We have used a series of all‐optical measurements to determine shallow acceptor, shallow donor, and midgap donor (EL2) concentrations. The validity of these measurements was then tested by comparing the predicted electron density to the measured Hall density. Resonant pumping of the first excited state of the polariton sharpened the photoluminescence spectrum sufficiently to allow a reliable measurement of the shallow donor to acceptor ratio. The absolute shallow acceptor, donor, and EL2 concentrations were determined by a series of absorption and luminescence measurements. The Fermi level and hence the carrier concentration was then deduced using the three‐level model of semi‐insulating behavior.