Physics M.C.Q [1M]

3. n = p

Explanation:

Intrinsic semiconductors are pure ones. Hence there is no question of impurity. Valence band is the range of energy of electrons which are in the valence shell of the atoms of a substance. Conduction band is the range of energy of electrons which are free and available for conduction.

At room temperature, some electrons of intrinsic semiconductors get excited and reach the conduction band. During this process, the electrons leave empty spaces in the valence shell. These empty spaces are known as holes in a semiconductor. Number of free electrons (n) is equal to the number of holes (p).

2. Statement 1-is true, Statement -2 is true and Statement -2 is correct explanation of Statement-1.

Explanation:

NAND or NOR gates are called universal gates or digital building blocks. Also, the repeated use of NAND (or NOR) gates can produce all the basis or any complicated gates can be formed using NAND (or NOR) gates. Hence, statement 1-is true statement -2 is true statement -2 is correct explanation of statement-1.

2. Good conductor

Explanation:

In good conductors, the valance band and the conduction band overlap each other resulting in a zero band gap energy. Whereas, bandgap in insulator is nearly 5 eV and that in semiconductors is nearly 1.1eV.

Hence the given material is a good conductor.

4. Overlapped.

Explanation:

In conductors, electrons are loosely bound to the nucleus hence, can detach easily at room temperature also. A large number of free electrons thus, available are conduction electrons.

The energy level of these electrons corresponds to the conduction level, hence valance level and conduction level are overlapped.

1. The notion that it is a whole lot easier to keep track of the missing particles in an "almost-full" band.

Explanation:

The concept of holes was introduced so as to occupy the empty spaces left by the missing electron. For example, in a valence band, when an electron jumps from valence to conduction band, an empty space is created in valence band which is occupied by a hole.

1. 0 eV

Explanation:

Forbidden band gap is the group of energy levels that cannot be occupied by the electrons, these energies lies in between conduction band and valence band. In pure conductors, electrons are loosely bound to the nucleus hence, can detach easily at room temperature also.

A large number of free electrons thus, available are conduction electrons. The energy level of these electrons corresponds to the conduction level, hence valence level and conduction level are overlapped.

Due to this, there is no forbidden band gap present in between conduction band and valence band hence, for metals the forbidden band gap is 0 eV.

1. They flow from positive terminal to negative terminal.

Explanation:

Electrons flow from negative to positive terminal of battery. whenever the electric current flows from positive terminal of battery to negative terminal of battery is called conventional current. the conventional current has same direction of flow of holes but opposite to direction of flow of from electron.

4. 0 + 0 = 1

Explanation:

In the Boolean algebra, binary addition is given as:

1 + 0 = 1

0 + 1= 1

1 + 1 = 1

Hence, 0 + 0 =1 is wrong.

4. Concentration of positive and negative charges near the junction.

Explanation:

During the formation of a junction diode, holes from p- region diffuse into n-region and electrons from n-region diffuse into p-region.

In both cases, when an electrons meets a hole, they cancel the effect at each other and as a result, a thin layer at the junction becomes free from any of charges carriers. This is called depletion layer.

There is a potential gradient in the depletion layer, negative on the p-side, and positive on the n-side.

The potential difference thus developed across the junction is called potential barrier.

1. Large velocity of the minority charge if the doping concentration is small.

Explanation:

When it comes to he breakdown in a reverse biased PN junction diode, it will probably happen only because of the accumulation of the higher charge at the biased region and large velocity of the minority charge if the doping concentration is small. This is the main cause the breakdown.

4. Relative widths of their energy gaps

Explanation:

According to band theory, distinction between conductors, insulators and semiconductors is based on relative width of energy gaps between valence band and conduction band.

1. Metallic conductors

Explanation:

When an electric field is applied across the metallic conductors the randomly moving electrons are subjected to electrical forces along the direction of the field. Due to this field, the electrons do not give up their randomness of motion, but they will be shifting towards higher potential. That means the electrons will drift towards higher potential along with their random motions.

In semiconductors, in addition to electrons, the travelling vacancies in the valence-band electron population (called 'holes'), act as mobile positive charges and are treated as charge carriers. Electrons and holes are the charge carriers in semiconductors.

Hence, electric current is due to drift of electrons in metallic conductors.

2. From the p-side to the n-side

Explanation:

Holes from p-type diffuse into the n-type and electrons from n-type diffuse into the p-type due to concentration gradient to form a depletion region which results in the rise of diffusion current flowing from p to n side.

4. Higher voltage level

Explanation:

In digital logic, higher voltage is defined as logic state '1' and lower voltage is defined as logic state '0'. The higher voltage need not necessarily be positive. For example, it is possible that state '0' is defined as −10 V and state '1' is defined as −5 V.

3. Gallium Arsenide

Explanation:

Direct Band gap: Defined when maximum energy in the valence band and minimum energy in the conduction band occurs at the same values of the crystal momentum i.e, a direct transition of electrons from valence to conduction band takes place.

4. All of the above

Explanation:

LEDs have longer life as compared to incandescent lamps. LED require low operational voltage and less power as compared to incandescent lamps. LEDs also have fast on-off switching capabilities.

2. 2.3V.

Solution:

If V is the potential difference between A and B, then according to the questions and using Kirchhoff’s law, we have

V - 0.3 = [(5+ 5)10³] × (0.2 × 1(10^-3))

⇒ V - 0.3 = [(5 + 5)10³] × (0.2 × 10^-3)

⇒ V - 0.3 = 10 × 10³ × 0.2 × 10^-3 = 2

⇒ V = 2 + 0.3 = 2.3V

3. The valence band is completely filled and the conduction band is partially filled.

2. Liquid Crystal Display

   Explanation:

   LCD stands for "Liquid Crystal Display".

LCD is a special thin flat panel that can let light go through it, or can block the light. (Unlike an LED it does not produce its own light).

The panel is made up of several blocks, and each block can be in any shape. Each block is filled with liquid crystals that can be made clear or solid, by changing the electric current to that block.

2. The current through the series Resistance (R_s) changes.

4. The resistance offered by the Zener changes.

Solution:

Symbolically zener diode represents like this:

In the forward bias, the zener diode acts as an ordinary diode. It can be used as a voltage regulator.

A zener diode when reverse biases offers constant voltage drop across in terminals as unregulated voltage is applied across circuit to regulate. Then during regulation action of a Zener diode, the current through the series resistance R_s changes and resistance offered by the Zener changes. The current through the Zener changes but the voltage across the Zener remains constant.

2. The photodiode has to be reversed biased.

Explanation:

Photo diode is used to detect light.

In reverse biased condition, the width of depletion region increases as the the applied reverse bias voltage increases across the diode. So, by applying a larger voltage, more of the incident photons are converted to electric current thereby increasing the efficiency.

In forward biased condition, the width of depletion region decreases so only a small portion of the incident photons get converted to electric current and hence the efficiency is less.

Hence photo diode in reverse biased condition detects light.

2. Reverse bias

Explanation:

We know that zener diode works on the reverse bias. When the reverse bias is equal to the break-down voltage, the voltage across the zener remains almost constant and the current increases rapidly.

4. NOR

Explanation:

The truth table for NOR gate is shown above which suggests that NOR gate is the logic gate which produces low output when any of the inputs is high.

2. non-ohmic resistance

Explanation:

We know that in the case of metallic conductors, the potential difference varies in direct proportion to the current flowing. The I−V graph of a ohmic conductor is a straight line. But, a p-n junction diode is not in according with Ohm's law. The current voltage characteristic curve of a p-n junction diode shows both forward bias as well as reverse bias characteristics as shown as in the graph.

3. Stabilisation

Explanation:

A zener diode is always operated in its reverse biased condition. A voltage regulator circuit can be designed using a zener diode to maintain a constant DC output voltage across the load in spite of variations in the input voltage or changes in the load current. The zener voltage regulator consists of a current limiting resistor RS​ connected in series with the input voltage VS​ with the zener diode connected in parallel with the load RL in this reverse biased condition.Hence we can say that ,zener diode is used for stabilization.

4. All of the above.

1. An electron hole

Explanation:

A hole is an absence of an electron in a particular place in an atom. Although it is not a physical particle, a hole can be passed from atom to atom in a semiconductor material.

1. It is a constant voltage device.

Explanation:

Zener diode is a p-n junction diode working in the breakdown region. It is used as a voltage regulator/stabilizer to provide a constant voltage from a source whose voltage may fluctuate over a wide range.

3. Depletion region

Explanation:

In depletion region negative charge carrier electrons are attached with their positive charge carrier holes. hence there is no free charge carriers exists.

2. Potential barrier across junction increases.

2. Increases continuously with the forward current of the diode, reaches a maximum and then decreases.

Explanation:

The intensity of emitted light increase with the foreword current more the light intensity.

2. Can be combined to produce OR, AND and NOT gates.

2. Hole

Explanation:

Whenever an electron jumps from a valence band to conduction band, an equal and opposite charge is left behind in the place of an electron. This is called a hole.

3. C.B. is empty and V.B. is filled with electrons.

Explanation:

As shown in the image, insulators have a very large forbidden gap. Their conduction band is empty, thus they cannot conduct electricity under normal room temperature and pressure conditions. The valence band is full.

2. 0.71eV.

1. Photocurrent is proportional to incident light intensity.

Explanation:

In a photodiode, photocurrent is directly proportional to intensiy of incident light. More the incident light intensity, more will be the current produced.

4. nh ≠ ne.

1. Large velocity of the minority charge carriers if the doping concentration is small.

4. Strong electric field in the depletion region if the doping concentration is large.

Solution:

Reverse biasing: Positive terminal of the battery is connected to the N-crystal and negative terminal of the battery is connected to P-crystal.

1. In reverse biasing width of depletion layer increases
2. In reverse biasing resistance offered R_Reverse = 10⁵Ω.
3. Reverse bias supports the potential barrier and no current flows across the junction due to the diffusion of the majority carriers.

(A very small reverse current may exist in the circuit due to the drifting of minority carriers across the junction)

4. Break down voltage: Reverse voltage at which break down of semiconductor occurs. For Ge it is 25V and for Si it is 35V.

So, we conclude that in reverse biasing, ionization takes place because the minority charge carriers will be accelerated due to reverse biasing and striking with atoms which in turn cause secondary electrons and thus more number of charge carriers.

When doping concentration is large, there will be a large number of ions in the depletion region, which will give rise to a strong electric field.

2. 6 eV

Explanation:

Atoms are neutral in the insulator, thus there are no valence electrons in the outer orbit. The electrons are tightly bound with the nucleus hence, at room temperature thermal energy is not enough to push the electrons into conduction band and hence, no electrons are available for conduction.

The energy required for electron to escape out from orbit and to over come the energy gap is thus of the order of 6 eV.

2. 1 hole is created

Explanation:

For one electron vacancy, an empty space or void is created. In order to fill that empty space, a charge carrier with a charge equal in magnitude but opposite polarity should occupy that space in order to maintain electrical neutrality. Such a charge particle is called a hole.

3. 0 eV

Explanation:

3. A-1 B-0 C-1

Explanation:

Logic gate OR is used for addition of the input signals and Logic gate AND is used for multiplication of the input signals. Hence, here inputs A = 1, B = 0 and C = 1 will yield the output Y = 1.

3. 1.1 = 0

Explanation:

In the Boolean algebra,

1.0 = 0

0.1 = 0

1.1 = 1

3. Missing electron in valence band

Explanation:

A hole is not itself a physical quantity but a missing electron in valence band. An electron from adjacent site jumps to fill this hole and thus creates a hole at its former site. So it seems as if the hole itself has moved.

1. Output Y exists when both inputs A and B exist.

3. Conduction band

Explanation:

The band gap in case of semiconductors is of order 3−4 eV. At absolute zero, the valence band is filled and conduction band is empty, so it behaves as an insulator. As the temperature is raised, electrons in the valence band gain sufficient energy to overcome the band gap and jump to the conduction band.

4. All of the above.

Explanation:

At absolute zero temperature, in a pure semiconductor, all electrons occupy the valence band and no electrons are present in the conduction band. The forbidden gap energy E_g​ is large.

1. NOT

Explanation:

Only one logic gate has one input terminal i.e. NOT gate.