What is a Diode? – Definition, Working, Types, Applications


What is a Diode? A diode is a two terminal electronics device which is made by semiconductor material. It is a unidirectional electronics component, means it only conducts in one direction. It is operated at specific voltage. The specific voltage is depending upon type of material used. The Diode is two bias one is forward bias and second reverse bias. It is only conduced in forward bias region, it means when diode is in forward bias it is “ON” state. When it is in reverse bias it will be “OFF”. In reverse bias, there is no conduction and no any current flow through it. The resistance in reverse bias is very high. in this tutorial we are going to understand what is a Diode? – Definition, Working, Types, Applications.

In ideal case, the resistance in forward bias is zero and infinite resistance in reverse bias. In real world the diode cannot be zero resistance and infinite resistance. A diode is also known as an electronics switch because it ‘ON’ in forward bias region and it is ‘OFF’ in reverse bias region. Due to this reason the It is also called rectifier because it convert alternating current (AC) to pulsating direct current (DC).

It only conduct when the threshold voltage is applied the threshold voltage for Silicon material is 0.7V and the threshold voltage for Germanium is 0.3V. After the threshold voltage it starts conducting beyond the threshold voltage it can’t conduct. So it required some threshold voltage.

What is a Diode?

A diode is two terminal semiconductor electronics component the only conducts current in one direction on a specific voltage. The ideal diode has zero resistance in forward and infinite resistance in reverse bias. But these conditions not exist in real world. So, practical it has some resistance in forward bias and high resistance in reverse bias.

The semiconductor materials are use in type of diodes fabrication. Silicon and germanium is the most usable material in diodes because it has low leakage current, low knee voltage and high operation temperature depending upon which type of semiconductor material use in diodes fabrication. The diodes start conduction after the threshold voltage is present in the forward direction (i.e. the “low resistance” direction). It is said to be “forward biased” when conducting current in this direction. When connected within a circuit in the reverse direction (i.e. the “high resistance” direction), the diode is said to be “reverse biased”.

We know the diode is a unidirectional electronics component, this means it can only conduct in only one direction. It only passes the current in one direction and that direction is forward bias. Diodes only works in forward direction.


In reverse direction, the current not flow through diode within a specified range. And this specific range the diode behaves like a open switch. Above this range, the reverse barrier breaks. The voltage at which this breakdown occurs is called the “reverse breakdown voltage”. The diode is use in various types of applications in our daily life. It is use in all electronics device where the low voltage is use.

Diode symbol

The P-N junction diode symbol and equivalent circuit is shown in below. The arrow show the direction of convectional current Flow in forward direction. From figure The +ve side is anode and -ve side is cathode. the connection of diode is done by connection of anode and cathode of this diode to battery terminal of +ve and -ve side respectively.



Diode Symbol


Diode Equivalent Circuit

Diode Construction

After the brief knowledge of what is diode and diode symbol. Let us come on another topic is its construction. Mostly the construction of diode we use only two semiconductors material silicon and germanium. The semiconductors are two intrinsic and extrinsic semiconductors. The intrinsic semiconductor are pure semiconductor and extrinsic are impure semiconductor. The p type material is constructed the adding trivalent (boron, indium, aluminum) impurity in intrinsic semiconductor and N type layer is constructed by adding pentavalent (Arsenic, Antimony, phosphorous) impurity in intrinsic semiconductor.

  • If we add trivalent impurities in a semiconductor (Silicon and germanium), a greater number of holes are present and it is a positive charge. The trivalent impurities forms P type layer.
  • If we add pentavalent impurities in semiconductors (silicon or germanium), due to excess electrons there is a negative charge. The pentavalent impurities forms N type layer.

The diffusion process is use in diode construction. When P and N type material are diffuse with each other, the PN junction diode form.

Ideal Diode

The ideal diode is said ideal diode when the current follow in only one direction these direction is forward direction and it fully block the current follow in reverses direction. It has some resistance in forward connection and in reverse condition it has infinite resistance which act like an insulator. The V-I characteristic is shown below.



Ideal Diode Characteristics
Operation Mode Forward biased Reverse biased
Current Through I>0 I=0
Voltage Across V=0 V<0
Looks like Short circuit Open circuit

N-type Semiconductor

N-type semiconductor is created by added pentavalent or donor impurity (arsenic, antimony, Phosphorus) in semiconductor material that produce large number of free electron in the semiconductor crystal is called in N-type semiconductor. It is also known as donor impurity because it provides free electrons. When we added pentavalent impurity it has five valance electrons. The four valance electron of pentavalent atom to form covalent bond with neighboring atom of silicon and the fifth is freely move in valance band to conduction band.

P-type Semiconductor

P-type semiconductor is created by added the trivalent or acceptor impurity (Gallium, Indium) in semiconductor crystal, is called P-type semiconductor. In P-type semiconductor crystal holes are forms so it is also known as acceptor. In trivalent has three valance elections. These three electrons forms covalent bonds with neighboring valance electrons of silicon and one valance electron of silicon is incomplete being short of one electron. This short of electron is called holes. Therefore each silicon atom added one hole.

Majority carrier and Minority carrier

In N-type semiconductor the electron are majority carrier and holes are minority carrier because the N-type is made by adding pentavalant impurity. The P- type semiconductor is has holes are majority carrier and elections are minority carries because the P-type semiconductor is made by adding trivalent impurity.



The diode is constructed by diffusion of N-type and P-type semiconductor. The due to difference concentration of electrons in N-type is diffuse from one side to another side and holes are also diffuse from P-type to N-type semiconductor.

Working of Diode

The P type is holes majority region and electron minority region. N type is an electron majority and holes minority region. Because of concentration difference the diffusion takes place in majority charge carriers and holes and electrons are recombining with each other. The recombination of electrons and holes at the junction, the positive and negative ion are present at near the junction. And this region is known as the depletion region. the biasing of diode is done by two methods which is described below. It is only work in forward bias.

The diode has two regions:-

  1. Forward region
  2. Reverse region

Forward Bias

It is forward bias when the +ve terminal of the battery is connected to the anode terminal and the –ve terminal of battery is connected to the cathode terminal. This connection is called forward bias connection. In forward bias the free electrons diffusing into the P-type region from the N-type and recombines with holes. In same way the holes diffuse in N-type from P-type and recombines with electron.

Reverse Bias

It is Reverse bias when the +ve terminal of the battery is connected to the cathode terminal and the –ve terminal of battery is connected to the anode terminal. This connection is called Reverse bias connection. In reverse bias the electron pull by – ve terminal the holes pull by –ve terminal so there is no combination of holes and electron in diffusion and no current flow. The diffusion is that region where the holes and electrons are recombine

Diode Definition, Symbol, Types of diodes, Uses - Characteristics of diode Diodes-symbol Forward bias

Types of Diodes and There Symbols

  1. Light Emitting Diode
  2. Laser diode
  3. Avalanche diode
  4. Zener diode
  5. Schottky diode
  6. Photodiode
  7. PN junction diode
  8. Tunnel diode
  9. Varactor diode


Diode Testing

We know the diode is only conduct in forward direction. It has some resistance and drops some voltage when it conducts. In reverse direction the it act like a insulator. It has very large resistance and large voltage drop. We use a multimeter for testing. We have to connect the millimeter terminal in forward direction and check multimeter display it has some voltage drop in reverse direction the multimeter display shows high voltage drop. Below the figure how to connect through digital multimeter.


Types of diodes and symbols -The colors of the LED are as follows:

Type of Semiconductor Wavelength Distance Color Forward Voltage at 20mA
GaAS 850-940nm Infra-Red 1.2v
GaAsP 630-660nm Red 1.8v
GaAsP 605-620nm Amber 2.0v
GaAsP: N 585-595nm Yellow 2.2v
AIGaP 550-570nm Green 3.5v
SiC 430-505nm Blue 3.6v
GalnN 450nm White 4.0v

Also read:- Zener Diode, Schottky Diode


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