Optical Devices MCQ Quiz - Objective Question with Answer for Optical Devices - Download Free PDF

Ans.(2)

Sol.

The main application of p-n junction diode is in rectification circuits. These circuits are used to describe the conversion of a.c signals to d.c in power supplies.

Ans.(3)

A. Correct: In semiconductor diodes, carriers can indeed be generated by photo-excitation, especially in devices like photodiodes or solar cells where light generates electron-hole pairs.

B. Correct: When a photodiode is exposed to light whose energy is greater than the band gap (E > E_g) of the semiconductor, electron-hole pairs are created due to photon absorption. This is a standard principle in photodetector operation.

C. Incorrect: Photovoltaic devices do not convert electricity into optical radiation. Instead, they convert optical radiation (light) into electricity, as seen in solar cells. Therefore, this statement is false.

D. Correct: A photodiode can be used as a photodetector, which is a device that detects optical signals (light) and converts them into electrical signals. This is a well-established application.

E. Incorrect: While Zener diode specifications do relate to voltage regulation, this statement is outside the context of photonic devices discussed in the other options and is not relevant to the comparison here.

Hence, the correct statements are A, B, D, and E, which corresponds to Option 3.

Concept

In the context of LEDs, "quantum efficiency" refers to how efficiently the device converts electrical energy into light energy. It is a measure of the effectiveness of the LED in emitting photons when an electric current is applied.

Solution

The term "quantum efficiency" in LEDs specifically refers to the ratio of the number of photons emitted into free space per second to the number of electrons injected into the LED per second. This can be mathematically represented as:

\( \text{Quantum Efficiency} = \frac{\text{Number of photons emitted}}{\text{Number of electrons injected}} \)

Hence, the correct answer is option 2.

• Forward-biased diode voltage drop:
  • Represents the voltage difference across the diode when current flows in the forward direction (anode positive, cathode negative).
• Caused by:
  • Internal resistance within the diode material.
  • Energy barrier created by the p-n junction.
     

Calculation:
Given:
Voltage  drop=2V;
E=15 V;
Apply KVL in the circuit,
\(E-V_D-V_R=0;\\ 15-2-I_f\times2.2=0\\ 13-I_f\times2.2=0\\ I_f\times2.2=13\\ I_f=\frac{13}{2.2}=5.91 mA;\)
so the answer is 5.91mA.

Advantages of LCD:

• They have low power consumption; A seven-segment display requires about 140 μW (20 μW/segment); This is a great advantage over LEDs which require about 40 mW per numeral
• They have a low cost

Disadvantages of LCD:

• LCDs are very slow devices; The turn-on and the turn off times are quite large
• The turn-on time is typical of the order a few milliseconds while the turn off time is ten milliseconds
• When used on DC their span is quite small, therefore, they are used with AC supplies having a frequency less than 500 Hz
• They occupy a large area
• They require an additional light source

Important Points:

• LED: Light-emitting diode.
• LASER: Light Amplification by stimulated Emission of Radiation.
• Heating Problems exist in LED lights in more compare to LASER lights.

The correct answer is electrical current into optical signal

Key Points

• LED (light-emitting diode)
  • The LED (light-emitting diode) is a PN junction device that emits light when a current passes through it in the forward direction, i.e. when LED is forward biased, it emits light.
  • In an LED, this energy lies in the visible region of electromagnetic radiation, and the photon released is perceived as light.
  • In reverse-biased mode, it works like a normal diode and does not emit light.
  • It is an optical semiconductor device that emits light when voltage is applied means it converts electrical energy into light energy. Hence, Option 3 is correct.
  • When the Light Emitting Diode (LED) is forward biased, the free electrons from the n-side and the holes from the p-side are pushed towards the junction.
  • Light-emitting diodes emit either visible light or invisible infrared light when forward-biased.
  • The LEDs which emit invisible infrared light are used for remote controls.
  • LEDs that can emit red, yellow, orange, green, and blue light are available for commercial use.

For an inverting amplifier, as shown above, the voltage gain is given by:

\(A_v=\frac{V_{out}}{V_{in}}\)

The ideal opamp will be having virtual ground and the current entering the op-amp will be 0.

Therefore, by applying KCL at the inverting terminal of opamp we get:

\(\frac{V_{in}-0}{R_1}=\frac{0-V_{out}}{R_f}\)

\(\frac{V_{out}}{V_{in}}=-\frac{R_f}{R_i}\)

A PIN diode consists of heavily doped p and n regions separated by a high resistivity material

PIN diode:

• A PIN diode is frequently used as a switching diode for frequencies up to GHz range, it is a microwave switch
• In PIN diode the intrinsic semiconductor is sandwiched in between high doped P and high doped N regions
• PIN diodes offer very small switching times
• The smaller switching time is due to the high resistivity of an intrinsic layer
• Due to increased depletion region, the covalent bonds break and increase the surface area for photosensitivity
• it’s photosensitive in reverse bias
• This property is used in fields of light sensors, image scanners, artificial retina systems

PIN diode represented as:

Concept:

When LED conducts, it is replaced by its cut-in voltage with a limiting resistor connected in series with it.

\(R = {V_S - V_γ \over I}\)

where, V_S = Source voltage

V_γ = Cut-in voltage

R = Limiting resistor 

I = Current

Calculation:

Given, V_S = 10V

V_γ = 2V

I = 20mA

\(R = {10 - 2 \over 0.02}\)

R = 400 Ω

Generally, optical devices are fabricated using direct bandgap semiconductors like GaAs and normal PN diode is fabricated by using Si. The most basic material for the fabrication of an LED is GaAs (Gallium Arsenide). 

Using GaAs as basic material, GaAs produces LEDs of different colors. This is as shown:

• An optocoupler (also called an opto-isolator, photocoupler, or optical isolator) is an electronic component that transfers electrical signals between two isolated circuits by using light.
• Opto-isolators prevent high voltages from affecting the system receiving the signal.

There are many different types of optocouplers are available based on their needs and switching capabilities. Depending on the use there are mainly four types of optocouplers are available.

• Opto-coupler which use Photo Transistor
• Opto-coupler which use Photo Darlington Transistor
• Opto-coupler which use Photo TRIAC
• Opto-coupler which use Photo SCR

The correct answer is Semiconductors.

Important Points 

• Solar cells are made up of Semiconductors.
• Two kinds of semiconductors, called p-type and n-type silicon, make up a solar cell.
• The p-type silicon is created by the addition of atoms, such as boron or gallium, which have one fewer electron than silicon in their outer energy level.
• Since boron has one fewer electron than is needed, an electron vacancy or "hole" is produced to form bonds with the surrounding silicon atoms.
• A solar cell consists of a p-type silicon layer mounted next to an n-type silicon layer.

Key Points 

• An electrical insulator is a substance through which the electron does not flow freely or the isolator atom has closely bound electrons whose internal electrical charges do not flow freely, under the control of an electric field, very little electrical current can flow through it.
• A superconductor is a surface that gets cooler than a "critical temperature" conducts electricity without resistance.
• Conductors are the structures or objects that cause electricity to pass through them.

Photo-Diode:

• A photodiode is a semiconductor p–n junction device that converts light into an electrical current
• The current is generated when photons are absorbed in the photodiode.
• The photodiode is a special type of diode which operates in reverse bias conditions.
• When the incident light on the photodiode increases, the reverse current in a photo-diode also increases.
• Diode current flows from n to p.
• Net current flowing is due to only minority charge carriers.
• The current flowing in the photodiode is due to diffusion current only.