The Tunnel diode is a highly doped semiconductor device which is commonly used for low voltage high frequency switching applications. It works on the principle of Tunneling effect.
It immediately conducts the diode when forward biased voltage is applied. The current is increases to its peak point value (Ip). The diode currents starts decreasing till it reaches its minimum value called valley point current (Iv) corresponding to valley voltage (Vv), when the forward voltage is increases.Current starts increasing again as in ordinary junction diode, for more voltage than valley voltage (Vv).
The region between point A and B is called negative resistance region. Current decreases with increase in applied voltage in negative resistance region that is tunnel diode possessing negative resistance region (Rn) in this region. Negative resistance of negative resistance region produces power instead of absorbing power. The negative resistance permits oscillations. So tunnel diode used as a very high frequency oscillator