Barry King wrote:

> > So if schottkies are so fast, why dont they make transistors out of them?
> > or do they?
> Schottky diodes are not necessarily all that fast.  The 1N5817,
> for example, is a Schottky rectifier, but its rated for use at 60Hz,
> which by switching regulator standards is SLOOOW.
>

Schottky barriers ARE made of a metal and semiconductor junction.  They are
intrisically faster than normal p-n junctions because of charge storage.  Like
any other junction, they can be physically laid out to optimize different
performance issue (power, switching speed, etc...)  But the Fermi level of the
metal make the barrier much easier to overcome in the forward biased mode.


>
> And there are diodes that are ultra-fast that are not Schottky
> diodes.
>
> However, the diodes used in switching power supplies must be BOTH
> very fast and low drop for best efficiency.  The huge quantities
> of switchmode power supplies being built has created a huge demand
> for fast Schottkys, that makes them common and therefore cheaper.
>
> I think the best way to build a transistor using metal / silicon
> junctions is called a MOSFET.  They DO make those :)

> Semiconductor Physics was a while ago, so I don't remember if the
> Schottky junction trick is applicable to bipolar transistors.
> Usually, low forward drop is not an issue in bipolar transistors,
> since the controlled current (Collector current) is not flowing
> across a regular forward-biased PN junction.

They do in fact use Schottky junctions in switching transistors.  In cuttoff
mode the collector-base junction is reversed-biased.  To switch to saturation,
you have to close the depletion region first.  By putting a Schottky barrier
diode from base to collector, you can dramatically improve this switching
operation.

>
> ------------
> Barry King, KA1NLH
> Engineering Manager
> NRG Systems "Measuring the Wind's Energy"
> Hinesburg, Vermont, USA
> barry@nrgsystems.com
> "The witty saying has been deleted due to limited EPROM space"