Semiconductor Electronics: Materials,Devices And Simple Circuits - Class 12 Engineering Physics - Extra Questions
What is the energy band gap of:
(i) silicon and (ii) germanium
What is a hole? Which type of doping creats a hole?
Identify the logic gate whose truth table is given below and draw its symbol:
| $$A$$ | $$B$$ | $$Y$$ |
| $$0$$ | $$0$$ | $$1$$ |
| $$0$$ | $$1$$ | $$1$$ |
| $$1$$ | $$0$$ | $$1$$ |
| $$1$$ | $$1$$ | $$0$$ |
The logic gate is NAND gate.
Draw the truth table of a NOR gate.
Which gates are called universal gates?
The use of zener diode is as ________.
What is a NAND gate? Give its logic symbol.
NAND gate is a Boolean operator which gives the value zero if and only if all the operands have a value of one, and otherwise has a value of one (equivalent to NOT AND)
Write one main use of Zener diode.
What is Zener diode? How is it used as a voltage regulator?
$$\textbf{Explanation:}$$
$$\bullet$$When a Zener diode is biased in the forward direction, it behaves like a conventional signal diode, with current increasing linearly with voltage,
$$\bullet$$When a reverse voltage is applied across the diode and it reaches the device's breakdown voltage, a massive current begins to flow through the diode.
$$\bullet$$Shunt Voltage Regulators like Zener Diodes are commonly used to control voltage across small loads. The reverse breakdown voltage of Zener diodes is acute, and the breakdown voltage remains constant over a wide range of currents.
$$\bullet$$As a result, the zener diode will be connected parallel to the load, causing the applied voltage to reverse bias it. The voltage across the load will remain constant if the reverse bias voltage across the zener diode exceeds the knee voltage.
$$\bullet$$When a Zener diode is biased in the forward direction, it behaves like a conventional signal diode, with current increasing linearly with voltage,
$$\bullet$$When a reverse voltage is applied across the diode and it reaches the device's breakdown voltage, a massive current begins to flow through the diode.
$$\bullet$$Shunt Voltage Regulators like Zener Diodes are commonly used to control voltage across small loads. The reverse breakdown voltage of Zener diodes is acute, and the breakdown voltage remains constant over a wide range of currents.
$$\bullet$$As a result, the zener diode will be connected parallel to the load, causing the applied voltage to reverse bias it. The voltage across the load will remain constant if the reverse bias voltage across the zener diode exceeds the knee voltage.
Construct a AND gate using only NOR gates only
REF.Image.
AND gate using NOR gate:-
proof:-
$$ \overline{A+A} = \bar{A}$$ similarly,$$ \overline{B+B} = \bar{B}$$
so, $$ \overline{\bar{A}+\bar{B}} = \overline{\overline{(A.B)}}$$ (Demorgain law)
$$ = A.B$$
A coil a current of $$1.0$$ ampere and a power of $$100$$. Find the Resistance.
Which device is used as a voltage regulator ?
Distinguish between 'paramagnetic' and 'ferromagnetic' substances.
Name the logic gate which generates high output when at least one input is high.
The number of silicon atoms per $$m^3$$ is $$5\, \times\, 10^{28}$$. This is doped simultaneously with $$5\, \times\, 10^{22}$$ atoms per $$m^3$$ of arsenic and $$5 \times\, 10^{20}$$ per $$m^3$$ atoms of indium. Calculate the number of electrons and holes. Given that $$n_i\, =\, 1.5\, \times\, 10^{16}\ /m^{3}$$. Is the material n-type or p-type?
Write the truth table for a NAND gate connected as given in Figure.
You are given two circuits as shown in Fig., which consist of NAND gates. Identify the logic operation carried out by the two circuits.
What happens when a forward bias is applied to a p-n junction?
When a p-n junction diode is forward biased, i.e. forward biasing voltage V is applied to the diode, this potential difference reduces the barrier potential $$V_{o}$$. The effective barrier potential reduces to $$(V_{o}-V)$$ and thickness of the depletion layer also decreases. The holes (majority carriers) in p-region and electrons (majority carriers) in n-junction acquire sufficient energy to cross over the potential barrier across the junction.
Identify the gates P and Q shown in figure. Write the truth table for the combination of the gates shown.
Name of equivalent gate representing this circuit and write its logic symbol
Here the gate $$P$$ is $$AND$$ gate and gate $$Q$$ is $$NOT$$ gate. The truth table for the combination of the gates is as shown in the figure.
The equivalent gate will be a $$NAND$$ gate and its truth is shown in the figure.
(a) Explain briefly the principle on which a transistor-amplifier works as an oscillator. Draw the necessary circuit diagram and explain its working.
(b) Identify the equivalent gate for the following circuit and write its truth table.
(a) Principle on which a transistor-amplifier works as an oscillator-
The transistor-amplifier oscillatory circuit consists of an inductive coil of inductance L connected in parallel with a capacitor of capacitance C. The frequency of oscillation in the circuit depends upon the values of L and C. The actual frequency of oscillation is the resonant.
The transistor-amplifier oscillatory circuit receives dc power from the battery and converts it into ac power for supply to the circuit. The oscillations occurring in the circuit are applied to the input of the electronic amplifier and due to amplifying properties of the amplifier, we get increased output of these oscillations which can be supplied to the circuit to meet the losses.
( b) The equivalent gate for the following circuit is AND gate and below is its Truth Table
(a) Describe briefly, with the help of a diagram, the role of the two important processes involved in the formation of a $$p-n$$ junction.
(b) Name the device which is used as a voltage regulator. Draw the necessary circuit diagram and explain its working.
(i) Diffusion
(ii) Drift
When p and n sides comes in contact then majority carries diffuse from high concentration to low concentration. Electrons flow from n to p junction and combines with holes near the junction. Similarly holes flow from p-side to n side and combine with electrons near the junction. As this happens, region near the junction gets depleted of carriers and gets charged as shown in figure. This diffusion of carriers leads to diffusion current.
The charge in depletion region creates electric field. This electric field leads to flow of minority carriers across depletion region. This is called the drift current.
The diffusion and drift currents are opposite in direction.
In equilibrium, they are equal to each other and p-n junction is formed.
(B) Zener diode is used as a voltage regulator.
The unregulated voltage is connected to zener diode through a series resistance such that the zener diode is reversed biased. If the input voltage increases, the current through the zener diode and resistance increases.
Thus the voltage drop over resistance increases without the change in voltage drop over zener diode. This is because in the breakdown region, zener voltage remain same irrespective of the current.
Similarly, if the input voltage decreases, voltage and current over resistance decreases without change in voltage across the zener diode.
How can electrical conductivity of metals, insulator and semiconductors be explained qualitatively?
What is the use of a Zener diode?
With the help of a diagram, show how you can use several NAND gates to obtain an OR gate.
We can use several NAND gates to obtain an OR gate using the above setup.
The Truth Table of the above setup is as shown below:| A | B | $$\overline A$$ | $$\overline B$$ | Y |
| 0 | 0 | 1 | 1 | 0 |
| 0 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 1 |
| 1 | 1 | 0 | 0 | 1 |
Write the names of two universal logic gates. What is integrated circuit? Write two advantages of it.
An incandescent lamp is operated at $$240 V$$ and the current is $$0.5A$$. What is the resistance of the lamp?
Write the names of logic gates related to figure $$P$$ and table $$Q$$.
Table:$$Q$$
| $$A$$ | $$B$$ | $$Y$$ | $$Y'$$ |
| $$0$$ | $$0$$ | $$0$$ | $$1$$ |
| $$0$$ | $$1$$ | $$1$$ | $$0$$ |
| $$1$$ | $$0$$ | $$1$$ | $$0$$ |
| $$1$$ | $$1$$ | $$1$$ | $$0$$ |
Find the output $$F$$ of the logic circuit given below.
Output of $$1st$$ OR gate $$= A + B$$
Output of $$2nd$$ OR gate $$= A + C$$
Output of AND gate $$= (A + B).(A + C)$$
$$= AA + BA + AC + BC$$
$$= A(1 + B + C) + BC$$
$$= A(1) + BC$$
$$= A + BC$$
Output of $$2nd$$ OR gate $$= A + C$$
Output of AND gate $$= (A + B).(A + C)$$
$$= AA + BA + AC + BC$$
$$= A(1 + B + C) + BC$$
$$= A(1) + BC$$
$$= A + BC$$
Draw the circuit diagram of a voltage regulator using zener diode.
REF.Image.
Zender diode as a voltage regulator
(a) NAND gates are also called universal gate. Why?
(b) Draw a logic symbol of OR gate.
(c) Write the value of output $$y$$ in the given circuit:
(d) Write the name of the diode used in voltage regulation.
(a) The NAND gate and the NOR gate can be said to be universal gates since combinations of them can be used to accomplish any of the basic operations.
(b) The logic symbol of OR gate is as shown in the figure.
(c) When same input is given to both input terminals of NAND gate, it acts as a NOT gate for the signal. Hence the output is 0.
(d) Zener diode is used in voltage regulation.
In which bias can a zener diode be used as voltage regulator?
What are universal gates? Why are they called so ?
A light bulb is rated at $$100 W$$ for a $$220 V$$ supply. Find the resistance of the bulb.
Write the logic symbol and truth table of NAND gate.
In digital electronics, a NAND gate which is a negative-AND gate, is a logic gate which produces an output which is false only if all its inputs are true. Thus its output is complement to that of the AND gate.
How will you obtain OR, AND gates from the NAND and NOR gates? Write symbol, Boolean formula and truth table.
(i) To obtain AND gates from NAND gates:
For obtaining AND gate from NAND gate the output of NAND gate is connected to the input (obtained from NAND gate by joining the two inputs).
For obtaining OR gate from NAND gate the inputs $$A$$ and $$B$$ are connected to the two NOT gates obtained from NAND gates. The input get inverted as $$\overline { A }$$ and $$ \overline { B }$$. These inputs are then fed to the NAND gate as shown in fig. drawn below.
Boolean formual: $$y=\overline { y } =\overline { \overline { A\cdot B } } =A\cdot B$$
(ii) To obtain OR gate from NAND gate:
For obtaining OR gate from NAND gate the inputs $$A$$ and $$B$$ are connected to the two NOT gates obtained from NAND gates. The input get inverted as $$\overline { A }$$ and $$ \overline { B }$$. These inputs are then fed to the NAND gate as shown in fig. drawn below.
Boolean formual: $$y=\overline { y } =\overline { \overline { A\cdot B } } =A\cdot B$$
| Input | Output |
| A | B | y | y |
| 0 1 0 1 | 0 0 1 1 | 1 1 1 0 | 0 0 0 1 |
For obtaining OR gate from NAND gate the input $$A$$ and $$B$$ are connected to the two NOT gates obtained from NAND gates. The input gate inverted as $$\overline { A }$$ and $$ \overline { B }$$. These inputs are then fed to the NAND gate as shown in fig, drawn belows.
Boolean formula: $$ y=\overline { \overline { A } } \overline { \overline { B } } =\overline { \overline { A } } +\overline { \overline { B } } =A+B$$
(iii) To obtain OR gate from NOR gate:
Boolean formula: $$ y=\overline { \overline { A } } \overline { \overline { B } } =\overline { \overline { A } } +\overline { \overline { B } } =A+B$$
| Input | Output |
| A 0 1 0 1 | B 0 0 1 1 | $$\overline { A }$$ 1 0 1 0 | $$ \overline { B }$$ 1 1 0 0 | y 0 1 1 1 |
To obtain OR gate from NOR gate the inputs are connected to NOR gate which in turn is again connected to NOR gate.
(iv) To obtain AND gate from NOR gate:
| Input | Output |
| A | $$\overline { A }$$ |
| 0 1 | 1 0 |
To obtain AND gate from NOR gate the inputs are connected to NOT gate obtained from NOR gate giving inverted $$ A\left( \overline { A } \right) $$ and inverted $$B\left( \overline { B } \right)$$.
Which is inturn are again connected is NOR gate.
Boolean formula: $$ \overline { y } =\overline { A } +\overline { B } =\overline { \overline { A } +\overline { B } } =A\cdot B$$
Which is inturn are again connected is NOR gate.
Boolean formula: $$ \overline { y } =\overline { A } +\overline { B } =\overline { \overline { A } +\overline { B } } =A\cdot B$$
By using Bolean Algebra prove that $$\bar{A}B + A\bar{B} + AB = A +B$$.
In the given logic circuit, name the logic gates 1 and 2 .
(a) Write the name of the combination of the gates.
Logic gate 1 is a $$"OR"$$ gate.
Logic gate 2 is a $$"NOT"$$ gate.
There combination is a $$"NOR"$$ gate.
Explain the formation of energy bands in solids. On the basis of energy bands distinguish between a metal, a semiconductor and an insulator.
$$\text{Formation of energy bands in solid:}$$
In a single isolated atom, the electrons in each orbit have definite energy associated with it. But in the case of solids all the atoms are close to each other, so the energy levels of outermost orbit electrons are affected by the neighbouring atoms.
When two single or isolated atoms are brought close to each other than the outermost orbit. The electrons in the outermost orbit of one atom experience an attractive force from the nearest or neighbouring atomic nucleus. Due to this, the energies of the electrons will not be at the same level, the energy levels of electrons are changed to a value which is higher or lower than that of the original energy level of the electron.
The electrons in the same orbit exhibit different energy levels.The grouping of this different energy levels is called energy band.
However, the energy levels of inner orbit electrons are not much affected by the presence of neighbouring atoms.
$$\text{Insultaor:}$$ Insulator is material, in which there is a large energy gap between the conduction band and the valence band, A large amount of energy is required to shift the electron from valence band to conduction band.
$$\text{Semiconductor:}$$Semiconductor is the material, in which there is a less energy gap required with respect to the insulator, When some amount of energy flow in the semiconductor, then it allows to flow the electron from valence band to conduction band.
$$\text{Conductior:}$$Conductor is the material in which, there is no energy gap between the conduction band and valence band, so no extra energy is needed to flow the electron from valence band to conduction band.
How is Zener diode used as voltage regulator?
Zener Diodes are widely used as Shunt Voltage Regulators to regulate the voltage across small loads. Zener Diodes have a sharp reverse breakdown voltage and breakdown voltage will be constant for a wide range of currents. Thus we will connect the zener diode parallel to the load such that the applied voltage will reverse bias it. Thus if the reverse bias voltage across the zener diode exceeds the knee voltage, the voltage across the load will be constant.
Draw a diagram of show how NAND gates can be combined to obtain an OR gate.(Truth table is not required).
OR gate can be formed using NAND gate as per the above circuit.
What is optoelectronic junction devices? Write any two names of optoelectronic junction devices.
Draw separate energy band diagrams for conductors, semi-conductors and insulators and label each of them.
Energy band diagrams for conductor, semiconductor and insulator are shown above.
In conductors, both the valance band and conduction band overlap each other. So there is zero band gap in a conductor.
In semiconductor, there is a small band gap approximately of $$1 \ eV$$.
In an insulator, there is a large band gap of nearly $$5 \ eV$$.
In conductors, both the valance band and conduction band overlap each other. So there is zero band gap in a conductor.
In semiconductor, there is a small band gap approximately of $$1 \ eV$$.
In an insulator, there is a large band gap of nearly $$5 \ eV$$.
What is the output of the combination?
the inputs are 1 and 0, hence, the output will be 0
In the NOR gate:
the inputs are again 1 and 0, if it were an OR gate, the output value would have been 1, but since this is a NOR gate, the output will be 0
In the NAND gate:
the inputs are 0 and 0, if it were an AND gate, the output would have been 0, but since it is a NAND gate, the output will be 1.
Hence the output of the given combination will be 1.
$$100\ mg$$ mass of nichrome metal is drawn into a wire of area of cross-section $$0.05{mm}^{2}$$. Calculate the resistance of this wire. Given density of nichrome $$8.4\times {10}^{3}$$ $$kg{m}^{-3}$$ and resistivity of the material as $$\rho =1.2\times { 10 }^{ -6 }\Omega m$$.
If the measured value of resistance $$R = 1.05 \Omega $$ wire diameter $$d = 0.60 mm$$, and length $$l = 75.3 cm$$, then find the maximum permissible error in resistivity, $$\rho = \frac{R(\pi d^2 /4)}{l}$$
Mention practical importance of Zener diode in a laboratory.
In a zener-regulated power supply a zener diode with $$V_{z}=6 $$ volt is used for regulation. The load current is to be $$4\ mA$$ and the unregulated input is $$10$$ volt. What should be the value of series resistor $$R_{s}$$ if the current through diode is five times the load current?
Find out output for given logic gate:
The figure shows the input waveforms A and B for 'AND' gate.
Draw the output waveform and write the truth table for this logic gate.
Three resistors each of $$2 \Omega$$ are connected together so that their total resistance is $$3 \Omega$$. Draw a diagram to show this arrangement and check it by calculation.
What is internal resistance and ideal voltmeter?
Draw energy band diagrams of $$n$$ -type and $$p$$ -type semiconductors at temperature $$\mathrm { T } > 0$$ K. Mark the donor and acceptor energy levels with their energies .
Given the ratio of the numbers of holes and the number of conduction electrons in an intrinsic semiconductor.
(a) What are logic gates?
(b) Draw a circuit diagram for dual-input AND Gate by using two diodes.
(a) logical gates: A gate is a digital circuit that is designed for performing a particular logical operation. As it work according to some logical relationship b/w input & output voltages, so it is generally known as a logical gates.
(b) Circuit diagram of AND gate by using two diodes.(Refer...image)
(b) Circuit diagram of AND gate by using two diodes.(Refer...image)
Why are NOR gates considered as universal gates?
Both gates are formed out of NOR gates:-
Draw the symbol of NAND gate and write down truth table for this. Why is it called a universal gate?
It is called universal gate, as it can be used to accomplish any operation & can provide inverter an OR gate or an AND gate.
The circuit shown below is working as a $$8\ V\ dc$$ regulated voltage source. When $$12\ V$$ is used as input, the power dissipated (in $$mW$$) in each diode is; (considering both zener dioses are identical)____________
Current $$i = \dfrac{12 - 8}{200 + 200}$$
$$= \dfrac{4}{400} = 10^{-2} A$$
Potential drop across each
diode $$= \dfrac{8}{2} = 4V$$
Power loss in each diode $$= VI = 4 \times 10^{-2} = 40 mW$$
Let $$\Delta \,E$$ denote the energy gap between the valence band and the conduction band. The population of conduction electrons (and of the holes) is roughly proportional to $$e^{-\Delta \, E /2kT}$$. Find the ratio of the concentration of conduction electrons in diamond to that in silicon at a room temperature 300 K. $$\Delta \,E$$ for silicon is $$1\cdot1\, eV$$ and for diamond is $$6\cdot 0\, eV$$.
The output of a $$ 2$$-input AND gate is fed to a NOT gate. Give the name of the combination and its logic symbol. Write down its truth table.
Name of combination : NAND gate logic symbol.
Write the truth table for the logic circuit shown below and identify the logic operation performed by this circuit.
The logic circuit perform OR-operation.
Truth Table
Truth Table
| $$ A $$ | $$ B $$ | $$ Y $$ |
| $$ 0 $$ | $$ 0 $$ | $$ 0 $$ |
| $$ 1 $$ | $$ 0 $$ | $$ 1 $$ |
| $$ 0 $$ | $$ 1 $$ | $$ 1 $$ |
| $$ 1 $$ | $$ 1 $$ | $$ 1 $$ |
Draw the necessary energy band diagrams to distinguish between conductors, semiconductors and insulators. How does the change in temperature affect the behaviour of these materials ? Explain briefly.
Energy bands : In a solid , the energy of electrons lie within certain range. The energy levels of allowed energy are in the form of bands, these bands are separated by regions of forbidden energy called band gaps.
Distinguish features :
(a) In conductors : valence band and conduction band overlap each other.
In semiconductors : Valence band and conduction band are separated by a small energy gap.
In insulators : They are separated by a large energy gap.
(b) In conductors : Large number of free electrons are available in conduction band.
In semiconductor : A very small number of electrons are available for electrical conduction.
In insulators : Conduction band is almost empty i.e., no electron is available for conduction.
Effect of temperature :
(i) In conductors : At high temperature , the collisions of electrons become more frequent with the atoms / molecules at lattice site in the metals as a result the conductivity decreases (or resistivity increases).
(ii) In semiconductors : As the temperature of the semiconducting material increases, more electrons hole pairs becomes available in the conduction band and valence band , and hence the conductivity increases or the resistivity decreases.
(ii) In insulators : The energy band between conduction band and valence band is very large , so it is unsurpassable for small temperature rise. So , there is no change in their behaviour.
Using truth table of AND gate and NOT gate , show that NAND gate is an AND gate followed by a NOT gate. Hence write the truth table of NAND gate. Why are NAND gates called 'Universal Gates' ?
Truth Table of NAND Gate
All primary gates likes AND , NOT and OR gates can be realised by using NAND gate only. Hence , it is a universal gate.
| $$ A $$ | $$ B $$ | Output of NAND Gate |
| $$ 0 $$ | $$ 0 $$ | $$ 1 $$ |
| $$ 1 $$ | $$ 0 $$ | $$ 1 $$ |
| $$ 0 $$ | $$ 1 $$ | $$ 1 $$ |
| $$ 1 $$ | $$ 1 $$ | $$ 0 $$ |
All primary gates likes AND , NOT and OR gates can be realised by using NAND gate only. Hence , it is a universal gate.
The following table gives the output of a two input logic gate.
If the output of this gate is fed as input to a NOT gate, name the new logic gate so formed.
| $$ A $$ | $$ B $$ | $$ Y $$ |
| $$ 0 $$ | $$ 0 $$ | $$ 1 $$ |
| $$ 1 $$ | $$ 0 $$ | $$ 1 $$ |
| $$ 0 $$ | $$ 1 $$ | $$ 1 $$ |
| $$ 1 $$ | $$ 1 $$ | $$ 0 $$ |
Clearly resulting gate will be an AND gate.
Write the truth table for the circuit shown in figure. Name the gate that the circuit resembles.
The circuit resembles AND gate. The Boolean expression of this circuit is $${V}_{0}=A.B$$ ie., $${V}_{0}$$ equals $$A$$ AND $$B$$
Identify the logic gate represented by the following circuit by writing its truth table :
This is a Exclusive-OR gate
Answer the following questions.
Show the output waveforms (Y) for the following inputs A and B of
(i) OR gate (ii) NAND gate
Output waveforms for the following inputs A and B of OR gate and NAND gate.
Why is a Zener diode considered as a special purpose semiconductor diode ?
Draw the I-V characteristics of a Zener diode and explain briefly how reverse current suddenly increases at the breakdown voltage.
Describe briefly with the help of a circuit diagram how a Zener diode works to obtain a constant dc voltage from the unregulated dc output of a rectifier.
A Zener diode is considered as a special purpose semiconductor diode because it is designed to operate under reverse bias in the breakdown region.
Zener diode is fabricated by heavy doping of its p and n sections. Since doping is high, depletion layer becomes very thin.
Hence , electric field $$ \left ( = \dfrac{v}{l} \right ) $$ becomes high even for a small reverse bias.
(Refer Fig.1) We know that reverse current is due to the flow of electrons (minority carriers) from $$ p \rightarrow n $$ and holes from $$ n \rightarrow p $$ . As the reverse bias voltage is increased, the electric field at the junction becomes significant . When the reverse bias voltage $$ V = V_z $$, then the electric field strength is high enough to pull valence electrons from the host atoms on the p-side which are accelerated to n-side. These electrons causes high current at breakdown.
Working :
The unregulated dc voltage output of a rectifier is connected to the Zener diode through a series resistance $$ R_s $$ such that the Zener diode is reverse biased. Now any increase / decrease in the input voltage results in increase / decrease of the voltage drop across $$ R_s $$ without any change in voltage across the Zener diode. Thus , the Zener diode acts as a voltage regulator.
Explanation of voltage regulator : (refer Fig.2)
If reverse bias voltage V reaches the breakdown $$ V_z $$ of Zener diode , there is a large change in the current . After that (just above $$ V_z $$ there is a large diode , in the current by almost insignificant change in reverse bias voltage. This means diode voltage remains constant.
For example: If unregulated voltage is supplied at terminals A and B , and input voltage increases , the current through resistor $$ R_z $$ and diode also increases. This current increases the voltage across $$ R_z $$ without any change in the voltage across diode. Thus we have a regulated voltage across load resistor $$ R_L $$.
Explain how a Zener diode maintains a constant voltage across a load.
In an intrinsic semiconductor the energy gap $$E_g$$ is $$1.2\ eV$$. Its hole mobility is much smaller than electron mobility and independent of temperature. What is the ratio between conductivity at $$600\ K$$ and that at $$300\ K$$? Assume that the temperature dependence of intrinsic carrier concentration $$n_i$$ is given by
$$n_i = n_o exp \left(- \dfrac{E_g}{2 k_B T} \right)$$
Where $$n_0$$ is a constant.
What happens to the potential barrier of a p-n junction diode when it is?
Draw the circuit diagram of OR-gate using diodes.
Explain the variation of conductivity with temperature for (i) a metallic conductor (ii) Ionic conductors and (iii) semiconductors
Conductivity of a metalic conductor $$\sigma=^{\dfrac{1}{p}=\dfrac{ne^2\tau}{m}}$$
Where m=mass of charge carrier,e=charge on each carrier.
T=relaxation time, n=number density of chargecarries.
i)With rise of temperatue, the collision of electrons with fixed lattice ions/atoms increases so that relaxation time (T) decreases.Consequently, the conductivity of metals decreases with rise of temperature.
ii) Conductivity of ionic conductor increases with increase of temperature because with increase of temperature, the ionic bonds break releasing positive and negative ions which are charge carriers in ionic conductors.
iii) In the case of a semiconductors, when temperature increases, covalent bonds break and charge carriers (electrons and holes) becomes free i.e. n increases, so conductivity increases with rise of temperature.
Where m=mass of charge carrier,e=charge on each carrier.
T=relaxation time, n=number density of chargecarries.
i)With rise of temperatue, the collision of electrons with fixed lattice ions/atoms increases so that relaxation time (T) decreases.Consequently, the conductivity of metals decreases with rise of temperature.
ii) Conductivity of ionic conductor increases with increase of temperature because with increase of temperature, the ionic bonds break releasing positive and negative ions which are charge carriers in ionic conductors.
iii) In the case of a semiconductors, when temperature increases, covalent bonds break and charge carriers (electrons and holes) becomes free i.e. n increases, so conductivity increases with rise of temperature.
$$Sn,C$$ and $$Si,Ge$$ are all group XIV elements. Yet, $$Sn$$ is a conductor, $$C$$ is an insulator while $$Si$$ and $$Ge$$ are semiconductors. Why?
Draw the circuit used for voltage regulation in a Zener diode and also explain its process in short.
Zener DiodeIn reverse biasing arrangement of P-N junction diode we saw that in the state of breakdown voltage the reverse current of the diode is not less but increases rapidly, but the potential at the ends of the diode remains approximately equal to the breakdown voltage meaning remains constant.
For any diode; breakdown voltage depends on the number of impurities mixed. Therefore, according to the quantity of the impure element any special breakdown voltage diode's fabrication can be done. When any diode is so formed for any special breakdown voltage that in reverse biasing also at breakdown voltage it works without being damaged; then such type of diode is called Zener diode. With the help of various quantities of impurity mixed today the diodes are available of less than 1 volt and in hundreds volt also. The sign of Zener diode is shown in the figure (16.27).
When the current flowing through the breakdown point from the Zener diode is increased highly still the potential difference at both its ends is constant. This property makes its use for voltage stabilization and voltage regulation. We know that in an ordinary dc power supply with the use of rectifier and equalizer filter, ac is changed to de. In such type of power supplies when there is change in load current, the value of do voltage also changes. Therefore, in such situations where current is to be obtained at constant dc voltage such power supply cannot be used. With the help of a Zener diode the output voltage of a power supply can be kept constant.
Figure (16.28) shows a power supply whose output voltage is assumed to be $$ V_{i} $$ and resistance $$ \mathrm{R}_{\mathrm{s}} $$ and a Zener diode are connected at its output terminals. A load resistor $$ \mathrm{R}_{\mathrm{L}} $$ is parallel to the Zener diode. The Zener diode is so selected whose breakdown voltage $$ V_{2} $$ is equal to the constant value of that dc voltage which isto be obtained at load resistor R_. Zener diode is so placed in the circuit that it is in reverse biasing state. If the value of $$ V_{i} $$ is more than $$ V_{z} $$ then the diode comes in breakdown region and the potential $$ V_{z} $$ remains at its ends. since $$ \mathrm{R}_{\mathrm{L}} $$ is parallel to the diode, hence on it also there would be $$ \mathrm{V}_{2} $$ potential. If the value of $$ \mathrm{R}_{\mathrm{L}} $$ is changed then also it will have $$ \mathrm{V}_{2} $$ potential. In this way the output voltage at both the ends of a Zener diode is stable. The resistance $$ R_{s} $$ is so selected that the diode is not damaged by the thermal energy due to the increase in the Zener current. This value of current is acknowledged by the datasheet given by the Zener diode manufacturing company.
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