Transistors are the very essential elements of electronics and electrical technology. The development of the bipolar transistor, BJT has resulted in many changes to the World. Transistors although not as noticeable as a current or voltage source – transistors are also an active circuit component. This is because transistors are able to amplify the power of a signal.

The overview of the bipolar transistor has enabled many technologies we take for granted today: everything from portable transistor radios to mobile phones, computers, remote operation, the functionality we take for granted in current day automobiles, etc. All of the daily electric items have all been imaginable by the development of the different transistors.

Today, transistors are available in many forms. There is the primary transistor in a leaded state, or it is known as a surface mount transistor. Nevertheless, transistors are also widely used within integrated circuits. Most digital ICs use field-effect technology, but many analog ICs use bipolar technology to provide the required performance.

Together with their field-effect transistor, FET, relatives that use a very different principle, the bipolar transistor forms the basis of most of today’s electronic equipment, either as discrete devices or within integrated circuits

What is a Transistor?

The transistors are the electric component used in circuits to switch electrical signals or power, amplifying, or permitting it to be used in a wider array of electronic devices. A transistor is made up of two PN diodes connected back-to-back, having three terminal pins (Emitter, Base, and Collector). The primary concept of a transistor is that it lets control the flow of current through one channel simply varying the intensity of a much smaller current that’s flowing through a second channel.


A transistor is kind of a semiconductor device that can both conduct and insulate. A transistor can perform as a switching device or, an amplifier device. It can convert audio waves into electronic waves and resistors, controlling electronic current. Transistors have a longer life, smaller in size, can operate on lower voltage supplies for more excellent safety, and required no filament current. The first transistor was fabricated with the metallic element germanium. A transistor acts the same function as a vacuum tube triode but using semiconductor junctions in place of heated electrodes in a vacuum chamber. It is the vital building block of modern electronic devices and found everywhere in modern electronic systems.

Transistors can be looked upon as a type of switch, as can many electronic components. They are used in a variety of circuits, and you will find that it is rare that a circuit built in a school Technology Department does not contain at least one transistor. They are central to electronics and there are two main types; NPN and PNP. Most circuits tend to use NPN. There are hundreds of transistors that work at different voltages, but they fall into these two categories.

Transistor Basics

A transistor is a three-terminal device and consists of three separate layers. Two of layers are slightly doped to give one type of semiconductor and there is the reverse type (two may be n-type and one p-type, or it can be opposite two p-type, and one n-type). They are designed so that the two similar layers of the transistor doped layer of the opposite type. As a result, these semiconductor devices are designated as either PNP transistors or NPN transistors, according to the way they are made up.

  • Base: It is responsible for activating the transistor.
  • Collector: It is the positive lead.
  • Emitter: It is the negative lead.

For the operation of the transistor, the base region must be fragile. In present transistors, the base could be approximately 1µm . Normally, the base region of the transistor is thin that is the key to the operation of the device

A small current flow between the base and the emitter; the base terminal can control a more significant current flow between the collector and the emitter terminals. It also has the three terminals for a field-effect transistor (Gate, Source, Drain). Transistors come in different sizes and shapes depending it’s applications The gate voltage control the current between source and drain. The basic diagrams of BJT and FET are shown in the figure below:

Types of Transistors:

Two types of transistors are available –

  • Bipolar Junction Transistors (BJT),
  • Field-Effect Transistors (FET).

Bipolar Junction Transistor:

A Bipolar Junction Transistor called BJT has three terminals connected to three doped semiconductor regions. It comes in two variants one is P-N-P other one is the N-P-N.

P-N-P transistor is made up of a layer of N-type semiconductor between two layers of P-type material. The base current, Ib entering the collector is amplified at its output.

That is when the PNP transistor is ON when its base is pulled low relative to the emitter. The PNP transistor’s arrows symbolize the direction of current flow when the device is in forward active mode.


N-P-N transistor consisting of a layer of P-doped semiconductor between two layers of N-doped material. By amplifying the current of the base, we get the high collector and emitter current.

That is when the NPN transistor is ON when its base is pulled low relative to the emitter. When the transistor is in an ON state, the current flow is between the collector and emitter of the transistor. Based on minority carriers in the P-type region, the electrons moving from emitter to collector. It allows the more significant current and faster operation; thus, most bipolar transistors used today are NPN.

transistor pin configuration
transistor pin configuration and layout

Field Effect Transistor (FET):

FET is a unipolar transistor.  N-channel or P-channel FET are used for conduction purposes. The three pins of FET are the source, gate, and drain. The elementary n-channel and p-channel FETs are shown above. For an n-channel FET, the device is made from an n-type semiconductor.

transistor types  - FET

This transistor manages the positive carriers and negative carriers concerning electrons (holes). FET channel is designed by moving positive and negative charge carriers. The channel of FET is fabricated using silicon. There are various types of FETs, MOSFET, JFET, etc. The applications of FETs are in a low noise amplifier, buffer amplifier, and analog switch.

Choosing a transistor for your application

If anyone wants to turn on a circuit or switch on a load simply, there are certain things you should consider. Determine if you’re going to bias or energize the transistor switch with a positive or negative current level (i.e., NPN or PNP type, respectively). An NPN transistor is run (or turned on) by a positive current biased at the base to control Collector’s current flow to Emitter. PNP type transistors are run by a negative current biased at the base to control the flow from Emitter to Collector.

Capacitor and Transistor and PCB board

For most general-purpose transistor applications, we need devices that are non-conducting with zero bias on the control input (base or gate). Such devices are BJTs or enhancement-mode MOSFETs.

In many cases, it does not matter. MOSFETs are possibly ten or twenty percent a lot more expensive than BJTs, however, they do not need base resistors which cost and inhabit costly board areas. They are slightly extra at risk of electrostatic damage (ESD) throughout handling. However, they do not draw base current as well as load circuits at DC (because they have relatively large input capacitance, they might trigger capacitive packing issues in greater frequency circuits). At once the gate threshold voltage (the value of Vgs at which a MOSFET starts to conduct) was numerous volts, so they cannot be used with extremely reduced supply voltages, however today the threshold voltages of several devices are comparable to the 0.7 V base turn-on voltage of a silicon BJT. However, the input of a BJT is a silicon diode. We can use its thermal residential properties to determine temperature level, as well as its high existing when over-driven to serve as a clamp or limiting circuit, so there are some circuits where we should have a BJT.

If one needs a transistor for a design. How do you choose the correct transistors?

It would be great to have a database of every transistor in the World attached to a spreadsheet. After entering limiting values of every critical parameter, we see a list of everyone that meets necessities. Unfortunately, such a list is impossible to compile – it is enormous and would change day by day as new transistors are introduced and old ones become obsolete.

Here is a summarized version of how to choose the right one for your applications

  • Polarity: NPN/N-channel or PNP/P-channel?
  • Type: BJT or FET
  • Operating voltage
  • Maximum current: Select a value ≥33% above the maximum expected collector/drain current.
  • Package: What package and pinout do you require

One has to concentrate on others parameters as well for choosing the suitable transistors.

  • Saturation voltage
  • On resistance
  • Noise figure
  • Transition frequency
  • Switching time
  • Capacitance

Applications of Transistor

Semi-conducting materials make the working of the transistor possible. . The majority of you must be familiar with electrically conductive as well as non-conductive materials. Metals are generally taken into consideration to be conductive. Things like plastics, wood, ceramics, as well as glass, are insulators or non-conductive. A group of scientists uncovered exactly how to examine specific sorts of crystals as well as use them as electronic control devices by exploiting their semi-conductive homes.

Capacitor and Transistor and PCB

Heat-operated switch

  • A thermistor is one of the most essential components in the circuit of a heat-operated switch. This is a type of resistor that replies to the adjacent temperature. Its resistance decreases when the temperature is high and vice versa.
  • The thermistor resistance drops, and a higher share of the supply voltage is dropped across R when applied to the thermistor. The base current rises, followed by an increase in collector current. As a result, the bulb glows, and siren sounds.
  • These particular circuits are mainly used in the fire alarm system.

Integrated Circuits (ICs)

  • Each and every IC hold a variety of components like – resistors, transistors, diodes, and capacitors. All of the elements combined with a wafer-thin silicon chip, known as the microchip.
  • The IC consumes less electrical energy, occupy a small space that reduces the circuit’s size, and can be built at a low cost

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