The diode equation describes the I-V (current vs voltage) characteristics of the p-n junction under forward and reverse bias:
I =Io[exp(qV/kT -1)].
k = Boltzmann constant
T = Kelvin temperature
q = charge of an electron
The reverse saturation current depends on the doping levels and the physical size of the diode. The e exponent, qV/kT, is V/.026 volts at room temperature.
When V is zero, the current is zero as expected.
When V is more than about .1 volt, the exponent term is a large number (around 47 or more) and the minus 1 can be ignored. Then the current is just an exponential function of V:
I =Io*exp(qV/kT ).
When V is more negative than about -.1 volt, the exponent term is close to zero (around 0.02 or less) and can be ignored. Then the current is just minus Io, the reverse saturation current:
I=-Io
The diode is said to operate like a one-way valve. A perfect valve would operate like an open circuit or infinite resistance when the voltage is negative, or a short circuit or zero resistance when the voltage tries to go positive. The current flows only in the positive direction.
A diode is not quite a perfect valve. With a negative voltage applied, the current is very near zero. With a positive voltage applied, a large current flows with very little resistance after a small voltage threshold is reached. The small threshold depends on the reverse saturation current, temperature, and the amount of current that is considered normal for forward operation of the diode.
At larger negative voltages, the p-n junction can break down and conduct a large current. This is reversible and non-destructive as long as the current is small enough to prevent heat damage. A diode designed to operate this way is called a Zener diode. There are two mechanisms that cause reverse bias junction breakdown, called avalanche and tunneling breakdown.
In avalanche breakdown, a strong electric field in the depletion region accelerates mobile charges to such a speed that they can impact silicon atoms and generate new electron-hole pairs, which themselves are accelerated to a high speed, and so on. Tunneling breakdown occurs when one side is very strongly doped and therefore has a very narrow depletion region through which quantum tunneling of majority carriers can occur.
A p-n junction can operate as a solar cell. Light shining on the semiconductor generates new electron-hole pairs. Minority carriers at the junction are swept to the opposite side, where they become majority carriers and escape recombination. The newly created particles move away from the junction, providing a current for electric power.
A light-emitting diode is a solar cell operating in reverse. An external power source feeds electron and hole majority carriers toward the junction. They are injected across the junction, where they recombine with the opposite type of carrier, releasing energy as light. The wavelength of the generated photons depends on the energy gap through which the particles fall, following the Planck equation, E=hv.
This is the fifth of several videos describing the p-n junction physics and electrical characteristics. The following viewing sequence is recommended.
P-N Junctions 1 -- Semiconductor P-N junction, electron/hole diffusion & drift, non-uniform doping profile: ua-cam.com/video/vcTR8RGjqpI/v-deo.html
P-N Junctions 2 -- Depletion region, electric field barrier, built-in potential, reverse bias: ua-cam.com/video/RjqLMU77Mec/v-deo.html
P-N Junctions 3 -- Forward bias, Boltzmann, majority & minority injected carrier concentration profiles: ua-cam.com/video/GHfXTlAowgs/v-deo.html
P-N Junctions 4 -- Injected carrier profile, diffusion length L, electron/hole currents vs X, Kirchhoff current law: ua-cam.com/video/Udxpe41cZQ4/v-deo.html
P-N Junctions 5 (this video) -- Electron/hole flow, exponential diode equation, diode as valve, photovoltaic solar cell, LED: ua-cam.com/video/XWSxCSql1wU/v-deo.html
If you are not already familiar with the concepts of mobile electrons and holes in semiconductors, start with the following videos:
Whole playlist covering electrons/holes, p-n junctions, NMOS/PMOS/CMOS technology, and silicon Miller indices: ua-cam.com/play/PLaqfpQcOfMt4RIP2WGy5uN1MOTHybtWFw.html
Electrons and holes in semiconductors: intrinsic/extrinsic silicon, donors & acceptors, mass action: ua-cam.com/video/QQ6Wr5VC3bA/v-deo.html
Semiconductor doping, donor/acceptor electron/hole, mass action, pH analogy: acid/base H+/OH- (optional): ua-cam.com/video/-GJemTcAgjg/v-deo.html
For energy band model (conduction band, valence band, Fermi level), see www.chu.berkeley.edu/wp-content/uploads/2020/01/Chenming-Hu_ch1-3.pdf
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.berkeley.edu/wp-content/uploads/2020/01/Chenming-Hu_ch4-1.pdf