Few-layer black phosphorus is a semiconductor material which was discovered in 2014. Phosphorus has both high mobility (1100-1140 cm²/V.s in armchair direction) and finite bandgap (~0.35 eV for bulk) compared to the other two-dimensional materials such as MoS₂ and graphene. A single layer of black phosphorus is named as phosphorene. Multilayer (ML) phosphorene has an adjustable direct bandgap that can be controlled either by changing the width of the ribbon or the number of layers.

In this work, first we calculate the band structure of ML phosphorene nanoribbons (PNRs) using the tight-binding method. Then, we analyzed the performance characteristics of SL- and ML-PNR field effect transistors (PNRFETs) with abrupt doping between the Source/Drain (S/D) region and the channel region by employing the non-equilibrium Green’s function (NEGF) formalism in real space. This type of FET with abrupt doping near S/D and channel regions is known as conventional FET. We found that the bandgap of armchair PNRs with 16 atoms across the width of PNR for single-layer (SL) and bilayer (BL) structures is 1.899 and 1.224 eV, respectively. We showed that increasing the number of layers reduces both the bandgap of the material and the turn on voltage of device, however, it increases the ON-current of device. Simultaneously, the off-current also increases due to a higher probability of tunneling in low bandgap devices. In addition, increasing the number of atoms across the width of PNR leads to an increase in the I_ds due to the decrease of the bandgap. Moreover, both on- and off-currents increase by decreasing the channel length in multilayer PNRFETs.

Next, we compared the performance characteristics of conventional and junction-less (JL) PNRFETs. Recently, JL-FET with high doping concentration in S/D and channel regions was proposed to replace the conventional FET. The JL silicon-on insulator FETs fabricated by Tyndall group show outstanding output characteristics with low Drain Induced Barrier Lowering (DIBL), ideal subthreshold swing, etc. There is no p-n junction between S/D and channel regions in a JL transistor. Instead, all regions are highly doped with the same amount of dopant to increase the conductivity of the channel material. In this case, the barrier in front of the electrons coming from source, is formed by applying suitable gate voltage. From experimental point of view, it should be much easier to dope all S/D and channel materials with the same doping concentration compared to the conventional FETs which needs abrupt doping. Thus, it should be a point of interest to have a performance comparison between conventional and JL phosphorene FETs. We investigated the performance of phosphorene FETs in terms of I_d-V_gs, I_d-V_ds characteristics, energy levels, subthreshold swing, DIBL, and I_ON/I_OFF.