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LT » Nano Group » Josephson junctions » Bloch Oscillating Transistor

Bloch Oscillating Transistor

Bloch oscillating transistor (BOT) is a novel type of low noise amplifier, the operation of which takes advantage of the band structure of a mesoscopic Josephson junction (JJ) in a resistively isolated environment. It is a charge-charge pulse converter where one single quasiparticle triggers a sequence of Cooper pairs, thus yielding a current gain determined by the conversion efficiency.

In general, the BOT can be considered as a complementary component of the SET, in the same fashion as the BJT (bipolar junction transistor) is complementary to the Field Effect Transistor. In other words, BOTs may replace SQUIDs in current read-out applications at large impedance levels on the order of 1 MΩ.

Typical device parameters for a BOT are: collector resistor of 100 kΩ and small 100x100 nm tunnel junctions giving a total device capacitance of 1-2 fF and charging energy of 1 K. Usually, the Josephson junction is made as a SQUID, giving additional tunability of the cricital current and enabling one to find the optimal operation point.

Animation of the operating principle of a Bloch Oscillating Transistor. Gain is given by 2N where N is the number of Bloch oscillations triggered by a quasiparticle.	Atomic force microscope image of a practical realization of the Bloch oscillating transistor. Using electron beam lithography and four-angle-evaporation, the same pattern is deposited several times, thus forming shadows. The conducting parts are indicated by a brighter color. The tunnel junctions, the dynamics of which govern the operation of the BOT are singled out by circles in the picture. The Josephson junction in the picture has been splitted into two in order to facilitate tuning of the ratio EJ/EC by magnetic flux. In this circuit, the optimum operating conditions were found in the absence of flux at the maximum value for EJ/EC = 0.3. The SQUID loop Josephson junctions are of Al-AlOx-Al while the SIN-junction is made from Al-AlOx-Cu. The Cr strip on the left has a resistance of 179 kΩ.
The BOT is a 3-terminal device consisting of a chromium resistor (collector), an SIN junction (base) and a Josephson junction or SQUID (emitter). The largest current gain dIC/dIB in a stable operating point we found is 30. The power gain at this point was also 30, when using a 100 kΩ resistor (close to optimal matching) as a collector load. The input impedance at the maximum gain follows approximately the rule Zin = 2βRC. From this we find an input impedance of 11 MΩ. In the space of mesoscopic devices, the BOT is thus found in the intermediate impedance regime, between the SET and SQUID (see figure).	The IV curve of the BOT and region of current and power gains are shown in the figure. The dynamic region for both the VC and IB space depends on the gain. For a gain of 30, the dynamic region is about 10 pA. The BOT is thus aimed for amplifying very small currents. Its sensitivity is confirmed by its good noise properties.

We have measured the current noise spectral density of the Bloch Oscillating Transistor (BOT) as a function of current gain. We find that the equivalent input noise that shows up in the output is less than the shot noise of the normal-insulating-superconductor tunnel junction At the optimal operating point we find a reduced input current noise of 1.0 fA/√Hz) and a corresponding noise temperature of 0.4 K

The current noise was converted to voltage noise in a 100 kΩ load at the collector. The voltage was then measured by two LI-75A low noise preamplifiers and the outputs fed into an HP 89410A vector signal analyzer and cross-correlated. After cross-correlation, the voltage noise of the amplifiers is practically eliminated and we are left with the BOT output noise, the back-action current noise of the LI-75A + any spurious noise source not accounted for.

The noise temperature vs. current gain is shown in the figure for the experiment (circles) and simulation (squares).

Related publications

• Noise properties of the Bloch oscillating transistor

R.K. Lindell and P.J. Hakonen

Appl. Phys. Lett. 86, 173507 (2005)

• Bloch oscillating transistor as the readout element for hot electron bolometers

J. Hassel, H. Seppä, J. Delahaye, and P.J. Hakonen

Proc. of SPIE 5498, 818 (2004)

• Bloch oscillating transistor - a new mesoscopic amplifier

J. Delahaye, J. Hassel, R.K. Lindell, M.A. Sillanpää, M.A. Paalanen, H. Seppä, and P.J. Hakonen

Physica E 18,15 (2003)

• Low-noise current amplifier based on mesoscopic Josephson junction

J. Delahaye, J. Hassel, R.K. Lindell, M.A. Sillanpää, M.A. Paalanen, H. Seppä, and P.J. Hakonen

Science 299, 1045 (2003)