Can anyone tell me what is a Schottky Diode? Scheme? Symbol? Where is it used? I mean in what type of circuits is it used? And for what is used?
I have searched online but not found what I am looking for.
Ordinary semiconductor diodes are a junction of N and P semiconductor material. It turns out you can make a diode from what is sort of half of a semiconductor junction.
Schottky diodes are a junction with one side a P or N semiconductor, but the other side just metal. The result still functions like a diode, but has the following differences relative to circuit design:
These are like diodes but only with a metal and an N-doped material instead of a P-N junction.
They are very useful for highspeed computer circuits, fast switching. Commonly used for designing rectifiers
Another common use of them is for voltage clamping because it has a steeper than that of an ordinary diode.
Tips: Some places to consider starting your search before asking
The most common type of diodes (doped silicon P-N junction diodes) have a minimum voltage drop, to overcome the junction potential i.e. energy well, for conduction of carriers. For silicon this is approximately 0.6-0.65 Volts, and is temperature dependent.
For certain applications, that ~0.65 Volt diode drop is unacceptable. The reasons include:
P = V x I. Thus, heat generated is proportional to this voltage
So logically, a simple answer should be to use some other semiconductor instead of Si... and this does work with some limitations: An alternative for low voltage applications has traditionally been the Germanium p-n junction diode: It's junction potential is approximately 0.15 Volts, much smaller than the ~0.65 Volts above. However, Ge diodes are largely disappearing from use due to issues where it loses out to silicon diodes: For instance high reverse leakage current, low forward current capacity, low reverse blocking voltage, and pathetic thermal stability.
The Schottky Diode falls somewhere between Si and Ge diodes in parameters, but is significantly different in the way it operates: The rectification function occurs between a doped semiconductor, almost always n-type, and a metal forming a "Schottky Barrier" to the semiconductor. Note that the complementary dopant type (p <--> n as the case may be) is absent in Schottky diodes.
The energy well voltage in the case of the metal-semiconductor barrier depends on which combination of semiconductor and metal is used to form the diode, and is typically much lower than that of a p-n junction diode (half the voltage, as noted by Olin in his answer).
The other big advantage is that reverse recovery time of a Schottky Barrier is pretty much infinitesimal, compared to the relatively sluggish p-n junction diode. That's the bit secret for high speed switching / rectification applications.
The downside of Schottky diodes is that the reverse leakage current is linked to the barrier voltage achieved - and rises drastically with decrease in this junction potential. Hence, while very low junction potentials are possible, for rectification purposes too low a voltage isn't a good thing.
Now, coming down to the questions: