Transistor Definition-

Transistors amplify current, the small output current from a logic IC can be amplified so that a light, relay or other high-current system can be controlled. A resistor is used in many circuits to convert the variable current into a variable voltage, so the transistor is used to amplify voltage. A transistor can be used as a switch (either completely on with full current or completely off with no current) and as an amplifier (always partially on).

Previously, an electronic device’s critical and essential element was a vacuum tube; it is an electron tube used to regulate electrical current. The vacuum tubes have operated but they are voluminous, require higher operating voltages, high power usage, lower output yield, and the use of cathode-emitting materials. So, that ended up as heat that shortened the tube’s own life. To solve these challenges, a transistor at Bell Labs was invented in 1947 by John Bardeen, Walter Brattain and William Shockley. The new device was a much more elegant solution to solve many of the vacuum tube’s basic limitations

Basics  of Transistor-

Transistor has three terminals named

  • Base- It activate the transistor
  • Collector- It is positive terminal
  • Emitter- It is negative terminal

The fundamental idea behind a transistor is that it helps you to regulate the current flow through one channel by adjusting the intensity of a much smaller current flowing through a second channel.

Symbols of Transistor-

In reality, transistors are three-terminal devices. Such pins are marked as collector (C), base (B) and emitter (E) on a bi-polar junction transistor (BJT). The NPN and PNP BJT circuit symbols are as follows:


The only difference between an NPN and PNP is the emitter’s arrow direction. The arrow on an on the PNP it points in and NPN points out.

Types of Transistors-

There are actually two types of transistors;

  • Bipolar junction transistor (BJT)
  • Field effect transistor (FET)

A small current flows between the base and the emitter; the base terminal can control a larger flow of current between the collector and the emitter terminals. It also has the three terminals for a field-effect transistor, they are gate, source and drain, and a voltage at the gate will regulate a source-drain current. The following figure shows the basic diagrams of BJT and FET:


Transistor Construction-

To work their magic, transistors depend on semiconductors. A semiconductor is not a pure conductor material (like copper wire) but also not an insulator (like air). A semiconductor’s conductivity— how quickly electrons can pass— depends on factors such as temperature or more or fewer electrons. Let’s look briefly at a transistor’s hood.

A Transistor as Two Diodes-

Transistors are like an extension of another element of the semiconductor: diodes. In a way, transistors are only two diodes bound together with their cathodes (or anodes):


The diode that links the base to the emitter is the crucial one here; it matches the arrow’s position on the schematic symbol and tells you how the current flows through the transistor.

The representation of the diode is a good starting point, but it is far from accurate. Do not base your understanding of the function of a transistor on that design (and definitely do not try to reproduce it on a breadboard, it will not work). There are a lot of strange things at the level of quantum physics controlling the interactions between the three terminals.

Working of Bipolar junction transistor (BJT) –

We say we’re using three layers of silicon instead of two in our sandwich. We can either construct a p-n-p sandwich (with a n-type silicon slice as the filling between two p-type slices) or a n-p-n sandwich (with the p-type between the two n-type slabs). When we connect electrical contacts to all three sandwich surfaces, we can construct a device that either amplifies a current or turns it on or off — in other words, a transistor. Let’s see how the n-p-n transistor operates.

So let’s give names to the three electrical connections, we know what we’re talking about. We will call the two contacts joined to the two parts of n-type silicon the emitter and the collector, and we will call the base the contact joined to the p-type silicon. When no current flows in the transistor, we know that the silicon p-type is short of electrons (shown here by the small plus signs, representing positive charges) and the two pieces of silicon n-type have additional electrons (shown by the small minus signs, representing negative charges).


Another way to look at this is to suggest that while the n-type has an electrons surplus, the p-type has holes where there should be electrons. The holes in the base usually act as a barrier, preventing any significant current flow from the emitter to the collector while the transistor is in “off” mode.

When the electrons and holes begin to move across the two junctions between n-type and p-type silicon, a transistor works.

Let’s attach some power to the transistor. Suppose we’re adding a small positive voltage to the base, negatively charging the emitter, and positively charging the collector. Electrons are drawn into the base from the emitter— and from the base into the collector. And the transistor is returning to its “on” condition:


The low current we turn on at the base makes a large stream of current between the emitter and the collector. The transistor works like an amplifier by converting a small input current into a large output current. But at the same time it also serves as a switch Little or no current flows between the collector and the emitter when there is no current at the base Switch on the base current and a massive stream of current. Thus the base current turns on and off the entire transistor. This type of transistor is technically called bipolar because there are two different types of electrical charge (negative electrons and positive holes) involved in making the current flow.

By thinking of it as a pair of diodes, we can also understand a transistor. The base-emitter junction is like a forward-based diode with the base positive and the emitter negative, with electrons moving in one direction across the junction (from left to right in the diagram) and holes moving in the opposite direction (from right to left). The junction of the base-collector is like a reverse biased diode. The collector’s positive voltage draws most electrons into and through the outer circuit (although some electrons recombine with base holes).

Working of Field effect transistor (FET)-

All transistors operate by controlling electrons ‘ motion, but not all do the same. Like a junction transistor, there are three separate terminals in a FET (field effect transistor), but they have the names source (analogous to the emitter), drain (analogous to the collector) and gate (analogous to the base). The n-type and p-type silicon layers are arranged in a slightly different way in a FET, and are lined with metal and oxide sheets. This gives us a MOSFET (Metal Oxide Field Effect Transistor) device.


Although there are extra electrons in the source and drain of the n-type, due to the holes in the p-type gate between them, they can not flow from one to the other. When we add a positive voltage to the gate, however, there is an electrical field that allows electrons to flow from the source to the drain in a thin path. This field effect” enables the movement of a current and activates the transistor:


For the sake of completeness, we should note that a MOSFET is a unipolar transistor because only one form of electrical charge (“polarity”) is involved in making it function.

Also Read- Difference between BJT and FET

Also Read- Frequency Response Of BJT Amplifiers

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