Floating-gate MOSFET

A floating gate transistor is an electronic transistor, which is used in non-volatile memory for permanent storage of information. It was developed in 1967 by Dawon Kahng and Simon Min Sze at Bell Laboratories and turned up to the early 2000s in integrated circuits in the flash memories, floating gate PROMs, EPROMs and EEPROMs, the elementary memory element dar.

Floating gate transistors are flash memories, the NAND flash in particular, are increasingly replaced by charge trap flash (CTF ). By avoiding interference effects, which are primarily caused by closely adjacent floating gate transistors, smaller feature sizes and higher storage densities per chip area than floating gate can be realized in CTFs.

General

Floating gate transistors, abbreviated as FGMOS, among the group of field effect insulated gate transistors ( IGFET ), and are usually made of the semiconductor material silicon. Besides one or occasionally several control electrodes ( V in the figure ), it comprises a " floating gate " (German, not connected control electrode ') which is electrically isolated. On this floating gate, a certain amount of electric charge is stored permanently, to a shift in the threshold voltage ( Vth) of the transistor leads. To read the information serve as conventional IGFET the terminals source (S) and drain (D ), while in addition the control connections are required to describe. The bulk (B ) port is usually connected to ground potential.

The introduction or removal of the electric charge (electrons) in the context of the writing operation to the electrically isolated by, for example, silica floating gate is performed by the quantum tunneling effect, or by the injection of hot carriers (English hot carrier injection) from the source - or drain out, with the aid of a control terminal ( engl. control gate). For the programming operation, a substantially higher voltage than that required for the normal read operation ( for example, 10 V with respect to 3.3 V ) is required.

In the absence of charge on the floating gate, the drain -source path is unaffected, and the transistor behaves as a normal MOSFET, the same type. For a typical FGFET type of an enhancement n-channel MOSFETs (English enhancement n - channel MOSFET), this means that the transistor is non-conducting and conducting, after exceeding the threshold voltage at a gate voltage of 0V. However, be applied to the floating gate charge shifts the threshold voltage of the transistor. Thus, the introduction of electrons ( negative charges ) in the floating gate results in an increase in the threshold voltage of these transistors. When the transistor during reading now driven normally, the drain-source path of the transistor remains in a high impedance state, that is, the transistor is not conductive, it is blocked.

These two states can be stored permanently in the simplest case, the information of a bit. Depending on the memory technology includes a transistor ( in the SLC memory cell) or a plurality of floating gates ( at the MLC memory cell). MLC memory cells can be more than one bit of information stored in a transistor by steps of amounts of charge, which increases the storage density. Common values ​​are two bits per floating gate transistor having four different levels of charge.

Applications

The primary application of these transistors is in the range of digital, non-volatile memory such as USB mass storage devices or SD memory cards. With a storage capacity of 4 GB of almost 35 billion floating-gate transistors are required. At high densities, the floating gate transistor cells are replaced by charge- trap flash cells.

In 1989, the company Intel a non-volatile memory cell ( ETANN ) developed in the framework of a research paper based on FGMOS for the storage of analog values ​​in the context of neural networks.