Light-emitting diodes (LED) operation

2.3.4 · Hardware › Diodes & Applications

WHAT is an LED?

Key facts:

• Light sirf tab emit hoti hai jab forward biased ho (electrons aur holes ko recombine karne ke liye junction ke andar push karna padta hai).
• Color material ki bandgap energy $E_g$ se fix hoti hai, current se nahi.
• Current brightness control karta hai, color nahi.

WHY does an LED emit light?

WHY (physics): Jab conduction band mein ek electron neeche girke valence band mein ek hole fill karta hai, toh use bandgap $E_g$ ke barabar energy lose karni padti hai. Woh energy kahin na kahin jaani chahiye:

• Indirect bandgap (Si, Ge): electron ka momentum ≠ hole ka momentum, isliye ek lattice vibration (phonon) bhi involve hona padta hai → energy heat ke roop mein leak hoti hai. Light ke liye bekar.
• Direct bandgap (GaAs, GaN): momenta match karte hain, isliye poori energy ek single photon ke roop mein bahar aati hai → light ke liye acha.

WHY forward bias? Sirf forward bias depletion barrier ko itna neeche karta hai ki majority carriers junction ke across flood ho jaayein aur opposite type ke carriers se milein. Reverse bias mein, carriers door kheench liye jaate hain — kuch recombine nahi hota, koi light nahi.

HOW to get the photon energy & color

Derivation from first principles

Step 1 — Energy released. Ek recombine hota electron exactly bandgap ke barabar energy lose karta hai: $E_{\text{photon}}=E_g$ Yeh step kyun? Band edges hi sirf allowed states hain; drop conduction-band bottom se valence-band top tak hota hai = $E_g$.

Step 2 — Photon energy frequency ke terms mein. Planck: $E_{\text{photon}}=hf$ Kyun? Quantum mechanics: light $hf$ energy ke packets mein aati hai.

Step 3 — Frequency se wavelength. Light $c$ speed se travel karti hai, aur $c=f\lambda$, isliye $f=c/\lambda$: $E_g=\frac{hc}{\lambda }$ Kyun? $f=c/\lambda$ substitute karne se energy seedha color se link ho jaati hai jo hum dekhte hain.

Step 4 — Emitted wavelength ke liye solve karein: $\boxed{\lambda =\frac{hc}{E_g}}$

The circuit side: current-limiting resistor

Ek LED ek diode hai → iska current voltage ke saath exponentially badhta hai. Ise seedha battery se connect karo toh yeh khud ko destroy kar leti hai. Isliye hum hamesha ek series resistor add karte hain.

Resistor ki value derive karein. Loop ke around Kirchhoff's Voltage Law: $V_S=I_{F}R+V_F$ Kyun? Supply voltage = resistor ke across drop + LED ke across forward drop.

$R$ ke liye solve karein: $\boxed{R=\frac{V_S-V_F}{I_F}}$

• $V_S$ = supply voltage
• $V_F$ = LED forward voltage (roughly iska color: red ≈ 1.8 V, green ≈ 2.2 V, blue ≈ 3.2 V)
• $I_F$ = chahiya gaya forward current (often 10–20 mA)

Worked Examples

Common Mistakes

Active Recall Flashcards

Connections

• PN Junction Diode — LED ek special p–n junction hai.
• Forward and Reverse Bias — LED ko kaam karne ke liye forward bias chahiye.
• Semiconductor Bandgap — $E_g$ color decide karta hai.
• Direct vs Indirect Bandgap — kyun GaAs glow karta hai aur Si nahi.
• Photodiodes and Solar Cells — reverse process (light → current).
• Ohms Law and KVL — series resistor size karne ke liye use hota hai.
• Planck Relation E=hf — $\lambda =hc/E_g$ ka quantum basis.

Concept Map