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A photodiode is a solid state device (as is a transistor) that produces current to flow in response to incoming EMR, i.e., a type of EMR sensor. They are used as sensors (e.g., Geiger-avalanche photodiodes) and photocells and solar cells are variants. A photodiode is subject to noise (thermal noise and photon noise) imposing limits on its signal-to-noise ratio (SNR). Dark current is another limiting factor, the small amount of current through the photodiode even when no EMR is incident.