Schottky Transistors

Schottky Transistors

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, which accounts for a significant portion of the propagation delay in the original TTL logic family. Storage time can be eliminated and propagation delay can be reduced by ensuring that transistors do not saturate in normal operation.

Contemporary TTL logic families reduce storage time by placing a Schottky diode between the base and collector of each transistor that might saturate, as shown in Fig.12. The resulting transistors, which do not saturate, are called Schottky-clamped transistors or Schottky transistors for short.

Fig.12: Schottky – clamped transistor (a) Circuit (b) Symbol

When forward biased, a Schottky diode’s voltage drop is much less than a standard diode’s, 0.25 V vs. 0.6 V. In a standard saturated transistor, the base-to-collector voltage is 0.4 V, as shown in Fig.13 (a).

Fig.13: Operation of a transistor with large base current (a) Standard Saturated Transistor (b) Transistor with Schottky diode to prevent saturation.

In a Schottky transistor, the Schottky diode shunts current from the base into the collector before the transistor goes into saturation, as shown in fig.13 (b). Fig.14 is the circuit diagram of a simple inverter using a Schottky transistor.

Fig.14: Inverter using Schottky Transistor

In Fig.14, the inverter using schottky transistor, works in the similar manner as that of BJT based inverter but the difference is in the logic level obtained at the output i.e., the voltage level in logic-'0' will be now 0.35V instead of 0.2V.