1. Diodes

A diode is a semiconductor component that allows current to flow only in one direction.

1.1 Circuit symbol of a diode

A diode will have two terminals that are polarized, meaning the two terminals are distinctly different.

The positive end of a diode is called the anode, and the negative end is called the cathode. Current always flow from anode to cathode in a diode. 

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1.1.1 Various types of diodes 

1.2 Current flow

The direction in which the diode allows current to flow is known as the diode's forward direction and the direction it opposes current flow is known as the diode's reverse direction.

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1.3 Types of semiconductor material

There are two main types of semiconductor materials:

• Intrinsic - where the semiconducting properties of the material occur naturally, means these materials are in their purest form by the nature of their existence.
• Extrinsic - the semiconducting properties of the material are manufactured by doping an agent to pure material.

1.4 Doping

Semiconductor doping is the process that changes an intrinsic semiconductor 

to an extrinsic semiconductor. During doping, very tiny amount of impurity atoms are intentionally added to intrinsic (extremely pure) semiconductor for the purpose of modulating electrical properties of a material (controlling the conducting properties of semiconductor).

Doping process produces two groups of semiconductors: the negative charge conductor (n-type) and the positive charge conductor (p-type).

1.5 Types of extrinsic semiconductor

There are two types of extrinsic semiconductors

• p-type semiconductor
• n-type semiconductor

1.6 n-Type Semiconductor

Silicon atom in its pure (intrinsic) state uses its four valence electrons (tetravalent) to form a covalent bond with its neighboring silicon atoms.

This crystal lattice arrangement of silicon leaves no free electrons for conduction. In order to have an extra free electron, the pure silicon is doped (added) with pentavalent (5 valence electrons) elements such as Phosphorus or Arsenic.

This results in large number of free electrons for conducting electricity in a doped semiconductor. Since electrons are negative charge carriers, the resultant material is called n-type (or negative type) semiconductor.

The impurity (Phosphorous) is called a donor because it donates electron. Note that the material remains electrically neutral.

   

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1.7 p-type semiconductor

In order to create p-type semiconductor, the pure silicon is doped with an element having 3 valence electrons such as Boron (B) or Gallium (Ga). As boron has 3 valence electrons, it donates a hole to accept an electron. 

Since these holes are considered as positively charged, its neighboring negatively charged electrons attract them. A semiconductor that has been doped with an acceptor is called a p-type semiconductor; "p" stands for positive. Notice that the material as a whole remains electrically neutral. In a p-type semiconductor, current is largely carried by the holes, which outnumber the free electrons. In p-type semiconductors holes are the majority carriers and the electrons are the minority carriers

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1.8 Summary

n-type semiconductors:

• Have extra free electrons;
• Electrons are the majority carriers;
• Impurity element is Phosphorous or Arsenic; have 5 valence electrons
• Free electrons are given by donor impurity

p-type semiconductors:

• Have extra free holes
• Holes are the majority carriers
• Impurity element is Boron (B) or Gallium (Ga); have 3 valence electrons
• Free holes are given by accepter impurity