What is Transistor? Definition and Concept behind It

What is a Transistor? Definition

A Transistor is a semiconductor device(generally has three terminals) that can amplify or switch the electrical or electronic signal by regulating or controlling current or voltage flowing through it. It allows us to control its operation by applying an external supply to it. A Transistor is an Active Device.

A transistor is a very important semiconductor device that plays a fundamental role in modern electronics. Transistors are key building blocks in a wide range of electronic circuits and devices including computers, telecommunication systems, audio amplifiers, and many other electronic devices. Transistors are typically made from semiconductor materials such as silicon or germanium. They consist of three layers of semiconductor material: the emitter, base, and collector. These layers are doped to create specific electrical properties.

Why it is known as an Active Device?

The term "active" indicates those electronic components that can provide gain or perform signal processing functions. Transistors have the ability to amplify weak signals, control the flow of current, and switch between different states (on and off) based on input signals or applied bias voltages. They can actively regulate the flow of electrons or holes through their various layers, enabling them to perform complex operations and provide active control over the signal. 

Unlike passive components such as resistors, capacitors, and inductors, which do not require an external power source to function, transistors require a power supply to operate and actively participate in signal amplification, switching, and modulation. By using an external power supply or biasing voltage, transistors can manipulate the current or voltage in a circuit, enabling them to act as amplifiers or switches.

A simple Description about construction and working of Transistor:

Silicon is a semiconductor, which means that its operational properties can be prepared by presenting impurities in the crystal structure. The silicon has 4 electrons in its valence shell, it is an outer orbit for electrons and it determines the many chemical properties of the atom. Atoms want 8 electrons in that shell, because of which they are made very stable.


 Therefore silicon easily creates covalent bonding to 4 neighboring silicon atoms to achieve those additional electrons. Now if we present those pure impurities in this pure silicon crystal, then we can change how this present holds. If we add phosphorus, in which its valence shell has 5 electrons, then the additional electron crystal structure is left free to rotate.


 This extra electron charges N-type negatively, from which the name comes. P-type is a positive charge because it is doped with boron, in which its valence shell has three electrons. This structure wants to achieve the ultimate electron and steals the electrons from its neighbor's atoms, this is a mobile positive charge, which is called a hole. Thus the conductivity of materials has increased because we have increased the number of mobile tariffs.


 When we arrange the N-type and P-type semiconductors in this way and attach the terminal to each, we make the world's most popular transistors. Transistor works on the N-type and P-type junction, due to the interaction of those free electrons and holes. The free electron in the P-Type will be migrated to fill those holes in the P-Type. They form a boundary layer that is called a decreasing layer which prevents more electrons that passing through each other due to negative charges. But, when a positive voltage is applied on the base, then it rejects the layer of that deficiency and permits electrons to flow through the circuit to complete it.

How a Transistor Switch Signals?

A transistor can be used as an electronic switch to turn a circuit on or off. There are two main types of transistors: One is the Bipolar Junction Transistor (BJT) and another one is the Field Effect Transistor (FET).

When a transistor is acting as a switch, it has two states: "On" and "Off".

In the "off" state, the transistor doesn't allow any current to flow through it. It's like an open gate that stops the flow of electricity.

In the "on" state, the transistor allows current to flow through it. It's like a closed gate that lets electricity pass through.

To control the transistor switch, we use a small signal or voltage. For a BJT, we send a small current to the base terminal to turn it on. For a FET, we apply a small voltage to the gate terminal.

When the control signal is present, the transistor turns on and allows current to flow through it, making the switch "on". When the control signal is absent, the transistor turns off and blocks the current, making the switch "off".

By using these On and Off states, we can control the flow of current through the transistor, which in turn controls the operation of the circuit connected to it.

How Does a Transistor Amplify Signals?

A transistor can make a weak signal stronger. It takes a small input signal and makes it larger without changing its shape. This is called amplification.

As we already know there are two types of transistors: bipolar junction transistor (BJT) and field-effect transistor (FET). So both of them has different working principle.

For a BJT:

When we give a small input signal to the base of the transistor, it controls a larger current flowing through it. Think of it like a valve. The small input signal acts like a handle, and when we turn it, more water (current) flows through the valve. So, the transistor amplifies the input signal by controlling a bigger current using the small input signal.

For a FET:

With a FET, when we give a small input voltage to the gate, it controls a larger current flowing through it. It's like adjusting a faucet. By changing the voltage, we can control how much water (current) flows through the faucet. Similarly, the FET amplifies the input voltage by controlling a bigger current using the small input voltage.

Read Also:

• What is Common Emitter Configuration in Transistor Amplifier?
• The Main Difference between Diode and Transistor
• Bipolar Junction Transistor(BJT) Characteristics
• Why NPN Transistors are mostly used than PNP Transistors?
• Main Difference Between PN Junction Diode and Zener Diode
• Difference between NMOS, PMOS, CMOS Transistor with Symbols

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