Difference between Diode and Transistor

In electronics, based on the direction of current flow, the elements or the components can be classified into two categories – bilateral and unilateral. The element that lets current pass through it in both directions i.e., the current does not depend on the polarity of the applied voltage, is called a bilateral element; for example – resistor, capacitor, inductor, etc. The element that lets the current pass through it in only one direction i.e., current depends on the polarity of the applied voltage, is called a unilateral element. Two of the examples of unilateral elements are – diode and transistor. In the following article, we will be discussing the difference between diode and transistor.

Difference between Diode and Transistor in tabular form

In the following table, we have pointed out the difference between diode and transistor.

Diode Transistor
The diode is a two-terminal device in which the flow of current depends on the polarity of the applied voltage on the two terminals. The transistor is a three-terminal device in which the flow of current between two terminals also depends on the voltage applied on the third terminal.
Diode has two terminals. Transistor has three terminals.
Terminals of the diode are called – Anode and Cathode. Terminals of a transistor are termed as – Collector, Base, and Emitter.
A diode has one depletion region. Transistor has two depletion regions.
Diode cannot control the amount of current flowing through it. Transistor can control the amount of current flowing through it.
Diode does not have any special mode of operation. Transistor has three modes of operation – common collector, common base, common emitter.
There are different types of diode such as – p-n junction diode, Zener diode, Schottky diode, LED or light-emitting diode, etc. Some of the examples of different types of the transistor are – BJT, JFET, MOSFET, etc.
A diode is primarily used where we need to control the direction of current i.e., rectification. Some of the examples of the application of transistors are – amplification, switching, rectification, etc.

What is Diode?

The diode is a two-terminal device that lets the current pass through it in only one direction. There are many types of diode. For the sake of simplicity, let’s first discuss the p-n junction diode. A p-n junction diode is made by infusing p-type semiconductor and n-type semiconductor together. Actually, a p-n junction diode is created by doping p-type and n-type material in the same piece of semiconductor but in two different parts. The two terminals of a diode are – anode and cathode. The terminal connected to the p-type semiconductor is called the anode and the terminal connected to the n-type semiconductor is called the cathode. The flow of current depends on the polarity of the applied voltage on these terminals.

Applying voltage to the terminals of the diode is called biasing. There are two modes of biasing – forward biasing and reverse biasing. If the anode is kept at a higher potential than the cathode then the mode of biasing is called forward biasing. In this mode, the diode acts as a short circuit and the current flows from anode to cathode. If the polarity of the applied voltage is reversed i.e., if the cathode is kept at a higher potential than anode then the mode of biasing is called reverse biasing and, in this case, ideally, the diode acts as a short circuit and no current flows through it.

The diode is the simplest nonlinear element. Different types of diodes are used in circuits for different purposes e.g.- p-n junction diode for rectification, Zener diode for over-voltage protection, LED or light-emitting diodes as low power light source, etc.

What is Transistor?

The transistor is a three-terminal device in which the flow of current between two terminals (collector and emitter) not only depends on the voltage applied to those terminals but also on the voltage applied to the third terminal (base).

Depending on the type of semiconductor material used, transistors can primarily be classified into two categories – PNP and NPN transistors. If an n-type semiconductor is sandwiched between two p-type semiconductors to form the transistor then it is called a PNP transistor. And if a p-type semiconductor is infused in between two n-type semiconductors then it is called an NPN transistor. Transistors have three modes of operation – Common Base (CB), Common Collector (CC), and Common Emitter (CE). These modes are applicable for both NPN and PNP transistors.

The invention of the transistor gave electronics a whole new direction. It has many applications such as an amplifier, digital switch, rectifier, voltage variable resistor, etc. As the current flowing through it can be controlled by a third terminal, the digital logic circuit can be made using transistors. Transistors were used in 3rd generation computers. After that, the Small-Scale Integration, Medium-Scale Integration, Large Scale Integration, and Very Large-Scale Integration of transistor reduced the size of the circuit and increased the speed as the stray capacitance reduced.

Conclusion

Diode and transistor both are non-linear unilateral elements. Transistors can be assumed to be a serial combination of two diodes. In electronics, both of them are very important.

Author
Subhrajyoti Choudhury
University of Calcutta, Kolkata

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