  Structural and electronic properties of silicene adsorbed by five kinds of
transition metal atoms (Cu, Ag, Au, Pt, and Ir) are systematically studied by
using first-principles calculations. We find that such adsorption can induce a
band gap at the Dirac point of doped silicene. Doped silicene can reach a band
gap up to 0.23 eV while keeping a relatively small effective mass of around 0.1
me, thus having high carrier mobility estimated to be 50000 cm2/Vs. P-type
doping and neutral state is realized in silicene by Ir and Pt adsorption,
respectively, while n-type doping is done by Cu, Ag, and Au adsorption. Based
on the knowledge above, a silicene p-i-n tunneling field effect transistor
(TFET) is proposed and simulated by both first-principles and semi-empirical
approaches. Silicene TFET shows high performance with an on-off ratio of 10^3,
a sub-threshold swing of 90 mV/dec, and an on-state current of 1 mA/{\mu}m.
Such an on-state current is even larger than that of most other existing TFETs.
