# Floating-gate transistor: Wikis

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The Floating Gate MOSFET (FGMOS) is a field effect transistor, whose structure is similar to a conventional MOSFET. The gate of the FGMOS is electrically isolated, creating a floating node in DC, and a number of secondary gates or inputs are 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 resisitve material, the charge contained in it remains unchanged for long periods of time. Usually Fowler-Nordheim Tunneling and Hot-Carrier Injection mechanisms are used in order to modify the amount of charge stored in the FG.

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.

## History

The first report of a Floating Gate MOSFET was made by Kahng and Sze,[1] 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 in FGMOS circuits 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,[2] 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:

1. Thomsen and Brooke's demonstration and use of electron tunneling in a standard CMOS double-poly process[3] allowed many researchers to investigate FGMOS circuits concepts without requiring access to specialized fabrication processes.
2. The νMOS, or neuron-MOS, circuit approach by Shibata and Ohmi[4] 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.
3. Carver Mead's adaptive retina[5] 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.

## Structure

A cross-section of a floating-gate transistor

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.

## Modelling

### Large signal DC

The equations modeling the DC operation of the FGMOS can be derived from the equations that describe the operation of the MOS transistor used to build the FGMOS. If it is possible to determine the voltage at the FG of an FGMOS device, it is then possible to express its drain to source current using standard MOS transistor models. Therefore, in order to derive a set of equations that model the large signal operation of an FGMOS device, it is necessary to find the relationship between its effective input voltages and the voltage at its FG.

### Small signal

An N-input FGMOS device has N-1 more terminals than a MOS transistor, and therefore, N+2 small signal parameters can be defined: N effective input transconductances, an output transconductance and a bulk transconductance. Respectively:

$g_{mi}=\frac{C_i}{C_T}g_m\quad\mbox{for}\quad i=[1,N]$

$g_{dsF}=g_{ds}+\frac{C_{GD}}{C_T}g_m$

$g_{mbF}=g_{mb}+\frac{C_{GB}}{C_T}g_m$

These equations show two drawbacks of the FGMOS compared with the MOS transistor:

• Reduction of the input transconductance.
• Reduction of the output resistance.

## 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.

## References

1. ^ D. Kahng and S.M. Sze, "A floating-gate and its application to memory devices," The Bell System Technical Journal, vol. 46, no. 4, 1967, pp. 1288-1295
2. ^ M. Holler, S. Tam, H. Castro, and R. Benson, "An electrically trainable artificial neural network with 10240 'floating gate' synapses," Proceeding of the International Joint Conference on Neural Networks, Washington, D.C., vol. II, 1989, pp. 191-196
3. ^ A. Thomsen and M.A. Brooke, "A floating gate MOSFET with tunneling injector fabricated using a standard double-polysilicon CMOS process," IEEE Electron Device Letters, vol. 12, 1991, pp. 111-113
4. ^ T. Shibata and T. Ohmi, "A functional MOS transistor featuring gate-level weighted sum and threshold operations," IEEE Transactions on Electron Devices, vol. 39, no. 6, 1992, pp. 1444-1455
5. ^ C.A. Mead and M. Ismail, editors, Analog VLSI Implementation of Neural Systems, Kluwer Academic Publishers, Norwell, MA, 1989

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.

## 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.

## 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.