| 摘要 |
<p>A composite information storage device which includes a variable threshold device and at least a fixed threshold device is disclosed. The device basically consists of a semiconductor substrate of one conductivity type having a pair of regions of opposite conductivity type disposed therein and defining a channel region therebetween. The variable threshold device is an MNOS (metal-nitride-silicon oxide-semiconductor) device while the fixed threshold device is an MOS device. Alternatively, the variable threshold device could be an MAOS (metal-aluminum oxide-silicon oxide-semiconductor) device. For one reason or another, the variable threshold device, when it is subjected to repeated reading, writing, and erasing cycles, tends to become a depletion mode device and consequently conducts with zero voltage on its gate when reading takes place. To obviate spurious conduction in an array which would effectively remove all the devices associated with a given bit line from the array, a fixed threshold device actuated by a common gate is formed using the same channel region to insure against spurious turn-on of a storage device when it is in the low threshold condition. The fixed threshold device is obtained by providing at least a single insulator layer of thickness sufficient to prevent tunneling therethrough and to provide a threshold voltage which causes the fixed threshold device to operate in the enhancement mode. By insuring that the device operates in the enhancement mode, when variable threshold devices which have tended to become depletion mode devices are interrogated, spurious paths are eliminated which would otherwise permit current to flow when zero volts are applied to a device associated with the same bit line which is not being interrogated. For convenience in fabrication, two fixed threshold devices are normally associated with the same channel region as the variable threshold device. Using this arrangement, the resulting cell lays out in an area which is only 20% larger than when no fixed threshold devices are incorporated.</p> |
