The floating-gate MOSFET (FGMOS), also known as a floating-gate MOS transistor or floating-gate transistor, is a type of
metal–oxide–semiconductor field-effect transistor
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(MOSFET) where the gate is electrically isolated, creating a floating node in
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
, 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 surrounded by highly resistive material, the charge contained in it remains unchanged for long periods
of time, nowadays typically longer than 10 years. Usually
Fowler-Nordheim tunneling
Field electron emission, also known as field emission (FE) and electron field emission, is emission of electrons induced by an electrostatic field. The most common context is field emission from a solid surface into a vacuum. However, field emissio ...
and
hot-carrier injection
Hot carrier injection (HCI) is a phenomenon in solid-state electronic devices where an electron or a “ hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term "hot" refers to ...
mechanisms are used to modify the amount of charge stored in the FG.
The FGMOS is commonly used as a floating-gate
memory cell, the
digital storage
Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, phonographic recording, magnetic tape, and optical discs are all examples of storage media. Biological molecules such as RNA and DNA are consid ...
element in
EPROM
An EPROM (rarely EROM), or erasable programmable read-only memory, is a type of programmable read-only memory (PROM) chip that retains its data when its power supply is switched off. Computer memory that can retrieve stored data after a power ...
,
EEPROM
EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or as ...
and
flash memory
Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both u ...
technologies. Other uses of the FGMOS include a neuronal computational element in
neural network
A neural network is a network or neural circuit, circuit of biological neurons, or, in a modern sense, an artificial neural network, composed of artificial neurons or nodes. Thus, a neural network is either a biological neural network, made up ...
s,
analog storage element,
digital potentiometers and single-transistor
DACs.
History
The first
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
was invented by
Mohamed Atalla
Mohamed M. Atalla ( ar, محمد عطاالله; August 4, 1924 – December 30, 2009) was an Egyptian-American engineer, physicist, cryptographer, inventor and entrepreneur. He was a semiconductor pioneer who made important contributions to ...
and
Dawon Kahng
Dawon Kahng ( ko, 강대원; May 4, 1931 – May 13, 1992) was a Korean-American electrical engineer and inventor, known for his work in solid-state electronics. He is best known for inventing the MOSFET (metal–oxide–semiconductor field-effe ...
at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
in 1959, and presented in 1960.
The first report of a FGMOS was later made by Dawon Kahng and
Simon Min Sze
Simon Min Sze, or Shi Min (; born 1936), is a Chinese-American electrical engineer. He is best known for inventing the floating-gate MOSFET with Korean electrical engineer Dawon Kahng in 1967.
Biography
Sze was born in Nanjing, Jiangsu, and gre ...
at Bell Labs, and dates from 1967. The earliest practical application of FGMOS was floating-gate
memory cells, which Kahng and Sze proposed could be used to produce
reprogrammable ROM (
read-only memory
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing s ...
).
Initial applications of FGMOS was digital
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered ...
, to store
nonvolatile data in
EPROM
An EPROM (rarely EROM), or erasable programmable read-only memory, is a type of programmable read-only memory (PROM) chip that retains its data when its power supply is switched off. Computer memory that can retrieve stored data after a power ...
,
EEPROM
EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or as ...
and
flash memory
Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both u ...
.
In 1989, Intel employed the FGMOS as an analog nonvolatile memory element in its electrically trainable
artificial neural network
Artificial neural networks (ANNs), usually simply called neural networks (NNs) or neural nets, are computing systems inspired by the biological neural networks that constitute animal brains.
An ANN is based on a collection of connected units ...
(ETANN) chip,
[M. Holler, S. Tam, H. Castro, and R. Benson, "An electrically trainable artificial neural network with 10240 'floating gate' synapses", ''Proceedings of the International Joint Conference on Neural Networks'', Washington, D.C., vol. II, 1989, pp. 191–196] 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 ''ν''MOS, 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.
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.
For applications where the charge of the FG needs to be modified, a pair of small extra transistors are added to each FGMOS transistor to conduct the injection and tunneling operations. The gates of every transistor are connected together; the tunneling transistor has its source, drain and bulk terminals interconnected to create a capacitive tunneling structure. The injection transistor is connected normally and specific voltages are applied to create hot carriers that are then injected via an electric field into the floating gate.
FGMOS transistor for purely capacitive use can be fabricated on N or P versions.
For charge modification applications, the tunneling transistor (and therefore the operating FGMOS) needs to be embedded into a well, hence the technology dictates the type of FGMOS that can be fabricated.
Modeling
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, 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
transconductance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproc ...
s, an output transconductance and a bulk transconductance. Respectively:
: