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FET stands for 'Field Effect Transistor' it is a three terminal uni polar solid state device in which current is control by an electric field.

FET can be fabricated with either N- Channel or P- Channel, for the fabrication of N-Channel JFET first a narrow bar of N-type of semiconductor material is taken and then two P-Type junction are defused on opposite sides of it's middle part, called channel. The two regions are internally connected to each other with a signal lead, which is called Gate terminal. One lead is called Source terminal and the other is called Drain terminal.Construction of FET

P-Channel JFET is similarly is constructed except that it use P- type of bar and two N- types of junctions.

Source:-

It is the terminal through which majority carriers are entered in the bar, so it is called Source.

Drain:-

It is the terminal through which the majority carriers leads the bar, so it is called the drain terminal.

Gate:-

These are two terminals which are internally connected with each other and heavily doped regions which form two PN-Junctions.

Working / Operation FET or JFET

Gate are always in reverse biased, hence the gate current IG is practically zero. The source terminal is always connected to end of the drain supply, which provides the necessary carrier, in N- Channel JFET Source terminal is connected to the negative end of the drain voltage source. The electrons flow from source to drain through the channel from D to S is started,

the current ID increases as VDS is increased from zero on ward. This relation ship between VDS and ID continuous till VDS reaches certain value called 'Pinch OFF' VPO.

When VDS is equal to zero and VGS is decreased from zero, the gate reverse bias increases the thinks of the region, as the negative value of the VGS is increase a stage cones when the two dip lections regions touch each other, in this conduction the channel is said to be Cut OFF.

JFET as Amplifier

One of the application of the JFET is an Amplifier, it amplified the weak signal connected in the Gate terminal , the input is always reversed biased, a small change in the reverse bias on the gate produce large change in the drain current, this fact make JFET capable of amplifing the weak signals

Fet Is A Current Controlled Device

Working / Operation

When negative signal is applied at in put of the amplifier, the gate bias is increase, duplication layer is decrease, Channel resistance is increase, ID is decreased, Drop across Load Resistor is decreases, and the positive signal is present at output through C2.
When the positive signal is applied at the input the action will be the wise versa
This seen that there is phase inveration between the input signal at the gate and the output signal at the drain.

Application of JFET

JFET is used at large scale in amplifiers circuits, analog switches; it is also used in AGC system, voltage regulators, buffer amplifiers.

MOSFET

The MOSFET is sub divided in to two types,

1. DE-MOSFET
2. E only MOSFET

DE- MOSFET

This MOSFET could be operating in both duplication and Enhancement mode. By Changing the Polarity o VGS, when VGS is negative for the N-Channel DE- MOSFET is operate in depletion mode, however with positive gate voltage it operates in an Enhancement mode.

E- Only MOSFET

This MOSFET Operates in the only Enhancement mode. It differs only in construction from the DE- MOSFET in that there exists no channel between the drain and source.

DE-MOSFET Construction

Like JFET it has source, Gate and Drain, However its gate is insulated from its conduction channel by an ultra thin metal oxide. Insulating film usually silicon dioxides (SiO2), because of this insulating property MOSFET is also known as Insulated Gate Field Effect Transistor (IGFET). In DE-MOSFET we can apply both the positive and negative voltages at gate terminal because the gate terminal is isolated from the channel.

DE-MOSFET Working / Operation

Depletion Mode

When VGS=0 electrons can flow freely from source to drain through the conduction channel, When a negative voltage is applied at gate terminal, it depletes the N- channel and its electrons by inducing positive charges in it. Grater negative voltage on the gate, grater is the reduction in the number of electrons in the channel which increase the conduction. In fact too much negative gate voltage cut off the channel, thus with negative gate voltage a DE-MOSFET behaves like a JFET, for this reason negative gate operation of DE-MOSFET is called Depletion mode Operation.

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Enhancement Mode

In circuit diagram the drain current flows from source to drain even with zero gate bias, when positive voltage is applied to the gate, the input gate capacitor is able to create pre- electrons in the channel which increase the ID. Pre- electrons are induced in the channel by the capacitor action, these electrons are added to the other ready electrons for the conduction, which increase the number of electrons and these electrons increase the conductivity of the channel.

As positive gate voltage increases the number of induced electrons is increased which increase the conductivity of channel from source to drain, this way the current is also increased. The positive gate operation of the DE-MOSFET is known as enhancement mode.

Application of MOSFET

MOSFET have wide application in field of electronics some of these application are given below.

1. As input amplifier in oscilloscope, electronic volt meter, and other measuring and testing equipment because they have high input resistance.
2. It is used In logic circuits for fast switching.
3. It is also used in TV receiver.
4. It is used in computer circuits.
5. In high frequency amplifiers.

In this article, we compare and contrast bipolar junction transistors (BJTs) and field effect transistors (FETs).

Though both are transistors and have 3 leads and achieve similar functions, they're fundamentally different in composition. Thus, there are several key differencesbetween the 2 transistors.

The table below pinpoints many of the differences between BJTs and FETs.

So the above table is a good, brief explanation of some of the differences between bipolar junction transistors (BJTs) and field effect transistors (FETs). Below we'll go over the table in more depth, so that you can get a better in-detailed explanation, if you feel the above lacked. We'll go in order.

So the first thing is how both transistors operate. BJTs are current-controlled devices. This means that BJTs are switched on by a current going through the base of the transistor. This base current then turns the BJT on, allowing for a much greater flow of current from the collector to the emitter of the transistor. FETs, on the other hand, are voltage-controlled. Voltage, not current, either turns the FET on or off. FETs have such high input impedance that they practically draw no current into the gate terminal. Instead they are entirely voltage-controlled.

The second difference is the input impedance. Input impedance is the amount of resistance that a transistor offers on its input terminal. For BJTs, this would be the base terminal; for FETs, this would be the gate terminal. BJTs offer much less resistance to their input terminal than FETs. Because of this much lower resistance, it draws current from the power supply powering the base. This is an effect called loading. Loading is when the power source circuit is affected by a second circuit, in this case the transistor circuit, which is drawing current from it. This small amount of currentdrawn, which then combines with the much larger current flowing from the other 2 leads can alter dynamics of the power source circuit. So BJTs offer less protection against this loading effect than FETs. FETs have very large input impedances, such as on the order of 10¹⁴ Ω, which is several teraohms (something you almost never hear about). With such high input impedance, the FETpractically draws no current to its input gate terminal. Therefore, since practically no current is drawn from the power supply circuit, the power supply circuit is not loaded down. It's as if the power supply circuit and the transistor circuit are well isolated and do not interfere with each other. Therefore, better power control is achieved with FETs with less interference of one circuit onto another.

Allow flash in safari. A third difference between BJTs and FETs is the gain (or transconductance). Transconductance is defined as the milliamp per volt ratio of the small change in the current output from an electronic device to the small change of voltage input. In other words, it is the gain of the transistor circuit. This is where BJTs have an advantage. BJTs have greater transconductance, meaning you are able to get more current output per unit power applied. The transconductance of FETs is much lower. So if you use the same amount of power at the input for both a BJT and FET transistor, the BJTtransistor will produce more gain. This is why BJTs are more popular for amplifier circuits. They produce gain than a FET can. This is why in the case of simple amplifier circuits, the use of a BJT is preferred and FETs are rarely used. For simple amplifiers, FETs are really only used only when it is desired for there to be extremely high input impedance.

In terms of manufacturing size, FETs can be manufactured to be much smaller than BJTs. This makes them moreefficient in commercial circuit design. Being that FETs are smaller, they take up less space on a chip. Thus, the sizeof a electronic product can be much smaller, which is what electronic design companies want a lot of times. Smaller devices, many times, can be more convenient, consumer-friendly, and FETs allow this. BJTs, on the other hand, require larger sizes generally than FETs.

In terms of expense, FETs, especially MOFSFETs, are more expensive to manufacture than BJTs. FETs normally are at a higher price point, but not significant enough to push away from them. This is just a slight drawback.

For a number of reasons, such as those listed above, FETs are more widely used and more popular than BJTs. FETs can be manufactured smaller and load the power supply less.

So while BJTs are used widely in hobby electronics and many times too in some consumer electronics and have the advantage of being able to produce higher gains than FETS, FETs still offer many advantages for large-scale commercial devices. When it comes to consumer products, FETs areoverwhelmingly preferred due tosize, high input impedance, as well as other factors. Intel, one of the largest chip makers in the world, uses practically only FET transistors to build its chips which power billions of devices in the world.

Thus, this is a brief overview of FETs vs BJTs.

Related Resources

JFET vs MOSFET (Transistors)
Types of Transistors
Difference between an NPN and a PNP Transistor
Transistor Schematic Symbols

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