A Schottky transistor is a combination of a transistor and a Schottky diode that prevents the transistor from saturating by diverting the excessive input current. It is also called a Schottky-clamped transistor.
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Standard TTL uses transistors as saturated switches. When a transistor in a TTL circuit is turned off, the stored charge in the base of the saturated transistor prevents the transistor from turning off instantaneously. The result is a delay between the applied input at the base and the corresponding voltage swing at the collector. Schottky transistors have found their application in solving this problem by preventing the stored base charge.[1]
When the input of a saturated transistor is changed, the output does not change immediately; it takes extra time, called storage time, to come out of saturation. In fact, storage time accounts for a significant portion of the propagation delay in the original transistor–transistor logic (TTL) logic family.
Storage time can be eliminated and propagation delay can be reduced by ensuring that transistors do not saturate in normal operation. Schottky TTL logic families do this by placing a Schottky diode between the base and collector of each transistor that might saturate. The resulting transistors, which do not saturate, are called Schottky-clamped transistors or Schottky transistors for short.
When forward biased, a Schottky diode's voltage drop is much less than a standard silicon diode's, 0.25 V versus 0.6 V. In a standard saturated transistor, the base-to-collector voltage is 0.4 V. In a Schottky transistor, the Schottky diode shunts current from the base into the collector before the transistor goes into saturation.
The input current which drives the transistor in active state sees two paths, one from base to Schottky diode to collector to emitter and the other from base to emitter. When transistor conducts it will develop 0.6 V across its base and emitter. The same voltage will appear across shunt path along the Schottky diode and C to E. Schottky will give 0.25 V drop so remaining 0.35 will occur at C to E branch. So transistor will not go in saturation because it has 0.2 volt across C to E branch in saturation.
In 1956, Richard Baker described some discrete diode clamp circuits to keep transistors from saturating.[2] The circuits are now known as Baker clamps. One of those clamp circuits used a single germanium diode to clamp a silicon transistor in a circuit configuration that is the same as the Schottky transistor.[3] The circuit relied on the germanium diode having a lower forward voltage drop than a silicon diode would have.
In 1964, James Baird filed a patent for the Schottky transistor.[4] He showed that it could be used in, for example, DTL circuits. The Schottky transistor has some significant features. The Schottky diode can be integrated on the same die, the diode has a compact layout, and the Schottky diode is faster than a conventional junction diode. Consequently, it improved the speed of saturated logic designs at little cost.