Semiconductor MCQ | Semiconductor Question and Answer with explanation

Ques.51. In an intrinsic semiconductor, the number of free electrons

  1. Equals the number of holes
  2. Is greater than the number of holes
  3. Is less than the number of holes
  4. None of the above

Answer.1. Equals the number of holes

Explanation:-

Intrinsic Semiconductor: A semiconductor in an extremely pure form is known as an intrinsic semiconductor. It is a semiconductor in which electrons and holes are solely created by thermal excitation. When the temperature of an intrinsic semiconductor is raised, the electrons reaching the conduction band due to thermal excitation leave an equal number of vacancies or holes in the valence band and so in an intrinsic semiconductor the number of free electrons is always equal to the number of holes. Thermal energy continues to produce new electron-hole pairs whereas other electron-hole pairs disappear as a result of recombination of holes and free electrons.

 

Ques.52. At the absolute zero temperature (−273° C), an intrinsic semiconductor has

  1. A few free electrons
  2. Many Holes
  3. Many free electrons
  4. No holes or free electrons

Answer.4. No holes or free electrons

Explanation:-

Pure semiconductors are known as intrinsic semiconductors. Frequently available semiconductors are germanium Ages and silicon Isis belonging to IV group in the periodic table. Each semiconductor has four valence electrons in their outermost orbit. To set stability, each of these atoms makes four covalent bonds with the surrounding four neighboring atoms in the semiconductor crystal. The two-dimensional representation of silicon intrinsic semiconductor at 0 K along with the energy band structure is as shown in Fig.

At 0K, all valence electrons are strongly bound to their atoms and are actively participating in the covalent bond formation. As a result, no free electrons are available for conduction and it acts as an insulator.

 

Ques.53. The number of free electrons and holes in an intrinsic semiconductor increase when the temperature

  1. Decrease
  2. Increase
  3. Stays Same
  4. None of the above

Answer.2. Increase

Explanation:-

At room temperature (T> 0 K), the valence electron acquires a sufficient amount of thermal energy. As a result, breakage of covalent bonds takes place releasing free electrons. These free electrons create a vacancy in its initial position in the crystal. This vacancy is known as a hole and is assumed to carry a positive charge equivalent to the charge of the electron. These free electrons, due to acquiring of sufficient thermal energy, cross the energy gap and enter into the conduction band from the valence band and occupy the energy levels in the conduction band. The electrons leaving the valence band create holes in its place. Thus, the valence band has holes while the conduction band has electrons. The corresponding crystal structure along with the energy band structure is as shown in Fig. 

valnce band

In the intrinsic semiconductor, the electrons and holes are generated in pairs and at any given temperature the volume concentration of electrons is equal to that of holes. With the rise in temperature, more and more electron-hole pairs are formed and more charge carriers are available for conduction. Thus the conductivity of intrinsic semiconductors increases with the increase in temperature (and hence the resistivity decreases with increase in temperature).

 

Ques.54. Which of the following cannot move?

  1. Holes
  2. Free Electrons
  3. Ions
  4. Majority carriers

Answer.3. Ions

Explanation:-

Consider the p-n junction at room temperature (say 300°K). The region to the left side of the junction is p-type. It has a large number of holes as majority carriers and the region to the right is n-type. It has a large number of electrons as charge carriers. There exists a large concentration gradient across the junction for the majority carriers. In order to reduce the concentration mismatch, the electrons from n- region tend to diffuse p region and holes from p regions tend to diffuse in n-region. Before diffusion, both the p and n regions are electrically neutral. But after a few carriers diffuse into the other region, there is the development of equal and opposite charges (immobile) on the two sides of the junction. Because as an electron from n region diffuses into p region it leaves behind positive ion and hole when moves to n region from p region, leaves behind negative ion. A layer of positive and negative ions across the junction forms a depletion layer.

depletion region

These immobile ions generate an electric field directed from n to p region. This field opposes majority carriers in both the regions to cross the junction. The carriers can cross the junction until the diffusion force is larger than the effect of this internal electric field. When the diffusion force exactly balances the electrostatic force, the further diffusion of majority charge carrier across the junction stops.

During diffusion, the electrons move from it to p-side and hence conventional current is from P to N. Similarly, because the diffusion of holes from P to N side the current is in the same direction. Thus the diffusion currents due to majority carriers in the two regions add up and give a net forward current IF from p.

The region across the P-N junction in which the potential changes from positive to negative is called the depletion region. Since this region has immobile (fixed) ions which are electrically charged, it is all called as the space-charge region. Outside this region on each side of the junction, the material is still neutral.

 

Ques.55. Which causes the barrier layer in a PN junction?

  1. Doping
  2. Recombination
  3. Barrier potential
  4. Ions

Answer.2. Recombination

Explanation:-

When a P-N junction is formed, the holes in the P-region diffuse into N-region and the electrons in the N-region diffuse into P-region. This process is called diffusion which happens for a short time as soon as the P-N junction is formed. After a few combinations of holes and electrons, a restraining force is developed which is known as the potential barrier. The potential difference created across the p-n junction due to the diffusion of electrons and holes is called the potential barrier. 

Barrier voltage

This potential barrier prevents further diffusion of holes and electrons. The barrier force development can be easily explained. That is, each recombination of hole and electron eliminates hole and electron. During this process, the negative acceptor ions in the P-region and positive donor ions in the N-region are left uncompensated. The additional holes trying to diffuse into N-region and additional electrons trying to diffuse into P-region are repelled by these negative and positive charges respectively. The region containing this uncompensated acceptor and donor ions is called the depletion layer.

 

Ques.56. When the reverse voltage increases from 5V to 10V, the depletion layer

  1. Become larger
  2. Become smaller
  3. Is unaffected
  4. Breaks down

Answer.1. Become larger

Explanation:-

When the external voltage applied to the P-N junction is in such a direction that the depletion (potential) barrier is increased, it is called reverse bias.

The negative terminal of the battery is connected to P-type and positive terminal to N-type. The reverse biasing establishes an electric field which acts in the same direction as the field due to depletion (potential) barrier. Thus, depletion (potential) barrier is increased and prevents the Flow of charge carriers across the junction. In effect, a high resistance path is established for the circuit, hence, the current flow is insignificant. By increasing the reverse voltage the width of the depletion layer is increased upto a certain point. If the reverse-biased voltage exceeds a certain level (usually 50 V or more), the diode “breaks down” and reverse current starts flowing from cathode to anode. The reverse voltage at which the diode breaks down is known as the peak reverse voltage (PRV), or peak inverse voltage (PIV).

 

Ques.57. When a diode is forward biased, the recombination of the free electrons and holes may produce

  1. Heat
  2. Light
  3. Radiation
  4. All of the above

Answer.4. All of the above

Explanation:-

Recombination of electron-hole pair Produce

  1. Heat energy
  2. Light energy
  3. Radiation energy

GENERATION AND RECOMBINATION

In semiconductors, a single event of covalent bond breaking leads to the generation of two charge carriers, an electron in the conduction band and a hole in the valence band (Fig.2. 13). The process may be represented as

Covalent Bond + Thermal Energy ⇒ (Electron + Hole)Pairs

As electron and hole are produced simultaneously, the process is called electro~-hole pair generation. In the process of generation, a covalent bond is broken and a bound electron is transformed into a free electron. Thermal energy is one of the agents which causes pair generation. Another agent is optical illumination. At any temperature T. the number of electrons generated would be equal to the number of holes produced. If n denotes the concentration of electrons in the conduction band and p is the concentration of holes in the valence band, then n = p 

After generation, the charge carriers move independently. The electrons move in the conduction band and the holes move in the valence band. Their motion is at random in the respective bands, as long as the external electric field is not applied.

It is likely that the electron in conduction band may lose its energy due to collision with other particles in the lattice and fall into the valence band. When a free electron falls into valence band, it merges with a hole. This process is called recombination. When a recombination event occurs, the free electron enters a ruptured covalent bond and re-bridges it. 

Therefore, recombination means that a free electron transforms into a valence electron and that a ruptured covalent bond is re-bridged. In the process, an electron-hole pair disappears and energy is released. The released energy is mainly in the form of thermal energy.

Electron + Hole ⇒  Covalent Bond + Energy

At a steady temperature, a dynamic equilibrium exists which balances the two processes of electron-hole pair generation and electron-hole recombination.

Just as thermal energy generates electron-hole pairs, light radiation can produce electron-hole pairs In a semiconductor. When a semiconductor material is irradiated with optical radiation, electron-hole pairs are generated if the frequency v of the radiation satisfies the condition.

Radiative Recombination, Emission of Photons

Radiative recombination, in which a hole reacts with an electron and produces a photon, is exactly the reverse of absorption (in the pure electron description it is the spontaneous transition of an electron from the conduction band to an unoccupied state in the valence band).

e + h ⇒ γ

Since a free electron and a free hole must find each other, the rate of radiative recombination at which electrons and holes are annihilated and photons are generated increases with the concentration of electrons and the concentration of holes.

Light Emitting Diode

Electron-hole pairs in semiconductors recombine by a variety of different mechanisms. In most cases, the energy will be released as heat (phonons), but a fraction of the recombination events may involve the emission of a photon. This process is termed “electroluminescence” (EL) and is widely used to produce solid-state light sources such as light-emitting diodes (LEDs). In order to produce LEDs or any other electroluminescent device, one must recombine significant populations of electrons and holes. Conventionally, this is achieved at an interface between a hole-doped and an electron-doped material. LED uses the radiative recombination in a forward biased p-n junction. 

 

Ques.58. A reverse voltage of 20 V is across the diode. What is the voltage across the depletion layer?

  1. 0 V
  2. 0.7 V
  3. 20 V
  4. None of the above

Answer.3. 20 V

Explanation:-

When the positive terminal of a dc source or battery is connected to the n-type and negative terminal is connected to the p-type semiconductor of a PN junction, as shown in Fig.. the junction is said to be in reverse bias. As the barrier potential is increased, it is very difficult for the majority carriers (holes in the P-type region and electrons in the N-type region) to diffuse across the junction. In the case, the applied reverse potential acts in such a way that it establishes an electric field which increases the field due to the potential barrier. Thus, the barrier potential at the junction is strengthened.

reversed biased PN

When the holes and electrons move away from the junction, the newly created ions increase the difference of potential across the depletion layer. The wider the depletion layer, the greater the difference of potential. The depletion layer stops growing when its difference of potential equals the applied reverse voltage. When this happens, electrons and holes stop moving away from the junction.

Since in reverse bias, the resultant field is strengthed i.e (electric field + potential barrier), but we can’t measure the voltage across the potential barrier hence the total voltage across the depletion layer is equal to the apply electric field i.e 20 V

 

Ques.59. The voltage where the avalanche occurs is called

  1. Barrier Potential
  2. Depletion layer
  3. Knee voltage
  4. Breakdown voltage

Answer.4. Breakdown voltage

Explanation:-

Reverse breakdown voltage

In a reverse-biased PN junction, a very small amount of current flows through the PN junction and this current is known as reverse saturation current. The reverse saturation current can flow only due to the movement of minority carriers across the junction. The amplitude of the reverse saturation current is independent of the applied reverse voltage. Whenever the reverse-bias voltage is increased to a very large value, the current through the junction is increased suddenly. The voltage, at which current increases abruptly, is called breakdown voltage. In this voltage, the crystal structure breaks down. In common applications, the junction breakdown should be avoided. When the excess reverse bias voltage is removed, the crystal structure becomes normal if overheating does not permanently damaged the crystal. There are two methods for junction breakdown due to the increase in reverse voltage. These two methods are

  1. Zener breakdown
  2. Avalanche breakdown

Zener Breakdown

Usually, the Zener breakdown occurs in heavily doped PN junctions. The heavily doped PN junction has a narrow depletion layer and it is reverse biased. Whenever the reverse voltage is increased, the electric field at the junction increases. Due to the increase in the electric field, covalent bonds break from the crystal structure. Consequently, a large number of minority carriers (electrons in the P-type region and holes in the N-type region) are generated and a large current flows through the junction in the reverse direction. This type of junction breakdown is called zoner breakdown.

Avalanche Breakdown

When the reverse-bias voltage is increased, a greater amount of energy is provided to minority carriers (electrons in the P-type region and holes in the N-type region) and they diffuse across the junction. Due to the further increase in reverse-bias voltage, the minority carriers get a large amount of energy. When the minority carriers have a collision with silicon or germanium atoms within the crystal structure, the minority carriers obtain sufficient energy to break a covalent bond and generate additional carriers (electron-hole pairs). Subsequently, these additional carriers get energy from the applied external voltage and generate more carriers. Hence, the reverse current increases very rapidly. If this cumulative process of carrier generation continues, a large current flows through the junction in the reverse direction. This type of junction breakdown is called avalanche breakdown. This is also known as avalanche multiplication.

Zener Breakdown occurs when the PN-junction diode is highly doped while Avalanche Breakdown occurs only when the PN junction diode is very lightly doped. Zener Breakdown has a negative temperature coefficient while Avalanche Breakdown has a positive temperature coefficient.

Though the end results of both the avalanche breakdown and the zoner breakdown are the same (generation of very high currents).

 

Ques.60. Suppose an intrinsic semiconductor has 1 billion free electrons at room temperature (25°C). If the temperature changes to 75°C, how many holes are there?

  1. Less than 1 billion
  2. More than 1 Billion
  3. 1 Billion
  4. Impossible to say

Answer.3. 1 Billion

Explanation:-

When heat energy creates an electron, it automatically creates a hole at the same time. Therefore, in an intrinsic semiconductor crystal always has the same number of holes and free electrons. if there are 1 billion free electrons, there are 1 billion holes.

Higher temperature increases the vibrations at the atomic level, which means that more free electrons and holes are created. But no matter what the temperature is, a pure intrinsic semiconductor has the same number of free electrons and holes.

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