A theory on the switching behavior of the metal-Si3N4-SiO2—semiconductor (MNOS) memory transistor is presented which is consistent with the experimentally observed facts. The theory treats the switching process as being initially predominantly direct band-to-band tunneling and then dominated by modified-Fowler-Nordheim tunneling. The large-signal mathematical treatment includes both of these tunneling terms. The resultant charge-transport equation is rather complex; a numerical method is needed to obtain an exact solution. However, a grossly approximate closed-form solution has been obtained, which Indicates that the transferred charge is initially a linear function of time and then logarithmic. This is similar to many of the previous theories. However, the coefficients in the current and charge solutions of the present theory contain the essential material, device, and operating parameters which are absent in previous theories. This makes the present model readily usable as a guide in the design and optimization of MNOS devices. Copyright © 1977 by The Institute of Electrical and Electronics Engineers, Inc.