Negative Resistance Effects in NbN HEB Devices

It is well known that HEB devices, whether phonon-cooled or diffusioncooled, exhibit unstable regions of their IV-characteristics. These phenomena occur when the device is not pumped by the LO, or when the LO power is decreased sufficiently compared with the power level required for optimum operation. The bias voltage is between the critical value at which the device stops being superconducting, and the point where a stable hotspot is formed. We have studied the instabilities occurring in unpumped NbN devices that are also being used in our THz mixer receivers. The aim is to understand the physical processes, which produce the instabilities in the device under these conditions. Eventually, this should also be useful for understanding the device models in the LO-pumped regime. Whereas other researchers have reported sidebands of the IF in diffusion – cooled devices, we find that a variety of different frequencies and waveforms may be produced in the NbN devices, with frequencies varying from about 300 Hz to several hundreds of MHz. A typical range is a few hundred kHz, however. During our measurements the device is mounted on a circuit board and enclosed in a small box in order to present well – defined circuit conditions. We then add chip components to the circuit and study the effect these have on the instabilities. We have identified some conditions for which the device is stabilized, which may be useful in practical mixer applications, and also present in this paper preliminary results for a model which can be used to interpret the physical processes which occur in the devices. In this model the device is in a metastable hotspot state in which heating and cooling recur periodically.