J. Delahaye, P. Hakonen, R. Lindell, M. Paalanen, and M. Sillanpää
Bloch Oscillating Transistor (BOT) is a new type of a mesoscopic transistor (three terminal device) in which a large supercurrent is controlled by a small quasiparticle current. The operating principle of a BOT utilizes the fact that, in a suitably biased Josephson junction Zener tunneling up to a higher band will lead to a blockade of Cooper-pair tunneling (Bloch oscillation). Bloch oscillation is resumed only after the junction has relaxed to the lowest band. Using a quasiparticle control current, this process can be made faster. Since, one quasiparticle triggers several cycles of Bloch oscillations, a high current gain can be achieved.
We have investigated the experimental realization of BOTs using four angle shadow evaporation: The base electrode is connected via a Cu-AlOx-Al SIN junction, the collector has a Cr-resistance of 100 kW, and on the emitter there is a tunable, SQUID-type Josephson junction with EJ/EC ~ 0.1 - 5. The maximum current gain, measured so far, is about 30. The input and output impedances were 1 MW and -30 kW, respectively. The dynamic range was found to be small, about 30 pA. Altogether, we have shown that a BOT is a good candidate for a low noise amplifier for applications at intermediate impedance levels. The fact that our results have been published in Science, reflects the international interest in such a device. We have also studied the noise properties of BOTs and shown that the equivalent input current noise can be made at least by a factor of five smaller than the shot noise calculated directly from the input base current.