  Flux transformers and superconducting quantum interference devices (SQUIDs)
are two key superconducting devices used in the cutting-edge magnetic-field
sensor systems, such as magnetocardiography (MCG) and magnetoencephalography
(MEG). They are superconductor integrated circuits enabled by two
superconductor elements, superconductor wire and Josephson junction, based on
Meissner effect and Josephson effect. The principles of SQUID sensors are
unfamiliar for electronics engineers who are trained with semiconductor
electronics, due to the superconductor physics. We present an introduction to
SQUID sensors, ranged from superconductor elements to the
superconductor-semiconductor hybrid system. It is demonstrated that, SQUIDs
inside are the resistor-inductor-capacitor (RLC) network driven by Josephson
currents, and share the common network equation with normal RLC circuits; SQUID
outside are magnetic-field-effect transistors (MFETs), and are dual to the
semiconductor FETs in signal conversions; SQUID magnetic-field sensors are
linear current amplifiers, as semiconductor FET electric-field sensors are
linear voltage amplifiers.
