Photodiode and Phototransistor

Purpose
Photodiodes and phototransistors are used to measure light intensity. The measured signal is used to control electric devices. A phototransistor is used in the instrument cluster (A1) to adjust the illumination or brightness of the display in accordance with the surrounding brightness.

Semiconductor, general
Photodiodes are semiconductor devices. The conductivity of semiconductors (e.g. silicon) is between that of metals and insulators. The conductivity of semiconductors depends greatly on
the temperature,
the lighting and
any impurities in the material.

The conductivity of a semiconductor can therefore be influenced by a known quantities of impurities to the material (doping). There are two main types of doping:

N-Doping
Known impurities are added to the material which create an excess of electrons. Since electrons are negatively charged, this is referred to as N-type conduction.

P-Doping
Known impurities are added to the material which introduce a lack of electrons, so called holes. Positively charged carriers are created; this is referred to as P-type conduction.

If a P-doped and an N-doped layer are arranged next to each other, a diode (barrier Junction) is created.
By moving electrons and holes, the conductivity of this arrangement depends greatly on the polarity of the voltage applied. The conductivity is very high if the N-material and P-material are connected to the negative and positive pole, respectively (conducting direction).

The conductivity is very low with the opposite polarity (reverse direction).

Photodiode
As a result of light indigence, additional electrons and holes are created in a diode. The conductivity of a diode operated in reverse direction increases proportionally to the light intensity. Photodiodes are manufactured such that this effect is particularity enhanced. Photodiodes are suitable for many purposes due to this linear correlation between light intensity and conductivity.

Phototransistor
If 3 doped layers are arranged in the order NPN or PNP a transistor is created.
A transistor therefore has 3 terminals (emitter, base = control input, collector). Transistors can be used to amplify and switch electric signals.
The NP or PN arrangement works in the same way as with photodiodes.
However, the correlation of the conductivity between the two outer layers (emitter and collector) is increased by the transistor effect. The transistor therefore reacts even more sensitively to light than photodiodes. Special phototransistors are generally manufactured without the base terminal since a control input is not required.