Opto/quantum device properties: Match each device with the defining physical attribute or emission property. List I List II A. LED 1. Heavy doping (degenerate p–n junction) B. Avalanche photodiode2. Coherent radiation C. Tunnel diode 3. Spontaneous emission D. LASER 4. Current gain via impact ionization

Introduction / Context:
LEDs, APDs, tunnel diodes, and LASERs are staple devices in electronics and photonics. This matching tests recognition of their core physical mechanisms.

Given Data / Assumptions:

• LED emission is spontaneous; LASER emission is coherent (stimulated).
• APD uses avalanche multiplication (current gain).
• Tunnel diode depends on heavy (degenerate) doping to enable quantum tunneling and negative resistance.

Concept / Approach:

Connect each device to the mechanism it is known for: spontaneous vs. coherent emission, avalanche gain, and tunneling due to heavy doping.

Step-by-Step Solution:

Verification / Alternative check:

Device I–V curves and optical spectra verify: LEDs have broad spectra; LASERs have narrow coherent output; APDs show multiplication factors M; tunnel diodes display negative resistance due to tunneling.

Why Other Options Are Wrong:

• Swapping LED with LASER confuses spontaneous versus stimulated emission.
• Attributing “current gain” to tunnel diodes ignores their primary quantum mechanism.

Common Pitfalls:

Assuming all photodiodes are alike; APDs are distinct because of avalanche multiplication and higher sensitivity requirements.

Final Answer:

A-3, B-4, C-1, D-2