Transistor: Working, Terminals & Characteristics

Transistor is a semiconductor device that is often used for amplifying or switching electronic signals and electronic powers. Transistors come in many shapes and sizes.

• Transistors are small electronic components with two main functions.
• A transistor acts as a switch to on and off electricity as well as amplifies signals.
• Small and low-power transistors are enclosed in a resin case to help protect the internal parts. 
• A transistor can be defined as a miniature device used to control or regulate the flow of electronic signals.

Transistors are considered one of the major components of most electronic devices today. The transistor was developed in the year 1947 by three American physicists John Bardeen, Walter Brattain and William Shockley.

• Base: Activate the transistor
• Collector: The positive lead of the transistor
• Emitter: The negative lead of the transistor

Key Terms: Amplifier, Transistor, Semiconductor Diode, Insulator, Voltage, FET, BJT, Schottky transistor, Inverted-T Field-effect Transistor (ITFET), MOSFET

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What is a Transistor?

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A transistor is a semiconductor device that can be used for both conducting and insulating electric current or voltage.

• A transistor acts as a switch and an amplifier.
• It is a miniature device used to control or regulate the flow of electronic signals.
• Transistors are the key components in most of the electronic devices present today.
• Transistors were developed in 1947 by three American physicists John Bardeen, Walter Brattain and William Shockley.
• A transistor acts as a switch to on and off electricity as well as amplifies signals. 

Transistor

Transistor as Switch

A switch should have on and off characteristics and to control it we need to give input to it as yes or no, 0 or 1, up or down.

• And since, a transistor has the collector, the base, and the emitter which means if the switch is “on” the current flows from the collector to the emitter.
• Input to the switch can be given through the base.
• If the base is given with 1, the switch is on. If the base is given with 0, the switch is off.

Transistor as Amplifier

A Transistor can be said to be a current booster.

• It takes in a very small weak signal through the base junction and the amplified signal will come out at the collector.
• The n-type material has electrons in abundance; the p-type material has holes that attract electrons.
• When the transistor is in its off state the holes in the base prevent electrons from flowing from the emitter to the collector. 
• The transistor starts operating when holes start moving across the n-p junction.
• So when we supply a small positive signal at the base and make the emitter negatively charged and the collector positively charged, electrons are driven from the emitter to the base to the collector and take the transistor to its “on” state.
• So, the small-signal supplied to the base causes a big current to flow from emitter to the collector, thus amplifying.

The video below explains this:

Transistor Detailed Video Explanation:

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Transistors Working

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A Bipolar Junction Transistor (BJT) comprises of three terminals (Emitter, Base and Collector). A BJT is a current-driven device wherein the two P-N junctions can be found within a BJT.

• One P-N junction is in between the emitter and base, while the second junction is in between the collector and base.
• A small amount of current flow is found to pass via the emitter to the base.
• It can control a large amount of current flow via the device from emitter to collector.
• As per the operation of BJT, the base-emitter junction is forward-biased, while the base-collector junction is in reverse-biased condition.
• When current is flowing via the base-emitter junction, a current is going to flow in the collector circuit.

To express the working of transistors, an example of an NPN transistor can be considered. 

Operation of NPN Transistor

The NPN device emitter is composed of an n-type material, thus the majority of carriers are electrons.

• When the base-emitter junction has been forward-biased, the electrons shift from the n-type region toward the p-type region.
• The minority carrier holes shift towards the n-type region.
• When they both meet, they combine and allow current to flow across the junction.
• However, if the junction is reverse-biased, then the holes and electrons shift away from the junction.
• It thus creates a depletion region between the two areas, allowing no current to flow through them.

Operation of NPN Transistor

• When the flow of current is between the base and emitter, the electrons tend to exit the emitter and flow into the base.
• The electrons, typically, combine when they reach the depletion region.
• However, the doping in this region is low, with the base being very thin.
• This causes the electrons to be able to travel across the region without recombining with the holes.
• Thus, the electrons drift towards the collector.

In this way, the electrons flow across the reverse-biased junction while the current flows in the collector circuit.

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Types of Transistors

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There is a variety of transistors including, Schottky transistor, MOSFET, inverted-T field-effect transistor (ITFET), JFET, thin-film transistor, insulated-gate bipolar transistor, high electron mobility transistor, fast-reverse epitaxial diode field-effect transistor (FREDFET), tunnel field-effect transistor, organic field-effect transistor (OFET), diffusion transistor, and more. Some of the most important types of transistors are:

Power Transistor

These types of transistors are used to make high-power amplifiers and power supplies.

Bipolar Junction Transistor (BJT)

It is a device in which the current flows between two terminals which are the collector and emitter. BJT is known for its current control. They are used for the amplification of signals, high-voltage switches, and digital circuits.

• P-N-P Transistor: PNP transistors are a type of Bipolar Junction Transistor (BJT) wherein one n-type material is placed between two p-type materials. In this type of configuration, the device is going to control the current flow. PNP transistor is a 2-crystal diode connected in series. Both the right side and left side of the diodes are called the collector-base diode and emitter-base diode, respectively.
• N-P-N Transistor: NPN transistor is a type of transistor wherein the p-type material is found between two n-type materials. It is utilised to amplify weak signals to strong signals. In an NPN transistor, the electrons are seen to shift from the emitter to the collector, thus resulting in the building of current in the transistor. 

There are three different types of configuration, which are the Common Base (CB), Common Collector (CC) and Common Emitter (CE).

Common Base (CB)

A Common Base configuration is a configuration where the base terminal of the transistor is common between input and output terminals.

Common Collector (CC)

In a Common Collector configuration, the collector terminals are observed to be common between the input and output terminals.

Common Emitter (CE)

In this type of configuration, the emitter terminal is common between the input and the output terminals.

CB, CC and CE Configurations

Small Signal Transistor

Small Signal Transistors are transistors that are used to amplify low-level signals but can also be used as a switch. Typical FE values for small signal transistors range from 10 to 500, with maximum ratings from about 80 to 600mA. This is useful in amplifiers, medical equipment, and measuring devices.

Switching Transistor

Switching transistors operate fully off or fully on. They are used in designing power supplies, electronic switches, and automation equipment.

Phototransistor

A phototransistor is an electronic switching and current amplification component which relies on exposure to light to operate. Since transistors are made from semi-conductive materials grown into crystals such as germanium or silicon, all transistors react to light. Phototransistors are used extensively to detect light pulses and convert them into digital electrical signals. These are operated by light rather than electric current.

Field-effect Transistor

A field-effect transistor is a transistor in which the voltage on one terminal creates a field that allows or disallows conduction between the other two terminals. It uses an electric field to control the flow of current in a semiconductor. The three terminals in a FET is Gate, Source and Drain.

Field Effect Transistor (FET)

The voltage present at the gate terminal is able to control the current between the source and the drain. FET is considered to be a unipolar transistor wherein N-channel FET or P-channel FET is utilised for conduction. The primary applications of FETs are found in low-noise amplifiers, buffer amplifiers and analogue switches.

Darlington Transistor

A Darlington transistor is an electronics component made via the combination of two BJT connected in such a way that it allows a very high amount of current gain. Darlington has two transistors joined together. They are useful for amplification and used to make pre-amplifiers for audio.

High-frequency Transistor

High-frequency transistors are used in high-frequency ranges like radio, communication equipment, television signal, and microwaves.

Unijunction Transistor

They are a three-lead electronic semiconductor devices with only one junction that acts exclusively as an electrically controlled switch.

​Metal Oxide Semiconductor FET

These are the transistors that are very important for logic circuit boards and computers. They are semiconductor devices that are widely used for switching purposes.

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Characteristics of Transistors

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The Characteristics of the transistor are the plots that help show the relationship between the current and the voltage of a transistor in a certain configuration. The types of characteristics are:

Input Characteristics

It helps to give the details about the change in input current, along with the variation in input voltage by retaining the output voltage to be constant.

• CB Configuration: Variation of emitter current, I_E with base – Emitter voltage, V_BE by holding the collector voltage to be constant, V_CB.
• CC Configuration: Variation in I_B as per V_CB with collector-emitter voltage V_CE constant.
• CE Configuration: Variation in I_B as per V_BE with V_CE constant.

Input Characteristics

Output Characteristics

Output characteristics are a plot of the output current along with the output voltage by holding the input current to be constant.

• CB Configuration: Variation of collector current, I_C along with V_CB by holding the emitter current I_E to be constant.
• CC Configuration: Variation in I_E against the changes in V_CE while I_B is constant.
• CE Configuration: Variation in I_C with changes in V_CE while I_B is constant.

Output Characteristics

Current transfer Characteristics

It shows the variation of the output current with the input current by holding the voltage to be constant.

• CB Configuration: Variation of I_C with I_E while V_CB is constant.
• CC Configuration: This shows the variation of I_E with I_B when V_CE is constant.
• CE Configuration: Variation of I_C with I_B while V_CE is constant.

Current transfer Characteristics

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Applications of Transistors

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The applications of a transistor are:

• Transistors are used for making memory chips.
• Transistors are used for making most of the parts of modern electronic devices like smartphones, smartwatches, etc.
• It is used for Microprocessors and Microcontrollers
• When a large current or voltage is needed to be controlled, the Darlington transistor can be used.
• Transistor switches can be used to switch and control lamps, relays, or even motors.

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Terminals of Transistors

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There are three parts of a transistor includes:

• Collector
• Base
• Emitter

Transistor Diagram

The transistor contains three pins which are labeled as E, B, and C. Herein, E, B, and C in a transistor stands for the Emitter, Base, and Collector. The pin of the transistor is an emitter, the middle is the base and the right side is the collector. However, not all transistors use this configuration which means that these configurations might be different for some transistors.

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Transistor Formula

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The formula of transistors are given below:

For the NPN transistor, the emitter current is the total of base and collector current.

i.e., I_E = I_C + I_B

For the PNP transistor, the collector current is the difference between the emitter current and the base current.

i.e., I_C = I_E - I_B

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Things to Remember

• Transistor is made from silicon or germanium and it acts as an amplifier or a switch.
• A transistor has three leads, which are base lead, collector lead, and emitter lead.
• One of the primary advantages of a transistor is that it can develop better efficiency circuits.
• One of the disadvantages of a transistor is that It lacks a higher electron mobility.

• There are three different types of configuration, the Common Base (CB), Common Collector (CC) and Common Emitter (CE).

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Previous Year Questions

• In the depletion region of an unbiased…..[AMUEEE 2007]
• Pure silicon at 300 K has equal electron…...[AMUEEE 2009]
• The intrinsic carrier concentration of silicon sample…..[JKCET 2018]
• When a p−n junction diode is connected in forward bias its barrier potential [JKCET 2008]
• Which of the following truth tables corresponds to NAND gate.[AMUEEE 2011]
• In the figures shown below….[EAMCET 2010]
• Consider the following statements A and B…..[EAMCET 2003]
• To get an OR gate from a NAND gate, we need…..[VITEEE 2013]
• The decimal number equivalent to a binary number 1011001 is [WBJEE 2012]
• The output Y of the logic circuit shown in figure is best represented as [VITEEE 2014]