This article covers different types of transistors and mentions applications of these transistor types.It describes different types of transistors viz. BJT,FET,MOSFET,UJT etc.
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A field-effect transistor or FET is a transistor, where the output current is controlled by an electric field. FET sometimes is called unipolar transistor as it involves single carrier type operation. The basic types of FET transistors are completely different from BJT transistor basics. FET is three-terminal semiconductor devices, with source, drain, and gate terminals.
The charge carries are electrons or holes, which flow from the source to drain through an active channel. This flow of electrons from source to drain is controlled by the voltage applied across the gate and source terminals.
Types of FET Transistor
FETs are of two types- JFETs or MOSFETs.
Junction FET
The Junction FET transistor is a type of field-effect transistor that can be used as an electrically controlled switch. The electric energy flows through an active channel between sources to drain terminals. By applying a reverse bias voltage to the gate terminal, the channel is strained so the electric current is switched off completely.
The junction FET transistor is available in two polarities which are;
N- Channel JFET
N channel JFET consists of an n-type bar at the sides of which two p-type layers are doped. The channel of electrons constitutes the N channel for the device. Two ohmic contacts are made at both ends of the N-channel device, which are connected together to form the gate terminal.
The source and drain terminals are taken from the other two sides of the bar. The potential difference between source and drain terminals is termed as Vdd and the potential difference between source and gate terminal is termed as Vgs. The charge flow is due to the flow of electrons from source to drain.
Whenever a positive voltage is applied across drain and source terminals, electrons flow from the source ‘S’ to drain ‘D’ terminal, whereas conventional drain current Id flows through the drain to source. As current flows through the device, it is in one state.
When a negative polarity voltage is applied to the gate terminal, a depletion region is created in the channel. The channel width is reduced, hence increasing the channel resistance between the source and drain. Since the gate-source junction is reverse biased and no current flows in the device, it is in off condition.
So basically if the voltage applied at the gate terminal is increased, less amount of current will flow from the source to drain.
The N channel JFET has a greater conductivity than the P channel JFET. So the N channel JFET is a more efficient conductor compared to P channel JFET.
P-Channel JFET
P channel JFET consists of a P-type bar, at two sides of which n-type layers are doped. The gate terminal is formed by joining the ohmic contacts at both sides. Like in an N channel JFET, the source and drain terminals are taken from the other two sides of the bar. A P-type channel, consisting of holes as charge carriers, is formed between the source and drain terminal.
A negative voltage applied to the drain and source terminals ensures the flow of current from source to drain terminal and the device operates in ohmic region. A positive voltage applied to the gate terminal ensures the reduction of channel width, thus increasing the channel resistance. More positive is the gate voltage; less is the current flowing through the device.
Characteristics of p channel Junction FET Transistor
Given below is the characteristic curve of the p channel Junction Field Effect transistor and different modes of operation of the transistor.
Cutoff region: When the voltage applied to the gate terminal is enough positive for the channel width to be minimum, no current flows. This causes the device to be in cut off region.
Ohmic region: The current flowing through the device is linearly proportional to the applied voltage until a breakdown voltage is reached. In this region, the transistor shows some resistance to the flow of current.
Saturation region: When the drain-source voltage reaches a value such that the current flowing through the device is constant with the drain-source voltage and varies only with the gate-source voltage, the device is said to be in the saturation region.
Break down region: When the drain-source voltage reaches a value that causes the depletion region to break down, causing an abrupt increase in the drain current, the device is said to be in the breakdown region. This breakdown region is reached earlier for a lower value of drain-source voltage when gate-source voltage is more positive.
MOSFET Transistor
MOSFET transistor as its name suggests is a p-type (n-type) semiconductor bar (with two heavily doped n-type regions diffused into it) with a metal oxide layer deposited on its surface and holes taken out of the layer to form source and drain terminals. A metal layer is deposited on the oxide layer to form the gate terminal. One of the basic applications of the field-effect transistors is using a MOSFET as a switch.
This type of FET transistor has three terminals, which are source, drain, and gate. The voltage applied to the gate terminal controls the flow of current from source to drain. The presence of an insulating layer of metal oxide results in the device having high input impedance.
Types of MOSFET Transistor Based on Operation Modes
A MOSFET transistor is the most commonly used type of field-effect transistor. MOSFET operation is achieved in two modes, based upon which MOSFET transistors are classified. MOSFET operation in enhancement mode consists of a gradual formation of a channel whereas, in depletion mode MOSFET, it consists of an already diffused channel. An advanced application of MOSFET is CMOS.
Enhancement MOSFET Transistor
When a negative voltage is applied to the gate terminal of MOSFET, the positive charge carrying carriers or holes get accumulated more near the oxide layer. A channel is formed from the source to the drain terminal.
As the voltage is made more negative, the channel width increases and current flows from source to drain terminal. Thus as the flow of current ‘enhances’ with applied gate voltage, this device is called Enhancement type MOSFET.
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Depletion Mode MOSFET Transistor
A depletion-mode MOSFET consists of a channel diffused between the drain to the source terminal. In absence of any gate voltage, current flows from source to drain because of the channel.
When this gate voltage is made negative, positive charges get accumulated in the channel.
This causes a depletion region or region of immobile charges in the channel and hinders the flow of current. Thus as the flow of current is affected by the formation of the depletion region, this device is called depletion-mode MOSFET.
Applications involving MOSFET as a switch
Controlling the speed of BLDC motor
MOSFET can be used as a switch to operate a DC motor. Here a transistor is used to trigger the MOSFET. PWM signals from a microcontroller are used to switch on or off the transistor.
A logic low signal from the microcontroller pin results in the OPTO Coupler to operate, generating a high logic signal at its output. The PNP transistor is cut off and accordingly, the MOSFET gets triggered and is switched ON. The drain and source terminals are shorted and the current flow to the motor windings such that it starts rotating. PWM signals ensure speed control of the motor.
Driving an array of LEDs:
MOSFET operation as a switch involves the application of controlling the intensity of an array of LEDs. Here a transistor, driven by signals from an external sources like microcontroller, is used to drive the MOSFET. When the transistor is switched off, the MOSFET gets the supply and is switched ON, thus providing proper biasing to the LED array.
Switching Lamp using MOSFET:
MOSFET can be used as a switch to control the switching of lamps. Here also, the MOSFET is triggered using a transistor switch. PWM signals from an external source like a microcontroller are used to control the conduction of transistor and accordingly the MOSFET switches on or off, thus control the switching of the lamp.
We hope we have been successful in providing the best knowledge to the readers about the topic of field-effect transistors. We would like the readers to answer a simple question – How are FETs different from BJTs and why they are more used comparatively.
Please your answers along with your feedback in the comment section below.
Photo Credits
A cluster of field-effect transistor by alibaba
N channel JFET by ebaying
P channel JFET by solarbotics
P channel JFET bar by wikimedia
P channel JFET characteristics curve by learningaboutelectronics
MOSFET transistor by imimg
Enhancement MOSFET transistor by circuitstoday
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There are many types of transistors in use. Each transistor is specialized in its application. The main classification is as follows.
The primary transistor is the BJT and FET is the modern version of transistor. Let us have a look at the BJTs.
Bipolar Junction Transistor
A Bipolar junction transistor, shortly termed as BJT is called so as it has two PN junctions for its function. This BJT is nothing but a normal transistor. It has got two types of configurations NPN and PNP. Usually NPN transistor is preferred for the sake of convenience. The following image shows how a practical BJT looks like.
The types of BJT are NPN and PNP transistors. The NPN transistor is made by placing a ptype material between two n-type materials. The PNP transistor is made by placing an ntype material between two p-type materials.
BJT is a current controlled device. A normal transistor which we had discussed in the previous chapters come under this category. The functionality, configurations and applications are all the same.
Field Effect Transistor
An FET is a three-terminal unipolar semiconductor device. It is a voltage controlled device unlike a bipolar junction transistor. The main advantage of FET is that it has a very high input impedance, which is in the order of Mega Ohms. It has many advantages like low power consumption, low heat dissipation and FETs are highly efficient devices. The following image shows how a practical FET looks like.
The FET is a unipolar device, which means that it is made using either p-type or n-type material as main substrate. Hence the current conduction of a FET is done by either electrons or holes.
Features of FET
The following are the varied features of a Field Effect Transistor.
Unipolar − It is unipolar as either holes or electrons are responsible for conduction.
High input impedance − The input current in a FET flows due to the reverse bias. Hence it has high input impedance.
Voltage controlled device − As the output voltage of a FET is controlled by the gate input voltage, FET is called as the voltage controlled device.
Noise is low − There are no junctions present in the conduction path. Hence noise is lower than in BJTs.
Gain is characterized as transconductance. Transconductance is the ratio of change in output current to the change in input voltage.
The output impedance of a FET is low.
Advantages of FET
To prefer a FET over BJT, there should be few advantages of using FETs, rather than BJTs. Let us try to summarize the advantages of FET over BJT.
| JFET | BJT |
|---|---|
| It is an unipolar device | It is a bipolar device |
| Voltage driven device | Current driven device |
| High input impedance | Low input impedance |
| Low noise level | High noise level |
| Better thermal stability | Less thermal stability |
| Gain is characterized by transconductance | Gain is characterized by voltage gain |
Applications of FET
FET is used in circuits to reduce the loading effect.
FETs are used in many circuits such as Buffer Amplifier, Phase shift Oscillators and Voltmeters.
FET Terminals
Though FET is a three terminal device, they are not the same as BJT terminals. The three terminals of FET are Gate, Source and Drain. The Source terminal in FET is analogous to the Emitter in BJT, while Gate is analogous to Base and Drain to Collector.
The symbols of a FET for both NPN and PNP types are as shown below
Source
The Source terminal in a Field Effect Transistor is the one through which the carriers enter the channel.
This is analogous to the emitter terminal in a Bipolar Junction Transistor.
The Source terminal can be designated as S.
The current entering the channel at Source terminal is indicated as IS.
Gate
Different Types Of Transistors
The Gate terminal in a Field Effect Transistor plays a key role in the function of FET by controlling the current through the channel.
By applying an external voltage at Gate terminal, the current through it can be controlled.
Gate is a combination of two terminals connected internally that are heavily doped.
The channel conductivity is said to be modulated by the Gate terminal.
This is analogous to the base terminal in a Bipolar Junction Transistor.
The Gate terminal can be designated as G.
The current entering the channel at Gate terminal is indicated as IG.
Drain
The Drain terminal in a Field Effect Transistor is the one through which the carriers leave the channel.
This is analogous to the collector terminal in a Bipolar Junction Transistor.
The Drain to Source voltage is designated as VDS.
The Drain terminal can be designated as D.
The current leaving the channel at Drain terminal is indicated as ID.
Types of FET
There are two main types of FETS. They are JFET and MOSFET. The following figure gives further classification of FETs.
Different Types Of Fetal Monitoring
In the subsequent chapters, we will have a detailed discussion on JFET and MOSFET.