NEET Study Notes for Electronic Devices: Electronic Devices are an essential part of our everyday lives. They are essentially the components that help to control the flow of electric current to serve the purpose of processing information and control system. Examples of Electronic devices include transistors and diodes.

• Electronic Devices is an important unit in NEET Physics Syllabus that covers essential topics like energy bands, semiconductors, Transistors, Diode, and Logic Gates. With a weightage of 6%, candidates can expect at least 2-3 questions from this unit in NEET 2022.
• With a thorough understanding of the topics from Electronic Devices, students will also be able to gain a better understanding of other units like Current Electricity, Magnetic Effects of Current, and Magnetism.

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NEET Study Notes for Electronic Devices: Important Topics

NEET Study Notes on Electronic Devices

Solids are made of atoms as well as molecules. However due to differences in the arrangement of molecules inside them, solids can be divided into two categories-

• Crystalline Solids- In Crystalline Solids, atoms and molecules are in a defined arrangement. Flat surfaces bound crystalline solids.
• Amorphous Solids- In Amorphous Solids, atoms and molecules are in an irregular not defined arrangement. Amorphous Solids in all directions have the same physical properties.

Band Theory of Solids- The quantum state of an electron inside solid metal is called the Band theory of solids. Band theory explains the behavior of electrons inside a molecule. In the energy orbits, electrons are filled. These form energy bands.

Classification of Energy Bands

Energy bands can be classified into the following;

• Valence Band- Electrons in the outermost shell are known as valence electrons. The energy levels in these valence electrons form energy band which is called the valence band. The highest occupied energy is in the valence band.
• Conduction Band- Valence electrons that are not tightly bound by the nucleus leave the outermost shell and become free electrons. In conductors, free electrons conduct current. These are called conduction electrons. The energy bands of conduction electrons are called conduction bands.

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Conductors

• The conductors have a band that is partially filled. On application of electric field, electrons can move freely. This is due to the vacant states available in uppermost bands.
• As the valence band and conduction band do not form a forbidden gap, there is overlapping of the band.
• Examples are Gold, Silver, Aluminium, and Copper/

Insulators

• In these, the uppermost bands are completely filled. This does not allow the electrons to move.
• An energy gap of 7eV separates the filled band and the unfilled band. This restricts the movement of electrons from the valence band to the conduction band.
• Examples are- Glass and Wood.

Semi-Conductors

• The valence band of semiconductors is completely filled however the conduction band is empty.
• The forbidden gap between valence and conduction band is 1eV. This makes the movement of electrons from one band to another easy Examples are Silicon and Germanium.
• There are two main types of semiconductors: Intrinsic and Extrinsic Semi-Conductors.

Intrinsic Semi-Conductors

The thermal vibrations of lattice liberate charge carriers in these semiconductors. The electron numbers and number of holes are equal in intrinsic semiconductors.

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Extrinsic Semi-Conductors

In these conductors, impurities induce charge carriers that are mobile Which may be positive or negative in addition to carriers liberated by thermal vibrations.

The two types of Extrinsic Semi-Conductors are:

• N-Type Semi-Conductor- N-type semiconductor is obtained when pentavalent impurity to an intrinsic semiconductor. P, As and Sb are some of the pentavalent impurity substances. In these semiconductors, the majority of carriers are electrons whereas holes are a minority.
• P-Type semiconductor- P-type semiconductor is obtained when a trivalent impurity is added to the intrinsic semiconductor. In these semiconductors, the majority carriers are holes whereas electrons are carriers in minority.

Points to remember

• Forbidden energy gap= space between the upmost valence band and the lowest part of the conduction band.
• In a conduction band of a semiconductor, there are no free electrons at absolute zero temperature.
• Some of the important compound semiconductors are gallium arsenide, lead sulfide, indium phosphide, etc.
• Holes exist only in semi-conductors but not conductors as they get deflected like electrons by electric and magnetic fields.

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P-N Junction Diode

A P-N Junction Diode is obtained when joining n-type to the p-type semiconductors. For the application of external voltage, two ends of the p-n junction are provided with metallic conductors. P-type semiconductors have more concentration of holes but less electron concentration in comparison to n-type semiconductors.

This concentration difference leads to a dense gradient which results in the majority of carrier diffusions.

Forward and Reverse Bias of P-N Junction Diode

Forward Bas- p-side of p-j junction connected to the positive terminal of a battery and n side to the negative terminal. Conduction takes place through this circuit and this diode is said to be forward-biased.

Reverse Bias- In this Reverse Bias, conduction does not take place in the p-n junction, the p-side is connected to the negative terminal of a battery whereas to the positive terminal n-side is connected.

• p-n junction or junction diode can be used as a rectifier to convert the a.c current to d.c. current This is called rectification.
• Zener diode Avalanche is used in voltage stabilization.
• The photodiode can be used in switching of light automatically or the light, fire, and burglar alarm.
• LED ( Light-emitting Diode) can be used for optical fiber communication.
• Zenode diode is also useful for voltage regulation as to when ziner diode operates in the breakdown region, it acts as a battery.

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NEET Study Notes on Transistors

Solid-state (three terminal) devices are called transistors. These can be compared to a triode. A junction transistor or (p-n-p) consists of a bar crystal, which can be either silicon or germanium. A layer of Silicon of n-type is placed between two layers of silicon of p-type. This makes it a p-n-p transistor.

Components of a Transistor

• Emitter- negative lead of transistor
• Collector- Positive lead of the transistor
• Base- Used in transistor activation

Characteristics of a transistor

Characteristics of a transistor explain the relationship between current and voltage of a transistor of a particular configuration. The two types of characteristics are:

• Input characteristics- These determine the change in input current along with the change in input voltage by considering the output voltage as constant.
• Output characteristics- Input current is kept constant and output current with the output voltage is plotted.
• Current Transfer characteristics- The change of output current with input current by constant input voltage is plotted.

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Transistor as an Amplifier

One of the major properties of the transistor is as an amplifier. Amplification can be described as the process by which the amplitude of a single linearly increases. The process is produced when current is transferred from a low to high resistance circuit.

Common Emitter Configuration

In border to use the transistor as an amplifier, the common emitter configuration is used. The figure depicts the way a transistor is set up when in an amplifier, it is connected in a circuit.

Here, the input is connected in forward biased whereas the output connects in reverse biased. On the base-emitter junction, the input signal is applied whereas through the emitter collection junction load, the output is taken. For amplification, DC voltage is applied in the input circuit. The load can be in combination of L, R, or C. A large voltage drop is caused due to load resistance of high value. In the collector circuit, the weak signal is amplified.

• Transistor can also be used as an oscillator. The frequency of the transistor as an oscillator is given by ,
• The main rules of the normal working of a transistor are emitter-base junction is biased forward which offers low resistance to flow of current.
• Collector base junction is biased reverse which provides high resistance to the flow of current.

What are Logic Gates?

A digital circuit that adheres to the logical relationship between input and output voltages is called a logic gate. The flow of information is controlled by a logic gate. The basic logic gates are- AND, NOT, and OR.

AND Gate

Output is 1 or high when all the inputs are high

 

NOT Gate

Output, not the input

 

OR Gate

If one of the inputs is high, output is high

 

NAND Gate

Combination of NOT and AND Gate, even if all inputs are low, the outputs are high NAND Gate is the building block of all digital circuits.

 

NOR Gate

Combination of NOT and OR gates, when all inputs are low, the output is high.

 

NEET Sample Questions on Electronic Devices

Question: What is the function of a transistor?

1. Switching device and a fixed resistor
2. Tuning device and rectifier
3. Rectifier and Fixed resistor
4. Variable resistor and switching device

Answer: Variable Resistor and Switching Device

Transistor is a semiconductor device that can be used commonly as an amplifier. The most important function of the transistor is for amplification and as a switch for operating computers, mobile phones, and other electrical circuits.

Question: What does the junction capacitance of p-n junction depend upon?

1. Applied Voltage only
2. Doping concentration only
3. Doping concentration as well as applied voltage
4. Barrier Potential only

Answer: Both Doping Concentration as well as applied voltage

Question: When the pure semiconductor is heated, what happened to its resistance?

1. Resistance decreases
2. Resistance increases
3. Temperature remains the same
4. None of the above

Answer: Resistance decreases when pure semiconductor is heated

Question: What is created when trivalent impurities are added to semiconductor?

1. Free electrons
2. Bound electrons
3. Valence electrons
4. Holes

Answer: Holes

Question: What makes the mobility of free electrons greater than holes?

1. Due to being light
2. Mutual collision is less
3. Carry negative energy
4. Require low energy to continue motion

Answer: The mobility of free electrons require low energy to continue the motion.

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