A diode conducts a small current in the forward direction up to a threshold voltage. This means that there is a small voltage across a conducting diode. This voltage for germanium is 0.3 and 0.7 for silicon.
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When voltage is applied across P-N junction, the *depletion region will become thinner and less resistive to the current passing through it. This leads to the collapse of depletion region due to the applied voltage. This certain minimum voltage required for a diode to conduct freely is called the forward voltage.