What Is Impact Ionization?

Impact ionization is a carrier generation process in semiconductors in which a charge carrier, an electron or hole, gains sufficient kinetic energy from an applied electric field to collide with a bound electron in the crystal lattice and promote it from the valence band to the conduction band, creating a new electron-hole pair in the process. The original high-energy carrier, the newly freed electron, and the newly created hole are all available as conduction carriers, so each impact ionization event increases the number of free carriers in the material. When operating conditions allow these secondary carriers to undergo further ionization events, the process becomes self-sustaining and the carrier population multiplies rapidly, a phenomenon known as avalanche multiplication. Impact ionization is central to the operation of several semiconductor devices and is the physical mechanism underlying avalanche breakdown in reverse-biased junctions.

The phenomenon was analyzed theoretically in the 1950s by Kenneth McKay and others working on germanium and silicon p-n junctions, and quantitative models relating ionization rates to electric field strength were developed alongside early transistor physics. It occurs in all semiconductor materials but its characteristics, particularly the ionization threshold energy, depend on the bandgap and band structure of the material, which is why wide-bandgap semiconductors such as silicon carbide and gallium nitride behave differently from silicon in high-field conditions.

Ionization Coefficients and Carrier Dynamics

The rate at which a carrier initiates ionization events as it travels through a high-field region is characterized by the ionization coefficient, denoted alpha for electrons and beta for holes, and defined as the number of electron-hole pairs generated per unit path length. Both coefficients are strong functions of electric field: below a threshold field they are negligible, but above it they increase rapidly, roughly exponentially, with field strength. The ratio k = beta/alpha is an important device design parameter because it governs the noise characteristics of avalanche multiplication; materials with k close to 0 or close to 1 differ substantially in the excess noise they introduce. Research on optimization of impact ionization in metal-oxide-semiconductor field-effect transistors published in Electronics examines how device geometry influences effective ionization ratios and the resulting trade-off between breakdown voltage and on-resistance.

Avalanche Multiplication and Breakdown

When the ionization coefficients are high enough that each generated carrier pair initiates further ionization before leaving the depletion region, the carrier population multiplies geometrically. The multiplication factor M, the ratio of output current to primary photocurrent in an avalanche photodiode, can exceed 100 in optimized designs. If the field continues to increase beyond the point where multiplication is self-sustaining without any external carrier injection, avalanche breakdown occurs: current through the junction rises without further voltage increase, limited only by external circuit resistance. The Springer chapter on impact ionization and avalanche breakdown in semiconductor device physics provides a thorough treatment of the carrier generation integral and its relationship to the avalanche breakdown condition.

Device Applications and Engineering Control

Engineers exploit impact ionization intentionally in avalanche photodiodes (APDs), where the internal gain from controlled multiplication increases detector sensitivity in optical communication and LIDAR receivers. In power semiconductor devices including IGBTs and power MOSFETs, impact ionization is a failure mechanism that must be suppressed through careful doping profiles and field-shaping structures to achieve the required breakdown voltage ratings. Research on non-local effects in avalanche multiplication published on arXiv describes how dead-space effects, the path a carrier must travel before accumulating enough energy for ionization, complicate simple local-field models and require Monte Carlo or non-local analytical treatments for accurate device design.

Applications

Impact ionization has applications in a range of fields, including:

• Avalanche photodiodes for optical fiber communication and single-photon detection
• LIDAR receivers in autonomous vehicle and atmospheric sensing systems
• Power electronics breakdown voltage design in MOSFETs, IGBTs, and diodes
• Radiation detection using multiplication-based detector architectures
• Wide-bandgap semiconductor devices for high-voltage power conversion