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Episode 111: Semiconductor Devices

LDRs (light dependent resistors) are semiconductor devices. From the Institute of Physics, in this learning episode students investigate the resistance of an LDR at different light intensities. This can be used as a basis to explain how photons liberate free electrons to reduce resistance in the device. It may also be related to the variation of resistance with temperature for a semiconductor thermistor.

The activities also offer an opportunity to look at semiconductor devices in general and the applications of semiconductor devices is also considered.

An LDR, thermistor or semiconductor diode can be used to challenge students to build circuits for applications such as:
• half-wave rectification using a single diode
• full-wave rectification using a bridge rectifier (4 diodes)
• a thermistor as a temperature sensing circuit (e.g. to switch on a fan/motor) when hot
• an LDR as a light sensor to sound an alarm when a light beam is broken (e.g. a burglar alarm)

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