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A short article on field effect transistors (FETs)


Field Effect Transistor

It is an active amplifier of semiconductor


In the case of a transistor, the size of the semiconductor junction layer between the emitter (E) and the collector (C) is changed by the base (B) current, and it is an active device that controls the amount of current together with the external applied voltage applied between E-C.

On the other hand, in the FET, the applied voltage of the gate (G) increases the impurity semiconductor layer opposite to the narrow channel between the source (S) and the drain (D), thereby limiting the current flowing between the channels.


In other words, the transistor provides an amplification proportional to the current input to the output and the FET provides an amplification that is inversely proportional to the voltage input to the output.

Usually, the amplification factor of a transistor is expressed as the ratio of the input current and the output current and used.

On the other hand, FET's input is not current, so if it is regarded as a general transistor, the amplification factor becomes infinite, and it is an inverted amplification.

However, depending on the design, proportional amplification rather than inverting amplification is possible. This is the reason why there are so many types of FETs and a lot of attention is needed for use.


However, the infinite amplification factor of the FET is very useful in the circuit. In the amplification of a fine signal, the infinite amplification factor does not distort the source signal.

That is, the required amount of output depends on the power and load applied to the S-D.


The channel size of the FET is adjusted by the voltage of the gate (G). At this time, it has the characteristic of a component's own capacity, that is, a very small capacitor. . The structure with this insulating layer eliminates the need for current to flow.

However, it takes some time to charge or discharge the charge due to the gate's own capacity in the operation of the voltage raising or lowering signal.

This is a problem that occurs even if the FET is made very small, but it has the effect of reducing the reaction speed of the device, which is important in signal amplification.

In other words, it is difficult to increase the frequency characteristics as compared to transistors. (recovery is late)

Application to memory

The gate's own capacity is simply a capacitor, that is, the state of charge or discharge can be memorized. That is, one FET is capable of a memory cell of the smallest size. However, since self-discharge occurs due to the characteristics of the insulating layer and the possibility of miniaturization, the state of the memory can be maintained only when the cell is recharged at a certain time unit. (However, since memory cells require circuit elements for addressing and write/read lines, it is a lie that one FET constitutes 1 bit)

In particular, if the insulation layer is well designed, the self-discharge decreases, and if the self-capacity is increased, the effect of self-discharge decreases, and if the circuit elements per cell are reduced, a memory with excellent integration can be made (ROM read-only memory).

Known content is high degree of integration besides logic and circuit configuration required for recharging.

Write Cell, Erase Cell

Writing the contents of the cell is charging the gate's own capacity

Erasing the contents of the self-capacity works by discharging all the charged charges of the self-capacitance applied to the gate.

Therefore, there is a difference between the time spent on reading and the time spent on memory (erasing or writing).

Use of memory

Memory cells due to the basic gate's own capacity are highly integrated, and the high amplification of FETs is advantageous for reference.

In other words, except for flash memory (also called solid state memory) and F/F cells used for cache and logic, it can be asserted that almost all types of random memory structures are composed of FETs.

Manufacture process

P-N stacking can be used in place of the insulating layer, but the insulating layer can be separately made of metal oxide or SiO2, so it can be processed into a device with excellent operational reliability and operating capacity, unlike a transistor composed of only semiconductors.

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