This page is written strictly according to the NCERT textbook and is designed for board-exam answer writing. Each topic is explained in a step-by-step, student-friendly manner so that learners can study independently.
On the basis of electrical conductivity, solids are classified into three main categories:
In an isolated atom, electrons have discrete energy levels. However, in a solid, these energy levels form continuous energy bands due to interaction between atoms.
The size of the forbidden gap determines whether a material behaves as a conductor, semiconductor or insulator.
An intrinsic semiconductor is a chemically pure semiconductor without any intentional impurity.
At room temperature, some valence electrons gain sufficient thermal energy to cross the forbidden energy gap and enter the conduction band, leaving behind holes in the valence band.
When a small amount of suitable impurity is added to a pure semiconductor, its conductivity increases significantly. Such a semiconductor is called an extrinsic semiconductor. This process is known as doping.
An n-type semiconductor is obtained by doping a pure semiconductor with a pentavalent impurity such as phosphorus or arsenic.
A p-type semiconductor is obtained by doping with a trivalent impurity such as boron or aluminium.
When a p-type semiconductor is joined to an n-type semiconductor, a p–n junction is formed.
Due to concentration difference, electrons diffuse from n-side to p-side and holes diffuse from p-side to n-side. This results in the formation of a region free of mobile charge carriers known as the depletion region.
An internal electric field is set up across the junction, producing a barrier potential which opposes further diffusion.
A semiconductor diode is a p–n junction device that allows current to flow easily in one direction and restricts it in the opposite direction.
Forward Bias: p-side connected to positive terminal and n-side to negative terminal. Depletion layer width decreases and current flows easily.
Reverse Bias: p-side connected to negative terminal and n-side to positive terminal. Depletion layer width increases and current is negligible.
A rectifier is an electronic device that converts alternating current (AC) into direct current (DC) using one or more semiconductor diodes.
In a half-wave rectifier, only one half-cycle of the input AC voltage is used. During the positive half-cycle, the diode is forward biased and conducts. During the negative half-cycle, the diode is reverse biased and does not conduct.
A full-wave rectifier uses both half-cycles of the AC input. It provides a smoother and more efficient DC output compared to a half-wave rectifier.
Final Exam Tip: In semiconductor answers, always write definition → principle → working → conclusion to score full marks.
A full-wave rectifier is an electronic device that converts alternating current (AC) into direct current (DC) by using both half cycles of the input AC voltage. Hence, the output obtained is more continuous as compared to a half-wave rectifier.
A full-wave rectifier works on the principle that a p–n junction diode conducts when forward biased and does not conduct when reverse biased. With proper arrangement of diodes, current through the load flows in the same direction during both half cycles of the AC input.
During the positive half cycle of the AC input, the upper end of the transformer secondary becomes positive with respect to the centre tap. Diode D1 becomes forward biased and conducts, while diode D2 becomes reverse biased and does not conduct. Current flows through D1, the load resistance RL, and the centre tap, producing a positive output voltage across the load.
During the negative half cycle, the lower end of the transformer secondary becomes positive with respect to the centre tap. Diode D2 becomes forward biased and conducts, while diode D1 becomes reverse biased. Current again flows through the load resistance in the same direction as before.
Board Exam Tip: Always write definition → principle → working → result to score full marks.