The transistor is an active component that is installed throughout the electronic circuit. These are used as amplifiers and switching equipment. As amplifiers, they are used in high and low-level, frequency stages, oscillators, modulators, detectors, etc. electronic circuits that need to perform a function. In digital circuits, they are used as switches. There are actually thousands of different types of transistors. Low, medium, and high-power transistors are found to operate with high and low frequencies, and with very high current or high voltage.
There are several types of transistors depending on the usage. Each transistor is specialized in its application. Transistors are mainly classified into two types namely Bipolar Junction Transistor (BJT) and Field Effect Transistor (FET).
Bipolar Junction Transistors (BJT)
A bipolar junction transistor is abbreviated as BJT. In this transistor, work is done by two P-N junctions. This BJT is just a normal transistor. A bipolar junction transistor has 3 terminals, base, collector, and emitter. Its two types of configurations are NPN and PNP. Generally, NPN transistors are preferred for convenience.
A small current entering the base region of the transistor causes a lot of currents to flow from the emitter to the collector region. An NPN transistor is one in which most of the current-carriers electrons are. The electrons that flow from the emitter to the collector form the basis of most of the current flowing through the transistor. Further types of charges, holes, are the minority. PNP transistors are the opposite. In PNP transistors, most of the current carriers are holes.
There are two main types of Bipolar Junction transistors:
- PNP Transistor.
- NPN Transistor.
PNP Transistor
PNP transistor is another type of bipolar junction transistor (BJT). It consists of an n-type semiconductor material sandwiched between two p-type semiconductor materials. In these transistors, most of the charge carriers are holes while the minority charge carriers are electrons.
In this transistor, conventional current flow is indicated by arrows (→). current flows from the emitter terminal to the collector terminal. This transistor will turn on once the base terminal is pulled LOW compared to the emitter terminal.
NPN Transistor
NPN is also a type of bipolar junction transistor (BJT) and consists of a P-type semiconductor material sandwiched between two N-type semiconductor materials. In an NPN transistor, the majority charge carriers are electrons while the minority charge carriers are holes. The flow of electrons from the emitter terminal to the collector terminal will cause current to flow within the base terminal of the transistor.
In transistors, a small amount of current supplied power at the base terminal can supply a large amount of current from the emitter terminal to the collector. Currently, the commonly used BJTs are NPN transistors, because the electron mobility is higher than the hole mobility.
Related Tutorial: Operations of PNP and NPN Transistor.
Field Effect Transistor (FET)
Field Effect Transistor is abbreviated as FET. FET is a three-terminal one gate, one source, and one drain unipolar semiconductor device. It is a voltage-controlled device, unlike a bipolar junction transistor. In a FET transistor, a voltage source on the gate controls the flow of current to the drain. The main advantage of a FET is that it has a very high input impedance, which can range from the resistance of mega ohms (MΩ) to very large values. It has many advantages such as low power consumption, and low heat dissipation, and FETs are highly efficient devices.
FETs have high input impedance so that work can be done efficiently even in the amount of working current. According to Ohm’s law, the current is inversely affected by the value of the circuit’s impedance. If the impedance is high, the current is very low. Because of this, both FETs draw very little current from a circuit’s power source.
The FET is a unipolar device, which means that it is fabricated using p-type or n-type semiconductor material as the main substrate. Because of this, the FET’s current is carried by electrons or holes.
Bipolar Junction Transistor is better than Field Effect Transistor because it provides more amplification. FETs are better, cause less loading, are cheaper, and are easier to manufacture.
Field Effect Transistor Terminals
FET is a three-terminal gate, source, and drain. The terminals of the FET correspond to the terminals of the BJT such as source to emitter, gate to the base, and drain to collector.
Source Terminal
- In field effect transistors, carriers enter the channel through the source terminal. It is denoted by S.
- It is similar to the emitter terminal of a bipolar junction transistor.
- The current entering the channel at the source terminal is represented as IS.
Gate Terminal
- In a field effect transistor, the current is controlled through the gate terminal channel. It plays an important role. It is denoted by G.
- The current through it can be controlled by applying an external voltage to the gate terminal.
- The gate is a combination of two internally connected terminals that are heavily doped.
- The channel conductivity is modified by the gate terminal.
- It is similar to the base terminal of a bipolar junction transistor.
- The current entering the channel at the gate terminal is represented as IG.
Drain Terminal
- In field effect transistors, carriers leave the channel through the drain terminal. It is denoted by D.
- It is similar to the collector terminal of a bipolar junction transistor.
- The drain to source voltage is denoted by VDS.
- The current leaving the channel at the drain terminal is represented as ID.
Features of Field Effect Transistor
The following are the different features of field effect transistors:
- Unipolar − It is unipolar because holes or electrons are responsible for conduction.
- High input impedance − FETs have high input impedance as the input current flows through them due to reverse bias.
- Voltage-controlled device − The output voltage of the FET is controlled by the gate input voltage, the FET is called a voltage-controlled device.
- Noise is low − No junction exists in the conduction path. Hence the noise is less as compared to BJTs.
- Gain is described as transconductance. Transconductance is the ratio of change in output current to change in input voltage.
- The output impedance of the FET is low.
Advantages of Field Effect Transistor
In order to prefer a FET over a BJT, there must be some advantages to using a FET instead of a BJT.
Applications of Field Effect Transistor
- FETs are used to reduce the loading effect in the circuit.
- FETs are used in many circuits such as buffer amplifiers, phase shift oscillators, and voltmeters.
There are two main types of field effect transistors:
- Junction Field Effect Transistor (JFET).
- Metal Oxide Semiconductor Field Effect Transistor (MOSFET).
Junction Field Effect Transistor (JFET)
Field Effect Transistor is abbreviated as JEFT. These are a type of FETs used as resistors, amplifiers, and switches that can be controlled electrically. It is a voltage-controlled device and does not use any biasing current.
Through the active channel, electrical energy flows between the source terminal and the drain terminal. If a reverse bias voltage supply is provided to the gate terminal, the flow of current will stop completely and the channel will be strained. Junction field effect transistors are generally classified into two types based on their polarity and they are:
- N-Channel junction field effect transistor
- P-Channel junction field effect transistor
N-Channel junction field effect transistor
The JFETs in which electrons are mainly used as charge carriers are called N-channel JFETs. Therefore, if the transistor is on, the current flowing through them is mainly due to the movement of electrons.
P-Channel junction field effect transistor
The JFETs in which holes are made mainly as charge carriers are called P-channel JFETs. Therefore, if the transistor is on, the current flow is mainly due to the holes.
Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
MOSFET or metal-oxide-semiconductor field-effect transistor is the most commonly used of all types of transistors. This includes the terminal of the metal gate. This transistor has four terminals namely source, drain, gate, and substrate or body.
Compared to BJT and JFET, MOSFETs have many advantages as it provides high input impedance as well as low output impedance. MOSFETs are mainly used in low-power circuits. These transistors are available in two types depletion and enhancement. Furthermore, these types are classified into P-channel and N-channel types.