In a simple memory application the threshold voltage VTof an MNOS transistor is switched by means of write and erase pulses of opposite polarities between two levels, representing logical ONEs and ZEROs. It is shown that by isolating the source and drain during the write pulse, the magnitude of the VTshift can be made dependent on the intensity of light on the MNOS device. By choosing the light intensity and the width of the write pulse properly, the device can be made to operate in either a digital or analog mode. The theory of this effect, applicable to both transistors and capacitors, is developed by generalizing the well-understood behavior of the MNOS memory device to include conduction in the silicon space-charge region. The operation of the device depends on the current-field relationships for the nitride, oxide, and silicon space-charge layers. It is shown how these three functions may be obtained from steady-state I-V measurements on the MNOS device. Good agreement is found between experimental and calculated charging curves for both transistors and capacitors.