This thesis work contributes to the analysis of the optoelectronic devices that are used for QKD systems. Chapter one discusses and introduces light sources and single photon avalanche diodes (SPADs), their advantages and issues. Then finally it analyses commercial QKD systems in order to know the performance requirements for SPADs in QKD systems. The light emitting diodes (LEDs) has for many years been ubiquitous in a wide range of applications. Chapter two analyses the way in which an approximate single photon source can be constructed from an LED without an excessive multi-photon production and with short pulse operation achieved by a reverse bias voltage after the LED excitation, to shorten the pulse tail associated with slow carrier recombination. The electronic circuits used for this task are the following two: one with an operational amplifier and the other with discrete electronic components. The single photons are detected with special photodiodes (single-photon avalanche diodes), because the capabilities of most commercial photodiodes means they cannot detect low light intensities. Chapter three analyses and characterises a relatively new single-photon avalanche diode type, which can detect single photons and reduce some undesired features such as a dead-time after each detection. It is based on a detector chip coupled with an on-chip quenching resistor which minimises stray capacitance effects commercially known as a negative feedback avalanche diode (NFAD).