shreeji academy of science

7/29/2019 Shreeji Academy of Science

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SHREEJI ACADEMY OF SCIENCE(A UNIQUE NAME FOR PHYSICS TUITION CLASS)

B-7/Kailash Apartment, Opp. Harinagar, Water Tank, Gotri, Vadodara

JEE/NEET MAIN 2013

SEMICONDUCTOR AND ELECTRONIC DEVICES

IMPORTANT FORMULAE

1. For a pure (intrinsic semiconductor)a) ne = nh =ni (intrinsic concentration) b) electrical conductivity is small as compare to

extrinsic c) current is due to motion of electrons as well as holes. Total current is the sum

of electron current and hole current. This current is very small. I = Ie + Ih

2. To increase the conductivity, a pure semiconductor is doped with impurity atoms. Adoped semiconductor is called extrinsic semiconductor for which ne nh.

3. An n-type semiconductor is formed when a pure semiconductor is doped with pentavalentimpurity such as phosphorous or antimony. For an n- type semiconductor, a) ne >> nhand nenh = ni

2b) conductivity and hence current is enormously increased. c) the majority

charge carrier are electrons and holes are minority carriers.

4. A p-type semiconductor is formed when a pure semiconductor is doped with a trivalentimpurity such as indium, gallium, zinc or aluminum. For p-type semiconductor a) nh >>

ne and nenh = ni2. b) Conductivity and hence current is enormously increased c) the

majority charge carrier are holes and electrons are minority carriers.5. P-N Junction Diode

Two points are noteworthy.


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Electrons are no longer the majority charge carriers in the small region of the Ntype material near the junction and the holes are not the majority charge carriersin the small region of the P-type semiconductor near the junction. These regions

are known as depletion region as they are deplete of their majority charge

carriers. The width of the depletion region is approximately 0.5 m. The varying electric potential at the region near the junction is called the

depletion barrier. Its value is about 0.7 V for Si and 0.3 V for Ge.

It can be seen from the band diagram of the P-N junction shown that the

charge carriers need about qVB energy to cross the junction and go into the otherregion of the diode.

6.

The current increases with the increase in the applied voltage as shown in thegraph. Initial increase in current is very less, but beyond a voltage known as cut

in voltage, current increases rapidly ( according to the fourth power ). Here,

current does not vary linearly as per Ohms law and hence resistance of thejunction is not given by it. The resistance of the junction is found as follows.

The dynamic resistance ( rfb ) ( fb = forward bias ) of the diode at any point is

given by


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The adjoining figure shows the -V graph for the reverse bias condition. The

current is negligible (of the order of A) for smaller values of the voltage due to

minority charge carriers.The electric current is constant and is known as reverse saturation current. There

is a sudden rise in the current on increasing the voltage beyond a certain pointknown as breakdown voltage. Normally, P-N junction diode is never used beyondthe reverse saturation current in the reverse bias mode.

In the reverse bias mode, the value of the dynamic resistance (rrb) is of the order

of 106.

7. P-N Junction Diode Rectifier


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8. Zener diode

9.

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