Abstract: A metal-insulator-semiconductor (MIS) diode having a suspended electrode forming a cavity between the electrode and the insulator is operated in the charge storage, i.e. inversion, mode to detect a fluid concentration. A reverse bias voltage is applied to the diode to form a charge inversion layer in the semiconductor, the bias is disconnected and subsequently reapplied. The charge that flows upon reapplication of the bias voltage is a measure of the concentration of a fluid in the cavity. An array of MIS diodes with multiplexed biasing and charge measurement permits detection of combinations of different fluids, even in the presence of potentially interfering chemical species.