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PN junction

2014-11-07 03:16  
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A PN junction diode is an electrical device that allow current to move through it in one direction with far greater ease than in the other.

Bipolar transistor circuit diagramBipolar transistor symbol Bipolar transistor circuit diagramBipolar transistor symbol

A semiconductor is a material whose natural resistivity lies between that of conductors and insulators , some example of semiconductor materials are  GERMANIUM, SILICON, SELENIUM .

A semiconductor diode is composed from two semiconductors type , from a PN junction .

What is it a PN junction ?

A PN junction resulted by placing a P type semiconductor in contact with a N type semiconductor  .

A P type material has a positive majority charge carriers ( holes) which are free to move and the N type material has mobile negative majority carriers ( electrons ) .

A semiconductor diode is based on a PN junction , the P type semiconductor is the anode and the N type semiconductor is the cathode .

As you can see in the figure bellow the if the battery is connected to the terminals  P terminal to + and the N terminal to – the diode conducts and the junction is direct polarized   . If the battery is reverse connected  the diode is blocked  and the junction is reverse polarized .

In the diagram bellow you can see how looks like the Silicon Diode I vs V characteristic curve.

Silicon diode I vs V characteristic curve

At breakdown  the current increases so greatly that the diode will be destroyed unless a high series resistance limits  . to prevent the damage of the diode we need to use a diode with a higher reverse voltage rating than any applied voltage   .

To understand better how a diode works imagine that the a diode is simple  switch  or a valve , when the diode conducts and is direct polarized the switch close the circuit and the current follow the entire circuit , but if the diode is reverse polarized the switch break the continuity of the circuit and the current don’t pass ( the resistance is high ) .

In this diagrams you can see that if the diode is direct polarized the circuit is closed and the lamp will flash , but if the diode is reverse polarized the circuit is not closed and the lamp will not flash .

direct polarized diode PN junction reverse polarized diode PN junction

As you can see the diode symbol is an arrow with an vertical line on the top of the arrow    . The direction indicated by arrow represents the direction of electron flow. This conventional symbol was chosen for used in  schematics circuits  to show that the current sense flow from the positive (+) side of the voltage source to the negative (-).

Diode symbol PN junction

This forward-bias  ( direct polarized diode ) voltage drop exhibited by the diode is due to the action of the depletion region formed by the P-N junction under the influence of an applied voltage .

forward bias polarized diode PN junction

If a reverse-biasing  ( reverse mode connection ) voltage is applied across the P-N junction the depletion region expands further resisting any current through it .

reverse bias diode PN junction

If no voltage  applied a thin depletion region exists around the region of the P-N junction preventing current flow.

If the voltage applied to the junction is too low the current will not follow through the circuit , so the minimum forward voltage depends of semiconductors type .  For exsample if the semiconductor diode is from silicon the forward voltage is 0.7 volts but if the used semiconductor is germanium the forward voltage is 0.3 volts .