Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2002-10-29
2004-10-26
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S252000, C338S03200R, C338S03200R, C428S692100
Reexamination Certificate
active
06809514
ABSTRACT:
This invention relates to a magnetic field sensor of semiconductor material.
Before considering the prior art, semiconductor properties will be discussed. Semiconductor magnetic field sensors operate using electrical transport effects, and broadly speaking, there are three important conduction regimes: unsaturated extrinsic, saturated extrinsic and intrinsic, and these occur at low, moderate and high temperature respectively. In the unsaturated extrinsic regime, there is insufficient thermal energy to ionise all impurities and the carrier concentration is temperature dependent because increasing the temperature ionises more impurities. Carriers are activated from dopant impurities of a single species, ie donors or acceptors. Conduction is due substantially to one kind of carrier in one band, ie electrons in the conduction band or holes in the valence band but not both. The saturated extrinsic regime is similar, but occurs at higher temperatures at which virtually all impurities have become ionised but insufficient thermal energy is available to ionise significant numbers of valence band states to create electron-hole pairs: here the carrier concentration is largely independent of temperature.
In the intrinsic regime, conduction has a substantial contribution from thermal ionisation of valence band states producing both types of carrier, ie electron-hole pairs, in addition to carriers of one type activated from impurities. Conduction is due to both kinds of carrier in both bands, ie electrons in the conduction band and holes in the valence band. Conductivity varies with temperature in this regime because the electron-hole pair concentration is temperature dependent. There is an intervening transition region between the extrinsic and intrinsic regimes where conduction is partially extrinsic and partially intrinsic giving rise to more of one type of charge carrier than the other, ie majority carriers and minority carriers: it is at or near ambient temperature in Ge depending on doping. The onset temperature of intrinsic conduction depends on band gap and dopant concentration; it can occur below ambient temperature, as low as 150K in narrow gap semiconductors with low doping.
Materials such as Si and GaAs with a saturated extrinsic regime at room temperature are preferred for magnetic field sensor applications despite their inferior mobility properties: this is because of the need for Hall effect or resistance to be largely independent of temperature. By analogy with Ge which if sufficiently purified is intrinsic at ambient temperature, weakly doped Si is sometimes referred to wrongly as intrinsic, such as in PIN diodes where the high resistivity I (“intrinsic”) region is in fact extrinsic at ambient temperature. The purest Si currently available is more than an order of magnitude too impure to be intrinsic at ambient temperature.
Magnetic field sensors involving semiconductor materials have been known for many years. They include:
(a) magneto-resistance sensors which change in electrical resistance in response to applied magnetic field, and
(b) Hall effect sensors which respond to a magnetic field by developing a voltage proportional to sensor current and field strength.
The electrical resistance R
M
of an extrinsic magneto-resistance sensor in a magnetic field B is given by:
R
M
=R
0
(1+&mgr;
2
B
2
) (1)
where &mgr; is charge carrier mobility and R
0
is sensor resistance in the absence of a magnetic field. The magneto-resistance contribution to Equation (1) is &mgr;
2
B
2
R
0
which varies as the square of both mobility and magnetic field.
A conventional Hall effect sensor arrangement consists of a rectangular block of semiconductor material carrying a longitudinal current in a transverse magnetic field: this produces a Hall voltage V
H
orthogonal both to field and current: for an extrinsic semiconductor arranged in this way, V
H
is given by:
V
H
=
E
y
t
y
=
1
ne
j
x
B
z
t
y
(
2
)
where
E
y
=Hall effect electric field;
t
y
=semiconductor thickness dimension across which V
H
is measured;
n=charge carrier concentration;
e=charge on each charge carrier (negative for electrons, positive for holes);
j
x
=current density in the semiconductor per unit cross-sectional area;
B
z
=magnetic field; and
indexes x, y, z indicate x y and z co-ordinate axes and directions of parameters to which they are suffixed.
For an extrinsic regime with one type of charge carrier, the Hall coefficient R
H
is defined as
R
H
=
E
H
j
x
B
z
=
1
ne
(
3
)
The situation is more complicated than Equation (3) indicates if the semiconductor is in an intrinsic regime with two types of charge carrier.
The conductivity &sgr; of a material is given by
&sgr;=ne&mgr;
c
(4)
where &mgr;
c
is the conductivity mobility.
A carrier mobility value &mgr;
H
referred to as the Hall mobility is obtainable by multiplying Equations (3) and (4) together, ie:
&mgr;
H
=&sgr;R
H
(5)
If conduction is extrinsic, the Hall mobility differs from the conductivity mobility by a numerical factor whose magnitude depends on the carrier scattering mechanism. However, Hall and conductivity mobilities follow the same general principles and will hereinafter be treated as equal and referred to as &mgr;. If conduction is intrinsic the expression for the Hall coefficient is more complex and is magnetic field dependent.
A large Hall voltage is desirable for ease of measurement; it can be achieved by using a high current density, which requires low resistivity to limit power dissipation and hence high carrier mobility. It is also desirable for magneto-resistance sensors to have high carrier mobility to reduce resistance and hence power consumption and to increase the sensitivity of magneto-resistance to magnetic field, which as has been said varies as the square of mobility from Equation (1). Narrow band gap semiconductors such as InSb or InAs best satisfy this mobility criterion. InSb has an electron mobility &mgr;
e
of 8 m
2
V
−1
s
−1
, nearly ten times that of GaAs, which is 0.85 m
2
V
−1
s
−1
and is in turn better than that of Si.
Despite their splendid mobility properties, narrow band gap semiconductors are not generally used for Hall effect or magneto-resistance sensors because they are intrinsic at ambient temperature. This results in low Hall coefficient and Hall voltage, and in Hall voltage and sensor resistance varying with temperature; it conflicts with an important requirement of a magnetic field sensor, namely that its response to magnetic field should be relatively insensitive to temperature change. Another consequence of the intrinsic regime is that Hall effect is non-linear with magnetic field (magneto-resistance varies as the square of magnetic field irrespective of regime). These problems have placed restrictions on use of narrow band gap semiconductors in magnetic field sensors operating at room temperature (290K) or above: in particular, they need to be heavily doped to reduce the temperature dependence of the carrier concentration (i.e. to make them extrinsic). This tends to defeat the object of using them, because it reduces their carrier mobility considerably counteracting their advantage.
Conventional magnetic field sensors are operated in the saturated extrinsic regime, where the carrier concentration is largely constant and does not produce unwanted changes in resistance and Hall effect. Temperature dependence of resistance and Hall effect arises however from mobility reduction with rise in temperature due to increased phonon scattering and onset of electron-hole scattering.
Prior art magnetic sensors based on silicon technology tend to be physically robust and are widely used in the motor industry in harsh environments: They are used for example in brushless compact disc (CD) drive motors where low noise is paramount. However, they suffer from the general problem of temperature dependent sensitivity, and moreover their sensitivity is inadequate for some applications.
I
Ashley Timothy
Elliott Charles T
Phillips Timothy J
Patidar Jay
Qinetiq Limited
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