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The gap (spacing) between the top of the valence band (EV) and the bottom of the conduction band (Ec) is called the energy band gap (Eg) or energy gap. 

Conduction band is the energy band which includes the energy levels of conduction electrons or free electrons. 

Valence band is the energy band which includes the energy levels of the valence electrons. It is the range of energies possessed by valence electrons. 

Energy band is a group of close by energy levels with continuous energy variation. 

(b), (c), (d)

(b) At V_i = 1V , it can be used as an amplifier.

(c) At V_i = 0..5V, it can be used as a switch turned off.

(d) At V_i = 2.5V, it can be used as a switch turned on.

1. Metals – electrons

Semiconductor – Electrons and holes 

2. Straight line graph 

3. Slope gives conductance

4. When the temperature increases, resistance is also increased and hence slope decreases.

Logic gates are basic building blocks of digital electronics, which process the digital signals in a specific manner. Logic gates are used in calculators, digital watches, computers, robots, industrial control systems, and in telecommunications. 

Logic gates follows curtain logical relationship between the input and output voltages. Therefore, they are generally known as logic gates — gates because they control the flow of information. The five common logic gates used are NOT, AND, OR, NAND, NOR. Each logic gate is indicated by a symbol and its function is defined by a truth table that shows all the possible input logic level combinations with their respective output logic levels. Truth tables help understand the behaviour of logic gates. These logic gates can be realised using semiconductor devices. 

Ge and Si are examples for semiconductors and have diamond like lattice structure. In its crystalline structure, every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest neighbour atoms, and also to take share of one electron from each such neighbour. These shared electron pairs are referred to as forming a covalent bond or simply a valence bond. 

As the temperature increases, more thermal energy becomes available to these electrons and some of these electrons may break–away (becoming free electrons contributing to conduction). The thermal energy effectively ionises only a few atoms in the crystalline lattice and creates a vacancy in the bond. The neighbourhood, from which the free electron (with charge –q) has come out leaves a vacancy with an effective charge (+q). This vacancy with the effective positive electronic charge is called a hole. The hole behaves as an apparent free particle with effective positive charge. 

In intrinsic semiconductors, the number of free electrons, n_e is equal to the number of holes, n_h. That is n_e = n_h = n_i where n_i is called intrinsic carrier concentration. Under the action of an electric field, these holes move towards negative potential giving the hole current, I_h. The total current, I is thus the sum of the electron current I_e and the hole current I_h: 

I = I_e + I_h ----------- (2) 

It may be noted that apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers. The recombination occurs due to an electron colliding with a hole.  

Classification of solids on the basis of energy bands : 

Various semiconductors available in practice could be: 

(i) Elemental semiconductors: Si and Ge 

(ii) Compound semiconductors: 

Examples are: 

• Inorganic: CdS, GaAs, CdSe, InP, etc. 

• Organic: anthracene, doped pthalocyanines, etc. 

• Organic polymers: polypyrrole, polyaniline, polythiophene, etc. 

(a) Metallic Conductors

(b) Insulators

(c) Semiconductors

It is easier to observe the change in the current with change in the light intensity, if a reverse bias is applied. Thus photodiode can be used as a photodetector to detect optical signals. 

Various semiconductors available in practice could be: 

(i) Elemental semiconductors: Si and Ge 

(ii) Compound semiconductors: 

Examples are: 

• Inorganic: CdS, GaAs, CdSe, InP, etc. 

• Organic: anthracene, doped pthalocyanines, etc. 

• Organic polymers: polypyrrole, polyaniline, polythiophene, etc. 

Most of the currently available semiconductor devices are based on elemental semiconductors Si or Ge and compound inorganic semiconductors. However, after 1990, a few semiconductor devices using organic semiconductors and semiconducting polymers have been developed signaling the birth of a futuristic technology of polymer electronics and molecular-electronics. 

Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. 

These different energy levels with continuous energy variation form what are called energy bands. The energy band which includes the energy levels of the valence electrons is called the valence band. The energy band above the valence band is called the conduction band. With no external energy, all the valence electrons will reside in the valence band. 

Oscillators are used to produce sinusoidal signal of any frequency including radio frequency signals. Hence they are used in electronic communication both at transmitter and receiver side.

It is a photovoltaic cell which is basically a p-n junction which generates emf when solar radiation falls on it. 

The generation of emf by a solar cell, when light falls on, it is due to the following three basic processes: generation, separation and collection— 

(i) generation of e-h pairs due to light (with h_n > E_g) close to the junction; 

(ii) separation of electrons and holes due to electric field of the depletion region. Electrons are swept to n-side and holes to p-side; 

(iii) the electrons reaching the n-side are collected by the front contact and holes reaching p-side are collected by the back contact. Thus p-side becomes positive and n-side becomes negative giving rise to photovoltage. 

An oscillator is an electronic circuit in which we get ac output of desired frequency without any external input signal. In other words, the output in an oscillator is self-sustained. 

A logic gate is a digital circuit that follows certain logical relationship between the input and output signals and works on the principles of Boolean algebra. 

A signal (current or voltage) which takes only discrete values is called digital signal.  

An electrical signal (current or voltage) which varies continuously with time is called an analogue signal. 

Positive feedback.  

When the transistor is in cut off state it works as an open switch. 

Eg of a photodiode

E_g = hC/λ_m 

Phosphorus can be used with germanium to form an n-type semiconductor. 

(i) The width of the depletion layer becomes small. 

(ii) The junction electric field becomes large. 

The impurity atoms added to pure semiconductors like germanium to increase their electrical conductivity are called dopants. 

The majority charge carriers in p-type semiconductors is holes.

No, because the voltmeter must have a resistance very high compared to the junction resistance, the latter being nearly infinite.

(i) 10 × 20 × 30 × 10^–3 = 6V

(ii) If dc supply voltage is 5V, the output peak will not exceed Vcc = 5V.

Hence, V₀ = 5V.

Insulators:

The valence band and the conduction band are separated by a large energy gap. The forbidden energy gap is approximately 6 eV in insulators. The gap is very large that electrons from valence band cannot move into conduction band even on the application of strong external electric field or the increase in temperature. Therefore, the electrical conduction is not possible as the free electrons are almost nil and hence these materials are called insulators. Its resistivity is in the range of 10¹¹- 10¹⁹ Ωm.

Metals:

In metals, the valence band and conduction band overlap. Hence, electrons can move freely into the conduction band which results in a large number of free electrons in the conduction band. Therefore, conduction becomes possible even at low temperatures. The application of electric field provides sufficient energy to the electrons to drift in a particular direction to constitute a current. For metals, the resistivity value lies between 10^-2 and 10^-8 Ωm.

Semiconductors:

In semiconductors, there exists a narrow forbidden energy gap (E_g < 3 eV ) between the valence band and the conduction band. At a finite temperature, thermal agitations in the solid can break the covalent bond between the atoms (covalent bond is formed due to the sharing of electrons to attain stable electronic configuration). This releases some electrons from valence band to conduction band. Since free electrons are small in number, the conductivity of the semiconductors is not as high as that of the conductors. The resistivity value of semiconductors is from 10^-5 to 10⁶ Ωm.

• Solar cells are widely used in calculators, watches, toys, portable power supplies, etc.
• Solar cells are used in satellites and space applications 
• Solar panels are used to generate electricity

A solar cell, also known as photovoltaic cell, converts light energy directly into electricity or electric potential difference by photovoltaic effect.

Barkhausen conditions for sustained oscillations The following condition called Barkhausen conditions should be satisfied for sustained oscillations in the oscillator

• The loop phase shift must be 0° or integral multiples of 2π. 
• The loop gain must be unity. |Aβ| = 1 Here; A → Voltage gain of the amplifier, b → feedback ratio; (fraction of the output that is fed back to the input)

Applications of oscillators:

• to generate a periodic sinusoidal or non sinusoidal wave forms. 
• to generate RF carriers. 
• to generate audio tones 
• to generate clock signal in digital circuits. 
• as sweep circuits in TV sets and CRO.

• Alarm system 
• Count items on a conveyer belt 
• Photoconductors 
• Compact disc players, smoke detectors 
• Medical applications such as detectors for computed tomography etc.

(i) Increase.

As the value of the base current increases, the collector current will increases proportionally.

(ii) Increases

Due to the increase in collector current, voltage drop across lamp will increase.

The forward resistance of an ideal p-n junction diode is Zero.

(c) NOR gate

Correct answer is (a) 101

• If the positive terminal of the external voltage source is connected to the pside and the negative terminal to the nside, it is called forward bias. 
• If the positive terminal of the battery is connected to the n-side and the negative potential to the p-side, the junction is said to be reverse biased. 

Biasing means providing external energy to charge carriers to overcome the barrier potential and make them move in a particular direction. The external voltage applied to the pn junction is called bias voltage.

At room temperature, a potential difference equal to the barrier potential is required before a reasonable forward current starts flowing across the diode. This voltage is known as threshold voltage or cut-in voltage or knee voltage (V_th).

Correct Answer: B) High potential at N side and low potential at P side

(b) β = \(\frac{a}{1-a}\)

Corrrect answer is (c) semiconductor

(a) positive feedback

When a transistor is used as an amplifier with positive feedback, it works as an oscillator

Correct answer is (a) logic

(b) 60 dB

Voltage gain in dB = 20 log₁₀ A = 20 log₁₀ (1000) = 20 x 3 = 60 dB.

(i) Emitter base junction is forward biased whereas base collector junction is reverse biased. 1,

(ii) Small change in the current Iu in the base circuit controls the larger current I. in the collector circuit. I_c = βIn

(iii) Elemental semiconductor's band gap is such that the emitted wavelength lies in IR region. Hence cannot be used for making LED.

Correct answer is (c) insulator

(c) insulator

At absolute zero, Si acts as an insulator due to the absence of free electrons in the conduction band.