LED — Revision Notes

⚡ 30-Second Revision

Principle: — Electroluminescence (electron-hole recombination $ightarrow$ photon emission).
Material: — Direct bandgap semiconductors (e.g., GaAs, GaN).
Biasing: — Always forward biased.
Energy-Wavelength Relation: — $E_g = h

u = rac{hc}{lambda}$.

Color: — Determined by $E_g$ (larger $E_g \rightarrow$ shorter $lambda$, e.g., blue).
Protection: — Requires a series current-limiting resistor.
Advantages: — High efficiency, long life, small size, fast switching.

2-Minute Revision

LEDs, or Light Emitting Diodes, are special p-n junction diodes designed to emit light. Their operation relies on the principle of electroluminescence. When an LED is connected in forward bias, electrons from the n-side and holes from the p-side are injected into the depletion region.

In the specific 'direct bandgap' semiconductor materials used for LEDs (like Gallium Arsenide or Indium Gallium Nitride), these injected carriers recombine, releasing energy in the form of photons. The energy of these photons, and thus the color of the emitted light, is directly determined by the bandgap energy ($E_g$) of the semiconductor material, following the relation $E_g = hc/lambda$.

A larger bandgap yields shorter wavelength (blue) light, while a smaller bandgap yields longer wavelength (red) light. LEDs are highly energy-efficient, durable, and have a long lifespan. Crucially, they must always be used with a current-limiting resistor to prevent damage from excessive current flow.

5-Minute Revision

The Light Emitting Diode (LED) is a fundamental optoelectronic device that converts electrical energy directly into light. It's essentially a p-n junction diode fabricated from 'direct bandgap' semiconductor materials, such as Gallium Arsenide (GaAs) for infrared, Gallium Phosphide (GaP) for green, or Indium Gallium Nitride (InGaN) for blue light.

The 'direct bandgap' property is critical because it allows for efficient 'radiative recombination' of electrons and holes, where the energy released is primarily in the form of photons rather than heat.

For an LED to operate, it must be connected in 'forward bias'. This means the positive terminal of the power supply is connected to the p-type side (anode) and the negative terminal to the n-type side (cathode).

This applied voltage overcomes the depletion region's potential barrier, injecting electrons into the p-region and holes into the n-region. These minority carriers then recombine with majority carriers, emitting photons.

The energy of these photons ($E_{photon}$) is approximately equal to the semiconductor's bandgap energy ($E_g$), and this energy dictates the light's wavelength ($lambda$) via the formula $E_g = hc/lambda$.

Therefore, different materials with different bandgaps emit different colors of light.

LEDs exhibit a characteristic forward voltage ($V_F$) and current ($I_F$). Their brightness is directly proportional to the forward current. Due to their exponential I-V characteristic, a 'current-limiting resistor' must always be connected in series with an LED to prevent excessive current and subsequent damage.

Key advantages of LEDs include their high energy efficiency, exceptionally long operational lifespan, compact size, fast switching capabilities, and robustness compared to traditional light sources. They are widely used in indicators, displays, and general illumination.

Prelims Revision Notes

1
Definition: — LED is a p-n junction diode that emits light under forward bias (electroluminescence).
2
Principle: — When forward biased, electrons from n-side and holes from p-side recombine at the junction. In direct bandgap semiconductors, this recombination releases energy as photons.
3
Direct Bandgap: — Essential for efficient light emission. Examples: GaAs, GaP, InGaN. (Indirect bandgap like Si, Ge release energy as heat).
4
Forward Bias: — Anode (+) to p-side, Cathode (-) to n-side. Current flows, light emits.
5
Photon Energy & Wavelength: — The energy of emitted photon ($E_{photon}$) is approximately equal to the bandgap energy ($E_g$) of the material.

* $E_g = h u = \frac{hc}{lambda}$ * Where $h = 6.63 \times 10^{-34},\text{J}cdot\text{s}$ (Planck's constant), $c = 3 \times 10^8,\text{m/s}$ (speed of light). * $1,\text{eV} = 1.6 \times 10^{-19},\text{J}$. * Shortcut: $lambda (\text{nm}) approx \frac{1240}{E_g (\text{eV})}$.

1
Color Determination: — Higher $E_g \rightarrow$ higher photon energy $ightarrow$ shorter wavelength (e.g., blue, UV). Lower $E_g \rightarrow$ lower photon energy $ightarrow$ longer wavelength (e.g., red, IR).
2
Materials for Colors:

* Infrared: GaAs, AlGaAs * Red/Orange/Yellow: GaAsP, AlGaInP * Green: GaP, InGaN * Blue: InGaN * White: Blue LED + yellow phosphor.

1
Current-Limiting Resistor: — Always required in series to protect the LED from excessive current due to its exponential I-V characteristic.

* $R_{series} = \frac{V_{supply} - V_F}{I_F}$, where $V_F$ is forward voltage drop of LED, $I_F$ is desired forward current.

1
I-V Characteristics: — Similar to a diode, but with a specific forward voltage for light emission.
2
Advantages: — High efficiency, long life, small size, fast switching, robustness, low power consumption.
3
Disadvantages: — Higher initial cost, sensitive to temperature (requires heat sinking for high power), specific voltage/current requirements.
4
Comparison with Photodiode: — LED emits light (electricity to light), Photodiode detects light (light to electricity).

Vyyuha Quick Recall

To remember LED characteristics: Light Emits Directly, Energy Varies Color.

Light Emits Directly: Refers to Direct bandgap semiconductors and Electroluminescence.
Energy Varies Color: Emphasizes that Energy bandgap determines the Color (wavelength) of light. Also reminds of eV unit for energy.