One of the most relevant features that a semiconducting channel material can offer when used in a field-effect transistor (FET) layout is its capability to enable both electron transport in the conduction band and hole transport in the valence band. In this way, complementary metal-oxide-semiconductor type applications become feasible once similar electron and hole drive current densities are achieved, and the threshold voltages are properly adjusted. In this article, we demonstrate pronounced ambipolar device characteristics of multilayer WSe2 FETs using different contact electrodes. Our study reveals that nickel electrodes facilitate electron injection while palladium electrodes are more efficient for hole injection. We also show, as an interesting demonstration, that by using nickel as the source contact electrode and palladium as the drain contact electrode, ambipolar device characteristics with similar on-state performance for both the electron and the hole branch can be achieved in WSe2 FETs. Finally, we discuss a unique technique based on the asymmetry in the ambipolar device characteristics to extract the Schottky barrier heights for such metal to WSe2 contacts.
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A truly Ohmic contact is only formed if the metal Fermi level lies above the conduction band or below the valence band of the semiconductor by an energy scale equal to a few kBT/q, where kB is the Boltzmann constant, T is the temperature, and q is the electronic charge.33
For a different material system, the ratio of electron conduction current to the hole conduction current could deviate considerably from unity as the effective masses for the electrons and the holes can be significantly different. For a 6 nm thick WSe2 the electron and the hole effective masses are very similar.31
Note that the channel length for the 5 nm thick flake was 4 times larger (8 μm), and the carrier density used was ∼5 times smaller, which account for a factor of ∼20 in the drive current density. For mono-layer flake, which has a significantly larger bandgap (1.65 eV), the Schottky barrier height is also larger for the hole injection. This results in an exponential decrease in the drive current density.
