8.3 Avalanche photodiodes and photomultipliers

Avalanche photodiodes and photomultipliers are high-gain detectors that amplify weak light signals. They use different mechanisms to multiply charge carriers, enabling detection of low light levels and even single photons.

These devices offer advantages like high sensitivity and fast response times. However, they also have drawbacks such as noise and high operating voltages. Understanding their operation is crucial for applications in low-light detection and photon counting.

Avalanche Photodiodes

Avalanche Multiplication Process

• Avalanche photodiodes (APDs) operate at high reverse bias voltages near the breakdown voltage
• Under high electric fields, photogenerated carriers accelerate and gain sufficient energy to ionize lattice atoms through impact ionization
  • Creates secondary electron-hole pairs, which also accelerate and cause further impact ionization
• Avalanche of charge carriers amplifies the photocurrent, resulting in internal gain (typically 50-1000)
• Multiplication factor M depends on the applied reverse bias voltage
  • Higher bias voltage leads to higher multiplication factor (Silicon APDs)

Key Performance Parameters

• Breakdown voltage
  • The reverse bias voltage at which the APD undergoes avalanche breakdown
  • APD is typically operated at a voltage slightly below the breakdown voltage for stable operation
• Gain-bandwidth product (GBP)
  • Represents the maximum achievable gain-bandwidth combination of an APD
  • As the gain increases, the bandwidth decreases due to increased avalanche build-up time
  • GBP is a constant for a given APD structure and material (InGaAs APDs have higher GBP than Silicon APDs)
• Excess noise factor (F)
  • Quantifies the increase in noise due to the statistical nature of the avalanche multiplication process
  • Excess noise factor depends on the ratio of hole-to-electron ionization coefficients (k)
    • Materials with k ≈ 0 (InP) have lower excess noise compared to materials with k ≈ 1 (Silicon)

Photomultipliers

Electron Multiplication Process

• Photomultipliers (PMTs) consist of a photocathode, a series of dynodes, and an anode in a vacuum tube
• Incident photons strike the photocathode, releasing electrons via the photoelectric effect
• Electrons are accelerated towards the first dynode by a high voltage
  • Upon striking the dynode, multiple secondary electrons are emitted through secondary emission
• The secondary electrons are accelerated towards the next dynode, causing further electron multiplication
• The process continues through a series of dynodes (typically 8-14), resulting in a large number of electrons collected at the anode

Dynode Characteristics

• Dynodes are electrodes designed to promote secondary electron emission
• Made of materials with high secondary emission coefficients (Copper-Beryllium, Gallium Phosphide)
• Dynode geometry and arrangement affect the electron collection efficiency and transit time spread
  • Venetian blind, box-and-grid, and linear-focused dynode structures are commonly used
• The applied voltage between dynodes determines the electron acceleration and multiplication factor

Performance Advantages and Limitations

• Photomultipliers offer extremely high gain (10^6 to 10^8) and low noise, making them suitable for detecting low light levels (single photon detection)
• Wide spectral response range, from ultraviolet to near-infrared, depending on the photocathode material (Multialkali, GaAsP)
• Fast response time (nanosecond scale) due to the short transit time of electrons between dynodes
• Limitations include sensitivity to magnetic fields, high operating voltages (1-2 kV), and relatively large size compared to solid-state detectors (APDs)