  High mobility and dilute two-dimensional electron systems exhibit metallic
behavior down to the lowest experimental temperatures. In studies of ionic
liquid gated insulating silicon, we have observed transitions to a metallic
state in low mobility samples at much higher areal carrier densities than found
for samples of high mobility. We have also observed a mobility peak in metallic
samples as the carrier density was increased beyond $10^{13} \text{cm}^{-2}$.
