Photodiodes and solar cells — Hinglish notes

2.3.5 · Hardware › Diodes & Applications

PN junction light pe respond KYUN karta hai?

Bandgap threshold KYUN? Ek electron jo valence band mein baitha hai woh "bonded" hai. Usse free karne ke liye tumhe kam se kam energy gap $E_{g}$ supply karni padegi. Ek photon exactly $h ν$ energy ka ek packet hai; agar woh packet bahut chhota hai, toh woh gap bridge nahi kar sakta aur bas guzar jaata hai (material uske liye transparent hai).

$E_{p h} = h ν = \frac{h c}{λ} \geq E_{g} \Rightarrow λ \leq λ_{ma x} = \frac{h c}{E _{g}}$

Junction EHPs ko current mein KAISE convert karta hai

Toh total diode current ordinary diode equation minus ek light-generated term ban jaati hai:

$I = normal diode I_{0} (e^{V / n V_{T}} - 1) - light I_{L}$

Minus sign KYUN? Photocurrent ek reverse (n→p external) current hai, forward diode current ke sign ke opposite. Zyada light ⇒ bada $I_{L}$ ⇒ poori I–V curve neeche shift ho jaati hai.

Mode 1 — PHOTODIODE (reverse-biased detector)

Reverse bias KYUN?

Depletion region ko wider karta hai ⇒ bada collection volume ⇒ zyada EHPs catch hote hain.
Wider ⇒ lower junction capacitance ⇒ faster response (high-speed optical comms).
Linear relation $I_{L} \propto$ optical power ⇒ ek clean measurement.

Mode 2 — SOLAR CELL (power generator, zero external bias)

Do key operating points, dono $I = I_{0} (e^{V / n V_{T}} - 1) - I_{L}$ se derived:

Worked Examples

Common Mistakes (Steel-manned)

Flashcards

Ek photon ko electron–hole pair create karne ke liye kya condition satisfy karni chahiye?

E_{p h} = h ν \geq E_{g}

(photon energy ≥ bandgap).

Cut-off wavelength ka formula aur engineer's shortcut?

λ_{ma x} = h c / E_{g} \approx 1.24/ E_{g} (eV) μ m

Reverse bias ek photodiode ki help KYUN karta hai?

Depletion region wider hoti hai (zyada collection, faster) aur junction capacitance kam hoti hai (faster response).

Dono devices ko cover karne wali full I–V equation?

I = I_{0} (e^{V / n V_{T}} - 1) - I_{L}

Solar cell ka short-circuit current?

I_{sc} = I_{L}

(

V = 0

set karo, diode term vanish ho jaata hai).

Solar cell ka open-circuit voltage?

V_{oc} = n V_{T} ln (I_{L} / I_{0} + 1)

; light ke saath sirf logarithmically badhta hai.

Responsivity ki definition?

R = I_{L} / P_{o pt} = η q λ / (h c)

; light ke watt per current.

Ek Si cell ka

V_{oc}

sirf ~0.5 V KYUN hota hai?

Kyunki yeh

n V_{T} ln (I_{L} / I_{0})

hai — ek bade ratio ka log phir bhi chhota number deta hai.

Fill factor kya hai?

F F = V_{m p} I_{m p} / (V_{oc} I_{sc})

; I–V curve kitna square/ideal hai.

Solar cell kis quadrant mein operate karta hai?

Fourth quadrant (V>0, I<0): power-deliver karne wala region.

Recall Feynman: 12-saal ke bachche ko explain karo

Ek chhoti pahadi par ek tiny gate imagine karo. Sunlight chhoti energy-bullets jaisi hai. Agar ek bullet strong enough ho, toh woh ek marble knock loose kar deti hai. Pahadi (built-in field) loose marbles ko hamesha ek side neeche roll karti hai, toh woh pile up ho jaate hain aur ek toy car (electricity!) push kar sakte hain. Agar tumhe sirf bullets count karni hain, toh pahadi ko hard tilt karo (reverse bias) aur dekho marbles kitni tez roll karte hain — yahi photodiode hai. Agar tumhe marbles se apni toy car power karni hai, toh unhe pile up hone do aur ek wheel ghoomne do — yahi solar cell hai. Same pahadi, do kaam. Aur ek weak, wimpy bullet (bahut red light) koi marble knock loose nahi kar sakti, chahe kitni bhi throw karo.

Connections

PN Junction Diode — same device physics, built-in field & depletion region.
Diode I-V Characteristic Equation — parent equation jisme se humne $I_{L}$ subtract ki.
Reverse Bias and Depletion Width — kyun reverse bias detection speed up karta hai.
Bandgap and Semiconductor Materials — $λ_{ma x}$ set karta hai; kyun Si vs GaAs vs Ge differ karte hain.
LEDs — inverse device (current → light instead of light → current).
Photoelectric Effect — $E_{p h} \geq E_{g}$ threshold ki quantum root.