In semiconductor diodes, the typical cut-in (threshold) voltages are different for germanium and silicon. Identify the correct pair for germanium and silicon, respectively, keeping common textbook values in mind for forward conduction onset.

Introduction / Context:
Cut-in (or knee/threshold) voltage is the approximate forward voltage where a diode begins conducting appreciable current. Germanium and silicon have different bandgaps, so their forward characteristics differ noticeably in low-current circuits and rectifiers.

Given Data / Assumptions:

• Room temperature operation.
• Common small-signal/rectifier diode behavior, not Schottky or special-purpose diodes.
• We are after typical nominal values.

Concept / Approach:
Due to a smaller bandgap, germanium diodes typically conduct at a lower forward voltage than silicon. Textbook nominal values widely used are about 0.3 V for Ge and about 0.7 V for Si under modest current levels.

Step-by-Step Solution:
1) Identify material: Ge vs. Si.2) Recall typical thresholds: Ge ≈ 0.3 V, Si ≈ 0.7 V.3) Match with the option set.
Verification / Alternative check:
Datasheets and lab experiments consistently show lower forward drops for Ge. Silicon’s higher drop is standard in rectifier and logic diode tutorials.

Why Other Options Are Wrong:

• 0.7 V and 0.3 V: reversed order.
• 0.6 V and 0.9 V: too high for Si and unusually high for Ge.
• 0.5 V and 0.7 V: Ge value is too high.
• 0.2 V and 0.4 V: unrealistically low for typical room-temperature conduction points.

Common Pitfalls:

• Confusing Schottky diodes (lower drops) with Ge/Si PN diodes.
• Assuming constant drop; actual forward voltage depends on current and temperature.

Final Answer:
0.3 V (Ge) and 0.7 V (Si) (Option A).