Fluctuations of the Characteristics of Semi-Conductors

Abstract

The aim of this research has been to investigate the fluctuations of characteristics of semiconductor devices as a source of the noise increase above the thermal level when the device is excited by an external signal. Special kinds of bridges and measuring techniques have been developed in order to examine this effect for different types of semiconductor devices such as resistors, diodes and transistors. It was first established that the d.c. excitation brings about an excess noise rising towards the zero frequency, which is well-known flicker effect. On the other hand a new result has been obtained on the application of a.c. excitation. It has been found in this case that the excess noise, when it arises, is concentrated around the applied signal frequency (84 kc/s) and has very similar properties to the d.c. excess noise. This shows that the excess noise rise is a consequence of a linear random modulation of the signal by the fluctuating characteristic and that the location of the excess noise spectra can be shifted along with the signal frequency. Theoretically, the problem has been approached through the analysis of a first order differential equation having a randomly time-varying parameter. The solutions obtained are new and they agree with the experimental results. The same has been examined by means of difference equations but no advantage has been obtained. However, a treatment of the problem in terms of random integral numbers produces a result in agreement with the experiment. The nonlinear modulation between signal and noise has also been examined experimentally and theoretically. The results show that this effect is not the cause of the observed excess noise. Some results have also been obtained of which no previous record was known such as excess noise in reverse biased diodes in relation to their instantaneous characteristics and also excess noise figures of complex transistor structures.