The fourth electron in Si is extra that remains isolated and creates an electron hole or electron vacancy.
An electron from a neighbouring atom can come and fill the electron hole, but leave an electron hole at its original position resulting in displacement of the electron hole in the direction opposite to that of the electron that filled it.
Under the influence of electric field, electrons would move towards the positively charged plate through electronic holes, but it would appear as if electron holes are positively charged and are moving towards negatively charged plate.
These types of semiconductors are called p-type semiconductors.
Applications of n-type and p-type semiconductors
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o n-type and p-type semiconductors finds a great use in manufacturing electronic components.
Diode is a combination of n-type and p-type semiconductors extensively used as a rectifier.
Transistors are manufactured by keeping a layer of one type of semiconductor between two layers of another type of semiconductor.
npn and pnp type of transistors are used to detect or amplify radio or audio signals.
The solar cell is an efficient photo-diode used for conversion of light energy into electrical energy.
Gallium arsenide( GaAs) semiconductors have very fast response and have transformed the design of semiconductor devices.