List of Experiments

Light Emitting Diode

Aim

Determination of the threshold voltage and study the V-I characteristics of LED.

Apparatus

  • LED (e.g., Red, Green, or Blue)
  • DC power supply (0-30V)
  • Resistor (e.g., 1 kΩ)
  • Multimeter (to measure voltage and current)
  • Connecting wires

Pre-Lab Questions

  1. What is an LED?
    An LED (Light Emitting Diode) is a semiconductor device that emits light when an electric current passes through it.
  2. What is threshold voltage in an LED?
    The threshold voltage is the minimum voltage required to forward bias the LED and initiate significant current flow, causing it to emit light.
  3. Why is a resistor used in series with an LED?
    A resistor limits the current flowing through the LED to prevent it from getting damaged due to excessive current.
  4. What happens if the voltage applied to an LED is below its threshold voltage?
    Below the threshold voltage, the LED does not conduct significantly, and no light is emitted.
  5. What are the characteristics of LED?
    An LED conducts current and emits light in forward bias, with the color depending on the material’s bandgap. It’s efficient, fast-switching, and long-lasting but sensitive to temperature and nonlinear in current-voltage behavior.
  6. Why direct bandgap materials can only be used for fabricating LED?
    Direct bandgap materials are used for fabricating LEDs because they allow electrons to recombine with holes efficiently, releasing energy as light (photons) due to aligned momentum states.
  7. Calculate the wavelength of LED radiation for a material with energy gap 5.1 eV
    \[ \lambda = \frac{hc}{E} \]

    Convert \( hc \) to a convenient constant in eV·nm:

    \[ hc \approx 1240 \, \text{eV·nm} \]

    Now substitute:

    \[ \lambda = \frac{1240}{5.1} \approx 243.13 \, \text{nm} \]

Theory

An LED is a p-n junction diode that emits light when forward-biased. When a voltage greater than the threshold voltage is applied, electrons recombine with holes, releasing energy in the form of photons. The V-I characteristics of an LED are non-linear: below the threshold voltage, the current is negligible; above it, the current increases exponentially with voltage. The threshold voltage depends on the material of the LED (e.g., 1.8V for red, 2.2V for green)

For a deeper understanding of the principles behind this experiment, refer to the LED theory

Circuit Diagram

A circuit diagram showing a DC power supply connected in series with a 1 kΩ resistor, an LED (anode to positive, cathode to negative), and a multimeter in parallel to measure VLED and in series to measure I.

Newtons-Rings Diagram

Procedure

  1. Set up the circuit on a breadboard with the LED, resistor, and connecting wires.
  2. Connect the multimeter in parallel across the LED to measure voltage (VLED).
  3. Connect another multimeter in series to measure current (I).
  4. Gradually increase the supply voltage (Vs) from 0V in steps of 0.2V using the DC power supply.
  5. Record the corresponding voltage across the LED and current through the circuit for each step.
  6. Note the voltage at which the LED begins to glow (threshold voltage).
  7. Continue increasing the voltage until the LED glows brightly (but avoid exceeding its rating).
  8. Plot the V-I characteristics on graph paper.

Observation Table

Sl. No Voltage (V) Current (mA)
10.00
20.25
30.50
40.75
51.00
61.25
71.50
81.75
92.00
102.25
112.50
122.75
133.00

Model Graph

LED

Precautions

  1. Do not exceed the maximum current rating of the LED (typically 20-30 mA).
  2. Ensure correct polarity of the LED (anode to positive, cathode to negative).
  3. Use a suitable resistor to limit current.
  4. Take readings carefully to avoid parallax error in the multimeter.
  5. Avoid prolonged high voltage to prevent LED burnout.

Applications

  1. Indicator lights in electronic devices.
  2. Display screens (e.g., TVs, monitors).
  3. Lighting (e.g., LED bulbs).
  4. Optical communication systems.
  5. Sensors and detectors.

Post-Lab Questions

  1. Differentiate between direct band gap and indirect band gap semiconductors.
    In direct bandgap semiconductors, electrons transition between bands with the same momentum, emitting light efficiently (e.g., GaAs). In indirect bandgap semiconductors, electrons require a momentum change via phonons, releasing heat instead of light (e.g., Si).
  2. What did you observe about the current below the threshold voltage?
    The current was negligible, and the LED did not glow.
  3. Why does the current increase rapidly after the threshold voltage?
    Beyond the threshold, the LED becomes forward-biased, allowing exponential current flow due to its semiconductor properties.
  4. Why LED operates only in forward biasing conditions
    An LED operates only in forward bias because this condition reduces the depletion region, allowing electrons and holes to recombine across the P-N junction, emitting light via electroluminescence
  5. How does the threshold voltage vary with different LED colors?
    It varies due to different bandgap energies (e.g., red ~1.8V, green ~2.2V, blue ~3V).
  6. Can an LED work in reverse bias?
    No, an LED does not emit light in reverse bias and may get damaged if the voltage exceeds its breakdown limit.
  7. What is the working principle of LED?
    An LED works by applying forward bias to a P-N junction, where electrons from the N-side recombine with holes from the P-side, releasing energy as photons (light) due to the direct bandgap of the material

Outcomes

  1. Successfully determined the threshold voltage of the LED (e.g., 1.8V for red LED).
  2. Studied and plotted the V-I characteristics, confirming the non-linear behavior of the LED.
  3. Understood the practical applications and limitations of LEDs in circuits.

Visual Explantion of Light Emitting Diode Experiment