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Introduction |
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In the chapter on current electricity, we had learnt about the flow of charges, conditions to produce the flow and how different materials respond to an external electric field. We had seen how materials are classified as conductors and insulators. |
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Energy Bands in Solids |
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Atoms are clustered together and are overlapped in solids. Therefore, the outermost valence atoms are overlapped. |
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Conductors, Insulators and Semiconductors |
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The electrons in the valence band are not capable of gaining energy from external electric field and hence do not contribute to the current. This band is never empty but may be partially or completely with electrons. |
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Intrinsic Semiconductors |
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The atoms in a crystal are strongly held by covalent bonds in space at tetrahedral angles. On receiving energy, a covalent bond breaks and an electron is free to move in crystal lattice. This electron leaves an empty space (shown as an open circle) called a hole. |
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P-type Semiconductor |
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When the impurity atom is a trivalent atom say indium, boron or aluminium, these atoms will replace the silicon atom as shown. |
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N-type Semiconductor |
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When the impurity atom is a pentavalent atom say arsenic, phosphorus, these atoms will replace the silicon atom as shown below. |
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P-N Junction |
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When a p-type semiconductor is brought into a close contact with n-type semiconductor crystal, the resulting arrangement is a PN junction or junction diode. |
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P-N Junction as Rectifier |
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Rectifier is a device which is used for converting alternating current/voltage into direct current /voltage. |
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Solar Cells |
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Junction diodes are of many types and solar cells are one such. Its working is based on production of potential difference by sunlight. In other words, it is a junction diode that can convert light energy into electrical energy. |
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Photodiode |
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Photodiode is essentially a P-N junction which works on the basis of electric conduction from light. When light falls on such diodes and if the wavelength of the light is such that the energy of the photon is sufficient to break a valence bond, a new hole - electron pairs are created. |
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LED |
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LED are light emitting diodes. It works in just the opposite way a photo diode works. Photodiodes receive light and hence conducts differently. But LEDs emit light using electric current. |
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Zener Diode |
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The symbol of zener diode is shown above. When we studied about the diode characteristics, it was found that under reverse bias, there is a small amount of current due to the drifting of the minority charge carriers. |
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Junction Transistor |
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It is a semiconductor device having two junctions and three terminals. The two types of transistors are p-n-p transistor and n-p-n junction transistors. |
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Biasing in Transistor |
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For proper working of a transistor, emitter base junction should be forward biased and CB junction should be reverse biased. |
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Logic Gates |
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There are digital circuits which either allow a signal to pass through or stop it. The circuits other wise called as gate allows the signal to pass only when some logical condition are satisfied. Under such condition the circuits are called logic gates. They are building blocks of any digital system. |
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Arithmetic Circuits |
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Arithmetic operations such as addition, subtraction and complex operations such as multiplication, division etc., are performed by using digital circuits called adders, which again consists of basic logical gates. |
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Summary |
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Energy band of a solid is the large number of energy levels confined in a small region of energy range of a given solid, constitute what is known as energy bands. |
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Conclusion |
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The two energy bands in solids are valence band and the conduction band. Depending on the gap between the two bands, solids are classified as conductors, insulators and semiconductors. |
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Numerical 01 |
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In an intrinsic semiconductor, the energy gap Eg is 1 eV. Its hole mobility is very much smaller than electron mobility and is independent of temperature. What is the ratio between conductivity at 600 K and that at 300 K? |
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Numerical 02 |
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Predict the effect on the electrical properties of a silicon crystal at room temperature (300 K) if every millionth silicon atom is replaced by an atom of indium. |
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Numerical 03 |
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A transistor is connected in common emitter configuration. The collector supply is 8 V and the voltage drop across a resistor of 800 W in the collector circuit is 0.5 V. If the current gain factor (a) is 0.96, calculate the base current. |
