Photodiodes: Light Detection and Measurement

Photodiodes: Converting Light to Electrical Signal

Photodiodes are semiconductor devices that detect photons and convert light intensity into measurable electrical current. They're fundamental to fiber optic communications, imaging systems, and optical measurement devices.

Photodiode Operating Physics

Photon Absorption and Generation

When a photon with sufficient energy strikes the semiconductor junction, it creates an electron-hole pair. The electric field at the junction separates these charges:

Quantum Efficiency

Quantum efficiency describes the percentage of incident photons converted to collectible electrons:

Photodiode Operating Modes

Photovoltaic Mode (Zero Bias)

No external voltage applied. The photodiode generates its own voltage:

Advantages:

• Self-powered operation
• Low noise
• No external bias supply needed

Disadvantages:

• Lower sensitivity
• Non-linear response (logarithmic)
• Slower response time (~ms)
• Limited dynamic range

Applications:

• Solar cells
• Light meters (used without amplification)
• Passive optical monitoring

Photoconductive Mode (Reverse Bias)

External reverse voltage applied to the photodiode creates wider depletion region:

Photocurrent generation:

Advantages:

• Higher sensitivity
• Linear response (photocurrent proportional to illumination)
• Faster response time (ns to µs)
• Larger dynamic range

Disadvantages:

• Requires bias supply
• Higher dark current (shot noise)
• Reverse voltage limitation (maximum ratings)

Typical Parameters (Reverse Bias):

• Dark current: 0.1-10 nA (dependent on bias and temperature)
• Response time: 1-100 ns (fast-recovery designs can reach <1ns)
• Capacitance: 10-100 pF (frequency response dependent)

Photodiode Transimpedance Amplifier Circuit

The standard circuit for practical photodiode measurements uses transimpedance amplifier topology:

Transfer Function:

Noise Performance:

Total referred-to-input noise:

Compensation Network (Cf):

Feedback capacitor Rf-Cf creates pole at:

Practical Photodiode Parameters

Responsivity

Describes sensor sensitivity to light:

Responsivity vs Wavelength for Silicon:

Spectral Response

Photodiode sensitivity varies dramatically with wavelength. Silicon detects up to ~1100 nm; beyond requires specialized materials:

Bandwidth and Response Time

Bandwidth depends on:

1. RC time constant (R_L × C_j)
2. Amplifier bandwidth (if used)
3. Intrinsic minority carrier transit time

Advanced Photodiode Types

Avalanche Photodiodes (APD)

Applied high reverse voltage causes impact ionization, creating gain:

Trade-offs:

• ✓ High sensitivity (10-100× more than PIN)
• ✓ Excellent signal-to-noise
• ✗ Temperature sensitive
• ✗ Requires precision high-voltage supply
• ✗ Excess noise at high gains

PIN Photodiodes

"Positive-Intrinsic-Negative" design with wide intrinsic region:

Advantages:

• Large depletion region = high QE
• Low capacitance possible (narrow intrinsic region)
• Linear response
• Simple operation

Optical Communication Application

Fiber optic receiver using photodiode:

Typical Data:

• Wavelength: 850nm or 1310nm or 1550nm
• Fiber loss: 0.5-10 dB/km (1550nm is lowest loss)
• Data rates: 10 Gbps to 400 Gbps (modern)
• Receiver sensitivity: -20 to -35 dBm typical

Design Example: Light Meter

Simple light intensity measurement system:

Component Selection:

1. Photodiode: BPW21 (PIN silicon)

• Peak wavelength: 950nm
• Responsivity: 0.65 A/W
• Capacitance: 85pF

1. Op-amp: LM358 (general purpose)

• Sufficient for audio-frequency light changes

1. Transimpedance feedback: Rf = 1 MΩ

Calculation for 1 lux illumination:

Typical silicon photodiode in bright daylight:

• 1 lux ≈ 0.15 µW/cm² × Sensor area

For 1cm² sensor at 950nm:

• Power ≈ 0.15 µW
• Photocurrent = 0.15 × 10⁻⁶ × 0.65 = 97.5 nA
• Output voltage = -97.5 nA × 1 MΩ = -97.5 mV

SNR Calculation:

Shot noise current: I_n = √(2qIp) = √(2 × 1.6×10⁻¹⁹ × 97.5×10⁻⁹) ≈ 5.6 pA

Voltage noise: V_n = 5.6 pA × 1 MΩ = 5.6 µV

SNR = 97.5 mV / 5.6 µV ≈ 17,400 (84 dB)

Interview Q&A

Q1: How does a photodiode differ from a phototransistor?

A: Photodiodes are 2-terminal devices (fast, low gain) while phototransistors are 3-terminal (slower, ~100-1000× gain). Photodiodes suit high-speed applications (fiber optics, fast pulse detection), while phototransistors work well for simple light sensing where speed isn't critical. Photodiodes require amplification for weak signals.

Q2: What is quantum efficiency and why does it matter?

A: Quantum efficiency is the percentage of photons converted to collectible electrons. Higher QE means more current output for the same optical power. It varies dramatically with wavelength—silicon peaks around 850-950nm. When designing detection circuits, you must account for QE to predict photocurrent magnitude.

Q3: Explain photovoltaic vs photoconductive modes. When would you use each?

A: Photovoltaic mode is unpowered—the diode generates its own voltage (solar cells, passive light meters). Photoconductive uses reverse bias for faster response and linear response. Use photovoltaic for autonomous operation; photoconductive for high-speed detection and amplified circuits.

Q4: How does dark current affect photodiode measurement accuracy?

A: Dark current is background current with no light present. It's especially problematic for weak signal measurements. At high reverse bias, dark current increases exponentially. Temperature strongly affects dark current (doubles every 5-7°C). For precision measurements, cooling and using low reverse bias are essential.

Q5: Why is transimpedance amplifier topology preferred for photodiodes?

A: Transimpedance amplifiers convert current to voltage with predictable gain (Rf), provide low impedance output, and minimize bandwidth limitations from photodiode capacitance. The virtual ground at the amplifier input keeps the photodiode at nearly zero voltage, minimizing capacitive effects and maximizing bandwidth compared to simple resistor loads.