Magnetic field sensor semiconductor devices

A semiconductor device incorporates a p-n-p-n structure of circular geometry, within which there may be formed a carrier domain which will rotate around the structure when an appropriate magnetic field is applied. The four regions of this structure are all bounded by a planar surface of the semiconductor, one end region being centrally disposed and the other forming an annular intrusion into the adjacent intermediate region, which is also of annular form and has contact made to it only outwardly of the annular intrusion. The device may be utilized in various ways in magnetic field sensors.

In two articles appearing at pages 608-611 of Volume 12 of "Electronics 
Letters", dated Nov. 11, 1976, there is disclosed a type of semiconductor 
device which may be used in magnetic field sensors. This type of device 
incorporates a p-n-p-n structure of circular geometry such that under 
appropriate biassing conditions the effective current flow in the 
structure will occur in a limited region, termed a carrier domain, which 
subtends only a small angle at the centre of the circle. If the device is 
subjected to a sufficiently strong magnetic field directed perpendicular 
to the plane of the circle, the carrier domain will rotate continuously 
around the centre of the circle at a rate dependent on the magnetic flux 
density and in a sense dependent on the sense of the field. Ideally 
rotation of the carrier domain would occur for any value of the flux 
density, but in practice it is found that there is a threshold value 
(dependent on the precise biassing conditions) below which rotation does 
not occur, because inescapable inhomogenities in the device structure tend 
to cause the carrier domain to stick at a particular angular position; for 
flux densities significantly above the threshold the rate of rotation of 
the domain is substantially linearly proportional to the flux density. To 
sense the rotation of the domain there are provided appropriate 
current-collecting contacts, from each of which can be derived a train of 
pulses having a recurrence frequency substantially linearly proportional 
to the magnetic flux density normal to the plane of the circle. Such a 
device thus has a significant advantage over more conventional types of 
semiconductor device used in magnetic field sensors, in that the form of 
the output signal lends itself readily to the employment of a digital 
readout system without any need for analogue-to-digital conversion. 
It is an object of the present invention to provide a semiconductor device 
designed to utilise the same basic operating principle as that of the type 
of device disclosed in the articles referred to above, but having a 
specific form such as to enable a more effective practical application of 
this principle to be achieved. 
According to the invention a semiconductor device comprises a semiconductor 
body having a substantially planar surface and incorporating first, 
second, third and fourth regions which are alternately of opposite 
conductivity types in the order stated and which all extend to said 
surface, the first region being contiguous only with the second region, 
the fourth region being contiguous only with the third region, the second 
and third regions being contiguous with each other, the first and second 
regions being of annular forms having a common axis perpendicular to said 
surface and the fourth region being in the form of a solid of revolution 
which is coaxial with and centrally disposed with respect to the first and 
second regions, and an electrode system providing separate connections for 
said four regions, the electrode contact for the second region being made 
only at points which are more remote than the first region from said axis, 
and the electrode contact for at least one of the second and third regions 
being non-uniform around said axis. 
In such a device the four specified regions constitute a p-n-p-n structure 
of circular geometry, and in operation the central p-n junction of this 
structure (i.e. that between the second and third regions) is biassed in 
the reverse direction while the first and fourth regions are current 
driven to operate as emitters; the structure can then be regarded as 
comprising two transistors of opposite types (respectively constituted by 
the first, second and third regions and the fourth, third and second 
regions) interconnected so as to give rise to regenerative feedback. This 
feedback causes the emitter current distribution for both transistors to 
be concentrated in a circumferentially limited part of the structure, thus 
forming a carrier domain, and if the device is subjected to a sufficiently 
strong magnetic field directed substantially parallel to the central axis 
of the structure (i.e. perpendicular to said planar surface) the 
interaction between the magnetic field and the flows of electrons and 
holes will cause the domain to rotate continuously around that axis. The 
rotation of the domain can be sensed by utilising the non-uniformity of 
the electrode contact to one of the second and third regions to derive a 
current which varies cyclically as the domain rotates, the frequency of 
the variation giving a measure of the flux density parallel to said axis. 
If it is required to detect the sense of the magnetic field, one of the 
second and third regions can be provided with plural contacts from which 
separate currents can be derived, these contacts being circumferentially 
spaced in such a manner that the phase relationship of the currents will 
indicate the sense of rotation of the domain. 
The form of a device according to the invention is such that, while using 
straightforward fabrication techniques, it is possible to make the p-n-p-n 
structure of smaller overall dimensions than is feasible with the type of 
device disclosed in the articles referred to above; it is thus possible to 
achieve higher sensitivity, i.e. an increase in the ratio of frequency of 
rotation of the carrier domain to magnetic flux density. Further, the form 
of a device according to the invention is such as should make it possible 
in practice to achieve somewhat lower threshold values of magnetic flux 
density than is the case with the previously disclosed type of device, 
since it should be less susceptible in this respect to the effects of 
certain types of inhomogeneity likely to arise in the manufacture of the 
device. It is also envisaged that some compensation for the effects of 
inhomogeneities, and hence some reduction of the threshold value of flux 
density, can be achieved by providing plural contacts for one or more of 
the regions of the p-n-p-n structure and using these contacts to establish 
differences in the biassing conditions for parts of the structure 
respectively disposed at different angular positions with respect to its 
central axis.

Referring to FIG. 1, the semiconductor body is in the form of a silicon 
chip having parallel planar main faces, only one of which (referenced 1) 
is shown in the drawing. The basic material 2 of the chip is n-type, of 
resistivity about two ohm-cm, and in the completed device this material 
serves to form the collector region of an n-p-n transistor structure and 
the base region of a p-n-p transistor structure. Various other regions 3, 
4, 5 and 6 are formed in the chip by diffusion of appropriate impurities 
through the face 1, the regions 3 and 4 being p-type and the regions 5 and 
6 being n-type of low resistivity; the region 3 is of substantially 
cylindrical form, while the regions 4, 5 and 6 are of annular forms 
coaxial with the region 3. In the completed device the regions 3 and 5 
respectively serve as the emitter regions of the p-n-p and n-p-n 
transistor structures, the region 4 serves to form the collector region of 
the p-n-p structure and the base region of the n-p-n structure, and the 
region 6 serves to provide a low resistance non-rectifying contact to the 
region 2. Suitably the region 3 may have a diameter of about 120 microns, 
the radial gap between the regions 3 and 4 may have a width of about two 
microns, the overall radial width of the region 4 may be about 110 
microns, the region 5 may have an inner diameter of about 170 microns and 
a radial width of about 40 microns, and the region 6 may have an internal 
diameter of about 400 microns and a radial width of about 30 microns. In 
FIG. 1, the depths of the regions, 3, 4, 5 and 6 are greatly exaggerated 
compared with their lateral dimensions; they may suitably have values of 
about three microns for the regions 3 and 4 and two microns for the 
regions 5 and 6. 
The diffusion processes for forming the regions 3, 4, 5 and 6 are carried 
out in two stages, the first using boron to form the region 3 and an 
annular p-type region corresponding in volume to the final regions 4 and 5 
(suitably with a final sheet resistance of about 100 ohms per square) and 
the second using phosphorus to form the regions 5 and 6 (suitably with a 
final sheet resistance of about five ohms per square) and leave the 
remainder of the annular p-type region constituting the final region 4. In 
each stage the lateral extent of the diffusion is controlled in 
conventional manner by effecting the diffusion through apertures formed in 
an oxide layer (not shown) on the face 1, the oxide layer being reformed 
after each stage. 
Referring now to FIG. 2, the device also includes an electrode system in 
the form of a series of metallic conductors 7 to 14, which are formed by 
deposition on the chip after completion of the diffusion processes. The 
conductors 7 to 14 are deposited partly directly on the face 1, through 
suitable apertures formed in the oxide layer, and partly on the surface of 
this layer, the arrangement being such that each conductor is in direct 
contact with one, and only one, of the regions 3, 4, 5 and 6, the 
positions of which are indicated by the broken lines in FIG. 2. Thus the 
conductor 7 is in contact with the region 3 over a central circular area, 
the conductor 8 is in contact with the region 5 over an area which is 
nearly a complete annulus but has a gap to accommodate the conductor 7, 
the conductors 9 and 10 are in contact with the region 4 over 
diametrically opposed areas, disposed outwardly of the region 5, which 
between them form nearly a complete annulus but leave gaps to accommodate 
the conductors 7 and 8, while the conductors 11 to 14 are in contact with 
the region 6 over small areas respectively disposed at angular intervals 
of 90.degree. around the region 6. The conductors 7 to 14 extend outwardly 
to conventional bonding pads (not shown) by means of which external 
connections can readily be made to the device. It will be appreciated that 
each of conductors 8-14 is non-uniform relative to their common axis 
extending through the center of region 3 perpendicular to face 1, i.e., 
none of those conductors extends completely around that axis. 
Using fabrication processes as indicated above, it will of course normally 
be convenient to manufacture a large batch of devices simultaneously from 
a single slice of silicon, the diffusion and deposition processes for the 
whole batch being carried out before the slice is divided into individual 
chips. 
It will be appreciated that in the device illustrated in FIGS. 1 and 2 the 
magnetically sensitive p-n-p-n structure is constituted by the regions 3, 
2, 4 and 5. As indicated above the operating conditions of the device 
involve the application of a reverse bias to the p-n junction between the 
regions 2 and 4 (which serves as the collector-base junction for both 
transistor structures), and the feeding of currents to the regions 3 and 5 
in such senses as to cause holes to be injected from the region 3 into the 
region 2 and electrons to be injected from the region 5 into the region 4. 
Where the structural parameters of the device have values as indicated 
above, the reverse bias voltage for the collector-base junction may 
suitably have a value of about three or four volts, and the currents fed 
to the regions 3 and 5 may suitably have values in the respective ranges 
5-10 and 10-15 mA. By virtue of the regenerative coupling between the two 
transistor structures, the actual minority carrier injection is 
concentrated in a domain of limited angular extent, which will be caused 
to rotate around the central axis of the structure if the device is 
subjected to a sufficiently strong magnetic field directed substantially 
parallel to this axis. Where the structural parameters of the device have 
values as indicated above, the threshold value of the magnetic flux 
density will typically lie in the range 0.02-0.2 tesla, and for flux 
densities significantly above the threshold the frequency of rotation of 
the domain will be linearly proportional to the flux density with a slope 
typically of the order of 100 kHz/tesla. 
As noted above, the existence of the threshold in respect of rotation of 
the carrier domain is due to unavoidable departures of the structure from 
perfect symmetry. In this connection an important feature of the design of 
the device illustrated in FIGS. 1 and 2 is the use of as symmetrical as 
possible a form of contact for the region 3. Ideally, an improvement in 
the design of the device could be made by arranging for the contact 
between the conductor 8 and the region 5 to be in the form of a complete 
annulus, and either providing some extra means of insulating the conductor 
7 from the conductor 8, for example by forming them in separate stages 
with an appropriate intervening deposition of insulating material, or 
replacing the conductor 7 by a central bonding pad; these alternatives 
would, however, have the practical disadvantage of entailing extra 
complication in the fabrication of the device. 
The provision of the plural contacts for the regions 2 and 4 allows some 
adjustment to be made of the threshold value of the magnetic flux density 
for a particular device, since by use of appropriate circuits the precise 
value of the reverse bias on the collector-base junction can be caused to 
vary somewhat around its circumference; this facility may in particular be 
used to compensate to some extent for structural inhomogeneities and hence 
minimise the threshold value for a particular device. One suitable circuit 
for this purpose is illustrated diagrammatically in FIG. 3. In this 
circuit the conductors 11 to 14 are connected to ground respectively via 
variable resistors 15 to 18, while the conductors 9 and 10 are connected 
to a negative supply terminal 19 respectively directly and via a variable 
resistor 20. If desired, this circuit can be modified by replacing the 
resistors with variable voltage sources. 
The rotation of the carrier domain will give rise to cyclic variations in 
the individual currents flowing through the conductors 9 to 14, so that 
the rotation can be sensed by deriving output signals from one or more of 
these conductors. Where, as will be the case in many applications, the 
sense of the magnetic field to which the device is subjected is known and 
it is required only to ascertain the strength of the field, only one of 
the conductors need be used for this purpose. A suitable arrangement which 
may be used in such a case is illustrated diagrammatically in FIG. 4. In 
this arrangement an output current derived from the relevant conductor is 
applied to a current-to-voltage converter 21, the output of which is fed 
to a squaring circuit 22 to produce a train of substantially rectangular 
pulses whose recurrence frequency is equal to the frequency of rotation of 
the carrier domain. The pulse train is fed to a counter 23 having an 
associated digital display unit 24. The counter 23 is reset at regular 
intervals (for example once every tenth of a second) by means of a signal 
derived from a clock generator 25, which also provides a signal to cause 
the unit 24 to display during each interval the contents of the counter 23 
at the end of the preceding interval. 
If required, the sense of the magnetic field can be ascertained by deriving 
output signals from two of the conductors 11 to 14, which must not be 
diametrically opposite each other, and detecting the sign of the phase 
difference between these two signals. In this connection it may be noted 
that if the sense of the field is the same as that in which the device is 
viewed in FIG. 2, the rotation of the carrier domain will be anticlockwise 
when viewed in the same sense. 
Besides affecting the threshold value of magnetic flux density, changes in 
the biassing conditions of the device will alter the precise value of the 
frequency of domain rotation for a given flux density; this value is also 
sensitive to variations in temperature because of consequent changes in 
the electrical properties of the semiconductor material. It may therefore 
be necessary to adopt appropriate stabilisation measures in respect of 
these factors if high accuracy is required in the measurement of field 
strength. 
As well as being utilised in a straightforward manner, such as described 
above, for the direct measurement of relatively strong steady magnetic 
fields, the device illustrated in FIGS. 1 and 2 has various other possible 
applications. For example it can be used for the measurement of weak 
magnetic fields, either steady or alternating, whose magnitudes are below 
the threshold value. One possible arrangement in this case is to apply a 
constant bias field having a flux density above the threshold value, and 
to monitor the change in the frequency of domain rotation resulting from 
the superimposition of the field to be measured on the bias field; where 
the unknown field is an alternating one, its magnitude can readily be 
sensed by the application of standard f.m. demodulation techniques to the 
output signal derived from the device. Another possible arrangement for 
measuring weak fields, which does not require the provision of a bias 
field, involves the application to the conductors 11 to 14 of a set of 
alternating voltage signals suitably phased so as to cause continuous 
rotation of the carrier domain in the absence of a magnetic field. With 
this arrangement there will be a finite delay between the applied voltage 
variation and the consequent variation of current at any of the conductors 
9 to 14, and this delay will be altered if the device is subjected to a 
magnetic field directed substantially parallel to the central axis of the 
p-n-p-n structure, the amount and sense of the alteration being dependent 
on the magnitude and polarity of the magnetic flux density; measurement of 
the field can thus be effected by making an appropriate phase comparison 
between the applied voltage pattern and an output signal derived from the 
device. 
In the applications discussed above the device is orientated with the axis 
of the p-n-p-n structure substantially parallel to the magnetic field to 
be sensed. If instead the device is subjected to a sufficiently strong 
magnetic field directed substantially perpendicular to this axis, it is 
possible to use the device to sense the orientation of the field in a 
plane perpendicular to the axis. In this case the magnetic field will not 
cause continuous rotation of the carrier domain, but will cause it to take 
up a static position angularly displaced by 90.degree. from the 
orientation of the field. The angular position of the domain can readily 
be ascertained by comparison of the magnitudes of output currents 
respectively derived from the conductors 11 to 14. 
More generally it may be noted that, if a sufficiently strong magnetic 
field is applied to the device at an arbitrary angle to the axis of the 
p-n-p-n structure, the carrier domain will either rotate continuously or 
take up a static position according to whether the arbitrary angle is less 
or greater than a critical angle. 
The magnetic field(s) applied to the p-n-p-n structure in operation will 
commonly be generated externally, but could instead be generated by the 
passage of current through a conductive pattern (not shown) formed on the 
chip in addition to the electrode system shown in FIG. 2. The device would 
then of course be responsive to variations of this current, leading to the 
possibility that it could be used in a signal translating system with the 
conductive pattern serving as a signal input connection. 
The device illustrated in FIGS. 1 and 2 can readily be augmented so that 
the chip incorporates further elements forming an integrated circuit. Such 
elements could, for example, constitute parts of an arrangement such as 
shown in FIG. 4, or parts of an automatic temperature compensating system 
designed to vary the bias conditions of the p-n-p-n structure in response 
to temperature changes so as to preserve a substantially constant relation 
between the frequency of domain rotation and the magnetic flux density.