THE SCHOTTKY DIODE
The Schottky diode (named after German physicist Walter H. Schottky; also known as hot carrier diode) is a semiconductor diode with a low forward voltage drop and a very fast switching action.
The cat's-whisker detectors used in the early days of wireless can be considered as primitive Schottky diodes.
A Schottky diode is a special type of diode with a very low forward-voltage drop.
When current flows through a diode there is a small voltage drop across the diode terminals.
A normal diode has between 0.7-1.7 volt drops, while a Schottky diode voltage drop is between approximately 0.15-0.45 volts.
This lower voltage drop can provide higher switching speed and better system efficiency.
The most important difference between p-n and Schottky diode is reverse recovery time, when the diode switches from non-conducting to conducting state and vice versa.
Where in a p-n diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from.
The switching time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for special high-capacity power diodes.
With p-n junction switching, there is also a reverse recovery current, which in high-power semiconductors brings increased EMI noise.
With Schottky diodes switching essentially instantly with only slight capacitive loading, this is much less of a concern.
It is often said that the Schottky diode is a "majority carrier" semiconductor device.
This means that if the semiconductor body is doped n-type, only the n-type carriers (mobile electrons) play a significant role in normal operation of the device.
The majority carriers are quickly injected into the conduction band of the metal contact on the other side of the diode to become free moving electrons.
Therefore no slow, random recombination of n- and p- type carriers is involved, so that this diode can cease conduction faster than an ordinary p-n rectifier diode.
This property in turn allows a smaller device area, which also makes for a faster transition.
This is another reason why Schottky diodes are useful in switch-mode power converters; the high speed of the diode means that the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing the use of small inductors and capacitors with greater efficiency than would be possible with other diode types.
Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 50 GHz.
Ref Wikipedia entry Schottky diodes
SCHOTTKY DIODE CLAMPED TRANSISTORS
A method of improving overall response time in a transistor is by adding gold doping to the processing flow.
The golf dopant causes a decrease in minority carrier lifetime which aids in the recombination process and shortens the saturation recovery time.
Unfortunately the transistor beta (ß) is adversley affected by gold causing slightly higher bias and offset currents.
It was not until the advent of the schottky clamp that a vast improvement in speed without input degradation was possible.
The schottky barrier diode's (SBD) is shown thus:
The schottky clamped transistor is formed by parelleling the base-collector junction of the NPN transistor with the schottky diode.
Without the clamp, as the base drive is increased the collector's voltage falls until
hard saturation occurs.
At this point the collector voltage is very near the emitter voltage and stored charges in the junctions causes slow recovery from saturation after base drive has been removed.
The forward voltage drop of the shottky diode is 0.4 volt less than the forward drop of silicon diodes.
This difference in forward drop is used by placing the diode across the base-collector junction.
The shottky diode becomes forward biassed when the collector voltage falls below the base voltage.
Excess base drive is then shunted into the collector circuit prohibiting the transistor from reaching classic saturation.
With almost no stored charge in either the SBD or the transistor there is a large reduction in storage time.
The transistor switching time is significantly reduced.
This was a significant advance for the TTL logic family, LS-TTL (low power schottky transistor transistor logic) being universally popular.
Ref Signetic Analogue manual 1977 pg694
Do not confuse with :-
The shockley diode which is a 4 layer PNPN device
(similar to an SCR - but without the gate connection)