This entry is part 6 of 9 in the series Semiconductors & PN Junction Diode

A PN junction diode is the first semiconductor device in semiconductor family. It is also known as unidirectional current device, because it allows current only in one direction through it.

When P-layer and N-layer are joined together, as shown in following figure, the PN junction is formed. Its internal structure is given below.

image 40 Vidyasagar Academy Akola — P-layer and N-layer are joined together to produce PN junction
(this diagram is only for understanding)

image 41 Vidyasagar Academy Akola — After joining depletion layer is produced containing positive & negative ions

image 42 Vidyasagar Academy Akola — Symbol of PN junction diode

When the layers are joined, the Arsenic atoms near the junction donate their fifth electron to the nearest Aluminium atoms in P-layer. Due to this exchange of charges, Arsenic atoms loose one electron and become positive ions (+). At the same time, Aluminium atoms accept one electron from Arsenic. So they become negative ions (-). Thus opposite types of charges are produced across the junction, as shown above.

These ions oppose further flow of electrons from N-layer to P-layer. The process of charge exchange stops automatically. This opposing force of ions across the junction is called potential barrier.

The area where these ions are present is known as depletion layer. For Silicon diode, the potential of this barrier is 0.6 to 0.7V and for Germanium it is 0.2 to 0.3V.