High-frequency noise mechanisms in electronic devices Which one of the following noise types becomes particularly significant at very high operating frequencies (for example, in microwave devices and fast transistors)?

Introduction / Context:
Different noise mechanisms dominate in different frequency ranges. Understanding which mechanism rises in importance at high frequencies is essential for RF and microwave design, low-noise amplifiers, and high-speed semiconductor devices.

Given Data / Assumptions:

• We are comparing common noise types: flicker (1/f), shot, thermal (Johnson), transit-time, and burst noise.
• Focus is on frequency dependence and physics in active devices (e.g., BJTs, FETs, diodes).
• Devices are biased in their normal operating regions without oscillation.

Concept / Approach:

Transit-time noise appears when the carrier transit time across the active region is comparable to the signal period. As frequency increases, the signal period decreases, causing phase lag and random fluctuations in current due to carrier transit statistics. This effect is negligible at low frequency but becomes prominent in microwave regimes. By contrast, flicker noise dominates at very low frequencies, and thermal and shot noise are broadband but do not specifically increase in relative importance solely because of very high frequency in the same way transit-time effects do.

Step-by-Step Solution:

Verification / Alternative check:

Microwave diodes (e.g., IMPATT, Gunn) and high-fT transistors exhibit behavior where carrier transit phenomena set performance limits and noise rises at high f.

Why Other Options Are Wrong:

Shot noise (b) is frequency-independent within the device bandwidth; flicker noise (c) dominates at low frequencies; thermal noise (d) is white and does not preferentially increase at high f due to the mechanism itself; burst noise (e) is low-frequency and process-related.

Common Pitfalls:

Assuming shot or thermal noise automatically dominates at all frequencies; forgetting that transit-time is tied to device geometry and carrier velocity.

Final Answer:

Transit-time noise