The floating gate transistor
is a kind of transistor
that is commonly used for non-volatile storage
such as flash
memory. Floating-gate transistors are almost always floating-gate MOSFETs
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 (since we have Vfg
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.
The first report of a Floating Gate mosfet was made by Kahng and Sze, and dates back to 1967. The first application of the FGMOS was to store digital data in EEPROM, EPROM and FLASH memories. However, the current interest incircuits started from developing large-scale computations in neuromorphic systems, which are inherently analog.
In 1989 Intel employed the FGMOS as an analog nonvolatile memory element in its ETANN chip, demonstrating the potential of using FGMOS devices for applications other than digital memory.
Three research accomplishments laid the groundwork for much of the current FGMOS circuit development:
- Thomsen and Brooke's demonstration and use of electron tunneling in a standard CMOS double-poly process allowed many researchers to investigate FGMOS circuits concepts without requiring access to specialized fabrication processes.
- The νMOS, or neuron-MOS, circuit approach by Shibata and Ohmi provided the initial inspiration and framework to use capacitors for linear computations. These researchers concentrated on the FG circuit properties instead of the device properties, and used either UV light to equalize charge, or simulated FG elements by opening and closing MOSFET switches.
- Carver Mead's adaptive retina gave the first example of using continuously-operating FG programming/erasing techniques, in this case UV light, as the backbone of an adaptive circuit technology.
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.