Photodiode

Photodiode

1. What a Photodiode Is

Photodiode = p-n junction operated reverse biased to detect light by converting photons to current. When photon $hν≥E_g$ hits depletion region, it creates electron-hole pair that is swept by built-in + applied field ⇒ photocurrent ∝ light intensity.

2. Structure and Bias

p  | depletion (intrinsic) | n   (PIN photodiode: p - i - n, i widens depletion for more absorption and speed)
Reverse bias widens depletion ⇒ larger collection volume, lower capacitance ⇒ faster.
Operation quadrants: 3rd quadrant (reverse bias) for photoconductive mode (fast, linear); 4th quadrant (zero bias) photovoltaic mode (solar cell).

3. Characteristics (exam graph)

• I-V: in dark, diode reverse leakage tiny ($I_0$); under illumination, curve shifts down by photocurrent $I_p = η e P/hν$ (linear with power $P$, quantum efficiency $η$).

in illuminated. At reverse bias $V<0$, $I≈ -I_p$ (current magnitude light-dependent).

4. Figures of Merit

• Responsivity

A/W (e.g., Si at 800 nm, η=0.8 ⇒ R≈0.52 A/W).

• Quantum efficiency $η$ = electrons per photon.
• Response time set by $RC$ and drift transit across depletion.

5. Uses

Light detection for fibre receivers (850/1300/1550 nm InGaAs), light meters, barcode readers, flame sensors.

6. Exam Points

• Draw structure (p-i-n) and band diagram with photon absorption — 5 marks, frequent.
• I-V characteristic shift under illumination — 5 marks, very frequent.
• Explain photocurrent ∝ intensity and reverse bias operation — 5 marks, frequent.
• Formula $R=η e/hν$ — 3 marks.