longer wavelengths and lower frequencies) are sufficient to trigger the device. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band - since the electrons don't have as far to jump, lower energy photons (i.e. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.Ī photoelectric device can be either intrinsic or extrinsic. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. This property makes them unsuitable for sensing rapidly flashing lights but is sometimes used to smooth the response of audio signal compression.Ī light-dependent resistor alternatively called an LDR, photoresistor, photoconductor, or photocell is a variable resistor whose value decreases with increasing incident light intensity.Īn LDR is made of a high-resistance semiconductor. The lag time when going from lit to dark environments is even greater, often as long as one second. Photoresistors also exhibit a certain degree of latency between exposure to light and the subsequent decrease in resistance, usually around 10 milliseconds. The photoresistivity of any photoresistor may vary widely depending on ambient temperature, making them unsuitable for applications requiring precise measurement of or sensitivity to light photons. Photoresistors are less light-sensitive devices than photodiodes or phototransistors: the two latter components are true semiconductor devices, while a photoresistor is a passive component and does not have a PN-junction. A photoresistor can be applied in light-sensitive detector circuits and light-activated and dark-activated switching circuits. The resistance of a photoresistor decreases with increasing incident light intensity in other words, it exhibits photoconductivity. A photoresistor (or light-dependent resistor, LDR, or photo-conductive cell) is a light-controlled variable resistor.
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