Floating-gate transistor


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A cross-section of a floating-gate transistor

The floating gate transistor is a kind of transistor that is commonly used for non-volatile storage such as flash, EPROM and EEPROM 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=Ccg/CT*Vcg+Cs/CT*Vs+Cd/CT*Vd+Cb/CT*Vb) 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.

Contents

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 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,[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

Schematic symbol for a 3-input FGMOS 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.

See also

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

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