Doping facing challenges. The formation of junctions

Doping is the key technology for semiconductor devices and it enables high on-currents and orders magnitude lower off-currents in transistors. However, with the device dimensions of only a few nanometers, the conventional impurity doping is facing challenges. The formation of junctions with extremely high doping gradients is practically difficult. At nanometer scale, a dopant concentration above its solid solubility limit would be required to achieve sufficiently low channel and contact resistances. In the case of wide bandgap semiconductors (e.g, GaN, SiC, etc), it is still difficult to obtain either p-type or n-type regions via impurity doping. This is because of the deep donor or acceptor levels or dopant passivation via complex formation and the chemical doping route is also not straightforward for many other semiconductor materials, such as carbon nanotubes (CNTs) and emerging 2-D materials graphene, phosphorene, and transition metal dichalcogenides (TMDs). For example, tunneling FETs (TFETs) based on ultrathin channels and 2-D materials are potential contenders for beyond-CMOS technology as they promise sub-60-mV/decade subthreshold slope. However, their full potential has not been still realized experimentally. The reason for their limited performance lies in the difficulty in realizing highly doped junctions with a steep profile and low defect density, which is critical to an efficient tunneling process.Various approaches have been proposed in recent years to influence the electron and hole concentrations by means other than chemical doping. In many of theseapproaches, electrostatic interaction between the semiconductor and a different material at the interface governs the carrier density referred to as ?electrostatic doping (ED).? These approaches include devices, such as Schottky barrier (SB) MOSFETs, charge plasma (CP)-based ultrathin body (UTB) devices, reconfigurable FETs based on silicon (Si) nanowires, FETs based on graphene, CNT and 2-D materials, and electron–hole bilayer (EHB)-based TFETs.II. ELECTROSTATIC DOPING CONCEPTED is a technique in which charge carriers (electrons or holes) are induced in a semiconductor material as a result of its band alignment near its interface with another (semi)conducting material. In the ED approach, the relative separation, between the Fermi level and the semiconductor energy bands, that governs the active doping concentration, which is controlled by the potential and the workfunction of the electrode adjacent to the semiconductor body rather than by the chemical impurities as in conventional doping. The electrostatic condition at the MS interface, which influences the band alignment, is a strong function of the metal workfunction (?m), the semiconductor’s energy bandgap (Eg), electron affinity (?s), and workfunction (?s). In addition, the applied electric field, if any, also influences the electrostatic properties near the interface 2.The ED approach is subdivided into three categories: 1) SB-based devices; 2) workfunction-induced doping; and 3) bias-induced doping.A. SCHOTTKY BARRIER BASED DEVICESSB-based devices are devices in which the current is limited by one or more Schottky contacts. An SB with height q?b is formed when ?m > ?s for an n-type semiconductor Fig. 1(a) and ?m < ?s for a p-type semiconductor, where q is the elementary charge. The SB height (SBH) q?b equals (?m ? ?s) for n-type and (Eg ? ?m + ?s) for p-type semiconductor. The presence of this potential barrier ?b at the MS interface results in the fundamental difference in the operation of SB devices from p-n junction devices. For the former, the MS interface, hence ?b, fully controls the majority unipolar current. The electron or hole emission in SB devices is governed by thermionic emission over the barrier and (thermionic) field emission through the barrier in contrast to p-n junction-based devices, where processes, such as drift-diffusion and band-to-band tunneling (BTBT) of