Test: Direct & Indirect Band Gap Semiconductors - Electronics and Communication Engineering (ECE) MCQ

Basic LED diode is represented as:

• The selection of emission of color from the LED is fairly limited due to the nature of the semiconductor used.
• Commonly available colors of LED are red, green, blue, yellow, amber, and white.
• Light Emitting Diode (LED) works on the principle of Electroluminescence.
• It is the conversion of electrical energy into light energy.
• In LED, most of the light is concentrated near the junction because charge carriers will be very close and less than the diffusion length.
• LEDs are generally fabricated with Direct Band Gap (DBG) semiconductors like GaAs, GaAsP, GaP.

Direct Band Gap (DBG) Semiconductors:
The semiconductor in which the top of the valence band and the bottom of the conduction band occur at the same value of momentum.

Since Silicon is an Indirect Band Gap semiconductor so electron cannot fall directly to the valence band but must undergo a momentum change as well as a change in energy.  So, energy is released as heat along with light. Hence, silicon is not suitable for the fabrication of LEDs.

Assertion Explanation-
A direct band-gap (DBG) semiconductor is one in which the maximum energy level of the valence band aligns with the minimum energy level of the conduction band with respect to momentum as shown in the figure.

Reason Explanation-
In a DBG semiconductor, direct recombination takes place with the release of the energy equal to the energy difference between the recombining particles. The probability of radiative recombination is high in the case of DBG semiconductors. Hence the efficiency factor of a DBG semiconductor is high. That’s why DBG semiconductors are always preferred for making optical sources.
Note- Best example of direct bandgap semiconductors is GaAs, InAs, InSb, GaN, InN, ZnO, CdSe, ZnS.

Light emitting diodes are made using the semiconductors that have direct bandgap.
The photon emitted when an electron transits from the bottom of conduction band to top of valence band falls into the visible light range.

However, in case of Indirect Bandgap semiconductors, the photon emitted does not fall into the visible light range.

Cadmium sulphide is best photo sensitive material.

In eV gap E_g = E/e = 3.62 x 10^-19/1.6 x 10^-19 = 2.26 eV