Physics of a diode (continued)

 The graph on the right shows represents relationship of current over voltage in a zener diode.  This graph is similar to the basic junction diode except the junction diode does not have the current drop shown on the left side of the graph (it remains zero). 

In most diodes the holes in the depletion layer won't open until a specific amount of voltage has been surpassed.  Since silicon is the material most diodes are made of, the voltage in most diodes at which the holes can be opened and current be produced is approximately 0.7V. 

Zener diodes are able to produce a current when the certain voltage is surpassed in the reverse-bias configuration.  Zener diodes made of silicon usually create current at around 6V in when it is reverse-biased.

Application of a diode in a circuit:
Consider each circuit schematic below (suppose the junction diode in these circuits is a silicon diode)

In this circuit, the anode (positive end) is connected to the negative end of the battery.  Therefore, charges in the diode layers are attracted to the outermost edges, so the depletion zone is at its largest, no current is flowing, and the diode is off.  A voltage VD still exists in the diode even when a current doesn't.  Because the orientation of the diode is opposite to the battery, the voltage going through the diode is the negative value of the battery, or -5V.

This schematic is identical to the one on the left except the diode orientation has been reversed.  Because the anode is now connected to the positive end of the battery, charges are interacting in the diode and current is flowing through.  Since the voltage across a diode when current is going through it is 0.7, we can determine the current through the circuit using Ohm's Law (V=IR).

The voltage through the resistor can be determined by subtracting the diode voltage from the source voltage.  Since the current through the circuit is uniform, the current through the resistor, diode, and the wires is:

I = V/R = (5V-0.7V)/(1000Ω) = 4.3mA


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