CBSE Class 12 Physics Electronic Devices Notes Set B

9. ELECTRONIC DEVICES

GIST

1. In metals, the conduction band and valence band partly overlap each other and there is no forbidden energy gap.

2. In insulators, the conduction band is empty and valence band is completely filled and forbidden gap is quite large = 6 eV. No electron from valence band can cross over to conduction band at room temperature, even if electric field is applied. Hence there is no conductivity of the insulators.

3. In semiconductors, the conduction band is empty and valence band is totally filled. But the forbidden gap between conduction band and valence band is quite small, which is about 1 eV. No electron from valence band can cross over to conduction band. Therefore, the semiconductor behaves as insulator. At room temperature, some electrons in the valence band acquire thermal energy, greater than energy gap of 1 eV and jump over to the conduction band where they are free to move under the influence of even a small electric field. Due

P-n junction diode

Two important processes occur during the formation of p-n junction diffusion and drift. The motion of majority charge carriers give rise to diffusion current.

Due to the space charge on n-side junction and negative space charge region on p-side the electric field is set up and potential barrier develops at the junction  electric field e- on p-side moves to n and holes from n-side to p-side which is called drift current.

In equilibrium state, there is no current across p-n junction and potential barrier across p-n junction has maximum value.

The width of the depletion region and magnitude of barrier potential depends on the nature of semiconductor and doping concentration on two sides of p-n junction.

Forward Bias

P-n junction is FB when p-type connected to the +ve of battery and n-type connected to –ve battery Potential barrier is reduced and width of depletion layer decreases.

Reverse Bias
P-n junction in RB p-type connected to the –ve battery and n-type connected to +ve Resistance of p-n junction is high to the flow of current.

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