GATE EC 2018 Set 1 — Question 40
Go beyond PYQs with Success TrackerAI-powered personalised practice and doubt support. Unlimited practice on eligible plans; AI usage limits apply.MCQ+2 / -0.67EasyBuilt-In PotentialP-N Junction & DiodesElectronic DevicesLED Principle & OperationOptoelectronic DevicesEnergy BandsSemiconductor Physics
Electronic Devices → Optoelectronic Devices → Built-In Potential
Last updated
Question
Red (R), Green (G) and Blue (B) Light Emitting Diodes (LEDs) were fabricated using junctions of three different inorganic semiconductors having different band-gaps. The built-in voltages of red, green and blue diodes are , and , respectively. Assume donor and acceptor doping to be the same ( and , respectively) in the and sides of all the three diodes.Which one of the following relationships about the built-in voltages is TRUE?
Correct answer
(B) V_R < V_G < V_B
Solution
The built-in potential of a junction is given by:Since the intrinsic carrier concentration is related to the band-gap energy by , we can express the built-in potential as:Given that the doping concentrations and are identical for all three LEDs, the built-in voltage is primarily determined by the band-gap energy . Specifically, a larger band-gap results in a higher built-in voltage.For LEDs, the band-gap energy is approximately equal to the energy of the emitted photons, which is inversely proportional to the wavelength (). The wavelengths of visible light follow the order:Consequently, the band-gap energies follow the reverse order:Therefore, the built-in voltages must follow the same order as the band-gaps:
Continue learning with Success Tracker
A step still unclear? Work through it with support
Use Success Tracker to ask about the reasoning, then try another GATE EC question to check your understanding.
AI-powered practice· Unlimited practice on eligible plans
- PYQs with solutions
- Attempt available previous-year questions, then compare your reasoning with the worked solution. Coverage varies by stream.
- Practice that adapts
- Choose a topic, work on weaker areas and bookmark questions to revisit. Your attempts feed your progress tracking.
- AI doubt support
- Ask follow-up questions about a step or concept while practising, instead of stopping at the final answer.
Unlimited practice is available on eligible plans. Free practice and AI usage have limits; check the current plan allowances before choosing.
This page stays readable without an account. AI responses can be wrong; check them against the solution and source material.
More questions on Optoelectronic Devices
2026 Set 1 Q14Consider carrier transport in a Zener diode in the breakdown region. Which is the dominant…2026 Set 1 Q18Consider the circuit shown in the Figure with and . Assume…2026 Set 1 Q25Consider a p-n junction diode when it is forward biased with 2 V. Which of the following is/are the…2026 Set 1 Q29Figure shows the output characteristics of two different Bipolar Junction Transistors (BJT), BJT 1…2026 Set 1 Q54Consider an LED based on a direct bandgap semiconductor material with energy bandgap eV.…