Fluctuation Noise in Semiconductor Space-Charge Regions

In addition to noise arising from the circuit, a semiconductor diodic junction must contain a basic noise source. This basic noise mechanism is due to fluctuations in the ionization state of the impurity atoms which occur at random relative to the probability of ionization, w=nD+/nD. In an n-type semiconductor, a neutral impurity that suddenly becomes ionized releases a mobile electron whose movement forms a noise current pulse; similarly for the elemental deionization event. The net result is a short-circuited noise current given approximately by the formula, 2 IIDA TV (1 -we) q(VB + Vc)]- TD+ w kT Using typical numbers, this formula indicates that the fundamental noise associated with a 0.3 μμfd depletion layer capacitance at 100 Mc is equivalent to the thermal noise of a 6-ohm series resistor at 40°K and a 0.1-ohm series resistor at 300°K. Experimental verification of this type of noise is not known. When it is observed experimentally, a new method for evaluating some of the basic properties of semiconductors will be available, and computations of minimum noise factor of parametric amplifiers will be possible.