EC-6.2 Drift, Diffusion and Carrier Transport
You can name the two mechanisms that move carriers, write the current each produces, and explain why an injected excess of carriers spreads out and dies away.
Before:EC-1. Circuit AnalysisEC-5. Physics and Materials for ElectronicsUnlocks:S4-1. Analog CircuitsS5-4. Linear and Digital Integrated CircuitsPE1-1. VLSI TechnologyPE1-2. Fiber Optic CommunicationPE2-1. CMOS Analog and Digital IC DesignEC-12. Electrical Energy, Machines and Power Electronics
Carriers move for exactly two reasons: a field pushes them, or there are more of them here than there. Drift is the first and diffusion the second, and every current in every semiconductor device is one, the other, or both. Diffusion is the part that surprises people, because nothing is pushing and the carriers move anyway, purely because random motion carries more of them out of a crowded region than into it. That is the diffusion equation from the mathematics module, appearing in its second home.
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Drift: what a field does to a carrier, and why it does not accelerate
A field pushes carriers along, but scattering interrupts them constantly, so they reach a steady average velocity rather than accelerating. That velocity is the mobility multiplied by the field, and the current follows from it.
NPTEL: Introduction to Semiconductor Devices · CourseConductivity, written from the carriers up
Both carrier types contribute, each in proportion to its concentration and its mobility, and adding the two gives the conductivity of the material. This is where the resistance of a doped region actually comes from.
Diffusion: movement with nothing pushing
Random thermal motion carries more carriers out of a dense region than into it, so a concentration difference produces a current on its own. The current is proportional to how steeply the concentration changes with position.
NPTEL: Solid State Devices · CourseThe Einstein relation, and why the two mechanisms are linked
Mobility and the diffusion coefficient are not independent: both come from the same scattering, and their ratio is fixed by temperature. Knowing one gives you the other without another measurement.
Generation and recombination, and carrier lifetime
Carriers are constantly created by thermal energy and destroyed by recombining, and in equilibrium the two rates match. An injected excess decays with a characteristic lifetime, which sets how fast a device can be switched off.
NPTEL: Introduction to Semiconductor Devices · CourseDiffusion length, and the distance an excess actually travels
An injected excess spreads and recombines at the same time, so it falls away over a characteristic distance rather than reaching everywhere. That distance is what decides how thin a transistor's base has to be.
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