Floating-gate transistor
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A floating gate transistor is any kind of transistor in which its driving terminal is electrically isolated from the rest of the device, i.e. there is no direct internal DC path from the input terminal to the other terminals, or the resistance is very big. The main advantages of the floating gate transistors are the big input resistance and the simplified driving characteristics of the device operating in voltage mode. There are mainly two kinds of floating gate transistors: the IGBT and the FGMOSFET.
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[edit] Structure
An FGMOS can be fabricated by electrically isolating the gate of a standard MOS transistor, so that there are no resistive connections to its gate. A number of secondary gates or inputs are then deposited above the floating gate (FG) and are electrically isolated from it. These inputs are only capacitively connected to the FG, since the FG is completely surrounded by highly resistive material. So, in terms of its DC operating point, the FG is a floating node.
[edit] Applications
The FGMOS floating gate transistor is commonly used for non-volatile storage such as flash, EPROM and EEPROM memory. Floating-gate MOSFETs are useful because of their ability to store an electrical charge for extended periods of time even without a connection to a power supply. Floating-gate MOSFETs are composed of a normal MOSFET and one or more capacitors used to couple control voltages to the floating gate. Oxide surrounds the floating gate entirely, so charge trapped on the floating gate remains there. The charge stored on the floating gate can be modified by applying voltages to the source, drain, body and control gate terminals, such that the fields result in phenomena like Fowler-Nordheim tunneling and hot carrier injection.
Some applications of the FGMOS are digital storage element in EPROM, EEPROM and FLASH memories, neuronal computational element in neural networks, analog storage element, e-Pots and single-transistor DACs.
