What is semiconductor ? Distinguish between intrinsic and extrinsic semiconductors ?
Semiconductor – A semiconductor is a solid whose electrical conductivity lies between the very high conductivity of metals and very low conductivity of insulators .
The most commonly known semiconductors are Germanium (Ge) and Silicon (Si).
Semiconductors are of two types –
i) intrinsic semiconductors.
ii) extrinsic semiconductors.
i) Pure germanium and silicon, in their natural form, are called intrinsic semiconductors.
ii) There is no impurity doped in intrinsic semiconductor.
iii) The electrical conductivity increases very slowly with temperature.
iv) The conductivity is zero at absolute zero.
v) The electrical conductivity of intrinsic semiconductor determined by the carriers which are generated by thermal excitation.
i) When a small amount of impurity (10^6 : 1) is introduced (doped) in a pure semiconductor, then it is called extrinsic semiconductor.
ii) The electrical conductivity increases rapidly will temperature.
iii) The conductivity is not zero at absolute zero.
iv) The electrical conductivity of extrinsic semiconductor is determined by the carriers which are generated by thermal excitation and excess charge carrier of impurity.
Remember that – extrinsic semiconductors are of two type
i) p – type semiconductor
ii) n- type semiconductor
What is p- type and n- type semiconductors ? Give examples.
n- type semiconductors :
If a small amount of trivalent impurity is doped with the pure germanium (on silicon) crystal, then the conductivity increases appreciably, then it is called p -type semiconductor as the majority charge carriers are holes.
Example : Trivalent aluminium (Al) is doped with pure germanium.
Doped Al has three valence electrons which make three covalent bonds with 3 adjacent Ge but there is a deficiency of electron to make 4th covalent bonds with the 4th Ge this create a “hole”. Here electrons and holes are mobile change carriers.
Majority mobile charge carriers are => “holes ”
Minority mobile charge carriers are “electrons”
Thus called p -type semiconductors. The impurity is called an “acceptor” because the impurity atoms accept electrons from the crystal.
Immobile charge carriers are – acceptor impurity.
n-type semiconductors :
If a small amount of pentavalent impurity is doped with the pure germanium (on silicon) crystal, then the conductivity increases appreciably, then it is called electrons which corries negative change type semiconductor as the majority charge currents are electrons which carries negative charge.
Examples: Pentavalent Arsenic (As) is doped with pure germanium.
Doped Arsenic (As) has five valence electrons which make four covalent bonds with four neighbouring Ge and the 5th extra electron is free. Here electrons, and holes are mobile charge carriers.
Majority mobile charge carriers are => “electrons.”
Minority mobile charge carriers are => “holes”
The impurity is called “donor ion” because the impurity atom donate electrons to the crystal.
Immobile charge carriers are = ‘donor ion’.
Difference between p- type and n- type semiconductor
1. Trivalent impurity is doped.
2. Majority charge carriers – hole .
3. Minority charge carriers – electron.
4. Impurity is called – acceptor ion.
1. Pentavalent impurity is doped.
2. Majority charge carriers – electron.
3. Minority charge carriers – hole.
4. Impurity is called – donor ion.
What is p-n junction diode?
When a p-type semiconductor is joined with a n-type semiconductor so that they form one continuous crystal structure, then it is called p-n junction diode.
How a potential barrier develops at the junction ?
As soon as the junction is formed, there is an immediate diffusion of the charge carriers across the junction due to thermal agitation. Some of the electrons in the n-region diffuse into the p-region while some of the holes in the p- region diffuse into n- region and combine.
Thus near the junction for the two sides there is formed “depletion region” as the region is depleted of mobile charge. But there are immobile charge carriers i.e. acceptor atoms in the p- side and donor atoms in the n- side set up a potential difference across the junction. This potential difference is called ‘potential barrier’.
Can a voltmeter read the barrier potential of a p-n junction diode?
As there is no mobile charge carriers in the depletion region, it only contains immobile charge carriers i.e. acceptor atoms (p- region) and donor atoms (n- region), the potential difference can not be measured by a voltmeter. When the voltmeter is connected at the two sides of the junction in the depletion region no charge carries will flow through the external circuit.
What are the different types of p-n junction diode ?
Different types of p-n junction diodes are
i) Ondinary p-n junction diode.
ii) Breakdown diode (Zener diode).
iii) Light- emitting diode (LED).
iv) Varacter diode.
v) p-n junction laser.
vi) Photo diode.
vii) Solar cell.
viii) Tunnel diode.
ix) Backward diode.
x) Switching diode.
xi) Schottky diode.
Biasing of a p-n junction diode
Connection of o battery (dc source) with the p-n junction diode is called biasing of a p-n junction diode.
Explain what happens when a dc bias voltage is applied between p and n portion of a junction :
Forward biased p-n junction
When the positive pole of a battery is connected to The p-side and the negative pole to the n-side of the junction, then it is known as forward biased p-n junction diode.
Circuit diagram –
The majority carriers( holes) in the p-region repelled by the “+ve” terminal of the battery, and majority carriers (electrons) in the n region repelled repeated by the ‘-ve’ terminal of the battery cross the junction and recombine with the opposite charge carriers, hence we get a current from p to n region through the junction.
in forward voltage in
Due to increase in forward current, the forward current increases.
Reversed-biased p-n junction :
When the negative pole of the battery is connected with the p- region and positive pole with the n -region of the junction, then it is called reverse- biased p-n junction.
Circuit diagram –
The majority carriers (holes) in the p- region and majority careers electrons) in the n -region attracted by the battery get away from the junction, thus we get no current due to majority carriers.
But we get a small amount of current (in μA range) due to flow of minority carriers.This current is not affected by the reverse voltage. This current is thus called reverse bias saturation current.