Electronics Questions & Answers ← browse Q&As

Please,help me understand how these diodes work and whats their function ? 1.Photodiode 2.Varacter diode 3.Tunnel diode

A photodiode is like a battery which generates a current in the forward direction when light shines on it. A varacter diode is like a capacitor whose capacitance varies with the reverse voltage across it. A tunnel diode is somewhat like an ordinary diode except that it has a negative resistance over part of the forward VI characteristic. i.e: with a low forward voltage it behaves like an ordinary diode but as the forward voltage increases, over a restricted range the current decreases (negative resistance) then as the forward voltage increases more the current increases, again like an ordinary diode.

Just to expand a bit on my answer, the use of a photo diode is (I think) obvious. A varacter diode could be used to adjust a tuned circuit resonant frequency by varying the dc voltage across the diode. I used a tunnel diode in the distant past, shortly after it was invented to make an oscillator, 1 MHz as I remember it

But how is capacitor formed in diode ? It forms during reverse biased condition right ?

Right. A reverse biased diode has two conducting layers separated by an insulating layer (which is a good description of a capacitor). The thickness of the insulating layer changes with the reverse bias voltage and so the capacitance changes. On forward bias the two conducting layers get thick enough that they touch and so current can flow. I'm sure a purist could go into much more detail but I think this is a reaonable first approximation.