Minority carrier motion in a semiconductor Created by Kapil Chhabria Summer 2002.

Motion of holes in an ‘n’ type semiconductor. Several factors determine the movement of holes (minority carriers) in an ‘n’ type semiconductor. Movement is determined by the following parameters: Drift Diffusion Thermal Recombination-Generation Thermal Recombination-Generation Other Phenomenon

Drift Current Due to the influence of an applied electric field, charged particles move either in the direction of the field (holes) or in the opposite direction (electrons). Such a flux of carriers results in a “drift current”. Home

Diffusion Current Diffusion is the process by which particles spontaneously redistribute themselves over the available space as a result of their spatial non uniformity. Moreover, the particles in question are not required to have any charge. If permitted to proceed uninterrupted, the particles redistribute themselves uniformly over the available volume. Home

Thermal Recombination and Generation The external stimuli that create excess carriers cause a state of non-equilibrium. If these stimuli are removed, excess carriers are removed by a process known as recombination. However, recombination is coupled with another process called generation. Recombination is the process in which a hole and an electron combine to their mutual annihilation. Generation is the process whereby an electron-hole pair is formed. Home

Simulation 1. Conditions: Room temperature: Temperature influences the diffusion rate by increasing the kinetic motion of particles. However, with increasing temperature drift mobility decreases which also influences diffusion rate. High Electric Field: Electric field is the driving force behind drift current. Low Doping Density: Lower doping density permits for a greater passage of particles. Hence, at low doping densities (of the majority carriers), both drift and diffusion rates are increased. Simulation.

Explanation of Simulation 1. The lifetime is fixed at 1μs. High Electric Field results in the dominance of drift current over diffusion. As a result, the entire Gaussian profile shifts position. Diffusion constant is small since mobility is high due to low temperature and low doping density. Hence, the Gaussian profile spreads with time.

Simulation 2. Conditions: High temperature: With increasing temperature, drift mobility decreases as does the diffusion coefficient. High Electric Field High Doping Density: At higher doping densities, mobility decreases as do the drift and diffusion rates. Simulation.

Explanation of Simulation 2. The lifetime is fixed at 1μs. High Electric Field results in the dominance of drift current over diffusion. Diffusion constant is small since mobility is low due to high temperature and high doping density. Hence, the Gaussian profile does not spread with time as much as it did in Simulation 1. Notice that though Simulations 1 and 2 are qualitatively similar, the former takes place ten times faster than the later due to greater diffusion rate.

Simulation 3. Conditions: Room temperature: High Electric Field. High Doping Density. Simulation.

Explanation of Simulation 3. The lifetime is fixed at 1μs. High Electric Field results in the dominance of drift current over diffusion. As a result, as seen in the simulation, the entire Gaussian profile shifts position. Diffusion constant is small since mobility is low due to very high doping density. Hence, the Gaussian profile does not spread as much as it did in Simulation 1 with time. Notice that though Simulations 1 and 3 are qualitatively similar, the former takes place approximately one hundred times faster than the later due to greater diffusion rate.

Simulation 4. Conditions: Room temperature. Very Low Electric Field. Low Doping Density: Lower doping density permits for a higher mobility, so both drift and diffusion rates are increased. Simulation.

Explanation of Simulation 4. The lifetime is fixed at 1μs. Very low Electric Field results in the dominance of diffusion over drift. As a result, as seen in the simulation, the Gaussian profile does not shift noticeably, though it does spread with time. Diffusion constant is high since mobility is high due to low doping density. Hence, the Gaussian profile does not spread as much as it did in Simulation 1 with time.