A Field Effect Transistor (FET) is a three-terminal semiconductor device. Its operation is based on a controlled input voltage. By appearance JFET and bipolar transistors are very similar. However, BJT is a current controlled device and JFET is controlled by input voltage. Field Effect Transistors come in 2 main types: JFETs and MOSFETs. JFETs and MOSFETs are very similar but MOSFETs have even higher input impedance values than JFETs. This causes even less loading in a circuit. FET transistors are classified into two types namely JFET and MOSFET.
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.
| BJTsvs FETs | ||
| How it operates | BJTs | FETs |
| BJTsare current-controlled. They require a biasing current to the baseterminal for operation. | FETsare voltage-controlled. They only require voltage applied to the gateto turn the FET either on or off. They do not require a biasing currentfor operation. | |
| InputImpedance | BJTsoffer smaller input impedances, meaning they draw more current from thepower circuit feeding it, which can cause loading of the circuit. | FETsoffer greater input impedance than BJTs. This means that theypractically draw no current and therefore do not load down the power circuitthat's feeding it. |
| Gain (Transconductance) | BJTsoffer greater gain at the output than FETs. | Thegain (or transconductance) of FETs are smaller than for BJTs. |
| Size | BJTsare larger in size and therefore take up more physical space than FETs normally. | FETscan be manufactured much smaller than BJTs. This is especiallyimportant for integrated circuits that are composed up of manytransistors. |
| Popularity | BJTsare lesspopular and less widely used | FETSare definitely more popular and widely used in commercial circuitstoday than BJTs |
| Cost | BJTsare cheaperto manufacture | FETs,especially MOSFETs, are more expensive to manufacture |
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.
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.
The Main Types Of Field Effect Transistor Are Mcq
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.
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FET, Field Effect Transistor Circuit Design Includes:
FET circuit design basicsCircuit configurationsCommon sourceCommon drain / source followerCommon gate
Field effect transistors are used in circuit design as they are able to provide very high input impedance levels along with significant levels of voltage gain.
Unlike the bipolar transistor which is a current controlled device, the field effect transistor is voltage controlled. This makes the way FET circuits are designed rather different to the way bipolar transistor circuits are designed.
However, circuits with current and voltage gain can still be designed and similar circuit formats are adopted.
FET circuit basics
When considering the use of a FET circuit, it is necessary to consider FET technology and the type of field effect transistor will be the most applicable.
Note on Field Effect Transistor Technology:
The field effect transistor, FET, is a three terminal device which provides voltage gain. Having a high input impedance the electric field in the vicinity of the input terminal called the gate modifies the current flowing in what is called the channel between terminals called the source and drain.
Read more about the Field Effect Transistor Device & How it Works
The FET has three electrodes:
- Source: The Source is the electrode on the FET through which the majority carriers enter the channel, i.e. at acts as the source of carriers for the device. Current entering the channel through the source is designated by IS.
- Drain: The Drain is the FET electrode through which the majority carriers leave the channel, i.e. they are drained from the channel. Conventional current entering the channel via the drain is designated by the letters ID. Also Drain to Source voltage is often designated by the letters VDS
- Gate: The Gate is the terminal that controls the channel conductivity, hence the level of voltage on the gate controls the current flowing in the output of the device.
Types Of Field Effect Transistor Pdf
FET circuit design parameters
When starting out on the design of a FET circuit, it is necessary to determine the basic requirements for the circuit. These will govern many of the decisions regarding the type of circuit topology to use and also the type of FET to use.
There can be a number of parameters required in the requirements for the transistor circuit design:
- Voltage gain: The voltage gain is often a key requirement. It is the output signal voltage divided by the input signal voltage.
- Current gain: This is the gain of the FET circuit in terms of current. It may be necessary to drive a high level of current into the load.
- Input impedance: This is the impedance that the previous stage will see when it is providing a signal to this FET circuit in question. FETs inherently have a high input impedance to the gate and therefore FETs are often used where this is of paramount importance.
- Output impedance: The output impedance is also important. If the FET circuit is driving a low impedance circuit, then its output must have a low impedance, otherwise a large voltage drop will occur in the transistor output stage.
- Frequency response: Frequency response is another important factor that will affect the FET circuit design. Low frequency or audio transistor circuit designs may be different to those used for RF applications. Also the choice of the FET and capacitor values in the circuit design will be greatly affected by the required frequency response.
- Supply voltage and current: In many circuits the supply voltage is determined by what is available. Also the current may be limited, especially if the finished FET circuit design is to be battery powered.
FET types for circuit design
As there are several different types of field effect transistor that can be used, it is necessary to define at least some of the FETs that can be used within the circuit design process.
The table below defines some of the different types and characteristics that can be encountered.
| FETs for Use in Circuit Design | |
|---|---|
| Characteristic | Details |
| N-channel | An N channel FET has a channel made from N-type semiconductor in which the majority carriers are electrons. |
| P-channel | An P channel FET has a channel made from P-type semiconductor in which the majority carriers are holes. |
| J-FET | The J-FET or junction FET is a form of FET where the gate is formed by using a diode junction onto the channel. The isolation is maintained by ensuring that the diode junction remains reverse biased when operated within the circuit. It is a key requirement of the FET circuit design to ensure the junction remains reverse biased for satisfactory operation. |
| MOSFET | This type of field effect transistor relies on a metal oxide later between the gate and channel. It offers a very high input resistance. |
| Dual-gate MOSFET | As the name implies, this form of MOSFET has two gates. In FET circuit design, this gives additional options. |
| Enhancement mode | Enhancement mode FETs are OFF at zero gate-source voltage. They are turned on by pulling the gate voltage in the direction of the drain voltage, i.e. towards the supply rail, which is positive for N-channel devices and negative for P-channel devices. In other words by pulling the gate voltage towards the drain voltage, the number of carriers in the active layer of the channel is enhanced. |
| Depletion mode | In a depletion-mode MOSFET, the device is normally ON at zero gate-source voltage. Any gate voltage in the direction of the drain voltage will tend to deplete the active area of channel of carriers and reduce the current flowing. |
When designing an FET circuit, it is first necessary to select the required type of FET. Factors including the basic type of FET including whether it is a junction FET or MOSFET or another type as well as the mode type and other factors all need to be determined to before it is possible to proceed with the circuit design.
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