A pn junction is the combination of a p- and n-type material.
The metallurgical junction is the imaginary boundary between the two layers.
When combined, electrons diffuse from the n- to the p-type region, and holes visa versa.
When they depart, the electrons leave behind positively charged donor ions. The holes likewise leave negative acceptor ions.
The positively and negatively charged ions cause an electric field to form. The field acts against diffusion currents to pull the electrons and holes back to their original regions. These are called drift currents.
Eventually, an equilibrium is reached when the diffusion and drift currents are exactly equal.
The electric field that now exists, however, establishes a built-in voltage, which acts to drive the bands of the two halves apart.
The difference in band energy levels makes it extremely difficult for charge carriers to cross the depletion region, effectively rendering the sample non-conductive.
Applying a positive voltage to the p-type side, known as forward biasing, pushes the bands back together by overpowering the internal electric field. Once a voltage equal to the built-in voltage is applied, the junction starts to conduct very rapidly.
If no external control (resistance) is present to slow the current flow, the junction will conduct so much that it overheats and burns.
This structure is called a diode, and its uses in modern devices are vast.
Without a doubt, that was the most difficult installment in the series to grasp conceptually, so well done!
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