  Monolayer molybdenum disulfide (MoS$_2$) nanosheets, obtained via chemical
vapor deposition onto SiO$_2$/Si substrates, are exploited to fabricate
field-effect transistors with n-type conduction, high on/off ratio, steep
subthreshold slope and good mobility. The transistor channel conductance
increases with the reducing air pressure due to oxygen and water desorption.
Local field emission measurements from the edges of the MoS$_2$ nanosheets are
performed in high vacuum using a tip-shaped anode. It is demonstrated that the
voltage applied to the Si substrate back-gate modulates the field emission
current. Such a finding, that we attribute to gate-bias lowering of the MoS$_2$
electron affinity, enables a new field-effect transistor based on field
emission.
