WTP Pepper,
1. Why is it called “Storage time”?
1. Base charge due to parasitics
Parasitics have nothing to do with storage time. Storage time occurs when a transistor in saturation is not able to turn off immediately because of the excess minority-carrier charge in the base region. Until this charge is removed, the transistor will not be able to respond to input signals. The time it takes from the time of the turn off signal to the time for the collector current to drop to 90% of its saturation value is the storage time.
2. What is stored?
Correct. Excess charge in the base region.
3. Why does the base current fall immediately from 430 uA to -69 uA?
3. I would build it for the exact values.
What does that mean? In reality, the base current is draining away the excess charge from the base region.
4. Why is the base current negative during the storage time period?
4. Excess minority carriers drawing -ve base current.
I hope you are saying the excess charge carriers are draining away when the BJT is trying to turn off.
5. Why does the collector current only decline slightly during the storage time period?
5. Uneven distribution of these minority cuurrent.
Nope, the Vbe controls the collector current, so as long as the Vbe is at the value it was when the transistor was in saturation, the collector current will stay at its same level, too.
6. Why is the collector current positive when Ib is negative?
6. Because it's not acting as a true current amplifiying transisitor during this period
The correct answer is that Ib is not controlling Ic. It never was. Vbe is really what is in control of Ic.
7. Why does the base emitter voltage remain at approximately 644mV?
The Early effect for a BJT in the saturaion region does not exist and has no effect. Since Vbe is controlling Ic, Ic will not drop until Vbe does.
8. Why is the wave shape of Ic very similar to that of Vbe? .
8. As it is similar to to Ib....
Look again. Ic is not following Ib. It is following Vbe.
In conclusion, Vbe controls Ic, not Ib controls Ic.
Ratch