Physics M.C.Q [1M]

2. At V_i = 1V , it can be used as an amplifier.
3. At V_i = 0.5V, it can be used as a switch turned off.
4. At V_i = 2.5V, it can be used as a switch turned on.

Solution:

According to above graph transfer characteristics of a base biased common emitter transistor, we note that.

1. When V_i= 0.4 V, output voltage remain same,there is no collection current. So, transistor circuit is not in active state.
2. when V_i = 1V (This is in between 0.6V to 2V), the transistor circuit is in active state and when input is increasing output is decreasing because when CE is used as an amplifier input and output voltages are 180º out of phase. Then it is used as an amplifier.
3. when V_i = 0.5V, there is no collector current. The transistor is in cut off state. The transistor circuit can be used as a switch to be turned off.
4. when V_i = 2.5V, the collector current becomes maximum and transistor is in a saturation state and can used as switch turned on state.

4. Holes move from base to emitter.

Solution:

Here, C = A.B and D = Ā.B

E = C + D = (A B) + (Ā .B)

Explanation The truth table of this arrangement of gates can be given 

3. Both electrons and holes.

2. Holes and electrons

Explanation:

Depletion region of a p-n junction is formed due to the shortage of holes and electrons.

1. p type → n type

Explanation:

The holes in the covalent bonds of p-type semiconductor. They appear to move towards n-type semiconductor. Because the electrons in the covalent bonds of n-type semiconductor (not free electrons) jump towards p-type semiconductor due to plenty of holes available in the covalent bonds at p-side. 

Hence, conventional current flow from p-type to n-type.

4. Base, collector and emitter.

3. Increases, decreases.

2. 0

Explanation:

NOT gate yields the reverse of the input signal in output, thus when an input signal 1 is applied to a NOT gate, its output is 0.

2. Boron.

4. Aluminium.

Explanation:

A p-type semiconductor is formed by doping an intrinsic semiconductor with a trivalent atom (atom having valency 3). As phosphorous and boron have three valence electrons, they can be doped with silicon to make a p-type semiconductor.

1. Conductors

Explanation:

Free electron density is more in conductors that is why they are useful in conducting electricity.

4. 4

Explanation:

The valency of semiconductor (Ge or Si) is four, hence it has 4 valence electrons in the outermost orbit of the Ge or Si-atom

1. High potential at N side and low potential at P side.

Explanation:

3. An unfilled covalent bond.

Explanation:

When the covalent bond breaks, electrons are freed from atom. The departure of electron from valence band creates vacancy in bond, this vacancy is known as hole. Hence, a hole is an unfilled covalent bond.

2. 10⁴Ω

Explanation:

It is known that the resistance of a dry human body is 10kΩ = 10⁴Ω.

3. Photo-voltaic effect

Explanation:

The generation of current or voltage in a material, when exposed to light is called photovoltaic effect.
The device which operates on this effect is called photo-voltaic device or solar cell. It is a p-n junction diode which converts solar energy into electrical energy.

1. Minority carriers

Explanation:

When a diode is reverse biased, its depletion region increases in width. Thus depletion region would correspond to an insulator. However still some current actually flows through the depletion region. This is due to the pushing of minority carriers across the depletion region causing very small reverse 'leakage current'.

4. OR.

4. All of the above

Explanation:

P-n junctions are an integral part of several optoelectronic devices. These include photodiodes, solar cells light emitting diodes (LEDs) and semiconductor lasers.

3. Both drift and diffusion

Explanation:

Three important phenomena occurs during formation of pn junction:

Diffusion, Formation of space charge, Drift.

1. Eliminates a.c. component.

1. Increases

Explanation:

The pure semiconductor has less number of thermally generated charge carriers. But when it is doped with pentavalent or trivalent impurity atoms, the number of charge carriers i.e. electrons and holes increases. So conductivity increases.

2. Cancels the potential barrier.

4. NOR

Explanation:

NAND gate is considered universal gate. As other gates can be formed from this gate.

4. 199

3. emitter

Explanation:

 It is a p-n-p, transistor. The Lead marked with the arrow is the emitter. It tells the direction of the flow of the holes i.e, the flow of conventional current. 

1. NAND gate

Explanation:

The output is 0 only when the two inputs are 1.

The output is 1 when any of the input is zero. 

This is the characteristic of a NAND gate.

4. Negative.

1. There are no mobile charges.
2. Equal number of holes and electrons exist, making the region neutral.

4. Immobile charged ions exist.

Solution:

On account of difference in concentration of charge carrier in the two sections of P-N junction, the electrons from N-rcgion diffuse through the junction into P-region and the hole from P-region diffuse into N-region.

​​​​​​​

Due to diffusion, neutrality of both N-and P-type semiconductor is disturbed, a layer of negative charged ions appear near the junction in the P-crystal and a layer of positive ions appears near the junction in N-crystal. This layer is called depletion layer.

The thickness of depletion layer is 1 micron = 10^-6m.

Width of depletion layer ∞ 1/Dopping

Depletion is directly proportional to temperature.

Important point: The P-N junction diode is equivalent to capacitor in which the depletion layer acts as a dielectric.

1. Electrons move from lower energy level to higher energy level in the conduction band.

3. Holes in the valence band move from higher energy level to lower energy level.

Solution:

As we apply electric field is applied across a semiconductor, the electrons in the conduction band acquire energy and get accelerated. They travel from lower energy level to higher energy level. While the holes in valence band travel from higher energy level to lower energy level, where they will be having more energy.

3. The number of free electrons for conduction is significant only in Si and Ge but small in C.

Explanation:

In Si and Ge at room temperature(300K); the energy band gap is low as the result of which electrons in the covalent bonds gain kinetic energy, they break the bond and move to conduction band.

A hole is also created in the valence band. So the number of free electrons for conduction is significant in Si and Ge.

The energy band gap in case of carbon is high as the result of which there are not significant number of electrons in the conduction band even at room temperature.

3. Germanium

Explanation:

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding.

As the valency of germanium is 4, it can share 4 electrons of neighboring atom to complete the octet.

Hence, Ge has covalent bond. NaCl is an example of an ionic bond and helium and copper both have valency 2 and 1 respectively.

4. All of these

Explanation:

Grown Junction Diode:

3. The electron concentration increases.

Explanation:

When a semiconductor is doped with a donor type such as arsenic or phosphorous, which has five valence electrons, the donor atom replaces the Si or Ge atom. As a result, four out of the five electrons of the donor atom form a covalent bond by sharing an electron with four atoms of silicon. However, the fifth electron is free to move. Also, due to the breaking up of covalent bonds at room temperature, equal number of electrons and holes are produced. Thus, the total number of holes in the n-type semiconductor is less compared to the number of free electrons.

2. Photoelectric effect

Explanation:

A transducer is an electronic device that converts light energy to electrical energy in television. When light falls on photosensitive element electric current is generated that is measured directly or after amplification. Similarly, photoelectric effect is the ejection of electrons from a metal or semiconductor surface when illuminated by light or any radiation of suitable wavelength.

3. Collision generated charge carriers

Explanation:

When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown.

Avalanche breakdown occurs in reverse bias when the applied voltage is high enough, the free electron may move fast enough to knock other electrons free, creating more free-electron-hole pairs (i.e., more charge carriers), increasing the current.

Thus, the current flow in a Zener diode is mainly due to collision generated charge carriers.

2. Density of charge-carriers increases, but their mobilities decreases.

Explanation:

On increasing the temperature of a semi-conductor material, the density of charge increases which apparently increases the hinderance and thus decreases their mobility.

1. Electrons crossover from emitter to collector.

3. Electrons move from emitter to base.

Solution:

Key concept: Transistor:

A junction transistor is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors or by sandwiching a thin layer of N-type semiconductor between two p-type semiconductors.

E - Emitter (emits majority charge carriers)

C - Collects majority charge carries

B - base (provide proper interaction between E and C)

In normal operation base-emitter is forward biased, i.e., the positive pole of emitter base battery is connected to base and its negative pole is connected to the emitter. And collector base junction is reverse biased, i.e., the positive pole of the collector base battery is connected to collector and negative pole to base. Thus, electron moves from emitter to base and crossover from emitter to collector.

1. Ionisation by collision.

Explanation:

Avalanche breakdown is caused by impact ionization of electron-hole pairs. A very little current flows under reverse bias conditions and depletion region increases. The electric field in the depletion region of a diode can be very high. Electron/holes that enter the depletion region undergo a tremendous acceleration.

As these accelerated carriers collide with the atoms, they can knock electrons from their bonds, creating additional electron/hole pairs and thus additional current. As these secondary carriers are swept into the depletion region, they too are accelerated and the process repeats itself.

1. Zero as no charges cross the junctions.

Explanation:

When a diode is connected in a Zero Bias condition, no external potential energy is applied to the PN junction. However if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the Forward Current.

Likewise, holes generated in the N-type material (minority carriers), find this situation favourable and move across the junction in the opposite direction. This is known as the Reverse Current and is referenced as IR. This transfer of electrons and holes back and forth across the PN junction is known as diffusion.

Then an Equilibrium or balance will be established when the majority carriers are equal and both moving in opposite directions so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of Dynamic Equilibrium.

4. Number density of current carriers increases, relaxation time decreases but effect of decrease in relaxation time is much less than increase in number density.

Solution:

With increase in temperature, the number density of current carries increases, relaxation time decreases but effect of decrease in relaxation is much less than increase in number density. So, the conductivity of a semiconductor increases with increase in temperature.

2. Y exists when A exists or B exists or both A and B exist.

Explanation:

In Boolean algebra, A + B = Y implies that Y exists when A exists or B exists or both A and B exist.

1. The valence band is partially empty and the conduction band is partially filled.

Explanation:

In semiconductors at room temperature the electrons get enough energy so that they are able to over come the forbidden gap. Thus at room temperature the valence band is partially empty and conduction band is partially filled.

1. Increasing intensity without changing frequency.

Explanation:

Intensity of photons is the number of photons passing through a cross sectional area per unit time. Hence changing the intensity would cause change in number of ejected photoelectrons emitted by them.

4. p - n junction is electrically neutral.

Explanation:

Generally Si is a semiconductor. In p type semiconductor acceptor level lies near the valence band and each donor atom contribute one electron. p-n junction electrically neutral because total charge is zero.

3. intrinsic

Explanation:

At room temperature, in impure semiconductors some free charge carriers are available for conduction due to impurity atoms. But in pure, i.e., intrinsic semiconductors, n = p.

Hence, at room temperature no free electrons are available for conduction. If the temperature is increased the covalent bonds will break and electrons will be freed, each electron will leave behind a hole and capture a new hole the process will continued and charge flows through intrinsic semiconductor.

Thus, its conductivity is only due to breaking of covalent bonds.

2. A vacancy created when an electron leaves a covalent bond.

Solution:

Concept of holes in the semiconductor:

1. When an electron is removed from a covalent bond, it leaves a vacancy behind. An electron from a neighbouring atom can move into this vacancy, leaving the neighbour with a vacancy. In this way the vacancy formed is called a hole (or cotter), and can travel through the material and serve as an additional current carriers.
2. A hole is considered as a seat of positive charge, having magnitude of charge equal to that of an electron.
3. Holes acts as a virtual charge, although there is no physical charge on it.
4. Effective mass of hole is more than an electron.
5. Mobility of hole is less than an electron.

2. D₂ is forward biased and D₁ is reverse biased and hence no current flows from B to A and vice versa.

Solution:

A symbol of the diode is represented like this: In this problem first we have to check the polarity of the diodes. -10 is the lower voltage in the circuit. Now p-side of p-n juction D₁ is connected to lower voltage and n-side of D₁ to higher voltage. Thus D₁ is reverse biased. Now, let us analyse 2^nd diode of the given circuit. The p-side of p-n junction D₁ is at higher potential and n-side of D₂ is at lower potential. Therefore D₂ is forward biased.

Hence, current flows through the jucntion from B to A.

4. Conduction band is empty and valence band is filled with electrons.

Explanation:

Insulators are the materials which do not conduct electricity. All the electrons are filled in valance band whereas the conduction band is empty. Moreover, the energy band gap in insulators is very large, thus electrons cannot jump from valence band to conduction band.