Turn-on time of the transistor is the time taken by the transistor from the instant the pulse is applied to the instant the transistor switches into the ON state and is the sum of the delay time (td) and rise time (tr).

Turn-On Time in Switching Mode

When a transistor is used in switching mode, the turn-on time refers to the time it takes for the transistor to transition from the off state to the on state. During this time, the transistor goes through several stages before it reaches its fully conducting state.

Explanation:

The turn-on time can be broken down into three components: delay time, rise time, and storage time. Let's understand each of these components in detail.

1. Delay Time:
The delay time is the time it takes for the transistor to respond to the input signal and begin the transition from the off state to the on state. It is primarily determined by the input capacitance and the current flowing into the base of the transistor. During this time, the input signal is being recognized and the transistor starts to turn on.

2. Rise Time:
The rise time is the time it takes for the transistor to complete the transition and reach its fully conducting state. It is primarily determined by the transistor's internal capacitances and the rate at which the base current increases. The rise time is the time taken for the transistor to switch from partially conducting to fully conducting.

3. Storage Time:
The storage time is the time it takes for the transistor to fully turn off after being in the on state. It is primarily determined by the charge stored in the base region of the transistor. During this time, the stored charge needs to be removed before the transistor can turn on again.

Sum of Delay Time and Rise Time:
The delay time and rise time are sequential stages in the turn-on process. The delay time represents the time taken for the transistor to start responding to the input signal, and the rise time represents the time taken for the transistor to complete the transition to its fully conducting state. Therefore, the sum of the delay time and rise time gives us the total turn-on time for the transistor.

Conclusion:
In switching mode, the turn-on time of a transistor is the sum of the delay time and rise time. This is because the delay time represents the response time of the transistor to the input signal, and the rise time represents the time taken for the transistor to reach its fully conducting state. By considering both these components, we can accurately determine the turn-on time of the transistor in switching mode.