FORWARD BIAS:

When the anode of the diode is connected with the positive terminal of the battery and the cathode is connected to the negative terminal of the battery, it is said as "forward biasing" and this is what happens inside: Since a diode is a simple p-n junction, few electrons in the portion of p-type (that came during the time of connection of p-type and n-type) are "sucked" by the positive terminal of the battery creating vacant spaces at the junction. This consequently allows more electrons in the n-type region to cross the junction and enter into those vacant spaces (holes), since the positive terminal of the battery is continuously "pulling" electrons from the p-type. On the other hand, electrons are being pushed by the negative terminal of the battery into the diode.

So, a constant "pull" of electrons by the positive terminal of battery from p-type and "push" of electrons from the negative terminal of the battery into the "n-type" region (consequently getting into the p-type region through the junction, as the vacant spaces creates) establishes the flow of current.

In such a way a current (flow of electron) starts flowing through the diode. But to be sure that the current is passing through the diode, the diode must be in "turn-on" stage. And this happens when the battery voltage exceeds the barrier voltage (i.e 0.7 volts for Silicon and 0.3 volts for Germanium)

REVERSE BIAS:

When the anode of the diode is connected with the negative terminal of the battery and cathode is connected with the positive terminal of the battery, this is what happens: The positive terminal of the battery "sucks" all the electrons in the n-type region (n-type being connected with the positive terminal of the battery), creating vacant spaces (holes). On the other hand, electrons are injected in the holes of the p-type region.

This makes the depletion region "wider", creating a more obstructing barrier. Consequently, it does not allow any electron to flow through the diode because positive terminal of the battery is "pulling" electrons from n-type region, in contrast with pushing electrons in the case of forward bias.

As a result no current flows through the diode when it is reverse biased.

DIODE CHARACTERISTIC CURVE:

The behavior of the diode, when it is forward or reverse biased:

In forward biasing, the diode starts passing current (or turns on) after a little voltage (that is obviously the barrier voltage: 0.7 volts). After that, increasing voltage results in the increase of current.

Looking at the reverse bias region of the graph, the diode does not conduct up to very high voltages but it starts conducing at extremely high voltage when it enters into "avalanche effect". (Later about avalanche effect)

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