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Difference Between Diode and Transistor

What are the Key Differences Between Diode and Transistor?

Diode and transistor are semiconductor-based electronic switches mostly used in every electronic device. Apart from that, they are quite different in every other way.

Before going into the list of differences between diode and transistor, we are going to discuss their basics.

P-type and N-type semiconductor materials are used for the construction of diode and transistors. Semiconductor are available in an intrinsic (pure) form where the amount of positive (hole) and negative (electrons) charges are same. They are made into extrinsic form by adding impurities to increase their conductivity. When impurities are added to a semiconductor crystal, P-type and N-type semiconductor materials are formed.

When a semiconductor is doped with a dopant having 5 valence-electrons, an N-type material is formed. Such semiconductor has free electrons in their valence shell. These electrons are free to move and serve as a charge carrier. Due to the presence of a higher number of electrons, they are named majority carriers. While the holes are minority carriers.

By doping the semiconductor with a dopant having 3 valence-electrons forms P-type material. Such material can accept or catch electrons. Therefore, P-type materials have holes. Holes are the absence of electrons. Due to the majority of holes, they are majority charge carriers in P-type material and electrons are minority carriers.

A PN-junction is a boundary between P-type and N-type material. Providing proper biasing or voltage between these junctions shrinks or expands this region to allow or block the flow of charges between the two layers.

Difference Between Diode and Transistor

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Diode

A diode is a semiconductor switch made from a combination of two layers of P-type and N-type material. There is only one PN junction in a diode having only two terminals i.e. Anode and Cathode.

Diode Symbol & StructureA diode has two operation modes i.e. Forward Bias and Reverse bias. In forward bias, the anode is supplied with a higher voltage than the cathode. Which results in an attraction between the PN junctions causing the charge carrier to easily flow between them. Thus in forward biasing, the diode conducts. While in reverse biasing, the Cathode voltage is higher than the anode which results in pulling apart the PN junction creating a depletion region thus breaking the path for charge flow. Therefore, diode blocks current flow in reverse biasing.

Diode BiasingA diode is a unidirectional switch that allows current in only one direction and blocks it in the reverse direction. Therefore it is mostly used in the rectification of AC into DC. However, it offers uncontrolled rectification i.e. the power rectified cannot be controlled.

A diode has many different types and each type is used for its special purpose. Some of these types are. Light-emitting diode (LED), Photodiode, Zener diode, Avalanche diode, Laser Diode, PIN Diode, Varactor and Tunnel Diode.

A diode has a wide range of applications used in electronics. Some of these applications are rectification, Voltage clipping and Clamping, Circuit Protection, Voltage regulation and multiplication, light source, etc.

Good to Know:  The name of Diode is derived from the combination of two words i.e. Di (Greek word meaning “Two”) and Ode as a short form of electrode = Diode. In other words, A diode has two electrodes as Anode and Cathode which only allow the current to flow in one direction known as forward bias. A diode offers a high resistance in one direction while it has low resistance on the other hand. That’s why it can only allow the flow of current in one direction only. 

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Transistor

The transistor is a semiconductor device made from 3 alternating layers of P-type and N-type material. In other words, either P-type is sandwiched between two N-types or the other way around. Or you could say that a transistor is made from two diodes joined back to back. Thus transistor has two types based on its construction i.e. PNP Transistor and NPN Transistor. The type of the transistor depends on its construction and also affects the type of majority carriers in it.

The word transistor is a combination of two words “transfer” and “resistor” meaning “transfer of resistor”. Its operation is based on the transfer of resistance between its terminals (from one circuit to another) to allow or amplify the charges between them.

The following figure given below shows the structure and a symbolic representation of a transistor.

BJT Transistor ConstructionThe three terminals of a transistor are named Emitter, Collector and Base. There are 2 p-n junctions in a transistor. The Emitter and Collector are made of the same type of semiconductor material. However, the emitter is heavily doped as compared to the collector to produce more charge carriers.

NPN Transistor - Construction & WorkingIf a transistor is properly biased (applying a gate signal), it will start conduction of majority carriers between emitter and collector. However, the gate signal is continuous and must not be removed during operation. A transistor does not conduct in the absence of the gate signal.

Therefore, a transistor has 3 regions of operation i.e. Active region, cutoff region and saturated region. The active region is used for amplification while the cutoff and saturated region is used for switching.

The transistor starts conduction when the Base-Emitter junction is in forward bias and the Collector-Base junction is in reverse bias. Therefore, it requires two voltage sources to operate.

The transistor is an active component and requires an extra power source to process the input signal. Whereas, the diode operates only on the input signal. However, the transistor can switch ON and OFF on command.

Good to Know:  The name of Transistor is derived from the combination of tow words i.e. Transfer and Resistance = Transistor. In other words, a transistor transfers the resistance from one end to the other. In short, a transistor has high resistance in the input section while low resistance in the output section.

Greek root di, meaning “two”, and ode, a shortened form of “electrode.”

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Transistors are mainly used for magnifying or amplification of small signals, audio amplifiers, switches etc.

Main Differences Between Diode and Transistor

The following comparison table shows the main differences between a diode and a transistor.

DiodeTransistor
A Semiconductor device that allows current flow in only one direction.A semiconductor device is capable of switching and transfer voltage between low resistance circuit and high resistance circuit.
The diode is made of two layers of P-type and N-type semiconductor.It is made of 3 layers of alternating semiconductor material (P-N-P and N-P-N).
It has two terminals called anode and cathode.It has three terminals named emitter, base and collector.
The diode has many types such as LED, Photodiode, Zener diode, Tunnel, Varactor, etc.Transistor has two major types Bipolar Junction (BJT) and Field Effect transistor (FET).
There is only 1 PN Junction.There are 2 PN junctions i.e. Collector-Base and Base-Emitter junctions.
It is mainly used for rectification of AC into DC.It is only used for switching and amplification.
It is a unidirectional switch.It is a switch as well as an amplifier.
It performs uncontrolled Switching.Transistor can perform controlled switching using the base signal.
A diode has only one depletion region.Transistor has two depletion regions.
A diode is a passive component.A transistor is an active component.
It only needs one voltage source to operate.It needs two voltage sources to operate.
Diodes are used for multiples purposes such as rectification, clipping, clamping, protection, voltage multiplier, voltage regulator, etc.It is used for switching and amplification. Best used in high-frequency applications.

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Properties and Characteristics of Diode & Transistor

The following different properties differentiate both Diode and a transistor having different characteristics and applications.

Construction

A diode is made from two layers of semiconductor. It is made from a combination of P-type and N-type material

The Transistor is made from three alternating layers of semiconductor. P-type material is sandwiched between N-type material to form an NPN transistor and N-type material is sandwiched between P-type material to form a PNP transistor.

Terminals

The diode has two terminals extended from P-region and N-region. The terminal connected with the P region is called Anode and while the terminal connected with N-region is called Cathode.

The transistor has 3 terminals. Each terminal is connected with each P or N region. The terminals connected with the outermost region are called collector and emitter while the terminal connected with the middle region is called the base.

The emitter region is heavily doped as compared to the collector region.

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PN Junctions & Depletion Region

A diode is made from a combination of only two layers that is why there is only one PN junction as well as only one depletion region.

The transistor is constituted of 3 alternating layers. Therefore, there are 2 PN junctions as well as 2 depletion regions.

Switching

The diode can perform switching but it is in an uncontrolled manner. It means it cannot switch on or switch off on command.

On the contrary, the transistor operates according to the signal applied at its base terminal. it can switch on and switch off according to the base signal. Therefore, it provides full control over switching thus providing a controlled power supply.

Active and Passive Component

A diode is a passive component because its output solely depends on its input and it does not require any extra power supply.

A transistor is an active component because its output depends on the input signal as well as its biasing. It requires an extra supply for its biasing.  

Applications

The application of diode is not limited to only switching as it has various different types having special uses. Generally, diodes are used for rectification, clipping, clamping, circuit protection, voltage regulation, voltage multiplication, Solar panels, LEDs, Voltage spike suppression, etc.

A transistor is a basic component of an electronic and logic circuit due to its high switching speed. Although it is also widely used for amplification and controlling of power supplied.

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