that the small-signal resistance is the reciprocal of the small-signal conductance just found, the diode resistance is independent of the ac current, but depends on the dc current, and is given as . Capacitance The charge in the diode carrying current is known to be , where is the forward transit time of charge carriers: The first term in the charge is the charge in transit across the diode when the current flows. The second term is the charge stored in the junction itself when it is viewed as a simple capacitor; that is, as a pair of electrodes with opposite charges on them. It is the charge stored on the diode by virtue of simply having a voltage across it, regardless of any current it conducts. In a similar fashion as before, the diode capacitance is the change in diode charge with diode voltage: , where is the junction capacitance and the first term is called the diffusion capacitance, because it is related to the current diffusing through the junction. Variation of forward voltage with temperature The Shockley diode equation has an exponential of , which would lead one to expect that the forward-voltage increases with temperature. In fact, this is generally not the case: as temperature rises, the saturation current rises, and this effect dominates. So as the diode becomes hotter, the forward-voltage (for a given current) decreases. Here is some detailed experimental data, which shows this for a 1N4005 silicon diode. In fact, some silicon diodes are used as temperature sensors; for example, the CY7 series from OMEGA has a forward voltage of 1.02V in liquid nitrogen (77K), 0.54V at room temperature, and 0.29V at 100 °C. In addition, there is a small change of the material parameter bandgap with temperature. For LEDs, this bandgap change also shifts their colour: they move towards the blue end of the spectrum when cooled. Since the diode forward-voltage drops as its temperature rises, this can lead to thermal runaway in bipolar-transistor circuits (base-emitter junction of a BJT acts as a diode), where a change in bias leads to an increase in power-dissipation, which in turn changes the bias even further. See also Bipolar junction transistor Semiconductor device modelling References Electronic device modeling