Noise in the present context is random and unpredictable fluctuations in voltage or in current. This is an unwanted property in any measurement system and normally determines its lower limit. Sometimes the magnitude of random fluctuations is higher than the signal to be handled, and in the case of device applications based on photodetector technology, this leads to incorrect information. If the fluctuations are of the same order as the signal, then, the signal is masked out and/or the desired information is perturbed. The noise consideration is of fundamental importance, particularly when the photoconductivity signal is very weak. An experimental physicist knows that after eliminating the random voltages originating from an improper experimental setup, there is always still noise that determines the measurement capability. In semiconductors, there are several reasons for random fluctuations in voltages.