  Although monolayer black phosphorus (BP) or phosphorene has been successfully
exfoliated and its optical properties have been explored, most of electrical
performance of the devices is demonstrated on few-layer phosphorene and
ultra-thin BP films. In this paper, we study the channel length scaling of
ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using
various contact metals to change transistor characteristics. Through studying
transistor behaviors with various channel lengths, the contact resistance can
be extracted from the transfer length method (TLM). With different contact
metals, we find out that the metal/BP interface has different Schottky barrier
heights, leading to a significant difference in contact resistance, which is
quite different from previous studies of transition metal dichalcogenides
(TMDs) such as MoS2 where Fermi-level is strongly pinned near conduction band
edge at metal/MoS2 interface. The nature of BP transistors are Schottky barrier
FETs, where the on and off states are controlled by tuning the Schottky
barriers at the two contacts. We also observe the ambipolar characteristics of
BP transistors with enhanced n-type drain current and demonstrate that the
p-type carriers can be easily shifted to n-type or vice versus by controlling
the gate bias and drain bias, showing the potential to realize BP CMOS logic
circuits.
