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A 37-channel DC SQUID magnetometer system has been built for biomagnetic studies. The SQUID loop of each magnetometer serves as the active sensing element, thereby eliminating the need for flux coupling circuits. The magnetometers are located approximately 3 cm above the outer dewar bottom. The SQUIDs are directly coupled to highly simplified read-out electronics using only five wires per channel; no helium temperature impedance matching circuits are required. Each channel can be independently inserted into or removed from the dewar. Using a novel electronic noise reduction technique the system white and 1 Hz flux density noise values are typically 5 and 10 fT Hz-1/2, respectively, including the noise contribution of the in-house fabricated dewar and the magnetically shielded room. The two parts of the data-processing system allow independent handling of the acquisition and analysing task. Two example measurements demonstrate the advantage of the electronic noise reduction method.

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Five silicon (Si) p ++ -n % -n ++ samples were grown at various doping concentrations (1.0 ' 10 17 -2.2 ' 10 17 cm %3 ) in an n % layer by using the reduced-pressure CVD technique. By using these samples, 30 ' 2 µm 2 single-drift (SD) impact-ionization avalanche transit-time (IMPATT) diodes were processed with Si-based monolithic millimeter-wave integrated circuit (SIMMWIC) technology. 1,2) The samples within a small process window exhibited a large negative differential resistance at approximately the avalanche frequency, as confirmed by small-signal S-parameter characterization. A model based on depletion width was given to explain the conditions for the appearance of the negative differential IMPATT resistance, which is the basis of millimeter-wave amplifier and oscillator applications. Furthermore, a measurement-based small-signal lumpedelement model was established to describe the IMPATT functionality from the circuit component aspect. This lumped-element model shows a negative differential resistance within a well-defined range in the given element parameters, which can explain the experimental observations.