A floating gate transistor, FGMOS, is the central storage element of flash memories. It is a non-volatile semiconductor memory that was already developed in the 1970s and used in several semiconductor devices.
A floating gate transistor, which belongs to the group of isolated gate FETs, has two gates unlike a normal field effect transistor: a control gate and a floating gate. These two gates are separated from each other by an oxide layer. The floating gate is deposited on the positively doped silicon substrate via another oxide layer. Two negatively doped silicon regions form the source and drain.
The floating gate forms a charge trap in which the electrical charge is stored. Since the floating gate has no terminal, hence the name floating, the charge cannot flow away from the floating gate. Thus, it normally prevents the charge from flowing to the N and P silicon layers. As an electric field, the charge on the floating gate forms a conductive channel between the drain and the source, through which the charge is read out. The charge on the floating gate is erased with a negative erase voltage.
The floating gate itself is controlled by a quantum mechanical effect, in which a tunnel is formed by a higher positive voltage, via which some electrodes from the control gate migrate to the floating gate. Charge trap flashes(CTFs) are similarly constructed and can be used to achieve a higher storage density.