I am currently studying how transistors react and behave to simple signals. I have the following circuit:
A square wave of approximately 83kHz is applied. The transistor used is a 2N5551. The resistor values were carefully chosen to try to avoid transistor saturation. However, with this setup, slow turn-off time is still experienced. Measurements were taken with a PC oscilloscope and are shown below. The full data can be found here.
The y-axis is voltage, the x-axis is time. All voltage levels are relative to ground. The 3-images are not aligned at the same point in time, but the clock signal should help with comparing the graphs. One full cycle is around 12 microseconds, and the on-time and off-time of the clock signal is 6 microseconds long. The clock signal was generated with a 555 timer, the duty cycle is very close to 50%.
As it can be observed in the Collector and Clock graph, when the clock turns to 0V, the transistor stays on for almost 3 microseconds before it fully turns off and the collector settles to 5V. This can also be observed in the Base and Clock graph, where the base of the transistor is at around 0.75V for almost 3 microseconds before it starts transitioning to 0V. The Collector and Base graph helps show that the transistor is not operating in the saturation region since Vb <= Vc. Vc seems to fall below Vb for a very short time, but I do not believe this is causing saturation. In fact, a Schottky diode would not help at all since the voltages are very close to each other. Hence, this should not be a storage-time issue. I have also ruled out the transistor input capacitance (of max 20pf) to be the problem as I believe I would see similar delays in turn-on time in that case. Additionally, the RC time constant would be 0.3 microseconds (5RC = 1.5us) at the input.
What could be the problem?
Following the accepted answer, adding a reverse diode accross the base resistor solved the issue. The updated graphs can be found in the same document, under the Solved Trace sheet. For convenience, the updated image of the Collector and Clock graph is shown below: