คำตอบที่ 95
In NOR gate flash, each cell resembles a standard MOSFET, except that it has two gates instead of just one. On top is the control gate (CG), as in other MOS transistors, but below this there is a floating gate (FG) insulated all around by an oxide layer. The FG sits between the CG and the MOSFET channel. Because the FG is electrically isolated by its insulating layer, any electrons placed on it are trapped there and, under normal conditions, will not discharge for a period of many years. When the FG holds a charge, it screens (partially cancels) the electric field from the CG, which modifies the threshold voltage (VT) of the cell. During read-out, a voltage is applied to the CG, and the MOSFET channel will become conducting or remain insulating, depending on the VT of the cell, which is in turn controlled by charge on the FG. The presence or absence of current flow through the MOSFET channel is sensed and forms a binary code, reproducing the stored data. In a multi-level cell device, which stores more than one bit per cell, the amount of current flow is sensed (rather than simply its presence or absence), in order to determine more precisely the level of charge on the FG.
A single-level NOR flash cell in its default state is logically equivalent to a binary "1" value, because current will flow through the channel under application of an appropriate voltage to the control gate. A NOR flash cell can be programmed, or set to a binary "0" value, by the following procedure:
an elevated on-voltage (typically >5 V) is applied to the CG
the channel is now turned on, so electrons can flow between the source and the drain
the source-drain current is sufficiently high to cause some high energy electrons to jump through the insulating layer onto the FG, via a process called hot-electron injection
