Photodiode

A photodiode converts light into electric current, and it does this fast enough to drive optical fiber receivers running at gigabit speeds. Every TV remote, solar cell controller, and pulse oximeter depends on this one device.

Core Concept

A photodiode is a p-n junction diode designed to operate in reverse bias. When photons hit the depletion region, they generate electron-hole pairs through the photoelectric effect. These carriers are swept out by the built-in electric field, producing a current proportional to light intensity.

The device is normally reverse-biased at 5V to 20V. This widens the depletion region, reduces junction capacitance, and speeds up carrier collection. The BPW34 is a common silicon photodiode used in lab and industrial sensors.

Two operating modes exist: photoconductive mode (reverse biased, fast, linear) and photovoltaic mode (zero bias, used in solar cells and precision measurements). Most signal-processing circuits use photoconductive mode for its wide bandwidth.

Key Equations

Photocurrent: I_ph = R_λ × P_opt where R_λ is responsivity in A/W and P_opt is incident optical power in W.

Total reverse current: I = I_dark + I_ph where I_dark is the dark saturation current (typically 1 nA to 10 nA for silicon).

Responsivity: R_λ = (η × q × λ) / (h × c) where η is quantum efficiency (0 to 1), q = 1.6×10^-19 C, λ is wavelength in m, h = 6.626×10^-34 J·s, c = 3×10^8 m/s.

Bandwidth: f_3dB = 1 / (2π × R_L × C_j) where R_L is load resistance and C_j is junction capacitance. A larger reverse bias reduces C_j and increases bandwidth.

Given:
  Responsivity R_λ = 0.5 A/W
  Incident power P_opt = 200 µW = 200×10^-6 W
  Load resistor R_L = 10 kΩ = 10×10^3 Ω
  Junction capacitance C_j = 20 pF = 20×10^-12 F

Why this formula:
  Photocurrent is proportional to incident power via responsivity.

Formula:
  I_ph = R_λ × P_opt
  V_out = I_ph × R_L
  f_3dB = 1 / (2π × R_L × C_j)

Substitution:
  I_ph = 0.5 × 200×10^-6
  V_out = 100×10^-6 × 10×10^3
  f_3dB = 1 / (2π × 10×10^3 × 20×10^-12)

Calculation:
  I_ph = 100 µA
  V_out = 100×10^-6 × 10000 = 1.0 V
  f_3dB = 1 / (2π × 200×10^-9)
        = 1 / (1.2566×10^-6)
        = 795.8 kHz ≈ 796 kHz

Final Answer:
  Photocurrent = 100 µA
  Output voltage across 10 kΩ = 1.0 V
  3 dB bandwidth = 796 kHz

Exam Tip: GATE frequently tests the difference between photoconductive and photovoltaic modes. In photoconductive mode, reverse bias is applied and speed is high but dark current adds noise. In photovoltaic mode, bias is zero so dark current is negligible, making it preferred for precision measurements. Also remember: responsivity R_λ has units A/W, not A/lux. Confusing these two units is a common mistake.

Key Properties

• Silicon photodiodes like BPW34 respond to wavelengths from 400 nm (visible) to 1100 nm (near-infrared), covering most practical optical communication bands.
• Responsivity of a typical silicon photodiode peaks around 0.5 A/W to 0.6 A/W at 850 nm wavelength.
• Reverse bias of 5V to 20V reduces junction capacitance from 50 pF (zero bias) to below 10 pF, increasing bandwidth significantly.
• Dark current is typically 1 nA at room temperature and doubles roughly every 10°C rise, limiting sensitivity at high temperatures.
• Rise time of a fast photodiode like BPW34 is about 100 ns at 5V reverse bias with a 50 Ω load.
• Quantum efficiency η ranges from 0.5 to 0.9 for silicon, meaning 50% to 90% of incident photons generate electron-hole pairs.

Quick Revision

• Photodiode operates in reverse bias for signal detection.
• Photocurrent I_ph = R_λ × P_opt, linearly proportional to light power.
• Larger reverse bias means wider depletion region, lower C_j, higher bandwidth.
• Photovoltaic mode: zero bias, low noise, used in solar cells and precision instruments.
• Photoconductive mode: reverse bias, fast response, linear over wide dynamic range.
• Dark current increases with temperature: doubles every 10°C.
• Bandwidth f_3dB = 1 / (2π R_L C_j): reduce R_L or C_j to increase speed.
• Exam trap: Students confuse responsivity (A/W) with sensitivity (A/lux). GATE problems give optical power in watts, so use R_λ in A/W, not the photometric unit.