The signal diode is a silicon doped semiconductor diode or a germanium doped diode but depending up on the doping material the characteristics of the diode varies. In signal diode the characteristics of the silicon doped diode is approximately opposite to the germanium doped diode.
Gunn Diode oscillator. Gunn diodes are used to construct gunn diode oscillator for producing microwaves with frequencies starting from 10 GHz to THz. It's a Adverse Differential Resistance system – additionally called as transferred electron machine oscillator – which is a tuned circuit consisting of Gunn diode with DC bias voltage utilized to it.
A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode that emits light when activated. When a suitable current is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons.
A photodiode is a transducer that takes light energy and converts it into electrical energy. If placed in a dark room, the photodiode is exposed to no light; therefore it creates no electricity. However, if light falls upon it, it takes the light energy and produces electric current in response.