An increase in voltage will lead to an increase in the current until it reaches peak current. In this respect , it is very similar to the unijuction transistor. It is called a tunnel diode because due to its extremely thin depletion layer, electrons are able to tunnel through the potential barrier at relatively low forward bias voltage (less than 0.05V). The forward resistance is very small because of its tunneling effect. What is meant by positive resistance and negative resistance? As voltage increase she current also increases till the current reaches Peak current. In fact, this constitutes the most useful property of the diode. primarily it’s the terribly high doping levels utilized in the tunnel diode its distinctive properties and characteristics. The symbol of tunnel diode is shown below. The tunnel diode characteristics and operation depend upon some of the subtle differences between a normal PN junction and structure of the tunnel diode itself.