Different Types of Transistors and Their Functions

The transistor is an active component and that is establishing all over electronic circuits. They are used as amplifiers and switching apparatus. As the amplifiers, they are used in high and low level, frequency stages, oscillators, modulators, detectors, and in any circuit need to perform a function. In digital circuits, they are used as switches. There are a huge number of manufacturers approximately the world who produces semiconductors (transistors are members of this family of apparatus), so there are exactly thousands of different types. There are low, medium, and high power transistors, for functioning with high and low frequencies, for functioning with very high current and or high voltages. This article gives an overview of what is a transistor, different types of transistors, and their applications.

What is a Transistor

The transistor is electronic equipment. It is made through a p and n-type semiconductor. When a semiconductor is placed in the center between the same type of semiconductors the arrangement is called transistors. We can say that a transistor is the combination of two diodes it is a connection back to back. A transistor is a device that regulates current or voltage flow and acts as a button or gate for electronic signals.

Types of Transistors
Types of Transistors

Transistors consist of three layers of a semiconductor device, each capable of moving a current. A semiconductor is a material such as germanium and silicon that conducts electricity in a “semi-enthusiastic” way. It’s anywhere between a genuine conductor such as a copper and an insulator (similar to the plastic-wrapped roughly wires).

Transistor Symbol

A diagrammatic form of n-p-n and p-n-p transistor is exposed. In-circuit is a connection drawn form is used. The arrow symbol defined the emitter current. In the n-p-n connection, we identify electrons flow into the emitter. This means that the conservative current flows out of the emitter as indicated by the outgoing arrow. Equally, it can be seen that for the p-n-p connection, the conservative current flows into the emitter as exposed by the inward arrow in the figure.

PNP and NPN Transistors
PNP and NPN Transistors

There are so many types of transistors and they each vary in their characteristics and each has its possess advantages and disadvantages. Some types of transistors are used mostly for switching applications. Others can be used for both switching and amplification. Still, other transistors are in a specialty group all of their own, such as phototransistors, which react to the amount of light shining on it to produce current flow through it. Below is a list of the different types of transistors; we will go over the characteristics that create them each up

What are the Two Main Types of Transistors?

Transistors are classified into two types like BJTs and FETs.


Bipolar Junction Transistor (BJT)

Bipolar Junction Transistors are transistors that are built up of 3 regions, the base, the collector, and the emitter. Bipolar Junction transistors, different FET transistors, are current-controlled devices. A small current entering the base region of the transistor causes a much larger current flow from the emitter to the collector region. Bipolar junction transistors come in two major types, NPN and PNP. An NPN transistor is one in which the majority of the current carriers are electrons.

Electron flowing from the emitter to the collector forms the base of the majority of current flow through the transistor. The further types of charge, holes, are a minority. PNP transistors are the opposite. In PNP transistors, the majority of current carrier holes. BJT transistors are available in two types namely PNP and NPN

Bipolar Junction Transistor pins
Bipolar Junction Transistor pins

PNP Transistor

This transistor is another kind of BJT – Bipolar Junction Transistors and it contains two p-type semiconductor materials. These materials are divided through a thin n-type semiconductor layer. In these transistors, the majority charge carriers are holes whereas the minority charge carriers are electrons.

In this transistor, the arrow symbol indicates the conventional current flow. The direction of current flow in this transistor is from the emitter terminal to the collector terminal. This transistor will be turned ON once the base terminal is dragged to LOW as compared with the emitter terminal. The PNP transistor with a symbol is shown below.

NPN Transistor

NPN is also one kind of BJT (Bipolar Junction Transistors) and it includes two n-type semiconductor materials which are divided through a thin p-type semiconductor layer. IN the NPN transistor, the majority charge carriers are electrons whereas the minority charge carriers are holes. The electrons flow from the emitter terminal to the collector terminal will form the current flow within the base terminal of the transistor.

In the transistor, the less amount of current supply at the base terminal can cause supply huge amount of current from the emitter terminal to the collector. At present, the commonly used BJTs are NPN transistors, as the electrons mobility is higher as compared with the mobility of holes. The NPN transistor with a symbol is shown below.

Field Effect Transistor

Field Effect Transistors are made up of 3 regions, a gate, a source, and a drain. Different bipolar transistors, FETs are voltage-controlled devices. A voltage placed at the gate controls current flow from the source to the drain of the transistor. Field Effect transistors have a very high input impedance, from several mega ohms (MΩ) of resistance to much, much larger values.

This high input impedance causes them to have very little current run through them. (According to ohm’s law, the current is inversely affected by the value of the impedance of the circuit. If the impedance is high, the current is very low.) So FETs both draw very little current from a circuit’s power source.

Field Effect Transistors
Field Effect Transistors

Thus, this is ideal because they don’t disturb the original circuit power elements to which they are connected to. They won’t cause the power source to be loaded down. The drawback of FETs is that they won’t provide the same amplification that could be gotten from bipolar transistors.

Bipolar transistors are superior in the fact that they provide greater amplification, even though FETs are better in that they cause less loading, are cheaper, and easier to manufacture. 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.


The JFET stands for Junction-Field-Effect transistor. This is simple as well as an initial type of FET transistors which are utilized like resistors, amplifiers, switches, etc. This is a voltage-controlled device and it doesn’t use any biasing current. Once the voltage is applied among gate & source terminals then it controls the current flow among the source & drain of the JFET transistor.

The Junction Field Effect Transistor (JUGFET or JFET) has no PN-junctions but in its place has a narrow part of high resistivity semiconductor material forming a “Channel” of either N-type or P-type silicon for the majority carriers to flow through with two ohmic electrical connections at either end normally called the Drain and the Source respectively.

Junction Field Effect Transistors
Junction Field Effect Transistors

There are two basic configurations of a junction field-effect transistor, the N-channel JFET and the P-channel JFET. The N-channel JFET’s channel is doped with donor impurities meaning that the flow of current through the channel is negative (hence the term N-channel) in the form of electrons. These transistors are accessible in both P-channel and N-channel types.


MOSFET or Metal-Oxide-Semiconductor Field-Effect Transistor is most frequently used among all kinds of transistors. As the name suggests, it includes the terminal of the metal gate. This transistor includes four terminals like source, drain, gate & substrate, or body.


As compared with BJT and JFET, MOSFETs has several benefits as it provides high i/p impedance as well as low o/p impedance. MOSFETs are mainly used in low power circuits especially while designing chips. These transistors are available in two types like depletion & enhancement. Further, these types are categorized into P-channel & N-channel types.

The main features of FET include the following.

  • It is unipolar because the charge carriers like either electrons or holes are accountable for transmission.
  • In FET, the input current will flow because of the reverse bias. Therefore the input impedance of this transistor is high.
  • When the o/p voltage of the field-effect transistor is controlled through the input voltage of the gate, then this transistor is named the voltage-controlled device.
  • In the conduction lane, there are no junctions present. So FETs have less noise as compared with BJTs.
  • The characterization of gain can be done with transconductance because it is the ratio of o/p change current and input voltage change
  • The o/p impedance of the FET is low.

Advantages of FET

The advantages of FET as compared with BJT include the following.

  • FET is a unipolar device whereas the BJT is a bipolar device
  • FET is a voltage-driven device whereas the BJT is a current-driven device
  • The i/p impedance of the FET is high whereas BJT has low
  • The noise level of FET is low as compared with BJT
  • In FET, thermal stability is high whereas BJT has low.
  • The gain characterization of FET can be done through transconductance whereas in BJT with a voltage gain

Applications of FET

The applications of FET include the following.

  • These transistors are used within different circuits to decrease the loading effect.
  • These are used in several circuits like Phase shift Oscillators, Voltmeters & Buffer amplifiers.

FET Terminals

FET has three terminals like source, gate, and drain which are not similar to the terminals of BJT. In FET, the Source terminal is similar to the Emitter terminal of BJT, whereas the Gate terminal is similar to the Base terminal & Drain terminal to the Collector terminal.

Source Terminal

  • In FET, the source terminal is the one through which the charge carriers enter the channel.
  •  This is similar to the emitter terminal of BJT
  • The source terminal can be represented with ‘S’.
  • The flow of current through the channel on the source terminal can be specified like IS.
    Gate Terminal
  • In a FET, the Gate terminal plays an essential role to control the flow of current throughout the channel.
  • The flow of current can be controlled through the gate terminal by providing an external voltage to it.
  • Gate terminal is a blend of two terminals which are internally connected and are doped heavily. The conductivity of the channel can be modulated through the Gate terminal.
  • This is similar to the base terminal of BJT
  • The gate terminal can be represented with ‘G’.
  • The flow of current through the channel at the Gate terminal can be specified as IG.

Drain Terminal

  • In FET, the drain terminal 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 flow of current moving away from the channel at the Drain terminal can be specified as ID.

Different Types of Transistors

There are different types of transistors available based on the function like the small-signal, small switching, power, high frequency, phototransistor, UJT. Some kinds of transistors are mainly used for amplification otherwise switching purposes.

Small Signal Types of Transistors

Small signal transistors are used mainly used to amplify low-level signals but can also function well as switches. These transistors available through an hFE value, that specifies how a transistor amplifies input signals. The range of typical hFE values is from 10 to 500 including the highest collector current (Ic) rating ranges from 80 mA to 600mA.

These transistors are available in two forms like PNP and NPN. The highest operating frequencies of this transistor have from 1 to 300 MHz. These transistors are used when amplifying small signals like a few volts & simply when a mill ampere of current is used. A power transistor is applicable once a huge voltage, as well as current, is used.

Small Switching Types of Transistors

Small Switching Transistors are used like switches as well as amplifiers. The typical hFE values for these transistors range from 10 to 200 including least collector current ratings which range from 10 mA to 1000mA. These transistors are available in two forms like PNP and NPN

These transistors are not capable of the small-signal amplification of transistors, which can include up to 500 amplification. So this will make the transistors more helpful for switching, although they may be used as amplifiers for providing gain. Once you require additional gain, then these transistors would function better like amplifiers.

Power Transistors

These transistors are applicable where a lot of power is used. The collector terminal of this transistor is allied to the base terminal of metal so that it works like a heat sink to dissolve surplus power. The range of typical power ratings mainly ranges from approximately 10 W to 300 W including frequency ratings which range from 1 MHz – 100 MHz.

Power Transistor
Power Transistor

The values of the highest collector current will range between 1A – 100 A. Power transistors are available in PNP & NPN forms whereas the Darlington transistor comes in either PNP or NPN forms.

High-Frequency Types of Transistors

High-Frequency Transistors are used especially for small signals that work at high frequencies and used in high-speed based switching applications. These transistors are applicable in high-frequency signals & should be capable of turning ON/OFF at extremely high speeds.

The applications of high-frequency transistors mainly include HF, UHF, VHF, MATV, and CATV amplifier as well as oscillator applications. The range of maximum frequency rating is about 2000 MHz & the highest collector currents range from 10 mA – 600mA. These are obtainable in both PNP & NPN forms.


These transistors are light-sensitive and a common type of this transistor looks like a bipolar transistor where the base lead of this transistor is removed as well as changed through a light-sensitive region. So this is the reason that a phototransistor includes simply two terminals in place of the three terminals. Once the outside region is kept shady, then the device will be turned off.


Basically, there is no flow of current from the regions of the collector to the emitter. But, whenever the region of light-sensitive is exposed toward daylight, then a small amount of base current can be produced to control a much high collector to emitter current.

Similar to normal transistors, these can be both FETs and BJTs. FETs are light-sensitive transistors, not like photo bipolar transistors, photo FETs utilize light to produce a gate voltage that is mainly used for controlling a drain-source current. These are very responsive to changes within light as well as more delicate as compared with bipolar phototransistors.

Unijunction Types of Transistors

Unijunction transistors (UJTs) include three-leads that work completely like electrical switches so they are not utilized like amplifiers. Generally, transistors work like a switch as well as an amplifier. However, a UJT does not give any kind of amplification due to its design. So it is not designed for providing enough voltage otherwise current.

The leads of these transistors are B1, B2 & an emitter lead. The operation of this transistor is simple. When voltage exists between its emitter or base terminal then there will be a small flow of current from B2 to B1.

Unijunction Transistor
Unijunction Transistor

The control leads in other types of transistors will provide a small additional current whereas, in UJT, it is quite opposite. The primary source of the transistor is its emitter current. The flow of current from B2 to B1 is simply a small amount of the whole combined current, which means that UJTs are not appropriate for amplification but they are suitable for switching.

Heterojunction Bipolar Transistor (HBT)

AlgaAs/GaAs heterojunction bipolar transistors (HBTs) are used for digital and analog microwave applications with frequencies as high as the Ku band. HBTs can supply faster-switching speeds than silicon bipolar transistors mostly because of reduced base resistance and collector-to-substrate capacitance. HBT processing requires less demanding lithography than GaAs FETs, therefore, HBTs can priceless to fabricate and can provide better lithographic yield.

This technology can also provide higher breakdown voltages and easier broadband impedance matching than GaAs FETs. In assessment with Si bipolar junction transistors (BJTs), HBTs show better presentation in terms of emitter injection efficiency, base resistance, the base-emitter capacitance, and cutoff frequency. They also present good linearity, low phase noise and high power-added efficiency. HBTs are used in both profitable and high-reliability applications, such as power amplifiers in mobile telephones and laser drivers.

Darlington Transistor

A Darlington transistor sometimes called a “Darlington pair” is a transistor circuit that is made from two transistors. Sidney Darlington invented it. It is like a transistor, but it has a much higher ability to gain current. The circuit can be made from two discrete transistors or it can be inside an integrated circuit.

The hfe parameter with a Darlington transistor is every transistor hfe multiplied mutually. The circuit is helpful in audio amplifiers or in a probe that measures a very small current that goes through the water. It is so sensitive that it can pick up the current in the skin. If you connect it to a piece of metal, you can build a touch-sensitive button.

Darlington Transistor
Darlington Transistor

Schottky Transistor

A Schottky transistor is a combination of a transistor and a Schottky diode that prevents the transistor from saturating by diverting the extreme input current. It is also called a Schottky-clamped transistor.

Multiple-Emitter Transistor

A multiple-emitter transistor is a specialized bipolar transistor frequently used as the inputs of transistor logic (TTL) NAND logic gates. Input signals are applied to the emitters. Collector current stops flowing simply, if all emitters are driven by the logical high voltage, thus performing a NAND logical process using a single transistor. Multiple-emitter transistors replace diodes of DTL and agree to a reduction of switching time and power dissipation.

Dual Gate MOSFET

One form of MOSFET that is particularly popular in several RF applications is the dual-gate MOSFET. The dual-gate MOSFET is used in many RF and other applications where two control gates are required in series. The dual-gate MOSFET is fundamentally a form of MOSFET where two gates are made-up along the length of the channel one after the other.

In this way, both gates influence the level of current flowing between the source and drain. In effect, the dual-gate MOSFET operation can be considered the same as two MOSFET devices in series. Both gates affect the general MOSFET operation and therefore the output. The dual-gate MOSFET can be used in a lot of applications including RF mixers /multipliers, RF amplifiers, amplifiers with gain control, and the like.

Avalanche Transistor

An avalanche transistor is a bipolar junction transistor designed for process in the region of its collector-current/collector-to-emitter voltage characteristics beyond the collector-to-emitter breakdown voltage, called the avalanche breakdown region. This region is characterized by the avalanche breakdown, an occurrence similar to Townsend discharge for gases, and negative differential resistance. Operation in the avalanche breakdown region is called avalanche-mode operation: it gives avalanche transistors the capability to switch very high currents with less than a nanosecond rise and fall times (transition times).

Transistors not particularly designed for the purpose can have reasonably consistent avalanche properties; for example, 82% of samples of the 15V high-speed switch 2N2369, manufactured over a 12-year period, were capable of generating avalanche breakdown pulses with a rising time of 350 ps or less, using a 90V power supply as Jim Williams writes.

Diffusion Transistor

A diffusion transistor is a bipolar junction transistor (BJT) formed by diffusing dopants into a semiconductor substrate. The diffusion process was implemented later than the alloy junction and grown junction processes for making BJTs. Bell Labs developed the first prototype diffusion transistors in 1954. The original diffusion transistors were diffused-base transistors.

These transistors still had alloy emitters and sometimes alloy collectors like the earlier alloy-junction transistors. Only the base was diffused into the substrate. Sometimes the substrate produced the collector, but in transistors like Philco’s micro-alloy diffused transistors, the substrate was the bulk of the base.

Applications of  Types of Transistors

The appropriate application of power semiconductors requires an understanding of their maximum ratings and electrical characteristics, information that is presented within the device datasheet. Good design practice employs datasheet limits and not information obtained from small sample lots. A rating is a maximum or minimum value that sets a limit on the device’s ability. Act in excess of a rating can result in irreversible degradation or device failure. Maximum ratings signify the extreme capabilities of a device. They are not to be used as design circumstances.

A characteristic is a measure of device performance under individual operating conditions expressed by minimum, characteristic, and/or maximum values, or revealed graphically.

Thus, this is all about what is a transistor and the different types of transistors and their applications. We hope that you have got a better understanding of this concept or to implement electrical and electronics projects, please give your valuable suggestions by commenting in the comment section below. Here is a question for you, what is the main function of a transistor?

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