Types of Transistors: Definition, Formula, Applications & Sample Questions

A transistor is a semiconductor device that is used to switch or amplify electronic signals in an electric circuit. There are mainly 2 types of transistors – field-effect transistors (FETs) and bipolar transistors (bipolar junction transistors, BJTs). There can be further many types of transistors based on their characteristics, advantages, and disadvantages.

• A field-effect transistor is a type of unipolar device with no PN junction in its current-carrying path. They can be classified as – N-channel and P-channel transistors.
• A bipolar transistor is a type of transistor using both holes and electrons as charge carriers. They can be classified as NPN and PNP transistors.
• There is another type of transistor – Insulated-gate bipolar transistors that have a voltage-driven MOSFET and a high-current transistor.

Key Takeaways: Circuits, semiconductors, transistors, current, Voltage, Pn junction, electronic device, p-type semiconductors, n-type semiconductors, electric circuit

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What is a Transistor?

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A transistor is a type of electronic device which is formed by p-type and n-type semiconductors. When a semiconductor is positioned in the middle of two semiconductors of the same type then this arrangement is called the transistor. 

• A transistor can also be defined as a simultaneous connection of two diodes.
• A transistor is a device that controls the passage of current or voltage and serves as a button or gate for electronic signals.
• It has three terminals namely emitter, base and collector.
• Transistors have had a significant impact on the electronic industry's revolution.
• It is capable of signal amplification as well as signal rectification. 

Working Principle of Transistor

A transistor is based on the operating principle that it allows you to control the flow of current via one path by varying the intensity of the smallest amount of current traveling through a second path.

The video below explains this:

Transistor Detailed Video Explanation:

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Types of Transistors

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There are many types of transistors that can be classified on the basis of their application. Some of these are – 

• Bipolar Junction Transistor
• Diffusion Transistor
• Avalanche Transistor
• Schottky Transistor
• Darlington Transistor
• Heterojunction Bipolar Transistor
• Field Effect Transistor
• Junction FET Transistor
• Dual Gate MOSFET
• Multiple-Emitter Transistor 

1. Bipolar Junction Transistor (BJT)

Bipolar Junction Transistors are those types of transistors that are made up of three regions: the base, collector, and emitter. Different from FET transistors, bipolar junction transistors are current-controlled devices.

• A little current flowing from the emitter to the collector region of the transistor causes a considerably bigger current to flow from the base to the emitter.
• NPN and PNP are the two main types of bipolar junction transistors.

Bipolar Junction Transistor

i) NPN Transistor

In these types of transistors, there are two p-type semiconductor materials. A thin n-type semiconductor layer separates these materials. The majority of charge carriers in these transistors are holes, whereas the minority are electrons.

• The current flows from the emitter terminal to the collector terminal in this transistor.
• When the base terminal is pulled to LOW in comparison to the emitter terminal, the transistor will switch on.

ii) PNP Transistor

PNP Transistor consists of two n-type semiconductor layers separated by a thin p-type semiconductor layer. The majority of charge carriers in an NPN transistor are electrons, whereas the minority are holes.

• The current flow within the base terminal of the transistor is formed by electrons flowing from the emitter terminal to the collector terminal.
• In a transistor, a low current supply at the base terminal can result in a large amount of current flowing from the emitter to the collector.
• NPN transistors are currently the most widely used BJTs because electron mobility is greater than hole mobility in them.

2. Field-Effect Transistors (FETs)

Field-Effect Transistors are those types of transistors that are made up of three regions: the gate, source, and drain. FETs are voltage-controlled bipolar transistors that are different from bipolar transistors. A voltage applied to the transistor's gate controls the current flow from the source to the drain.

• Field-Effect transistors have extremely high input impedances, ranging from a few mega ohms (M) to very high values.
• Due to their high input impedance, these types of transistors only receive a little amount of current. (The current is inversely impacted by the value of the circuit's resistance, according to Ohm’s Law.)
• The current is very low if the impedance is high.
• As a result, FETs draw extremely little current from the power source of a circuit.
• FETs can be further classified as JFETs and MOSFETs.

Field-Effect Transistors (FETs)

i) Junction Field-Effect Transistor (JFET)

These are the type of FET transistors, which are used in resistors, amplifiers, switches, and other applications. This gadget is voltage-controlled and does not require any biassing of current. The current flow between the source and drain of the JFET transistor is controlled by the voltage supplied between the gate and source terminals. 

Junction Field-Effect Transistor

• The Junction Field Effect Transistor (JFET) lacks PN-junctions in favor of a narrow section of high resistive semiconductor material producing a Channel of either N-type or P-type silicon with two ohmic electrical connections at either end.
• It is generally referred to as the Drain and Source.
• The N-channel JFET and the P-channel JFET are the two main configurations of a junction field-effect transistor.

ii) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

MOSFET is the most commonly used transistor. As the name implies, it incorporates the metal gate's terminal. The source, drain, gate, and substrate, or body, are the four terminals of this transistor.

MOSFETs

• MOSFETs have various advantages over BJT and JFET including a high input (i/p) impedance and a low output (o/p) impedance.
• MOSFETs are primarily employed in low-power circuits, particularly in chips.
• These transistors come in two varieties: depletion and enhancement.
• These types are further divided into P-channel and N-channel types.

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Other Types of Transistors

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Some other major types of transistors are – 

• Avalanche Transistor: It is a type of Bipolar Junction Transistor that processes the region of collector-current/collector-to-emitter voltage area. It has avalanche-mode operations that change between high currents in less than nanoseconds.
• Diffusion Transistor: These are a type of BJT that is created via dopant diffusion into a semiconductor substrate. Example: the micro-alloy diffused transistor of Philco.
• Darlington Transistor: It's a transistor circuit with two distinct transistors in it. It is more capable of gaining current. Its circuit can also be contained within an integrated circuit.
• Schottky Transistor: Schottky Transistors are types of transistors that have been integrated with a Schottky diode. By introducing that type of diode, the transistors are prevented from saturating due to the diversion of high input current.
• Heterojunction Bipolar Transistor: These transistors are utilized in higher-frequency analog or digital microwaves. It has a faster switching speed and a higher lithographic yield. They have a higher efficiency of emitter injection.
• Multiple-Emitter Transistor: The emitters are used with input signals in these transistors. It has the ability to reduce switching time and power consumption.

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Applications of Transistors

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Transistors are commonly used for switching applications or for both amplification and switching.

• Phototransistors are a type of transistor that produces current flow in response to the amount of light that reflects on them.
• When a tiny amount of current is supplied through the base, Bipolar Junction Transistors (BJT) can generate a higher current flow from the emitter to the collector.
• Field-Effect Transistors (FETs) operate as voltage-controlled devices.
• In RF mixers/multipliers, RF amplifiers, and other applications where two controlled gates must be coupled in series, dual-gate MOSFETs are used.

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Things to Remember

• A transistor is a type of electronic device which is formed by p-type and n-type semiconductors.
• It controls the passage of current or voltage and serves as a button or gate for electronic signals.
• Bipolar Junction Transistors are made up of three regions: the base, collector, and emitter. 
  • In PNP transistors, two p-type semiconductor materials are separated by a thin n-type semiconductor layer.
  • In NPN transistors, two n-type semiconductor materials are separated by a thin p-type semiconductor layer.
• Field-Effect Transistors are those types of transistors that are made up of three regions: the gate, source, and drain. 
  • JFETs are used in resistors, amplifiers, switches, and other applications. 
  • MOSFET is the most commonly used transistor. The source, drain, gate, and substrate, or body, are the four terminals of this transistor.

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Previous Year Questions

1. Which logic gate does the following truth table represent?… [JKCET 2012]
2. If an emitter current is changed by 4mA, the collector current… [JEE Main 2021]
3. The impurity atoms with which pure silicon should be doped to make… [JEE Advanced 1988]
4. The following figure shows a logic gate circuit with two inputs A and B… [NEET 2010]
5. The solids which have negative temperature coefficient of resistance are… [NEET 2020]
6. The device that can act as a complete electronic circuit is… [NEET 2010]
7. In intrinsic semiconductor at room temperature number of electrons… [MET 2012]
8. Pure silicon is a/an… [JIPMER 2018]
9. The half-life of the radioactive radon is 3.8 days… [JEE Advanced 1981]
10. A light emitting diode (LED) has a voltage drop of 2V… [BCECE 2008]
11. The minimum potential difference between the base and emitter required… [BCECE 2008]
12. For detecting intensity of light we use… [BCECE 2010]
13. The current gain of a transistor in common emitter mode is 49… [BCECE 2003]
14. A pure Ge specimen is doped with Al. The number density of acceptor atoms is… [GUJCET 2008]
15. LCD stands for… [COMEDK UGET 2012]
16. In a reverse-biased p-n junction when the applied bias voltage… [DUET 2011]
17. For a transistor, the current ratio is defined as the ratio of…
18. In a common emitter configuration, a transistor has beta = 50…
19. The output Y of the logic circuit shown in figure is best represented a…
20. To get an OR gate from a NAND gate, we need…