctive two-dimensional structure, Graphene exhibits a field enhancement effect that induces band bending near its edge, rendering it a highly promising candidate as an emitter [2]. Furthermore, its controllability of the fermi level offers a prospect for electrical state modulation, thereby augmenting its potential for highspeed switching and rectification devices when combined with a gate-like structure [3]. In this context, we have conducted a numerical semiconductor device modeling of a three-terminal vacuum channel field effect transistor and developed a simulator for it. The device comprises two metallic electrodes that serve as the source and drain, a metal-oxide-graphene structure for gate bias modulation, an emitter of Graphene monolayer, and the vacuum channel (Fig. 1). The absence of scattering in the vacuum channel, apart from electron-electron scattering, facilitates high-speed transport of emitted electrons without velocity saturation. Our simulations confirm that the emission current, primarily driven by Fowler-Nordheim tunneling, can be modulated through gate voltage modulation. We have simulated that the cut-off frequency is expected to fall within the range of 0.4 ~ 1.7 THz, while the maximum oscillation frequency is anticipated to be within the range of 6.4 × 102 ~ 1.9 × 104 THz under high source-drain voltage for the given structure. Furthermore, we have conducted an investigation on the screening effect within the graphene channel and explored the limits of effective emission current controllability through the gate structure. Our findings suggest that the screening length plays a critical role in determining the device characteristics like I-V curve and frequency response. The screening length is highly dependent on external voltage conditions, more fundamentally the induced charge concentration in the graphene channel. We have also confirmed that the emission current controllability decreases exponentially with increasing emitter length, consistent with prior research on the screening effect of Graphene [4]. In addition, we successfully conducted a simulation on a similar device structure utilizing a Graphene bilayer emitter. We observed that, even in the same device structure, the device characteristics differed somewhat from those when using a Graphene monolayer emitter. In this case, the screening effect between the two Graphene layers played a significant role in determining the device characteristics. As a result of this effect the controllability of the emission current through gate bias modulation was weakened, and the emission current decreased compared to when the Graphene monolayer emitter was used. Additionally, the field enhancement effect was also weakened because the induced charge concentration compensating for the externally applied voltage was distributed to the two Graphene layers respectively. Consequently, we have successfully developed a numerical simulator for semiconductor device utilizing semi-metallic emitters. With this simulator, we were able to simulate vacuum device structures employing both Graphene monolayer emitter and Graphene bilayer emitter. Our simulations have conclusively demonstrated that the given device structure is capable of operating under high-frequency conditions.