What is a transistor and what are its types?

What is a transistor?

The transistor is an active component and a semiconductor device that is used as an amplifier to amplify the high- and low-level signals and utilized in oscillators, detectors, modulators as well as it can be employed as an electrically controlled switch in digital circuits. So, in simple words, it can be said that a transistor is a miniature form of a device that can be used to regulate or restrict the flow of electronic signals.

Transistor has become the most important element in modern electronics. Transistor was developed in the year of 1947 by three American physicists, named Walter Brattain, John Bardeen, and William Shockley.

• Layers of a Transistor:

A transistor is comprised of three layers of semiconductor materials, typically terminals. These terminals help to make a connection with an external circuit and flow of the electric current. The current through the transistor in a pair of terminals is controlled by the voltage or current applied to the other pair of terminals. The three layers of the transistor are mentioned below:

1. Base: This terminal is used to activate a transistor
2. Collector: This layer is called the positive lead of a transistor
3. Emitter: This is the negative lead of a transistor

Types of transistors:

There are various types of transistors and they are classified based on their structure. Each type of transistor has its own characteristics and different application. On a broader scale, the transistor is classified in the below categories

Transistors are majorly classified into two broad categories: BJT and FET

• Bipolar Junction Transistor (BJT): BJTs are basically composed of three terminals, the base, the collector, and the emitter. In this device, ‘Bipolar’ means both electrons and holes which are essentially required for current flow and the term ‘Junction’ means PN junction. BJTs are basically current-controlled devices. If a little amount of current is applied to the base of a BJT transistor, a large amount of current flows from the emitter terminal to the collector. These transistors are basically operated in three regions:
• Cut-off Region: In this region, no current flows through the transistor and the transistor acts as an open switch. Here the transistor stays in an ’OFF’ state.
• Active Region: In this region, the transistor operates as an amplifier.
• Saturation Region: Transistor acts as a closed switch in this mode of operation and it remains in a fully ‘ON’ state.

BJTs are classified into two major categories:

• PNP Transistor: This is one kind of Bipolar Junction Transistors – BJT and it comprises two p-types of semiconductor layers. These materials are separated by a lean layer of n-type semiconductors. In this type of transistor, holes are the majority charge carriers and the minority carriers are the electrons.

PNP transistor is in, active or ON state when the current at the base terminal is kept low with respect to that of the emitter. Hence, in the PNP transistor, the base is always negative with respect to the emitter and collector terminals. The current that enters the base is amplified before reaching the collector terminal. The emitter-base junction is forward biased as the collector-base junction is reversed biased.

The PNP transistor is denoted as ‘PNP’ and the diagram below is the symbol of the PNP transistor. The conventional current flow is indicated by the arrow symbol and the current will be directed from the emitter terminal to the collector terminal of the transistor.

Below is the structure of a PNP transistor where the forward-biased emitter pulls the electrons to the source (battery) so that the current can flow from the emitter to the collector terminal.

1. NPN Transistor: This is also one kind of bipolar transistor that is comprised of two n-type semiconductor materials and a p-type semiconductor is fused between the two n-type semiconductor layers in this configuration. Unlike the PNP transistor, here the majority of charge carriers are electrons and the minority carriers are holes. The flow of electrons from the emitter terminal to the collector terminal causes the current flow within the base terminal of the transistor. This is a device that is controlled by current. In the modern era, the most commonly used BJTs are NPN transistors. A negative-positive-negative transistor is symbolized by the abbreviation ‘NPN’. The below figure depicts the NPN transistor:

The standard equation for the current flowing in this transistor is:

I_E = I_B +I_C

     The construction structure of the NPN transistor is given below:

• Field Effect Transistor (FET): The Field-Effect-Transistor (FET) is another major and important classification of transistors which is a unipolar device. Like BJT it is also having three terminals. The terminals are:

Source: This is the terminal of FET through which the majority of carriers enter the bar. Hence, it is named as Source.

Drain: Through this terminal, the majority of carriers lead the bar.

Gate: These are two heavily doped terminals that are internally connected which form the PN-junctions.

FETs are broadly classified into two categories:

1. JFET
2. MOSFET

• JFET: The abbreviation of JFET is Junction Field Effect Transistor. These are the simple and major types of transistors that are majorly used as amplifiers, switches, and resistors. JFET is a voltage-controlled device and it doesn’t require any biasing current. In this type of transistor, the flow of electric current between the source and the drain is controlled by the voltage applied between the gate and source.

JFET is of two types. Those are n-channel FET and p-type FET. In N-channel FET, to the n-type substrate, a p-type material is added. In the same manner, in the P-type FET, to the p-type substrate, an n-type material is added.

• N-Channel JFET: This kind of FET is mostly used. In this type of transistor, the current flow is due to electrons. For current flow, a channel is formed between the source and drain when a voltage is applied between gate and source.

For the sake of understanding the fabrication process of N-channel JFET, a narrow bar of N-type semiconductor is considered here, and P-type material is produced on the opposite side of the bar by a diffusion process. Both the sides are joined to make a single connection through the gate terminal which is depicted in the figure below:

At the two ends of the n-type semiconductor bar, an ohmic contact is made and these form the source and the drain. Both the terminals may be interchanged. The symbolic representation of N-channel JFET is shown below:

• P-Channel JFET: In this kind of JFET, holes are the majority charge carriers and are responsible for current flow. It is made of a P-type silicon channel which consists of two N-type silicon terminals situated at both sides. The gate terminal is connected to the N-type material, whereas the source and drain terminals are connected to the edges of the P-type channel.

The electric current (holes) can flow freely within the central P-channel even when no voltage is applied to the gate terminal of a P-channel JFET. This is the reason why JFETs are called ‘normally ON’ devices as the current can pass across the source to drain even without any voltage.

The construction of P-channel JFET is depicted below figure:

The figure below shows the various parts of a P-channel JFET

MOSFET: MOSFET is abbreviated by Metal Oxide Semiconductor Field Effect Transistor which is very popular in the transistor family. It comprises four terminals, Source (S), Drain (D), Gate (G), and Body (B). MOSFET can be employed in both analog and digital circuits. The general construction of MOSFET is shown below in the figure.

As the Gate region of MOSFET is totally insulated from the Source-Drain region, it is often called Insulated Gate FET. In this transistor, the body or substrate is the main Semiconductor (Silicon) in which the fabrication of FET is done. The MOSFET functionality depends on the variations in electric current that occurred in the channel width as well as the flow of the charge carriers; either electrons or holes. Through the source terminal, the charge carriers enter and come out from the drain. The below figure shows the symbol of MOSFET.

Voltage is applied on an electrode called the gate which controls the width of the channel. MOSFET functions in two modes:

• Depletion Mode
• Enhancement Mode

Depletion Mode: In this mode of operation, the channel has maximum conductance when no voltage appears across the gate terminal, whereas when the voltage is either positive or negative across the gate, the conductivity in the channel decreases.

Enhancement Mode: No conductivity is present when there is no voltage across the gate terminal. When the voltage across the gate is at its maximum, the conductivity of the device is enhanced.

Depletion and Enhancement types are also available in two types; N-channel and P-channel types.

N-channel MOSFET: In this type of MOSFET, N-channel is present between source and drain. Here, the source and drain terminals are doped heavily by n-type material which is placed in the p-type semiconductor layer, or substrate which is heavily doped.

Here, electrons are only responsible for the current flow between the source and drain zone and the current flow is typically controlled by the gate voltage. The below figure depicts the symbol of N-channel MOSFET.

P-channel MOSFET: Here P-channel is present between the source and drain. P-type materials are used for doping the source and drain terminals, whereas the N-type materials are used for doping the substrate. Here, holes are the majority carriers that cause the flow of current and the current flow is controlled by the voltage applied across the gate terminal.

The symbol of P-channel MOSFET is shown below:

Working Principle of Transistor

There are various types of transistors available, but for the sake of understanding, a common emitter configuration of NPN transistor is considered here. In this type of transistor, the emitter region is wider and highly doped as the number of majority carriers (electrons) is huge here. The collector region is comparatively less doped and so here the number of electrons is not as much as in the emitter region. The base region is quite thin as is lightly doped and diffused in between emitter and collector. The number of holes (majority carriers) is quite little here.

Now, in this situation, if we connect a battery in between the emitter and collector, the emitter region will be connected to the negative terminal of the battery. Hence, the emitter-base junction is forward biased and the collector-base is reverse biased. In this mode of operation, no current flows through the transistor device. In actual practice, as the emitter region is heavily doped, the concentration of free electrons in this region is large. On the other hand, the base region is thin and there are very few holes. The collector region is comparatively wider and moderately doped.

When a voltage is applied between the emitter and collector, the entire voltage is dropped in two places; one is at the emitter-base junction at the forward barrier potential region which is about 0.7V in the case of silicon transistor and the other drop occurs at the base-collector junction at reverse barrier region. No matter what is the applied voltage, the voltage drop across the emitter-base junction always remains constant and is 0.7V, and the rest of the voltage drops across the base-collector junction. As a result, the collector voltage can never overcome the forward potential and no free electron can cross the barrier in the emitter region and come to the base region. At this condition, the transistor behaves like an off switch.

When a positive voltage is applied across the base terminal, the base-emitter junction is forward biased and now it is able to overcome the forward potential barrier and hence the electrons in the emitter region will easily cross the junction and get in touch with the base region where recombination occurs due to the presence of few numbers of holes in that place. But the free electrons which are escaping from the emitter region get some kinetic energy due to the electric field present in that junction. As the base region is thin, the electrons coming from the emitter don’t get enough time for recombination and cross the reverse-biased depletion layer and finally go to the collector zone. The free electrons in the base region are the majority carriers and hence their flow is not obstructed as a reverse barrier present in the base-collector zone. As a result, the electrons flow from the emitter to collector and collector to emitter region. There are only a few holes present in the base region with which the electrons coming from the emitter region recombine and produce a smaller amount of base current.

Making of transistor

There are two major procedures for making transistors:

a) Alloy Diffused Transistor: There are varieties of methods through which a transistor can be produced. In this process, a thin wafer of Germanium is used to make the base terminal of the transistor. Two pellets of Indium are diffused to either side of a wafer of the N-type base as it is a trivalent material with three electrons at its valence shell to make the collector and emitter terminals of the transistor.

b) Silicon Planar Transistor: Its construction is more complex compared to the alloy diffusion technique. This process requires many separate steps like depositing the oxide layers on the silicon wafer and etching away unwanted silicon areas. To shape the required patterns or layers, the steps are repeated with many variations to form a transistor or integrated interconnected circuits.

Applications of Transistors

• Transistors operate as a switch in digital and analog circuits or both for switching and amplification purpose. Our cell phone uses the transistor amplifier which is a widely used application.

• Used for signal amplification

• Used in the power supply part as a power amplifier in the devices like AC to DC adapters, Mobile phone chargers, SMPS, Tape recorders, Power supply section of TV, etc.

• Transistors are employed to build various integrated circuits like Microprocessors, Microcontrollers, Logical blocks, etc. and the logical operations in microcontrollers are done using transistors. In each chip of a microprocessor, it requires a billion transistors.

• Used in almost every electronic device starting from computers to aircraft.

• Transistor’s high power radio frequency (RF) is often used in radar or hand-held two-way radios.

• Darlington pairs of transistors are sometimes used in light-sensing and touch-sensing devices.