Switching transistors are usually driven into saturation, which has an adverse effect on the switching speed. This effect is eliminated, or nearly so, by the use of Schottly diodes at the inputs. It is equally possible to add a diode to a transistor (across its base-collector junction) as shown in the diagram to increase its switching speed.
When the transistor is driven into conduction. Its base current will soon be limited because the diode has a lower transfer potential than the base-collector, junction so that part of the current will flow through the diode. When the transistor is switched off, it will, therefore, require less time to reach the non-conducting state. The effect is seen clearly in the photograph. Signal I is the input signal at a frequency of 166 kHz. Signal 2 is the Inverted collector signal with the diode added. It is evident that , owing to the diode, the collector returns to the high-level state much more rapidly.