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Semiconductors are those substances whose electrical properties are between those conductors and dielectrics. For example, germanium and silicon, of which are the main examples of semiconductors. Semiconductors are substances in which the conduction of electric current takes place only under certain conditions. These are of two types – such as intrinsic semiconductors and excipient semiconductors.

Pure semiconductors are those pure semiconductors in which no impurity is mixed. In such a situation, semiconductors are called intrinsic or pure semiconductors. In the natural state, pure germanium and silicon are intrinsic semiconductors. Apart from this, degenerate semiconductors are those in which the conductivity of pure semiconductors is very less.

If the amount of any substance having valency 5 or 3 is added as an impurity to pure semiconductors, then it increases the conductivity of semiconductors significantly. Such materials are called degenerate semiconductors or impure semiconductors. So friends, if you like the given information, then download Semiconductors Class 12 Notes PDF to know more on this topic.

Semiconductors Class 12 Notes PDF 2023 – Semiconductor Electronics Notes

1. Energy Bands in Solids:

In the case of solids, there are single energy levels where the atoms are arranged in a systematic space lattice and therefore the atom is highly influenced by the neighbouring atoms for a single isolated atom.

The closeness of atoms will be resulting in the intermixing of electrons of the neighbouring atoms, of course, for the valence electrons, not strongly bounded by the nucleus, in the outermost shells.

Because of the intermixing the number of permissible energy levels will be increased or there will be significant variations in the energy levels. Therefore, instead of single energy levels related to the single atom in the case of a solid, there will be bands of energy levels.

1.1. Conduction Band, Valence Band & Forbidden Energy Gap:

The band created by a series of energy levels including the valence electrons can be defined as a valence band. The valence band can be explained as a band which is inhabited by the valence electrons or a band which is having highest inhabited band energy.

The conduction band can be explained as the least unfilled energy band. The forbidden energy gap can be defined as the separation between the conduction band and the valence band. There will be no permitted energy state in this gap and therefore electrons can’t occupy the forbidden energy gap.

1.2. Conductors, Semiconductors and Insulators:

Conductors are the materials which can conduct the charge carriers easily. Insulators are materials in which the free flow of charge carriers is not possible. And semiconductors are materials which have conductivity between insulators and conductors.

According to the forbidden band, the insulators, semiconductors and conductors can be explained as mentioned below:

1.2.1. Insulators:

The forbidden energy band will be very wide in the case of insulators. Because of this information electrons will not be jumped from the valence band to the conduction band. The valence electrons will bond very tightly to their parent atoms in insulators. An increase in temperature will be enabling some electrons to move to the conduction band.

1.2.2. Semiconductors:

The forbidden band will be very small in semiconductors. Silicon and Germanium can be taken as examples of semiconductors. A semiconductor material will be one having electrical characteristics lying between insulators and good conductors.

If a little amount of energy has been supplied, the electrons can jump from the valence band to the conduction band easily. As an example, if the temperature will be enhanced, then the forbidden band will be reduced such that some electrons are liberated into the conduction band.

1.2.3. Conductors:

There will be no forbidden band and the valence band and conduction band overlap each other, in the case of the conductors. Here a lot of free electrons will be available for the conduction of electricity. A small potential difference across the conductor will be causing the free electrons for including the electric current.

The very relevant point in conductors is that because of the absence of a forbidden band, there will be no structure for establishing holes. The summation of current in conductors will be simply a flow of electrons.

2. Semiconductors

Hence, a semiconductor can be defined as a substance which is having resistivity in between conductors and insulators.

Semiconductors will be having the properties mentioned below.

1. They will be having less resistivity than insulators and more than conductors.
2. The resistance of the semiconductor will be reduced with the increase in temperature and vice versa.
3. If suitable metallic impurity such as arsenic, gallium etc. will be added to semiconductors, their current conducting characteristics varies appreciably.

2.1. Impact of the temperature of Semiconductors:

The semiconductor crystal will be acting like a proper insulator since the covalent bonds are pretty much strong and no free electrons are available. At room temperature, some of the covalent bonds will be broken due to the thermal energy given to the crystal.

Due to the breakage of the bonds, some electrons will be free which were inhabited in the production of these bonds. The non-appearance of the electron in the covalent bond can be represented by a small circle.

Therefore Hole can be shown as an empty place or vacancy left behind in the crystal structure. Since an electron is having a unit negative charge, the hole will similarly be having a unit positive charge.

2.2. Mechanism of conduction of Electrons and Holes:

If the electrons are released on the breakage of the covalent bonds, they will be moving randomly through the lattice of the crystal. If an electric field has been applied, these free electrons will be having a uniform drift which is opposite to the direction of the field applied.

This can be referred to as the electric current. If a covalent bond is broken, a hole has been produced. One hole is produced when one electron sets free. This thermal energy will be producing electron-hole pairs-there will be as many holes as free electrons. These holes can go through the crystal lattice in a random fashion such as free electrons.

The holes drift in the direction of the applied field if an external electric field is applied. Hence, this can be referred to as electric current. There will be a strong tendency of semiconductor crystals for producing covalent bonds. Hence, a hole will be attracting an electron from the neighbouring atom.

Now a valence electron from a nearby covalent bond will be coming for occupying the hole at \[\text{A}\]. This will cause the production of holes at \[B\]. The hole will be therefore successfully shifted from \[A\] to \[B\]. This hole will be moving from \[\text{B}\] to \[C\] from \[C\] to \[D\] and so on.

This movement of the hole in the non-appearance of an applied field will be random. But the hole gets drifted along the applied field if an electric field has been applied.

2.3. Generation and Recombination of Carrier:

The electrons and holes will be produced in pairs. The free electrons and holes will move within the crystal lattice in an irregular manner. There is always a chance that an electron which is free will be meeting with a hole, in such a random motion. If a free electron encounters a hole, it will recombine to re-establishing the covalent bond.

Both the free electron and hole will be destroyed in the process of recombination and cause the emission of energy as heat. The energy produced thereby, may in turn get re-absorbed by another electron for breaking its covalent bond. In this manner, a new electron-hole pair will be generated.

Hence, the method of splitting covalent bonds and the recombination of electrons and holes will be taking place simultaneously. If the temperature will be increased, the rate of generation of electrons and holes will be enhanced. This in turn increases, and the densities of electrons and holes will get increased.

Because of this the conductivity of the semiconductor will be increased or resistivity decreased. This will be the reason that semiconductors is having a negative temperature coefficient of resistance.

2.4. Pure or Intrinsic Semiconductor and Impure or Extrinsic Semiconductors:

The intrinsic semiconductor can be defined as a semiconductor in which electrons and holes are solely produced by thermal excitation or a semiconductor in an extremely pure form is referred to as a pure or intrinsic semiconductor. The number of free electrons will be always equivalent to the number of holes in an intrinsic semiconductor.

2.4.1. Extrinsic Semiconductors:

The electrical conductivity of intrinsic semiconductors will be enhanced by the addition of some impurities in the method of crystallization. The added impurity will be very small on the order of one atom per million atoms of the pure semiconductor. This kind of semiconductor can be referred to as an impurity or extrinsic semiconductor.

The method of infusing impurity into a semiconductor can be defined as doping. The doping material will be either pentavalent atoms such as bismuth, antimony, arsenic, or phosphorus which are having five valence electrons or trivalent atoms such as gallium, indium, aluminium, and boron which are having three valence electrons.

The pentavalent doping atom can be referred to as a donor atom since it is donating one electron to the conduction band of a pure semiconductor. The doping materials can be referred to as impurities as they change the structure of semiconductor crystals which are pure.

2.4.2. N–Type Extrinsic Semiconductor:

If a little amount of pentavalent impurity has been added to a pure semiconductor crystal while the crystal growth, the produced crystal can be referred to as an N-type extrinsic semiconductor.

The following points should be remembered in the case of N-type semiconductors:

• The electrons will be the majority carriers while positive holes are minority carriers, in N-type semiconductors.
• Even though the N-type semiconductor is having an excess of electrons but it will be electrically neutral. This will be because of the fact that electrons are produced by the addition of neutral pentavalent impurity atoms to the semiconductor. That is, there will be no addition of either positive or negative charges.

2.4.3. P–Type Extrinsic Semiconductor:

If a little amount of trivalent impurity has been added to a pure crystal while the crystal grows, the produced crystal can be referred to as a P-type extrinsic semiconductor.

The following points should be remembered in the case of P-type semiconductors:

• The majority carriers are positive holes but minority carriers are the electrons in P-type semiconductor materials.
• The P–type semiconductor will be remaining electrically neutral as the number of mobile holes under all conditions remains equivalent to the number of acceptors.

2.5. P–N Junction Diode:

If a P-type material has joined to the N-type intimately, a P-N junction will be created. Joining the two pieces a P-N junction will not be formed due to the surface films and other irregularities creating a major discontinuity in the crystal structure.

Hence, a P-N junction will be created from a piece of semiconductor (say, germanium) by diffusing P-type material to one-half side and N-type material to the other half side. If a P-type crystal has kept in contact with an N-type crystal so as to produce one piece, the assembly required will be defined as a P-N junction diode.

2.5.1. Forward Bias:

The junction diode can be defined as forward-biased if external d.c. a source has been connected to the diode with p–section connected to the positive pole and n–a section connected to the negative pole.

2.5.2. Reverse Bias:

The junction diode can be defined as reverse-biased if an external d.c. the battery has been connected to the junction diode with the P–section connected to the negative pole and n–the section connected to the positive pole.

Note:

P–N JUNCTION can be defined as a device which will be offering low resistance when forward biased and act as an insulator if reverse biased.

Symbol:

2.6. Junction Diode as Rectifier:

An electronic device which will be transforming a.c. power into d.c. power can be defined as a rectifier.

2.6.1. Principle:

If forward biased, the Junction diode will be offering a low resistive path and high resistance if reverse biased.

2.6.2. Arrangement:

The a.c. supply will be given across the primary coil (\[\text{P}\]) of step down transformer. The secondary coil ‘\[\text{S}\]’ of the transformer will be in connection to the junction diode and load resistance \[{{\text{R}}_{\text{L}}}\]. The output d.c. voltage will be received across \[{{\text{R}}_{\text{L}}}\].

2.6.3. Theory:

Assume that when the first half of a.c. input cycle occurs, the junction diode will be forward-biased. The conventional current will be flowing in the direction of arrow heats. The upper end of \[{{R}_{L}}\] will be at a positive potential with respect to the lower end.

The magnitude of output across \[{{R}_{L}}\] while the first half at any moment will be proportional to the magnitude of current through\[{{R}_{L}}\], which in turn will be proportional to the magnitude of the forward bias and which will be dependent upon the value of a.c. input at that time ultimately.

Hence, output across \[{{R}_{L}}\] will be changing with respect to a.c. input. During the second half, the junction diode will be getting reverse biased and therefore no–output will be there. Hence, a discontinuous supply will be received.

2.7. Full Wave Rectifier:

A rectifier will be rectifying both halves of a.c. input can be referred to as a full wave rectifier.

2.7.1. Principle:

Junction Diode will be given a low resistive path if forward biased and a high resistive path if reverse biased.

2.7.2. Arrangement:

The a.c. supply has been supplied across the primary coil (\[P\]) of step down transformer. The two ends of S–the coil (secondary) of the transformer will be connected to the P-section of junction diodes \[{{D}_{1}}\] and \[{{D}_{2}}\].

A load resistance \[{{R}_{L}}\] will be connected across the n–sections of two diodes and central tapping of the secondary coil. The d.c. the output will be received across secondary.

2.7.3. Theory:

Assume that while the first half of the input cycle upper end of the s-coil will be at \[+ve\] potential. The junction diode \[{{D}_{1}}\] will be getting forward biased, while \[{{D}_{2}}\] will be getting reverse biased. The conventional current because of \[{{D}_{1}}\] will flow along the path of full arrows.

If the second half of the input cycle comes, the conditions will be in the opposite manner. Now the junction diode \[{{D}_{2}}\] will be conducting and the conventional current will be flowing along the path of the dotted arrows.

The output during both the half cycles will be of similar nature as the current during both the half cycles flows from right to left through load resistance \[{{R}_{L}}\]. The right end of \[{{R}_{L}}\] will be at \[+ve\] potential in accordance with left end. Hence, the output will be continuous in a full wave rectifier.

Special usages of P-N junction Diode:

(a) Zener Diode:

The Zener diodes can be defined as a heavily doped P-N junction diode where the operation of the device can occur only in the reverse biased condition. Here as both p and n sides are heavily dope, the depletion region developed will be very thin.

In these diodes, when the applied reverse bias voltage reaches the breakdown voltage of the Zener diode, we can see a huge variation in the current. At this time, the reverse voltage will be almost constant even though there is a significant change in the current occurs. Hence this kind of diode can be used as a voltage regulator.

2.8. Transistor:

This will be a three-section semiconductor, where three sections are combined so that the two at extreme ends will be having similar kinds of majority carriers. At the same time, the section that is separating them will be having the majority carriers in opposite nature. The three sections of the transistor can be defined as emitter (\[E\]), Base (\[B\]) and collector \[(C)\].

Semiconductors Class 12 Notes PDF Download – Symbol

2.8.1. Action of n-p-n Transistor:

Fig. mentions that the n-type emitter will be forward biased when connected to \[-ve\] pole of \[{{V}_{EB}}\](emitter-base battery) and the n-type collector will be reverse biased when connected to it to \[+ve\] pole of \[{{V}_{CB}}\](collector-base battery).

The majority of carriers (\[{{e}^{-}}\]) in the emitter will be repelled towards the base because of the forward bias. The base will be having holes as majority carriers but their number density is small because it is doped very lightly (\[5%\]) when we compare it to the emitter and collector.

Because of the probability of \[{{e}^{-}}\] and hole combination in the base will be small. Most of \[{{e}^{-}}\] (\[95%\]) cross into the collector region where they will be swept away by \[+ve\] terminal of battery \[{{V}_{CB}}\].

In correspondence to each electron that is taken by the collector, an electron will enter the emitter from \[-ve\] pole of collector – base battery. When  \[{{I}_{e}},{{I}_{b}},{{I}_{c}}\] be emitter, base and collector current respectively than by the use of Kirchoff’s first law, we can say that,

\[{{I}_{e}}={{I}_{b}}+{{I}_{c}}\]

2.8.2. Action of p–n–p Transistor:

The p–type emitter will be forward biased when we connect it to \[+ve\] pole of the emitter–base battery and the p–type collector will be reverse biased when it is connected to \[-ve\] pole of the collection-base battery. Here, the majority of carriers in the emitter, that is holes will be repelled towards the base because of forward bias.

As the base is lightly doped, it will be having low number density of \[{{e}^{-}}\]. If hole enters base region, then only \[5%\] of \[{{e}^{-}}\] and hole combination will be occurring. Most of the holes will be reaching the collector and are swept away by \[-ve\] pole of \[{{V}_{CB}}\] battery.

2.9. Common base Amplifier:

Here, the base of the transistor will be common to both the emitter and collector.

(a) Amplifier circuit by the use of n-p-n transistor: The emitter will be forward biased by the use of an emitter bias battery (Vcc) and because of this, the resistance of the output circuit will be large.

The low input voltage will be applied across emitter–base circuit and the amplified circuit will be received across collector – base circuit. When\[{{I}_{e}},{{I}_{b}},{{I}_{c}}\] be the emitter, base and collector current then,

\[{{I}_{e}}={{I}_{b}}+{{I}_{c}}\]…\[(i)\]

If current \[{{I}_{c}}\]  is flowing in the collector circuit, a potential drop \[{{I}_{c}}{{R}_{c}}\] will be happening the resistance connected in collector – base circuit and base-collector voltage will be,

\[{{V}_{cb}}={{V}_{cc}}-{{I}_{c}}{{R}_{c}}…(ii)\]

(b) Amplifier circuit by the use of p–n–p Transistor:

• If the positive half cycle of input a.c. the signal voltage is coming, it supports the forward biasing of the emitter–base circuit. Due to this, the emitter current increases and consequently the collector current increases.
• As \[{{I}_{c}}\] increases, the collector voltage \[{{V}_{c}}\] decreases.
• Since the collector is connected to the negative terminal of \[{{V}_{CC}}\] a battery of voltage \[{{V}_{CB}}\], therefore, the reduction in collector voltage shows that the collector is becoming less negative. This represents that while the positive half cycle of input a.c. signal voltage occurs, the output signal voltage at the collector will also vary through the positive half cycle.
• In the same way, when a negative half cycle of input a.c. signal voltage occurs, the output signal voltage at the collector also varies through the negative half cycle. Hence, the input signal voltage and the output collector voltage are in the same phase in the common base transistor amplifier circuit.

2.10. Common Emitter Amplifier:

Amplifier Circuit by the Use of n–p-n Transistor:

1. The input (emitter-base) circuit will be forward biased with the battery \[{{V}_{BB}}\] of voltage \[{{V}_{EB}}\], and the output (collector-emitter) circuit will be reversed biased with the battery \[{{V}_{CC}}\] of voltage \[{{V}_{CE}}\]. Because of this, the resistance of the input circuit will be low and that of the output circuit will be high. \[{{R}_{c}}\] will be a load resistance connected in collector circuit.
2. If a.c. signal voltage has not been applied to the input circuit but the emitter-base circuit being closed, assume that \[{{I}_{e}},{{I}_{b}}\] and \[{{I}_{c}}\] be the emitter current, base current and collector current respectively. Then in accordance with Kirchhoff’s first law

\[{{I}_{e}}={{I}_{b}}+{{I}_{c}}\]

3. If the positive half cycle of input a.c. the signal voltage is considered, it will be supporting the forward biasing of the emitter-base circuit. Because of this, the emitter current will be increasing and as a result, the collector current will get increased. Because of which, the collector voltage \[{{V}_{c}}\] will be decreased.
4. Since the collector has been connected to the positive terminal of \[{{V}_{CE}}\] battery, therefore the reduction in voltage at the collector means the collector will become less positive, which shows that negative in accordance with the initial value. This shows that while the positive half cycle of input a.c. signal voltage occurs, the output signal voltage at the collector will be varying through a negative half cycle.
5. If the negative half cycle of input a.c. the signal voltage is considered, it will be opposing the forward biasing of the emitter-base circuit, because of this the emitter current will be decreasing and therefore collector current will get decreased; As a result, the collector voltage \[{{V}_{c}}\] gets increased. That is, the collector will be more positive. This shows that when the negative half cycle of input a.c. signal voltage occurs, the output signal voltage will be varying through the positive half cycle.

3. Analog Signals

Analogue signals can be defined as signals which are varying continuously with respect to time. A typical analogue signal has been represented in the diagram below. A circuit which has been used for the generation of analogue signals can be defined as an analogue electronic circuit.

Digital Signals

Signals which are having either of the two levels, \[0\] or \[1\], can be defined as digital signals.

Logic Gates

A Gate can be explained such that a digital circuit which is either stopping a signal or permits it to pass through it. A logic gate will be an electronic circuit which will be creating logical decisions. The logic gate will be having one or more inputs but only one output.

Logic gates are considered the basic building blocks for every digital system. Variables used at the input and output will be \[1’s\] and \[0’s\].

The three basic logic gates are mentioned below:

1. OR gate
2. AND gate
3. NOT gate

5.1. OR Gate:

OR gate can be considered as electronic equipment that is combining \[A\] and \[B\] for providing \[Y\] as output. Two inputs are \[A\] and \[B\] and the output is \[Y\] in this figure. In Boolean algebra OR is shown as \[+\].

Truth Table: This can be defined as a table which is providing an output state for all possible input combinations.

Logic operations of the OR gate have been provided in its truth table for all possible input combinations.

Input		Output
A	B	Y
0	0	0
0	1	1
1	0	1
1	1	1

5.2. AND Gate:

There are two or more inputs and one output in an AND gate. In Boolean algebra AND has been denoted as a dot (.).

Truth Table

Input		Output
A	B	Y
0	0	0
0	1	0
1	0	0
1	1	1

5.3. NOT Gate:

NOT gate can be considered as an electronic device which is having one input and one output. This circuit will be known as the same as the output is NOT similar one as the input.

Boolean expression for NOT gate is \[Y=\overline{A}\].

Truth Table:

Input	Output
A	Y
0	1
1	0

5.4. NOR Gate:

A NOR gate will have two or more inputs and only one output. Here, NOR gate is a NOT-OR gate actually. When a NOT gate has been connected at the output of an OR gate, we will be getting NOR gate as represented in the diagram below and its truth table in the table.

Truth Table:

A	B	${{Y}^{‘}}$	Y
0	0	0	1
0	1	1	0
1	0	1	0
1	1	1	0

Boolean expression for NOT gate will be \[Y=\overline{A+B}\] and this can be read as \[Y\] equals \[A\] OR \[B\] negated. A NOR function will be the reverse of the OR function.

Truth Table:

Input		Output
A	B	Y
0	0	1
0	1	0
1	0	0
1	1	0

5.5. NAND Gate:

A NAND gate will be having two or more inputs and only one output. A NAND gate is actually a NOT–AND gate. When a NOT gate has been connected at the output of an AND gate, we will be getting a NAND gate as represented in the diagram and its truth table has been shown in the table.

A	B	${{Y}^{‘}}$	Y
0	0	0	1
0	1	0	1
1	0	0	1
1	1	1	0

Boolean expression for NAND gate, will be \[Y=\overline{A\cdot B}\] and is read as \[Y\] equals \[\text{A}\]and \[\text{B}\] negated.

The logical symbol of the NAND gate has been represented in the diagram and its truth table in the table.

Truth Table:

Input		Output
A	B	Y
0	0	1
0	1	1
1	0	1
1	1	0

Similar to the NOR gate, the NAND gate will also be useful to realize all basic gates: OR, AND and NOT. Therefore it is also called Universal Gate.

Universal Gate:

A universal gate can be defined as a logic gate which can be used to perform any Boolean function with no utilization of any other type of logic gate. Examples of universal gates are NOR gate and NAND gate.

Integrated Circuits:

The conventional process of creating circuits will be to choose components such as transistors, diodes, R, L, C etc., and connect them using soldering wires in the required manner. The commonly used technology will be the Monolithic Integrated Circuit.

These kinds of circuits contain logic gates. When we depend upon the level of integration (that is, the number of circuit components or logic gates), the ICs can be defined as Small Scale Integration, SSI (logic gates 1000).

Semiconductor Electronics Class 12 Notes PDF

Some devices like vacuum diodes were manufactured and used to control the flow of electrons in a circuit before the discovery of transistors. Nowadays, the usage of these devices is obsolete. These devices were bulky, spent high power, and functioned at high voltages.

Also, these devices had limited life and reliability. Physicists concluded in the year 1930 that the solid-state semiconductors and their junction presented the opportunity of monitoring the flow of charge exports through them.

Modern solid-state semiconductors are known as LEDs, otherwise light-emitting diodes. These LEDs ingest low power in very few amounts, operate at low voltages, hold a durable life, and have high reliability.

We Are Going to Study About These Following concepts in Chapter 12:

• Elementary ideas of semiconductor Physics
• Semiconductor devices (bipolar junction transistors and junction diodes)
• Semiconductors and their applications

CBSE Class 12 Physics Chapter 14 Notes Semiconductor Electronic Material Devices and Simple Circuits

In order to define Semiconductors, which are used in solid-state electronic devices such as transistors, diodes, etc. Semiconductors are acted as the fundamental core materials.

It is our complete freedom to choose any of the materials like metal, semiconductor, or insulator. These elements can be treated as the semiconductors such as Ge, Si, or compounds like CdS or GaAs.

Revision Notes Class 12 Chapter 14

In this chapter, students are going to face questions about calculating the output frequency of a half-wave and full-wave rectifier. Also, they have to answer some questions on finding the input signal and base current of a CE-Transistor amplifier.

In this revision notes, Vedantu can offer help to the students to find out the output AC signal if two amplifiers are connected in series. When a condition arises about the fabrication of a p-n photodiode, will it detect wavelengths? These kinds of questions are also given in the notes.

There are some other practice questions based on this chapter that contain finding several holes and electrons in atoms of silicon, Indium, and arsenic.

In this way, chapter 14 of semiconductor electronic material devices and simple circuits will be covered with concepts as well as questions and answers.

Semiconductor Electronic: Material, Devices And Simple Circuits Class 12 Notes Class 12 Notes Chapter 14

1. Metals: They possess very low resistivity or high conductivity.
ρ ~ 10^-2.10^-8 Ωm, σ ~10². 108 Sm^-1
2. Semiconductors: They have resistivity or conductivity intermediate to metals and insulators.
ρ ~ 10^-5. 10⁶ Ωm, σ ~ 10⁺⁵ .10^-6 Sm^-1

Types of Semiconductors

Types of semiconductors are given below:

(i) Elements Semiconductors: These semiconductors are available in natural form, e.g. silicon and germanium.
(ii) Compound Semiconductors: These semiconductors are made by compounding the metals, e.g. CdS, GaAs, CdSe, InP, anthracene, polyaniline, etc.
3. Insulators: They have high resistivity or low conductivity.
ρ ~ 10¹¹. 10¹⁹ Ωm, σ ~ 10^-11. 10^-19 Sm^-1
4. Energy Band: In a crystal due to interatomic interaction, valence electrons of one atom are shared by more than one atom in the crystal. Now, the splitting of energy levels takes place. The collection of these closely spaced energy levels is called an energy band.
5. Valence Band: Valence band are the energy band which includes the energy levels of the valence electrons.
6. Conduction Band: The conduction band is the energy band above the valence band.
7. Energy Band Gap The minimum energy required for shifting electrons from the valence band to the conduction band is called the energy band gap (E_g ).
8. Fermi Energy It is the maximum possible energy possessed by free electrons of a material at absolute zero temperature (i.e. 0K)

Class 12 Physics Chapter 14 Revision Notes

The following subtopics are present in chapter 14:

Students can go through the important points to revise during the final examination:

• Diode:

(i) By changing the externally applied voltage, the junction barrier of the p-n junction diode can be changed.
(ii) Diodes along with the help of a capacitor or a suitable filter, can rectify AC voltage to a DC voltage.
(iii) After a certain (breakdown) voltage, the current suddenly increases in a Zener diode. This property has been used to obtain voltage regulation.
(iv) Applications of diodes: Photodiodes, Solar cells, Light emitting diode and Diode Laser, etc.

• Logic gates:

(i) NAND and NOR gates are known as universal gates.
(ii) Output for OR gate will be False only when both the inputs are False.
(iii) Output for AND gate will be True only when both the inputs are True.
(iv) NAND gate is a combination of AND and NOT gates.
(v) NOR gate is a combination of OR and NOT gates.

• Extrinsic semiconductor:

(i) Semiconductors which are doped with some impurities are known as extrinsic semiconductors.
(ii) Extrinsic semiconductors are of two types: nn-type and pp-type.

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