What is the immediate consequence of light interacting with silicon in this detector?

• A. Absorption without ionization
• B. Ionization
• C. Emission of X-rays
• D. Material phase change

Answer: (B)

When light enters silicon in a detector, it promotes electrons from the valence band to the conduction band, creating free electron–hole pairs. This is ionization in a solid, and it’s the immediate consequence because those newly generated charge carriers are what the detector collects to produce a signal. In silicon, photons with energy above the band gap (about 1.12 eV at room temperature) cause this carrier generation.

If the light had energy below the band gap, absorption could occur without generating carriers, but for typical detector illumination the key effect is ionization. Emission of X-rays or a phase change would require much higher energies or different conditions, which aren’t the immediate result of ordinary light interacting with silicon.