Photodiodes and Hall Sensors

In photoconductive mode a photodiode is reverse biased. Incident light generates charge carriers, producing a reverse photocurrent whose magnitude increases with incident optical power over a useful range. There is a small dark current even without illumination.

A family of current–voltage curves shows how illumination changes reverse current. A spectral-response curve shows sensitivity against wavelength. Choose a device that responds strongly at the signal wavelength and avoid interpreting one quoted responsivity as valid across the entire spectrum. If responsivity is given in A W⁻¹, multiply it by optical power to estimate photocurrent.

A scintillator absorbs energy from an incident particle or photon and emits a small flash of light. A photodiode detects that light electrically. The scintillator’s emission spectrum should overlap the photodiode’s high-response wavelength range.

A detector system may need to distinguish a weak signal from dark current, ambient light and electrical noise. Shielding from unwanted light and subtracting a measured dark baseline can improve interpretation. This is an example of a chain of conversions rather than the photodiode directly measuring every kind of nuclear particle.

A Hall sensor gives an electrical response to a magnetic field. Changes in orientation relative to a reference magnetic field can be used to monitor attitude. The detailed microscopic operating principle is not required for this AQA option.

For a tachometer, magnets attached to a rotating shaft pass the sensor and produce pulses. Frequency depends on both rotation rate and the number of magnets. A single magnet gives one pulse per revolution; four magnets give four. Counting for a longer time reduces the fractional effect of a one-pulse uncertainty.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.