New mobility-measurement technique on inverted semiconductor surfaces near the conduction threshold

A new experimental technique has been demonstrated which has a sensitivity of ${10}^8$ electrons/${\mathrm{cm}}^2$ in a semiconductor surface inversion channel. Combined with surface-conductivity measurements, the electron (or hole) conductivity mobility can be determined unambiguously near the conduction threshold, which is not possible by previous measurement methods of conductance or transconductance versus perpendicular surface electric field (or gate voltage). The new technique measures the input admittance or impedance of the surface channel and interprets the result by a distributed-transmission-line model. The distributed capacitance element associated with the inversion layer electrons is a direct measure of the inversion electron density, $C_N=\frac{q^2{N}_{\mathrm{INV}}}{k_{B}T}$. Electron mobilities in the $n$-type surface inversion layers on $p$-type silicon with high and low densities of oxide charge scattering centers have been measured down to ${10}^8$ electrons/${\mathrm{cm}}^2$. A monotonic mobility decrease with decreasing electron densities has been observed in contrast to previous published data which showed unexplained mobility peaks near the conduction threshold.