In semiconductor physics, carrier behavior under high electric fields is a critical area of study due to its implications for device performance and reliability. When an electric field is applied to a semiconductor, charge carriers—electrons and holes—accelerate, gaining kinetic energy. At low fields, carrier velocity increases linearly with the field, following Ohm’s law. However, as the field intensifies, several nonlinear phenomena emerge, including velocity saturation, impact ionization, and hot electron effects. These mechanisms influence device operation and can lead to breakdown if not properly managed.
At moderate electric fields, carrier mobility remains relatively constant, and the drift velocity is proportional to the field strength. However, as the field exceeds a critical threshold, typically on the order of 1–10 kV/cm for silicon, carriers begin to scatter more frequently with optical phonons, which dissipate their energy. This scattering limits the maximum velocity carriers can attain, a phenomenon known as velocity saturation. For silicon, the saturation velocity is approximately 1×10^7 cm/s for both electrons and holes. In compound semiconductors like GaAs, velocity overshoot can occur due to the lower effective mass of electrons, but eventually, saturation is reached at higher fields. Velocity saturation is particularly significant in short-channel transistors, where high fields are common, as it limits current drive and affects switching speed.
Impact ionization becomes dominant at even higher electric fields, usually above 100 kV/cm in silicon. Here, carriers gain sufficient energy to create additional electron-hole pairs by colliding with the lattice. The newly generated carriers can themselves accelerate and cause further ionization, leading to an avalanche multiplication effect. The ionization rates for electrons and holes are described by empirical models such as Chynoweth’s law, which relates the ionization coefficient to the electric field exponentially. Impact ionization is a key mechanism in avalanche breakdown, where uncontrolled multiplication causes a sharp increase in current, potentially damaging the device. In bipolar transistors and certain diodes, this effect is harnessed for controlled avalanche operation, but in MOSFETs, it contributes to parasitic substrate currents and reliability issues like hot carrier degradation.
Hot electron effects arise when carriers acquire energies significantly higher than the thermal equilibrium value. These "hot" carriers can overcome potential barriers, leading to phenomena such as gate oxide injection in MOSFETs. When high-energy electrons tunnel into the oxide, they create trapped charges and interface states, gradually degrading device parameters like threshold voltage and transconductance. Over time, this wear-out mechanism can cause circuit failure. Techniques such as lightly doped drain (LDD) structures and graded junctions are employed to mitigate hot electron effects by reducing peak electric fields near the drain.
The interplay of these mechanisms determines the breakdown characteristics of semiconductor devices. In planar devices, breakdown is often triggered by impact ionization leading to avalanche multiplication. However, in nanoscale transistors, tunneling currents and hot carrier effects dominate at lower voltages due to aggressive scaling. Punch-through breakdown, where the depletion regions of source and drain merge, is another concern in short-channel devices. Proper doping profiles and device design are essential to prevent premature breakdown.
Material properties also play a crucial role in high-field behavior. Wide bandgap semiconductors like GaN and SiC exhibit higher breakdown fields due to their larger bandgaps, making them suitable for high-voltage applications. However, they are not immune to hot carrier effects, which can still degrade reliability over time. In heterostructures, the presence of interfaces introduces additional scattering mechanisms and localized high-field regions, complicating carrier dynamics.
Understanding these phenomena is essential for optimizing device performance and ensuring long-term reliability. While power devices (covered under G91) are explicitly designed to handle high fields, conventional transistors and integrated circuits must also account for these effects to prevent unintended consequences. Advanced simulation tools and characterization techniques are employed to study high-field transport and predict device behavior under stress conditions.
In summary, carrier behavior under high electric fields is governed by velocity saturation, impact ionization, and hot electron effects, each contributing to device performance and reliability challenges. These mechanisms are critical considerations in modern semiconductor design, particularly as devices continue to scale downward and operate at higher frequencies and power densities. By carefully engineering materials and device architectures, it is possible to mitigate adverse effects and extend the operational lifespan of semiconductor components.