Electronic devices variable resistors

Less effective was the use of a power supply and variable resistors, controlled by human intervention [2] or by slow electromechanical devices [3, 4]. More effective was the combination of a controlled power supply with an electronic control circuit, which worked fast and unattended. This was realized by Hickling [5], who called the instrument for the first time potentiostat. The name is nowadays somewhat misleading as scientists do not want to keep the... 

If a semiconductor element with negative differential conductance is operated in a reactive circuit, oscillatory instabilities may be induced by these reactive components, even if the relaxation time of the semiconductor is much smaller than that of the external circuit so that the semiconductor can be described by its stationary I U) characteristic and simply acts as a nonlinear resistor. Self-sustained semiconductor oscillations, where the semiconductor itself introduces an internal unstable temporal degree of freedom, must be distinguished from those circuit-induced oscillations. The self-sustained oscillations under time-independent external bias will be discussed in the following. Examples for internal degrees of freedom are the charge carrier density, or the electron temperature, or a junction capacitance within the device. Eq.(5.3) is then supplemented by a dynamic equation for this internal variable. It should be noted that the same class of models is also applicable to describe neural dynamics in the framework of the Hodgkin-Huxley equations [16]. 

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