What Are Silicon Avalanche Diodes (SADs)?

Silicon avalanche diodes (SADs) are semiconductor devices built on reverse-biased silicon p-n junctions that exploit the avalanche multiplication of charge carriers to achieve either high-gain photodetection or stable voltage-reference behavior. When the reverse bias exceeds the breakdown voltage, a single electron-hole pair can trigger a cascade of impact ionizations, multiplying the current by factors of 10 to 10⁶ depending on the operating regime and device design. This multiplication mechanism distinguishes avalanche diodes from ordinary silicon rectifiers and photodiodes.

The physics underlying avalanche operation was established by McKay and McAfee in 1953, who applied Townsend's earlier ionization theory from gas discharges to semiconductor junctions. Subsequent development of guard-ring structures, planar processing techniques, and active quenching circuits progressively eliminated the microplasma defects that caused premature, spatially localized breakdown, enabling reliable and reproducible devices. The historical development of silicon avalanche and Geiger-mode photodiodes traces this progression from early junction observations to modern single-photon detectors.

Avalanche Breakdown Mechanism

Avalanche breakdown occurs when the electric field in the depletion region of a reverse-biased junction is strong enough that free carriers gain sufficient kinetic energy between collisions to ionize the crystal lattice on impact, each impact producing a new electron-hole pair. Those new carriers in turn undergo impact ionization, generating an exponentially growing current. The ionization coefficient, which describes the probability of ionization per unit path length, is a strong function of electric field and depends on whether electrons or holes are the initiating carriers.

Silicon's ionization coefficients for electrons and holes are unequal, which affects the noise characteristics of the resulting multiplication. Devices designed for linear analog gain, such as avalanche photodiodes (APDs) for fiber-optic receivers, operate below breakdown voltage at multiplication factors of 10 to 100. The excess noise factor, a measure of statistical gain fluctuation, rises with multiplication and sets a practical limit on usable gain for APD operation.

Operating Regimes

Silicon avalanche diodes are operated in two distinct regimes depending on the application. Below breakdown voltage, the device operates in linear mode: absorbed photons generate carriers that are multiplied by the electric field, producing an output photocurrent proportional to the input optical signal. This is the regime used in APDs for optical communications at 850 nm, where silicon has good quantum efficiency.

Above breakdown voltage, the device operates in Geiger mode: a single absorbed photon or thermally generated carrier can trigger a self-sustaining avalanche that grows until an external quenching circuit reduces the bias below breakdown and resets the device for the next detection event. This single-photon avalanche diode (SPAD) mode achieves detection efficiencies exceeding 50% in the 400–900 nm wavelength range and timing resolution below 100 picoseconds. The rp-photonics reference on single-photon avalanche diodes provides a detailed treatment of Geiger-mode operating parameters including dead time, afterpulsing probability, and dark count rate.

Voltage Reference and Transient Suppression Applications

Not all silicon avalanche diodes are photodetectors. When operated in steady-state breakdown, a silicon avalanche diode clamps the voltage across its terminals at the breakdown voltage, providing a stable voltage reference. This is the functional basis of the Zener diode in its higher-voltage variants, where avalanche rather than Zener tunneling is the dominant mechanism above approximately 6 V. Transient voltage suppressors (TVS) are high-area avalanche diodes designed to clamp voltage spikes from electrostatic discharge or lightning coupling, absorbing large pulse energies over microsecond timescales. The PMC study of electronic interfaces for SPADs illustrates the circuit design considerations that couple avalanche diode operating characteristics to quenching and readout electronics.

Applications

Silicon avalanche diodes have applications in a wide range of fields, including:

• Single-photon detection for LiDAR ranging and time-of-flight imaging
• Photon-counting detectors in fluorescence microscopy and quantum key distribution
• Voltage reference elements in precision analog circuits
• Transient voltage suppression for electrostatic discharge protection
• Gamma and X-ray photon counting in medical imaging detectors