Semiconductor P-N junction, electron/hole diffusion & drift, non-uniform doping profile

The p-n junction is a semiconductor structure in diodes, transistors, solar cells, and LEDs. The junction behavior is based on mobile electrons in the n-type region and mobile holes in the p-type region, and their interactions across the junction.
Under equilibrium without any external voltage bias, a region surrounding the junction is depleted of all mobile electrons and holes, and contains only immobile donor and acceptor atoms as charged particles. The fixed charged regions result in an electric field that holds back the majority electrons and holes but sweeps across any minority carriers to the opposite side.
Applying an external reverse bias (positive voltage to the n region), causes the depletion region to get larger and increases the barrier against majority carriers crossing the junction. Only a tiny, usually negligible, current flows because of scant minority carriers at the junction.
Conversely, applying an external forward bias (positive voltage to the p region) narrows the depletion region and decreases the barrier against majority carriers, to the point where a significant number of majority carriers have enough thermal energy to cross the junction to the other side. This is called minority carrier injection.
The numbers of injected minority carriers increases exponentially with the applied forward voltage. This gives the p-n junction diode its exponential current characteristics.
This is the first of several videos describing the p-n junction physics and electrical characteristics. The following viewing sequence is recommended.
P-N Junctions 1 (this video) -- Semiconductor P-N junction, electron/hole diffusion & drift, non-uniform doping profile: • Semiconductor P-N junc...
P-N Junctions 2 -- Depletion region, electric field barrier, built-in potential, reverse bias: • P-N junction, depletio...
P-N Junctions 3 -- Forward bias, Boltzmann, majority & minority injected carrier concentration profiles: • P-N junction forward b...
P-N Junctions 4 -- Injected carrier profile, diffusion length L, electron/hole currents vs X, Kirchhoff current law: • P-N junction, injected...
P-N Junctions 5 -- Electron/hole flow, exponential diode equation, diode as valve, photovoltaic solar cell, LED: • P-N junction, electron...
If you are not already familiar with the concepts of mobile electrons and holes in semiconductors, start with the following videos:
Electrons and holes in semiconductors: intrinsic/extrinsic silicon, donors & acceptors, mass action: • Electrons and holes in...
Semiconductor doping, donor/acceptor electron/hole, mass action, pH analogy: acid/base H+/OH- (optional): • Semiconductor doping, ...
The first video in the series shows why diffusion and drift of mobile charge carriers occur with non-uniform doping. A piece of silicon is doped with boron, an acceptor atom that produces holes, with linearly decreasing concentration from left to right.
You might expect the hole concentration to match the boron concentration, but this is not a stable equilibrium state, because of diffusion. When you have a material made of mobile particles, the particles tend to spread from areas of higher concentration to lower concentration, for example, an odor spreading through a sealed room.
After diffusion runs its course, you might expect the holes to settle into a flat, even distribution. But this is not a stable state either because even as the holes move, the charged boron atoms are immobile. This gives rise to areas of net static charge as the hole and boron distributions are mismatched, resulting in an electric field within the material.
Although the electric field is difficult to detect in the laboratory, it is as real and an electric field applied externally. Charge particles move under the force of the field. Particle movement caused by an electric field is called drift.
Under equilibrium, the hole concentration profile has a slope causes diffusion toward the more lightly doped side, but this is exactly balanced by drift toward the left caused by the electric field resulting from charge mismatch. Although holes are constantly in motion, the total net flow is zero. This is true at every point along the block of silicon.
The next video in the p-n junction series shows the p-n junction doping profile, static charge concentration profile, electric field profile, and built-in potential graph, and shows the effects of reverse bias: • P-N junction, depletio...
For energy band model (conduction band, valence band, Fermi level), see www.chu.berkel...
For a detailed analysis of p-n junctions (Poisson’s equation, capacitance, charge storage, tunneling/avalanche junction breakdown, quasi-equilibrium boundary, laser diodes, Schottky diodes, optoelectronics), see www.chu.berkel...
CMOS inverter operation: • CMOS Tech: NMOS and PM...

[@norliegh]

Incredible Video! Thank you for making these. ❤