Purpose/Function of Photodiode and Phototransistor

Task
Photodiodes and phototransistors are used to measure the light intensity in order to actuate the measuring signal of electrical devices.
A phototransistor is used in the instrument cluster (A1) to adjust the illumination or the brightness of the display to the brightness of the surroundings.

Semiconductor, general
Photodiodes are semiconductor components. On semiconductors, (e.g. silicon) the conductivity is between that of the metals and isolators. The conductivity of semiconductors depends largely on
- the temperature
- the lighting
- and any contaminants due to stray atoms

The conductivity can be influenced by selective contaminants (doping). A distinction is generally made between two types of doping:
- N-doping
Material is contaminated by stray atoms which generate an excess of electrons. As electrons are negatively charged they are called N-line.
- P-doping
The material is contaminated by stray atoms which generate a shortage of electrons, holes as they are known. Positive load carriers are created as a result, they are called P-line.

If a P - and a N - doping layer are brought together, this results in a diode (barrier junction). By shifting electrons and holes the conductivity of this arrangement largely depends on the polarity of the voltage applied.
The conductivity is very high if the N-material is connected to the negative pole and the P-material is connected to the positive pole (forwards direction).
The conductivity is very low for the other polarities (non conducting direction).

Photodiode
Additional electrons and holes are produced on a diode due to the incidence of light. The conductivity of diode switched to a non-conducting direction increases proportional to the light intensity. Photodiodes are manufactured so that this effect is particularly marked.
Photodiodes are ideally suited to measuring due to the linear relationships between light density and conductivity.

Phototransistor
If 3 doped layers are arranged in the arrangement NPN or PNP this results in a transistor.
A transistor therefore has 3 connections (emitter, basis control input, collector). Electrical signals can be amplified and switched using a transistor.
The NP or PN arrangement therefore acts as a photodiode.
The dependency of the conductivity of a transistor between the two outer layers (emitter and collector) is however still increased further due to the transistor effect. As a result the transistor responds more sensitively to light than photodiodes. Special photodiodes are generally produced without the basic connection as no control input is needed.