On the interpretation of tunnelling into an amorphous semiconductor

Several recent measurements of the tunnel conductance G(V) in metal-insulator-amorphous semiconductor film junctions have shown, at V = 0, the $\text{G(0) ∝}\text{exp}\text{(-BT}{}^{\mathrm{<mtext>-</mtext><mtext>1</mtext><mtext>4</mtext>}}\text{)}$ temperature dependence characteristic of the variable-range phonon-assisted tunnelling model of Mott. It is suggested, however, that observation of this temperature dependence for a bias $\text{V》kT}\text{e}$ indicates failure to achieve tunnel injection and internal equilibrium in the amorphous film. At equilibrium in the low temperature case, effective electron-phonon coupling can lead to the proportionality $\text{G(V)∝}\text{∫}\text{0}\text{eV}\text{?(E)}\text{d}\text{E}$ where ?(E) is the energy density of states, rather than to G(V) ∝ ?(eV) as is commonly assumed.