Tunnel Diode is the P-N junction device that exhibits negative resistance. When the voltage is increased then the current flowing through it decreases. It works on the principle of Tunneling effect. Metal-Insulator-Metal (MIM) diode is another type of Tunnel diode, but its present application appears to be limited to research environments due to inherit sensitivities, its applications considered to be very limited to research environments.
There is one more diode called Metal-Insulator-Inslator-Metal (MIIM) diode which includes an additional insulator layer. The tunnel diode is a two terminal device with n-type semiconductor as cathode and p-type semiconductor as anode. The tunnel diode circuit symbol is as shown below.
It's a diode, a kind of semiconductor device (usually with 2 terminals as the name suggests). Unlike a regular pn diode, it conducts both ways. This is due to a peculiarity of the manufacturing process: the impurity (dopant) concentration is intentionally high, that the semiconductor becomes a bit more like a conductor (small resistance):
Semiconductors are the only materials that exhibit this weird behaviour: If you add impurities in a metal, it won't significantly alter its resistance (when you inject DC). Ohm's law reminder: R=V/I