The unity gain frequency (fT) of a transistor is the frequency at which the transistor's current gain (hfe or hfe) drops to 1. At this frequency, the transistor's output current is approximately equal to its input current.

When comparing two transistors with different hfe values, the transistor with the lower hfe will have a higher bandwidth.

Here's why:

1. Introduction to bandwidth:
- Bandwidth refers to the range of frequencies over which a device or system can operate effectively.
- In the context of transistors, bandwidth refers to the range of frequencies over which the transistor can amplify signals without significant distortion or loss.

2. Unity gain frequency and bandwidth:
- The unity gain frequency (fT) of a transistor represents the upper limit of its frequency response.
- Above the unity gain frequency, the transistor's gain decreases, and it cannot effectively amplify signals.
- The bandwidth of a transistor can be defined as the frequency range from 0 Hz (DC) to the unity gain frequency (fT).

3. Relationship between hfe and bandwidth:
- The current gain (hfe or hfe) of a transistor affects its bandwidth.
- A higher current gain means that the transistor can amplify signals more effectively, resulting in a wider bandwidth.
- Conversely, a lower current gain limits the transistor's ability to amplify signals, resulting in a narrower bandwidth.

4. Impact of higher hfe on bandwidth:
- A transistor with a higher hfe will have a higher unity gain frequency (fT).
- This higher fT indicates that the transistor can amplify signals up to higher frequencies.
- As a result, the bandwidth of the transistor will be wider, as it can effectively amplify signals over a larger frequency range.

5. Impact of lower hfe on bandwidth:
- A transistor with a lower hfe will have a lower unity gain frequency (fT).
- This lower fT indicates that the transistor's amplification capability decreases at lower frequencies.
- As a result, the bandwidth of the transistor will be narrower, as it can effectively amplify signals only within a limited frequency range.

In conclusion, when comparing two transistors with different hfe values, the transistor with the lower hfe will have a higher bandwidth. This is because a lower hfe limits the transistor's amplification capability at lower frequencies, resulting in a narrower frequency range over which it can effectively amplify signals.