ABSTRACT
Contents 8.1 Introduction.................................................................................................208 8.2 How to Extract SBs and Band Gaps ......................................................... 210
8.2.1 SBs in Nano-Devices ..................................................................... 210 8.2.2 Ambipolar Device Characteristics .............................................. 213 8.2.3 Utilising Temperature and Gate Voltage Dependence to
Identify the Flat Band Voltage ..................................................... 215 8.2.3.1 Method of SB Extraction .............................................. 215 8.2.3.2 Device Fabrication ......................................................... 217 8.2.3.3 Determining ΦSB for MoS2 ........................................... 217
8.1 Introduction Conventional three-terminal devices like metal-oxide-semiconductor eldeect transistors (MOSFETs) are oen characterised by an n/p/n or p/n/p doping prole [1]. Figure 8.1a displays how degenerately doped source and drain segments may be attached to the gated p-doped channel region of an n-type eld eect transistor (n-FET) marked by the two vertical dashed lines. Ec and Ev are the conduction and valence band edge of the semiconductor, respectively. EFs and EFd denote the position of the Fermi level in source and in drain. A positive drain voltage moves EFd relative to EFs downwards. An ‘ohmic’ contact with a constant resistance is created in case (a) if the doped source and drain regions are (i) not under gate control, (ii) display a linear dependence of current as a function of drain voltage and (iii) are attached to the channel region such that ideally no carriers are reected back into the contacts. In contrast, Figures 8.1b and 8.2 illustrate the case of metal source/drain contacts with a much larger spacing between the Fermi level and the conduction band edge if compared to Figure 8.1a, indicating a large mismatch of the carrier concentrations in the contact and the channel. Most importantly, the absence of a band gap in case of a metal contact allows easy injection of both carrier types, that is, electrons and holes, while the band gap in case of Figure 8.1a prohibits hole injection to a large extent.
