Photonic Semiconductors

Light meets semiconductors. LEDs, laser diodes, photodiodes, phototransistors, photothyristors, solar cells, and the photoresistive predecessors. The covert channel of choice when RF is loud and wires are bugged; laser mics off a window pane, IR exfil from an air-gapped LED, solar-trickle bugs that listen for years.

Catalog

Symbol	Photo	Component
		LED. Forward-biased junction that emits visible photons. Status indicator on every device; also an exfil channel, blinking faster than the eye can see and picked up by a phone camera in the same room.
		IR LED. Same junction as the LED but in the 850–950 nm band. Remote controls, IR data links, the LED-keyboard exfil channel.
		Laser diode. Diode with a polished cavity that produces stimulated, coherent emission. Laser microphones, free-space optical comms, retroreflective glints.
		Photodiode. Reverse-biased junction; incident photons release a measurable current. IR receivers, fiber-optic taps, the receiver in any LED-based exfil channel.
		Phototransistor. Photodiode with a transistor’s gain. Standard receiver paired with an LED in slot sensors, counters, and beam-break alarms.
		LDR. Cadmium-sulphide cell whose resistance falls with light. Older dawn-and-dusk triggers, dead-drop sensors, the lo-fi predecessor of the photodiode.
		Optocoupler. LED and phototransistor in one sealed package. Galvanic isolation across the dielectric gap; modem couplers, line-tap isolators, mains-side trigger inputs.
		Solar cell. Photodiode optimized for power. Trickles enough current to keep a low-power bug listening indefinitely under indirect light.
		Photothyristor. Light-triggered SCR. Niche; firing high-power loads from an isolated optical pulse.

References

• Semiconductors for the underlying junction physics.