Differences of LDR and Photodiode
The photodiode uses ordinary PN junction diode which has two terminals namely cathode and anode. And in phototransistor, the normal transistor is used. The only difference between the transistor and the phototransistor is that the phototransistor does not have the base terminal.LED's can also be used as photodiodes as they can both emit and detect light from their junction. All PN-junctions are light sensitive and can be used in a photo-conductive unbiased voltage mode with the PN-junction of the photodiode always “Reverse Biased” so that only the diodes leakage or dark current can flow.As explained above, the main component for the construction of LDR is cadmium sulphide (CdS), which is used as the photoconductor and contains no or very few electrons when not illuminated. ... This causes the free electrons or holes to conduct electricity and thus dropping the resistance dramaticallyPhotons (light particles) activate phototransistors and are used in virtually all electronic devices that depend on light in some way. A phototransistor is a bipolar device that is completely made of silicon or another semi-conductive material and is dependent on light energy.