Local Joule Heating Research Articles

Thermal Effects at Superconducting Point Contacts

In this paper we have calculated $I\ensuremath{-}V$ curves of superconducting weak-link constriction junctions by assuming that there is a region of normal material which tends to spread with increasing power levels. The causes for the spreading of the normal resistance are twofold. One is the increase of the current-density distribution and the other is the increase of localized Joule heating at the contact as the total current is increased. The resultant rise in temperature of the link above the bath temperature, over the range of the $I\ensuremath{-}V$ characteristic, is found to be significant. Using material constants that are representative of bulk Nb, we found that the calculated $I\ensuremath{-}V$ characteristic is very similar to several experimentally observed Nb point-contact curves. The spreading normal-resistance analysis has suggested a model to explain the $I\ensuremath{-}V$ characteristic of a superconductor-normal-metal ($S\ensuremath{-}N$) point-contact system. A calculation has indicated that large excess temperatures are also present at the contacts when biased in the millivolt region. These findings have prompted us to review several published experiments with $S\ensuremath{-}N$ contacts.

Ionization Model for Negative Capacitance in Low-Mobility Semiconductors such as Amorphous Chalcogenides

A model to explain negative incremental ac capacitance has been developed, under the assumption that excess electrons and holes are generated in approximately equal numbers due to fairly general ionization processes such as impact ionization, localized Joule heating, or photoionization. Impact ionization is treated specifically. The motion of the excess carriers in a two-terminal bulk device gives rise to space charges which enhance the field near the electrodes. The finite times required for redistribution of the space charge and motion of the carriers through the device give rise to a lag τ of the external circuit current behind the voltage. The magnitude of the current component that lags by π/2 is proportional to ω if ωτ≪1. Thus if ωτe and ωτh≪1, where τe and τh are the electron and hole transit times, negative contributions to the ac capacitance are expected. A relation between the negative capacitance and the current-voltage characteristic is given, and this relation can provide an estimate of the order-of-magnitude of mobility for the lower-mobility charge carrier. Possible application of this model to the negative incremental capacitance observed in amorphous semiconductor switching devices is discussed, and data in the literature for a chalcogenide alloy are shown to fit the model, with an exponential dependence upon voltage. An effective carrier drift mobility of 3×10−4 cm2/V·sec is obtained.