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Semiconductor electronics is considered as the backbone of the modern digital technologies.
- Semiconductors are materials with unique electrical properties that allow them to be used as both conductors and insulators under certain conditions.
- Semiconductor Electronics was discovered as a part of experiments in the 1930s.
- This led to the realization that certain solid-state semiconductors and their junctions could control the number and direction of flow of charge carriers through them.
- This paved the way for the development of modern-day solid-state semiconductor electronics.
- Semiconductors are now one of the most essential components of all major electronic devices.
- Semiconductors are materials that are neither good conductors nor good insulators.
- Their low cost, reliability, and compact structure make them the most preferred in the designing of electrical devices and appliances.
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Key Terms: Semiconductors, Diodes, PN Junction, Intrinsic Semiconductor, Charge Carrier, Extrinsic Semiconductor, Electron, Hole, Rectifier, Integrated Circuit
What is a Semiconductor?
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A semiconductor is a type of material whose resistivity is between a conductor (silver, copper, etc) and insulator (glass, diamond) which is
The table below is a summary of the resistivity and conductivity of conductors, semiconductors & insulators.
| Material | Resistivity | Conductivity |
|---|---|---|
| Conductor | 102 - 10-8(ohm m) | 102 - 108 5m-1 |
| Semiconductor | 105 - 106 (ohm m) | 105 - 106 5m-1 |
| Insulator | 1011 - 106 (ohm m) | 10-11 - 10-19m-1 |
The video below explains this:
Semiconductors and Insulators Detailed Video Explanation:
Properties of Semiconductors
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Semiconductors have two important properties :
- They can control the number of charge carriers (electrons and holes), thus controlling conductivity.
- They can produce unidirectional current
Electrons and holes in a semiconductor can vary with change in intensity of heat, light, voltage.
Examples of Semiconductor Electronics
Examples of Semiconductors
Some of the examples of Semiconductors are given below -
- Elemental semiconductors - Silicon ( Si), Germanium (Ge)
- Compound Semiconductors - GaAs. InP , CdS, etc.
- Organic Semiconductor - doped pthalocyanines, anthracene
- Inorganic Semiconductor - polythiophene, polypyrrole
Semiconductor Electronics Class 12 Important Notes PDF
Semiconductor Electronics Class 12 Important Notes
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| Important Links Related to Semiconductor Devices | ||
|---|---|---|
| Light Emitting Diodes | Diode Vs Rectifier | Semiconductor Diode |
| Rectifier | Semiconductor Formula | Extrinsic Semiconductors |
| Diode Uses | Intrinsic Semiconductors | Zener Diodes |
| Classification of Metals, Conductors & Insulators | Varactor Diode | Photodiodes |
Energy Bands in Crystals
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The energy of an electron in an isolated atom depends upon the orbit it is revolving in as per Bohr’s Theory. However, the atoms in a solid material lie very close to each other. This has a significant impact on the nature of electron motion within a solid object.
- In a crystal each electron has a unique position.
- Hence, no two electrons have the same pattern of charges around them.
- Consequently, they will have a different energy level even when they belong to the same subshell of an isolated atom.
- Their energy levels are slightly greater or smaller than their original energies in an isolated state with continuous variation which is known as energy bands.
Insulators, conductors and semiconductors can be differentiated based on their energy bands.
- In metals, the valence and conduction band lie very close to each other and sometimes even overlap which allows free movement of electrons.
- In case of insulators, the conduction band and valence band is separated by a large gap which discourages movement of electrons.
- However, this gap is smaller in semiconductors which encourages some electrons to enter the conduction band by crossing the gap.
- This results in limited electrical conductivity. Eg. Silicon or germanium semiconductor.
Valence Bands
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- Range of energies possessed by valence electrons in a crystal.
- it contains valence electrons.
- These valence bond electrons do not participate in electrical conductivity.
- Conduction band electrons are responsible for electrical conductivity.
What is FEG?
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FEG stands for Forbidden Energy Gap. It is denoted by Eg.
- This is the distance between the conduction and valence band.
- This band has no energy as no electron stays in this band.
- The electrons traveling from the valence band to the conduction band, only pass through this band.
Eg is calculated :
Eg = Ec – Evi
Eg=(CB)min –(VB)max
The electron in the valence band may gain energy and jump to the conduction band; the electrons can not be found in the region between the valence band and the conduction band is forbidden for the electron. Those energy levels are together known as FEG.
Valence Band, Conduction Band and FEG
Characteristics of FEG
Some of the interesting characteristics of FEG are listed below:
- The forbidden energy gas is very large.
- The electrons in the valence band are held tightly to the atoms.
- The FEG for an insulator is 10 eV.
- Some insulators at higher temperatures show some conduction.
- The inuslator resistivity is of the order of 107 Ώm.
Types of Semiconductors
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Semiconductors are classified into two types based on the number of electrons and holes. These are:
- Intrinsic
- Extrinsic
Let us understand the two types in detail below.
Intrinsic Semiconductors
Intrinsic semiconductors are those that are pure in nature and free from impurities. In this case, the property of the material determines the number of electrons and holes present in it as opposed to the presence of impurities. It has an equal number of holes and free electrons. Examples: Germanium and silicon are some of the most commonly used intrinsic semiconductors.
Thus, ne = nh = ni
Here,
ni = intrinsic carrier concentration
ne = number of electrons
nh = number of holes
Thus for Pure Si (Z=14) and for Pure Ge (Z=32). Both have 4 valence electrons. All 4 valence electrons are involved in covalent bond formation in Si or Ge crystal.
Covalent Bond Formation in Si or Ge crystal
Extrinsic Semiconductors
The electrical conductivity of intrinsic (pure) semiconductor is dependent on its temperature. However,at room temperature, its conductivity is very poor.
- The addition of certain impurities can increase the conductivity of the intrinsic (natural) semiconductors.
- A very small amount (in part per million ppm) of impurities atoms can increase the conductivity of intrinsic semiconductors many times.
- The semiconductor thus obtained is called an Extrinsic Semiconductor.
- The process of addition of impurity is called doping and the impurity atoms are called dopants
- The impure semiconductor thus formed is called a “doped” semiconductor.
An Extrinsic Semiconductor can be of two types based on the type of impure atom.
n-type semiconductor doped with Pentavalent impurity atom
Here ne >> nh that is the number of electrons is greater than the number of holes. Examples include Phosphorus (P), Antimony (Sb), Arsenic (As).
p-type semiconductor doped with Trivalent impurities atom
Here nh>> ne that is the number of holes is greater than the number of electrons. Examples include Boron (B), Aluminium ( Al), Indium (In).
Extrinsic Semiconductors
P-N Junction
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An interface or a boundary that is between p-type and n-type, the two types of semiconductor material, is known as a p-n junction. The positive side of the semiconductor, which is the p side, has an excess of holes while the negative side of the semiconductor, which is the n side, has an excess of electrons. P-N junctions are formed through the process of doping.
Doping in Semiconductors
Semiconductor Diode
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A semiconductor device that allows current to flow easily in one direction is termed as diode. When a diode allows current to pass through it is known as forward-biased. When its discourages the flow of current and acts as an insulator, it is known as reverse-biased. A Semiconductor Diode is a p-n junction that has metallic contacts at both of its ends in order to apply external voltage. It is a p-n junction diode. It has the ability to conduct current in only one direction
Semicunductor Diode
Diodes can be of many types. Some have been mentioned below -
- Small Signal Diode.
- Large Signal Diode.
- Zener Diode.
- Light Emitting Diode (LED).
- Constant Current Diodes.
- Schottky Diode.
- Shockley Diode.
- Step Recovery Diodes
- Tunnel Diode
- Varactor Diode
- Laser Diode
Application of Junction Diode as a Rectifier
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An electrical device that converts alternating current into direct current with the help of a diode is called a Rectifier.There are two types of rectifier:
Let us look at each of these in detail.
Types of Rectifiers
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Given below are the different types of rectifiers.
Half Wave Rectifier
A half-wave rectifier is defined as a type of rectifier that only allows the one-half cycle of an AC voltage and gives the pulsating DC voltage. There are two diodes in the half-wave rectifier. Half wave rectifiers having two diodes which helps them to rectify the AC voltage to DC voltage with the help of a filter its convert pulsating DC with generated from the half-wave rectifier will convert into constant DC but some amount of filter circuit is there in the filter.
Half Wave Rectifier
Full Wave Rectifier
Full-wave rectifiers have four diodes where the first two diodes will conduct in the positive half cycle and other two diodes will conduct in the negative half cycle. It will give full pulsating DC the sinusoidal wave is complete and with the help of the capacitor or inductor we will filter and convert pulsating DC into constant DC.
Full Wave Rectifier
Application of Full Wave Rectifier and Half Wave Rectifier
The use of a half-wave rectifier can help us achieve the desired dc voltage by using step-down or step-up transformers. Moreover, to power up the motor and LED that works on DC voltage, full wave rectifiers are used.
Integrated Circuits
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An Integrated Circuit can be defined as a small chip that are made out of semiconductor materials upon which an entire circuit can be mounted. The monolithic Integrated Circuit is among the most commonly used today.
There are primarily two types of circuits:
- Digital Integrated Circuits
- Analog Integrated Circuits
Integrated Circuits
Junction Transistor
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A Junction Transistor or Bipolar Junction Transistor is a semiconductor device that helps transfer a weak signal from a low resistance circuit to a high resistance circuit. A Junction Transistor helps in regulating and amplifying the electrical signals such as current or voltage. Transistors are a unique device as they allow control over of how much current can flow through a circuit, which they achieve by controlling the voltage across two of the three transistor leads.
Junction Transistors are a combination of two junction diodes and the three-layer structure of these can be of either of the combinations:
- P-N-P type of semiconductor: an n-type of semiconductor is between two layers of p-type semiconductor, or
- N-P-N type of semiconductor: p-type of semiconductor is between two layers of n-type semiconductor.
Junction Transistor
Digital Electronics and Logic Gates
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Digital Electronics is that branch of electronics that deals with the study of digital signals, circuits and the devices required to produce them. Simply put, logic gates are electronic circuits. They perform the function of a gate by controlling the flow of information on the basis of logical relations. The signal passes through the digital circuit only if the logical relations are satisfied.
Logic Gates
Logic gates are the basic building blocks of digital electronics. It is a digital circuit which follows a logical relationship between input and output.
Things to Remember
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- A semiconductor is a type of material whose resistivity is between a conductor (silver, copper, etc) and an insulator (glass, diamond)
- The energy of an electron in an isolated atom depends upon the orbit it revolves around.
- An electrical device that converts alternating current into direct current with the help of a diode is called a Rectifier.
- There are two types of rectifiers - half-wave rectifiers and full-wave rectifiers.
- There are primarily two types of circuits: Digital Integrated Circuits and Analog Integrated Circuits
- A Junction Transistor or Bipolar Junction Transistor is a semiconductor device that helps transfer a weak signal from a low-resistance circuit to a high-resistance circuit.
- Digital Electronics is a branch of electronics that deals with the study of digital signals, circuits, and the devices required to produce them.
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Previous Year Questions
- Determine the type of gate. [JIPMER 2019]
- Which Logic gate is represented by the truth table? [JKCET 2012]
- Calculate ß when the emitter current is changed. [JEE Main 2021]
- What is the impurity atom to be doped for p type semiconductors? [JEE Advanced 1988]
- What is pure silicon. [JIPMER 2018]
- What is an intrinsic semiconductor. [MET 2012]
- Device that acts as a complete electronic circuit. [NEET 2010]
- Which have negative coefficient of resistance. [NEET 2020]
- Determine the logic gate? [NEET 2010]
- Determine the semiconductor device type. [NEET 1998]
- When is an npn transistor used as an amplifier. [NEET 1996]
- What causes increase in width of depletion region. [NEET 2020]
- A p-n junction diode is also used as? [NEET 1999]
- The value of photo electromotive force is proportional to? [NEET 2005]
- Which is p-type semiconductor? [NEET 2013]
- Calculate the voltage gain and power of the amplifier. [NEET 2017]
- The electrical network is equivalent to? [NEET 2017]
- The pn junction is forward biased when? [NEET 1988]
- At absolute zero, Si acts as? [NEET 1988]
- Calculate the power gain of the amplifier. [NEET 2010]
- A p-n-p condcutor conducts when? [NEET 2003]
- Calculate the voltage and power gain of the amplifier. [NEET 2016]
- Calculate the minimum frequency of radiation to be absorbed by the material. [NEET 2008]
- A piece of Cu and Gr are cooled from room temperature to 80 K, then? [NEET 1992]
- Calculate the current through the diode? [NEET 2011]
- Why are C and Si intrinsic semiconductors? [NEET 2012]
- For an ideal diode, what is thevalue of current? [NEET 2016]
- Depletion layer consists of? [NEET 1999]
Sample Questions
Ques. Why is the valence band in semiconductors partially empty and the conduction band is partially filled at room temperature?(2 marks)
Ans. At zero Kelvin, the conduction band is empty and the valence band is completely filled in semiconductors. No electron from the valence band can cross over to the conduction band at this temperature. But at room temperature, some electrons in the valence band jump over to the conduction band due to a small forbidden gap i.e. 1 eV.
Ques. What happens to the resistance of a pure semiconductor when heated.(1 mark)
Ans. The resistor decreases.
Ques. How many valence electrons do trivalent impurities have?(1 mark)
Ans. 3 valence electrons can be found in trivalent impurities.
Ques. What is the relation between temperature and the number of free electrons in a semiconductor?(1 mark)
Ans. N is directly proportional to t3/2.
Ques. Why should a photodiode be operated at a reverse bias? (All India 2008)
Ans. As fractional change in minority charge carriers is more than the fractional change in majority charge carriers, the variation in reverse saturation current is more prominent.
Ques. Give the logic symbol of NOR gate. (All India 2009)
Ans.
Ques. The figure shows the V-I characteristic of a semi conductor device. Identify this device. Explain briefly, using the necessary circuit diagram, how this device is used as a voltage regulator. (Comptt. Delhi 2011)
Ans.
(i) The semiconductor diode used is a Zener diode.
Ques. The given inputs A, B are fed to a 2-input NAND gate. Draw the output wave form of the gate.
Ans.
Ques. Draw the transfer characteristic curve of a base biased transistor in CE configuration. Explain clearly how the active region of the VD versus V, curve in a transistor is used as an amplifier. (Delhi 2011)
Ans. For using the transistor as an amplifier we will use the active region of the V0 vs. V, curve. The slope of the linear part of the curve represents the rate of change of the output with input. It is negative, that is why as input voltage of the CE amplifier increases its output voltage decreases and the output is said to be out of phase with input.
Ques. Explain how a depletion region is formed in a junction diode. (Delhi 2011)
Ans.
As soon as a p-n junction is formed, the majority charge carriers begin to diffuse from the regions of higher concentration to the regions of lower concentrations. Thus the electrons from the n-region diffuse into the p-region and where they combine with the holes and get neutralised. Similarly, the holes from the p-region diffuse into the n-region where they combine with the electrons and get neutralised. This process is called electron-hole recombination.
The p-region near the junction is left with immobile -ve ions and n-region near the junction is left with +ve ions as shown in the figure. The small region in the vicinity of the junction which is depleted of free charge carriers and has only immobile ions is called the depletion layer. In the depletion region, a potential difference VB is created, called potential barrier as it creates an electric field which opposes the further diffusion of electrons and holes.
- In forward biased, the width of depletion region is decreased.
- In reverse biased, the width of depletion region is increased.
Ques. Explain, with the help of a circuit diagram, the working of a p-n junction diode as a half-wave rectifier. (All India 2013)
Ans. Rectifier. A rectifier is a circuit which converts an alternating current into direct current.
p-n diode as a half wave rectifier. A half wave rectifier consists of a single diode as shown in the circuit diagram. The secondary of the transformer gives the desired a.c. voltage across A and B.
In the positive half cycle of a.c., the voltage at A is positive, the diode is forward biased and it conducts current.
In the negative half cycle of a.c., the voltage at A is negative, the diode is reversed biased and it does not conduct current.
Thus, we get output across RL during positive half cycles only. The output is unidirectional but varying.
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