Quasi-Ballistic Transport

Quasi-Ballistic Transport is the operating regime of modern short-channel transistors where carriers experience only a few scattering events crossing the channel — positioned between purely diffusive transport and ideal ballistic flow, it describes the physics of leading-edge 5nm and 3nm node devices.

What Is Quasi-Ballistic Transport?

• Definition: Transport characterized by a small but nonzero number of scattering collisions during channel traversal, resulting in performance between the diffusive and ballistic limits.
• Backscattering Coefficient: The key parameter is r, the fraction of carriers injected from the source that backscatter and return to the source rather than crossing to the drain. Lower r means higher current.
• Current Formula: On-state current equals ballistic current multiplied by (1-r)/(1+r), so even a backscattering coefficient of 0.3 reduces current to roughly 54% of the ballistic limit.
• Physical Picture: Most injected carriers make it across with one or two phonon collisions; a minority scatter backward early in the channel and are lost from the current.

Why Quasi-Ballistic Transport Matters

• Dominant Regime: Advanced logic transistors at 5nm and below operate primarily in the quasi-ballistic regime — making backscattering physics the central quantity to optimize rather than classical mobility.
• Model Requirement: Standard drift-diffusion TCAD cannot correctly predict current in this regime; quasi-ballistic compact models or Monte Carlo simulation are needed for accurate device analysis.
• Process Target: Process improvements that reduce backscattering near the source — through better source/drain abruptness, reduced interface roughness, or channel strain — directly translate to higher drive current.
• Contact Resistance Interaction: As channel backscattering decreases, external parasitics such as contact resistance and access-region resistance become relatively more important performance limiters.
• Temperature Sensitivity: Higher operating temperature increases phonon density and raises the backscattering coefficient, worsening quasi-ballistic efficiency and degrading hot-chip performance.

How It Is Analyzed and Optimized

• Scattering Theory: The virtual source model and McKelvey flux theory provide compact analytical frameworks for extracting backscattering coefficients from measured I-V characteristics.
• Monte Carlo Simulation: Full-band stochastic simulation directly counts scattering events per carrier trajectory, providing the most physically complete picture of quasi-ballistic behavior.
• Channel Engineering: Strained silicon and SiGe channels increase injection velocity and reduce phonon scattering rates, improving ballisticity without changing gate length.

Quasi-Ballistic Transport is the real-world physics of cutting-edge transistors — understanding and minimizing backscattering near the source is the central challenge of device engineering at 5nm and below.