Skip to main content. Leave this field blank. Search form Search. Formation of a PN-Junction Overview Joining n-type material with p-type material causes excess electrons in the n-type material to diffuse to the p-type side and excess holes from the p-type material to diffuse to the n-type side. A diode PN junction in an electrical circuit allows current to flow more easily in one direction than another.
Forward biasing means putting a voltage across a diode that allows current to flow easily, while reverse biasing means putting a voltage across a diode in the opposite direction. The voltage with reverse biasing doesn't cause any appreciable current to flow. This is useful for changing AC current to DC current. It has other uses in manipulating electronic signals as well. If a voltage is applied across the diode in such a way that the n-type half of the diode was connected to the positive terminal of the voltage source and the p-type half was connected to the negative terminal, electrons from the external circuit would create more negative ions in the p-type region by "filling the holes" and more positive ions would be created in the n-type region as electrons are displaced toward the positive terminal of the voltage source see Figure 2.
Hence, the depletion region would increase and the voltage between the p-type and n-type regions would also increase as the total charge on each side of the junction increases in magnitude until the voltage across the diode equals and opposes the applied voltage and cancels it out, ceasing the current through the circuit.
This process happens nearly instantaneously and results in essentially no current flow through the circuit when voltage is applied in this direction across the diode.
This is known as a reverse-biased p-n junction. When the voltage is applied in the opposite direction across the diode, the depletion region begins to shrink see Figure 3. In a reverse-biased diode, the electrons and holes would be pulled away from the junction, but a forward-biased scenario ensures that the electrons and holes move toward the junction as they are repelled from the positive and negative terminals of the voltage source respectively.
Hall, Phys. Miles and D. Muller and T. Sah, R. Noyce, and W. IRE 45 , Shockley and W. Read, Phys. Sze, Physics of Semiconductor Devices , 2nd ed. Depending on the impurity and semiconductor used, the difference in energy level between conduction and valence bands can be large enough to emit the energy as a photon of light. This is a light emitting diode, or LED. Watch this video to find out more about how LEDs work.
Diodes can also be made so that the junction will absorb photons of light. When a photon of light is absorbed, it provides energy that can cause electrons from the valence band of the p-type semiconductor to be promoted to the conduction band. The electrons flow towards the n-type semiconductor in the junction.
This creates a potential difference across the diode and allows it to generate an EMF.
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