We report controlled silicon doping of Ga2O3 grown in plasma-assisted molecular beam epitaxy. Adding an endplate to the Si effusion cell enables the control of the mobile carrier density, leading to over 5-orders of magnitude change in the electrical resistivity. Room temperature mobilities >100 cm2/V s are achieved, with a peak value >125 cm2/V s at a doping density of low-1017/cm3. Temperature-dependent Hall effect measurements exhibit carrier freeze out for samples doped below the Mott criterion. A mobility of 390 cm2/V s is observed at 97 K.