Floating Gate MOSFET

Floating Gate MOSFET

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The Floating Gate MOSFET (FGMOS) is a field effect transistor, whose structure is similar to a conventional MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...

. 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 resistive material, the charge contained in it remains unchanged for long periods of time. Usually Fowler-Nordheim Tunneling and Hot-Carrier Injection
Hot carrier injection
Hot carrier injection 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 the effective temperature used to model carrier density, not to the...

 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
EPROM
An EPROM , or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages...

, EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

 and flash
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

 memories, neuronal computational element in neural network
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...

s, analog storage element, e-Pots and single-transistor DACs
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...

.

History


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 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, 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 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 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 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 in order 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 in order to create a capacitive tunneling structure. The injection transistor is connected normally and specific voltages are applied in order 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.

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 transconductance
Transconductance
Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...

s, an output transconductance and a bulk transconductance. Respectively:







Where is the total capacitance seen by the floating gate. These equations show two drawbacks of the FGMOS compared with the MOS transistor:
  • Reduction of the input transconductance.
  • Reduction of the output resistance.

Simulation


Under normal conditions, a floating node in a circuit represents an error due to the fact that the initial condition is unknown unless it is somehow fixed. This generates two problems: first, is not straight forward to simulate these circuits; and second, an unknown amount of charge might stay trapped at the floating gate during the fabrication process which will result in an unknown initial condition for the FG voltage.

Among the many solutions proposed for the computer simulation, one of the most promising methods is an Initial Transient Analysis (ITA) proposed by Rodriguez-Villegas , where the FGs are set to zero volts or a previously known voltage based on the measurement of the charge trapped in the FG after the fabrication process. A transient analysis is then run with the supply voltages set to their final values, letting the outputs evolve normally. The values of the FGs can then be extracted and used for posterior small signal simulations, connecting a voltage supply with the initial FG value to the floating gate using a very-high-value inductor.

Applications


The usage and applications of the FGMOS can be broadly classified in two cases. If the charge in the floating gate is not modified during the circuit usage, the operation is capacitively coupled.

In the capacitively coupled regime of operation, the net charge in the floating gate is not modified. Examples of application for this regime are single transistor adders, DACs, multipliers and logic functions, variable threshold inverters,

Using the FGMOS as a programmable charge element, it is commonly used for non-volatile storage such as flash
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

, EPROM
EPROM
An EPROM , or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages...

 and EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

 memory. In this context, floating-gate MOSFETs are useful because of their ability to store an electrical charge for extended periods of time without a connection to a power supply. Other applications of the FGMOS are neuronal computational element in neural network
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...

s, analog storage element and e-Pots.

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