Noise in Optoelectrohic Devices

Abstract

It has long been recognised that a system in thermal equilibrium exhibits internal fluctuations which result from the kinetic energy of molecules and electrons. For example, an isolated resistor at room temperature has internal charge fluctuations which give rise to a randomly fluctuating voltage at its terminals. Even though it may be electrically neutral in the sense that its average voltage is zero, it contains a very large number of atoms whose outer electrons may occasionally gain enough thermal energy to break free and move about, causing local fluctuations in the internal charge density. An electron that has broken free from its particular host atom executes a random walk through the resistor until being recaptured, and its average kinetic energy while free is given by \(\tfrac{1}{2}m{\bar V^2} = \tfrac{3}{2}kT\) where m is its mass and \({\bar V^2}\) its mean square velocity. Since the mass of an electron is very small its mean square velocity is very large, and thermal fluctuations in charge density in a resistor give rise to a very rapid variation in terminal voltage. This type of noise is often referred to as ‘white’ noise, since it has a frequency spectrum which is flat out beyond any frequencies of interest in electrical systems. It is more properly called thermal noise.