ULTRAVIOLET ERASING TYPE SEMICONDUCTOR MEMORY DEVICE

摘要

Each memory cell of an EPROM contains two MOSFETs and a data of each memory cell is read out by detecting a current difference between the two MOSFETs by using a differential amplifier. In such constitution as described above, even when the data is erased by irradiating an ultraviolet ray, a stable output of the differential amplifier can be obtained and, therefore, confirmation of an initialized state can be facilitated. Specifically, a channel width W_A of one of the two MOSFETs constituting the memory cell is formed narrower than a channel width W_B of the other. By such arrangement as described above, in an initialized state in which the ultraviolet ray is irradiated, a data signal current value I_HA of the MOSFET having the channel width W_A becomes smaller than a data signal current value I_HB flowing in the MOSFET having the channel width W_B. Accordingly, the output of the differential amplifier is fixed in accordance with a current magnitude relation of I_HAHB, to thereby define a data "0". On the other hand, in writing a data "1", charges are injected in a floating gate electrode of the MOSFET having the channel width W_B to raise a threshold voltage Vt and, then, the MOSFET is set in an "off" state.