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Diodes are semiconductor components that allows current to flow through them only in "one direction". The context "One direction" used to specify how diodes conducts electricity means current can only flow when connected in correct polarity in circuits but blocks current totally when connected in the wrong polarity. They conduct current electricity with low resistance or blocks current with infinite resistance depending on the direction current flows through them. Diodes  are polarised components unlike resistors which isn't polarized. They consists of a cathode and an anode. The anode is the positive terminal of a diode and the cathode is the negative terminal of the diode. For current to flow through a diode in a circuit, positive voltage must be applied to the anode and negative voltage at the cathode.

       circuit symbol of diode.

We can say that a diode is an electric switch that allows current to flow through it in one direction. Diodes are made from semiconductors. They consists of an N-type semiconductor merged directly to a  P-type semiconductor thus forming a junction. N-type semiconductors allows only negative current(electrons) to flow through them and P-type semiconductor allows only positive charge (holes) to flow through them.

Fabrication of diodes.

Diodes are made from semiconductors like silicon. Silicon is gotten from sand and it's the most abundant element on earth after oxygen. Pure silicon is a metaliod or semiconductor which doesn't allow current to flow through it except at high temperature. 

At room temperature the valence electrons in silicon aren't mobile enough to conduct electricity but when it temperature is increased above room temperature electrons become mobile enough to conduct electricity. Elements like arsenic, indium and boron contains charge carriers (Positive charge carriers and negative charge carriers), which can conduct electricity. 

To make silicon conduct electricity at room temperature it is doped with foreign elements which can either increase it positive charge carriers or increase it negative charge carriers. When silicon is doped(mixed) with foreign elements like indium or boron it produces a P-TYPE semiconductor. But if it's doped with  antimony or arsenic it produces an N-TYPE semiconductor. 

A P-TYPE semiconductor has high number of holes or protons also called positive charge carriers than electrons also known as negative charge carriers. So when a P-TYPE semiconductor is connected to a power source it allows only positive charge to flow through it. An N-TYPE semiconductor has an higher number of electrons or positive charge carriers than holes. When an N-TYPE semiconductor is directly marged to a P-TYPE semiconductor it becomes a P-N junction diode.

When current is applied to the junction diode with the negative terminal of the battery connected to the N-TYPE and the positive terminal of the battery connected to the N-TYPE, current flows through the simple circuit without any resistance. But if the  connection is reversed no current flows through the circuit. Conduction modes of diodes. Diodes conducts electricity in two modes, this makes them as an electric switch which switch on the flow of water or switch off the flow of water. Thus diodes can only exist in circuits performing either OFF or ON.

•Forward bias.

When a negative voltage is applied to the cathode of a diode and a positive voltage is applied to the anode current flows freely without any obstruction, in this mode the diode is said to be in forward bias.

 •Reverse bias.

When a positive voltage is applied to the cathode of a diode and a negative voltage is applied to the anode it blocks current from flowing because of the N-type semiconductor can't conduct positive charge and the P-type semiconductor can't conduct negative charge, so the diode is said to be connected in reverse bias. Diodes are semiconductor components which can be made to produce light, regulate current, produce a beam of coherent light and convert signals and photons in current or volatage depending on how they are doped. Examples are LEDs, Zener diodes, LASER diodes, Shockley diodes, avlanche diodes.