Devices for microwave mixing/detection — identify the exception Which of the following is generally <em>not</em> used as a microwave mixer or detector diode?

Introduction / Context:
Microwave mixers and detectors rely on diode nonlinearity to translate frequencies (mixing) or to rectify small RF signals (detection). Several diode types are optimized for these functions. Others, while useful in microwave hardware, are not intended for mixing/detection due to their charge-storage behavior and switching orientation.

Given Data / Assumptions:

• Common microwave diode families considered: crystal (point-contact), Schottky, backward (tunnel-like behavior near zero bias), and PIN diodes.
• Task: identify which is not typically used as mixer or detector.

Concept / Approach:
Schottky diodes (metal–semiconductor) and historical crystal diodes are the workhorses of microwave mixers and detectors due to fast response and strong nonlinearity at low junction capacitance. Backward diodes are excellent zero-bias detectors because of their steep conductance near zero volts. In contrast, PIN diodes are designed for RF switching and attenuation: their intrinsic region stores charge, making them behave like controllable resistors at RF when forward-biased. This stored charge and their design focus make them unsuitable as primary mixer/detector elements compared with Schottky/backward diodes.

Step-by-Step Solution:

Verification / Alternative check:
Microwave receiver front-ends and power meters commonly specify Schottky or backward diodes for detection and conversion; PIN diodes appear in T/R switches and step attenuators instead.

Why Other Options Are Wrong:

• Crystal/Schottky: standard mixer/detector choices.
• Backward diode: widely used as a zero-bias detector in microwave instruments.

Common Pitfalls:
Assuming any RF diode can serve as a detector; overlooking the importance of low junction capacitance and fast response for mixing/detection.

Final Answer:
PIN diode