A diode is an electrical device allowing current to move
through it in one direction with far greater ease than in the other. The most
common kind of diode in modern circuit design is the semiconductor diode,
although other diode technologies exist. Semiconductor diodes are symbolized in
schematic diagrams such as Figure below.
When placed in a
simple battery-lamp circuit, the diode will either allow or prevent current
through the lamp, depending on the polarity of the applied voltage.
forward bias reverse bais
For silicon diodes, the typical forward voltage is 0.7 volts, nominal. For germanium diodes, the forward voltage is only 0.3 volts. The chemical constituency of the P-N junction comprising the diode accounts for its nominal forward voltage figure, which is why silicon and germanium diodes have such different forward voltages. Forward voltage drop remains approximately constant for a wide range of diode currents, meaning that diode voltage drop is not like that of a resistor or even a normal (closed) switch. For most simplified circuit analysis, the voltage drop across a conducting diode may be considered constant at the nominal figure and not related to the amount of current.
Actually, forward voltage drop is more complex. An equation
describes the exact current through a diode, given the voltage dropped across
the junction, the temperature of the junction, and several physical constants.
It is commonly known as the diode equation.
In general, diodes
tend to permit current flow in one direction, but tend to inhibit current flow
in the opposite direction. The graph
below shows how current can depend upon voltage for a diode.
But instead of the
actual voltage-current curve for the diode shown in the thin, lighter red,
curved line below, we're going to imagine that the diode has the voltage-current
curve shown in the thicker, dark red lines below.
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