1.5 Magnetic MOS Field-Effect-Transistor

The inversion layer of a metal-oxide-semiconductor field-effect-transistor (MOSFET) can be used as the active region of a magnetic sensor [14]. This active region can exploit the Hall effect for Hall based sensors, or the carrier deflection, if the device has a split-drain (See Figure 1.3). A split-drain MOSFET structure is called MAGFET if it is used as a magnetic sensor.

One of the main advantages of a MAGFET is its compatibility with the CMOS process without the need for post-processing steps. Very low power consumption can be achieved with this technology. The ability of integrating the bias and control circuitry on the same chip makes this device structure particularly attractive. However, the most serious drawback of the MAGFET is its $1/f$ noise. Also, the electron mobility of the inversion layer is relatively low, which results in limited sensitivity of the whole structure, making it only useful for large magnetic induction in the 100 mT range.

Figure 1.3: Split-drain MOSFET structure.

All these features point out that the MAGFET structure is not the proper sensor selection for a range of applications. However, if the solid-state magnetic sensor is cooled down to 77 K, many figures of merit are greatly improved [16]. The $1/f$ noise is lowered and the electron mobility of the inversion layer is increased, which results in increased sensitivity and allows detection in the sub-mT range. Cooling down the solid-state sensor will impact other figures, such as weight and cost. However, cooling down to 77 K (liquid Nitrogen temperature) is cheaper as compared to 4.2 K (liquid Helium temperature), the operating temperature used for most superconducting quantum interference devices (SQUID) which are capable of resolving the picotesla fields occurring in biomagnetometry [12].

Additionally, manufacturing the MAGFET in CMOS technology implies the use of silicon as the detector material. Although InSb or GaAs offer higher mobilities in n-type materials than Si, some other properties of InSb and GaAs make the choice of silicon straightforward [5,24,33].