Page 294 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

274 MICROSENSORS

A large variety of Hall effect devices are commercially available (e.g. from Siemens)
with different designs, packages, and output signals. For example, a general-purpose linear
Hall effect IC (RS 304–267) is available in a 4-pin dual-in-line (DIL) plastic package
to detect magnetic flux densities of ±40 mT. The supply voltage is 4 to 10 V direct
current (DC) with a current output in the milliampere range and a sensitivity of about
10 V/T. Linear Hall effect devices are also commercially available and these have an
integrated transconductance amplifier to provide a linear output voltage. For example,
the RS 650–532 (RS Components Ltd) comes in a 3-pin in-line surface-mounted package
16
with its thick film resistors laser-trimmed in a hybrid (ceramic) circuit to give a voltage
sensitivity of 75 ± 2 V/T.
One problem associated with a Hall plate device is that the offset voltage becomes
significant at low magnetic flux densities and, therefore, an alternative is to use a magne-
toresistive device (see next section) that has a higher sensitivity. This offset problem can
be reduced by spinning the Hall plate and averaging the signal (Bellekom 1998), but a
more practical solution is to employ a magnetoresistor or, when needing a standard IC
process, to employ a magnetodiode or magnetotransistor instead (see Section 8.5.3).

8.5.2 Magnetoresistive Devices

The resistance of a semiconducting material is influenced by the application of an external
magnetic flux density B z. In this effect, known as magnetoresistivity, the resistance R of
a slab of the material (see Figure 8.38 for basic layout) depends on the Hall angle 9\\ and
is given by
2
2
R = R 0(1 + tan #H) and OH « k^ B\ (8.45)
is the resistance of the slab at zero flux density. The Hall angle is the angle by which
R 0
the direction of the current I x is rotated as a result of the Lorentzian force that acts on
the charge carriers and is related to the mobility n of the carrier. The ideal, theoretical
value is corrected by a geometrical factor k g that depends on the actual aspect ratio of
the slab. Ideally, the slab should be much wider than longer and the Hall voltage must
be shorted out.
2
The low carrier mobility in silicon (n n ~ 1600 cm /Vs) makes the effect rather small
and so other materials are used that exhibit a giant magnetoresistive effect. For example,
2
InSb has a very high electron mobility of 70000 cm /Vs and when doped with NiSb can
produce a high sensitivity of about 700%/T as illustrated in Figure 8.41.
The strong temperature-dependence of the response of a giant magnetoresistor needs
compensation. Magnetoresistors are relatively inexpensive to make but are not compat-
ible with an IC process like a Hall effect IC or magnetodiode/transistor. Therefore, the
integration of a giant magnetoresistor into a silicon MEMS structure would be diffi-
cult. More recently, research has been directed toward perovskite materials that exhibit
a so-called 'colossal' magnetoresistance and may also be used as the dielectric mate-
rial in submicron ICs because of their high dielectric constant and high breakdown field
strength.

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Hybrid technologies are described in Section 4.6.