Abstract:
A magnetic field measuring system is disclosed. The magnetic field measuring system includes a substrate, a conductive well formed in the substrate, the well having a first side with a first length, a first contact electrically coupled to the conductive well at a first location of the first side, a second contact electrically coupled to the conductive well at a second location of the first side, wherein the distance between the first location and the second location is less than the first length, a stimulus circuit coupled to the first contact and the second contact, and a sensor for identifying a property indicative of the length of a current path from the first location to the second location through the conductive well.

Description:
FIELD 
       [0001]    The present invention generally relates to devices that are used to measure magnetic field intensity and direction and more particularly to magnetoresistive devices that use changes in resistance to determine the magnetic field intensity and direction. 
       BACKGROUND 
       [0002]    Magnetoresistive devices use change in the electrical resistance of a material between two electrodes in the presence of a magnetic field to determine the strength of the magnetic field. A certain amount of electrical resistance is present in the material without any magnetic field being applied. When a magnetic field is applied the electrical resistance changes. 
         [0003]    The change in the electrical resistance corresponds to the strength of the magnetic field. This change in the electrical resistance is the result of the Lorentz force applied to the charge carrying particles. When a charge carrying particle is in the presence of a magnetic field, the Lorentz force applied to the particle is expressed as 
         [0000]        F=q[E +(ν× B )],
   where F is the Lorentz force in Newtons,   q is the charge of the charge carrying particle in coulombs,   ν is the instantaneous velocity in m/s,   E is the electric field in v/m, and   B is the magnetic field in Tesla. The “×” is the vector cross-product between ν and B.   
 
         [0009]    To ensure proper sensitivity to the magnetic field, it has been a goal of the magnetoresistive device makers in the prior art to maximize the change in the electrical resistance of the magnetoresistive devices when a magnetic field is applied. For example, in semiconductor magnetoresistive devices the mobility of the semiconductor material affects the velocity of the charge carrying particles and thereby affects the Lorentz force applied to those particles. In order to maximize change in resistance, semiconductor materials with high mobilities are often chosen. 
         [0010]    The relationship between the change in the resistance in a magnetoresistive device, the applied magnetic field, and the mobility is non-linear. The change in the resistance is proportional to (1+(μB) 2 ), where μ is the mobility. This non-linear relationship presents challenges in determining the direction of the magnetic field when a magnetoresistive device is placed in a magnetic field. For example, the same amount of change in the electrical resistance is experienced whether the magnetic field is coming out of a plane or going into the plane, i.e., positive and negative magnetic fields. 
         [0011]    Also, sensitivity to changes in the magnetic field is disadvantageously affected by the non-linear relationship described above. Furthermore, when the magnetic field is parallel to the plane of the magnetoresistive device, change in the electrical resistance is not as easily determinable. This difficulty is due to the vector cross-product of the B field and the velocity of the charge carriers, as provided in the Lorentz force equation. When the magnetic field is parallel to the plane of the magnetoresistive device, the cross product becomes zero since the angle between the two vectors is either 0° or 180°. 
         [0012]    Therefore, a need exists to address the stated shortcomings of the prior art. Particularly, a need exists to measure the magnetic field as a result of change in the electrical resistance of a magnetoresistive device with improved sensitivity, including situations where a further need exists for determining the direction of the magnetic field. 
       SUMMARY 
       [0013]    In one embodiment, a magnetic field measuring system is disclosed. The magnetic field measuring system includes a substrate, a conductive well formed in the substrate, the well having a first side with a first length, a first contact electrically coupled to the conductive well at a first location of the first side, a second contact electrically coupled to the conductive well at a second location of the first side, wherein the distance between the first location and the second location is less than the first length, a stimulus circuit coupled to the first contact and the second contact, and a sensor for identifying a property indicative of the length of a current path from the first location to the second location through the conductive well. 
         [0014]    In another embodiment, a method for measuring a magnetic field is disclosed. The method includes forming a conductive well in a substrate, the well having a first edge with a first length, electrically coupling a first contact to the conductive well at a first location of the first edge, electrically coupling a second contact to the conductive well at a second location of the first edge, wherein the distance between the first location and the second location is less than the first length, identifying a property indicative of the length of a current path from the first location to the second location through the conductive well, and determining the strength of a magnetic field passing through the conductive well based upon the identified property. 
         [0015]    The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  depicts a block diagram of a system having blocks in communication with a magnetoresistive device; 
           [0017]      FIGS. 2-5  depict top plan views of magnetoresistive devices for single-ended measurements including current sources and designed to detect perpendicular magnetic field lines; 
           [0018]      FIGS. 6-9  depict top plan views of magnetoresistive devices for single-ended measurements including voltage sources and designed to detect perpendicular magnetic field lines; 
           [0019]      FIG. 10  depicts a top plan view of side-by-side magnetoresistive devices for obtaining differential measurements including current sources and designed to detect perpendicular magnetic field lines; 
           [0020]      FIG. 11  depicts a top plan view of magnetoresistive devices for differential measurements including current sources and matching of the magnetoresistive devices and designed to detect perpendicular magnetic field lines; 
           [0021]      FIG. 12  depicts a top plan view of side-by-side magnetoresistive devices for differential measurements including a voltage source and designed to detect perpendicular magnetic field lines; 
           [0022]      FIG. 13  depicts a top plan view of magnetoresistive devices for differential measurements including a voltage source and matching of the magnetoresistive devices and designed to detect perpendicular magnetic field lines; 
           [0023]      FIG. 14  depicts a top plan view of magnetoresistive devices in a full bridge designed to detect perpendicular magnetic field lines; 
           [0024]      FIG. 15  depicts a top plan view of a magnetoresistive device for single-ended measurements including a current source and designed to detect in-plane magnetic field lines; 
           [0025]      FIG. 16  depicts a cross sectional view of a magnetoresistive device for single-ended measurements including a current source and designed to detect in-plane magnetic field lines; 
           [0026]      FIG. 17  depicts a top plan view of a magnetoresistive device for single-ended measurements including a current source and designed to detect in-plane magnetic field lines; 
           [0027]      FIG. 18  depicts a cross sectional view of a magnetoresistive device for single-ended measurements including a current source and designed to detect in-plane magnetic field lines; 
           [0028]      FIG. 19  depicts a top plan view of a magnetoresistive device for single-ended measurements including a current source and designed to detect in-plane magnetic field lines with an opposite direction as compared to the in-plane magnetic field lines depicted in  FIG. 17 ; 
           [0029]      FIG. 20  depicts a cross sectional view of a magnetoresistive device for single-ended measurements including a current source and designed to detect in-plane magnetic field lines with an opposite direction as compared to the in-plane magnetic field lines depicted in  FIG. 17 ; and 
           [0030]      FIG. 21  depicts a cross sectional view of a magnetoresistive device for single-ended measurements including a current source and designed to detect in-plane magnetic field lines with the current source in an opposite direction to as compared to the current source of  FIG. 20 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
         [0032]    Referring to  FIG. 1 , there is depicted a representation of a magnetoresistive system generally designated  10 . The magnetoresistive system  10  includes an I/O device  12 , a processing circuit  14  and a memory  16 . The I/O device  12  may include a user interface, graphical user interface, keyboards, pointing devices, remote and/or local communication links, displays, and other devices that allow externally generated information to be provided to the magnetoresistive system  10 , and that allow internal information of the magnetoresistive system  10  to be communicated externally. 
         [0033]    The processing circuit  14  may suitably be a general purpose computer processing circuit such as a microprocessor and its associated circuitry. The processing circuit  14  is operable to carry out the operations attributed to it herein. 
         [0034]    Within the memory  16  are various program instructions  18 . The program instructions  18  are executable by the processing circuit  104  and/or any other components as appropriate. 
         [0035]    The magnetoresistive system  10  further includes a stimulus/response circuit  22  and at least one magnetoresistive device  24  that is stimulated by the stimulus/response circuit  22 . In one embodiment, the stimulus/response circuit  22  provides a stimulus to the magnetoresistive device  24  and also provides a response to the processing circuit  14 . The processing circuit  14  is connected to the stimulus/response circuit  22  to request stimuli and sense responses from the stimulus/response circuit  22 . 
         [0036]    Referring to  FIG. 2 , a top plan view of an exemplary embodiment of a magnetoresistance device  100  which may be incorporated into the magnetoresistive system  10  is shown. The basic layout of the magnetoresistance device  100  includes a well or diffusion area  104  inside a substrate  102 . The substrate  102  can be a positively or a negatively doped substrate. The well  104  can be an N-well, a P-well, an N-diffusion, or a P-diffusion area. The well  104 , however, is of an opposite doping type than the substrate  102 . Ohmic contacts  106  and  108  are electrically connected to the well  104 . A current source  122  is connected to the ohmic contact  106 , while the ohmic contact  108  is connected to the electrical ground. 
         [0037]    The well  104  is shown as a rectangle which also has a depth extending into the substrate  102 . The well  104  can have different shapes provided that ohmic contacts  106  and  108  are placed asymmetrically with respect to the well  104 . By way of example, in the exemplary embodiment of  FIG. 2 , the ohmic contacts  106  and  108  are placed on the top edge  110  of the well  104 . The ohmic contacts  106  and  108 , however, could have been placed asymmetrically on another edge of the well  104 , e.g., bottom edge  112 . It is also envisioned that the ohmic contacts  106  and  108  can be placed on different edges. For example, in one embodiment, the ohmic contact  106  can be placed on the left edge  118  while the ohmic contact  108  can be placed on the top edge  110  of the well  104 . In one embodiment, the ohmic contacts  106  and  108  are separated by a distance  107  which may be selected to be less than a width  105  of the rectangular well  104 , and the size of the ohmic contacts  106  and  108  (e.g., the width of the ohmic contacts  106  and  108  as measured along the edge  110 ) may be selected to be less than the distance between the ohmic contacts  106  and  108 . In one embodiment the ohmic contacts  106  and  108  are point contacts. 
         [0038]    Connections between the ohmic contact  106  and a current source  122  and the ohmic contact  108  and an electrical ground initiate current flow along a mean current path  120 . The asymmetrical placements of the ohmic contacts  106  and  108  results in a curved mean current path  120  in the absence of a magnetic field. With the configuration shown in  FIG. 2 , the mean current path  120  is mainly within an area proximate the top edge  110 . Direction of the mean current path  120  is shown by way of the arrow  114 , i.e., flowing from the ohmic contact  106  to the ohmic contact  108 , which is consistent with the direction of the current supplying the current source  122 . 
         [0039]    The magnetoresistance device  100  shown in  FIG. 2  is suitable for magnetic fields that are perpendicular to the plane in which the well  104  extends, i.e., fields that cross the well  104 . These perpendicular fields can be crossing out of the plane of the well  104  (coming out of the plane) or into the plane (going into the plane). By way of example, the magnetoresistance device  100  is depicted in  FIG. 3  with an out-of-plane magnetic field. The symbolic circles with dots at the centers of the circles, identified by reference numeral  126 , indicate a magnetic field that is perpendicular to the plane of the well  104  and is coming out of the plane. For reference purposes, the mean current path  120 , the mean current path with a zero magnetic field, is shown in dashed lines. In the presence of the out-of-plane magnetic field  126 , the Lorentz force causes a new curved mean current path  124  to be established between the ohmic contacts  106  and  108 . The new mean current path  124 , however, extends farther away from the top edge  110  than the mean current path  120 . Thus the path length of the mean current path  124  is longer than the path length of the mean current path  120 . 
         [0040]    The increased length of the mean current path of  124  as compared to the mean current path  120  can be used to determine the strength of the magnetic field  126 . Generally, the mean current path  124  provides a higher resistance because of the longer distance required for the charge carriers to travel through the well  104 . The higher resistance results in a larger voltage drop across the ohmic contacts  106  and  108 . 
         [0041]    By way of another example, the magnetoresistance device  100  is depicted in  FIG. 4  with an “into-plane” magnetic field  130  as represented by the symbolic circles with crosses at the centers of the circles. The magnetic field  130  is perpendicular to the plane of the well  104  and is going into the plane as viewed in  FIG. 4 . For reference purposes, the mean current path  120 , the mean current path with a zero magnetic field, is shown in dashed lines. In the presence of the into-plane magnetic field  130 , the Lorentz force causes a new mean curved current path  128  to be established between the ohmic contacts  106  and  108 . The new mean current path  128  is closer to the top edge  110  than the mean current path  120 . Thus the path length of the mean current path  128  is shorter than the path length of the mean current path  120 . The decreased length of the current path of the new mean current path  128  as compared to the mean current path  120  can be used to determine the strength of the magnetic field. Therefore, an opposite direction of the magnetic field that is perpendicular to the plane of the well  104 , shown in  FIGS. 3 and 4 , causes the current paths to elongate in one case and to shorten in the other case. 
         [0042]    While the direction of the magnetic field, i.e., coming out of the plane or going into the plan of the well  104 , can result in lengthening or shortening of the mean current path  120 , reversing the direction of the current source can also result in lengthening or shortening of the mean current path  120 . By way of example, the magnetoresistance device  100  is depicted in  FIG. 5  with an into-plane magnetic field  130  and with a current source  132  that provides a current that follows in a reversed direction as compared to the current source  122  shown in  FIG. 4 . The reversed current source  132  generates a mean current path  134  that is substantially equivalent in length to the mean current path  124  of  FIG. 3 . That is, reversing the direction of the current source and reversing the direction of perpendicular magnetic field lines have substantially the same effect on the current paths. 
         [0043]    The current reversal is also evidenced in the direction  116  of current flowing along the mean current path  134  between the ohmic contacts  108  and  106 . Referring to  FIG. 5 , the dotted mean current path  128  indicates the mean current path of  FIG. 4 . The solid mean current path  134  indicates the mean current path with the current source  132  in presence of a magnetic field  130  that is perpendicular to the plane of the well  104  and is going into the plane. The voltage drop across the ohmic contacts  108  and  106 , corresponding to the mean current path  134 , can be used to compare to the previous measured voltage across the ohmic contacts  106  and  108 , i.e., with respect to the mean current path  128  in  FIG. 4 , to determine the magnetic field intensity. The difference between the two voltage measurements is proportional to the intensity of the magnetic field crossing the well  104 . That is, the quantity (V 128 −V 134 ) is proportional to B. 
         [0044]    In addition to determining the intensity of the magnetic field, the change in voltage across the ohmic contacts  106  and  108  can be used to ascertain the direction of the magnetic field. Referring to  FIGS. 3 and 4 , the direction of the magnetic field can be ascertained by determining if the voltage across the ohmic contacts  106  and  108  increases or decreases when a magnetic field is applied. An increase in the voltage drop across the ohmic contact  106  and  108  indicates the magnetic field is coming out of the plane. 
         [0045]      FIGS. 6-9  depict single ended measurement techniques involving the magnetoresistive device  100  of  FIG. 2  configured with a voltage source  152 .  FIG. 6  depicts the magnetoresistive device  100 . A sense resistor  150  is connected in between the magnetoresistive device  100  and the voltage source  152 . 
         [0046]    Connections between the ohmic contact  106  and the sense resistor  150  and the ohmic contact  108  and the electrical ground establish the mean current path  120 . Asymmetrical placement of the ohmic contacts  106  and  108  results in the curved mean current path  120  in the absence of a magnetic field with the direction of the mean current path indicated by reference numeral  114 . The sense resistor  150  is provided to measure current flowing through the magnetoresistive device  100 . Measuring the voltage drop across the sense resistor  150  and dividing the voltage drop by the resistance of the sense resistor  150  results in the current flowing through the sense resistor  150 , and hence through the magnetoresistive device  100 . This method of measuring current is also used in relationship with  FIGS. 8 and 9  for determining the magnetic field intensity, as described below. 
         [0047]    In  FIG. 6 , there is no magnetic field applied to the magnetoresistive device  100 . The magnetoresistance device  100  is depicted in  FIG. 7  with an out-of-plane magnetic field. The symbolic circles with dots at the centers of the circles, identified by reference numeral  126 , indicate a magnetic field that is perpendicular to the plane of the well  104  and which is coming out of the plane. For reference purposes, the mean current path  120 , the mean current path with a zero magnetic field, is shown in dashed lines. In the presence of the out-of-plane magnetic field  126 , the Lorentz force causes a new mean curved current path  124  to be established between the ohmic contacts  106  and  108 . The new mean current path, however, extends further away from the top edge  110  than the mean current path  120 . Thus the path length of the mean current path  124  is longer than the path length of the mean current path  120 . The increased length of the mean current path of  124  as compared to the mean current path  120  can be used to determine the strength of the magnetic field  126 . 
         [0048]    The magnetoresistance device  100  is depicted in  FIG. 8  with the into-plane magnetic field  130 . For reference purposes, the mean current path  120 , the mean current path with a zero magnetic field, is shown in dashed lines. In the presence of the into-plane magnetic field  130 , the Lorentz force causes a new curved mean current path  128  to be established between the ohmic contacts  106  and  108 . The new mean current path  128  is closer to the top edge  110  than the mean current path  120 . Thus the path length of the mean current path  128  has a shorter path than the path length of the mean current path  120 . The decreased length of the mean current path of  128  as compared to the mean current path  120  can be used to determine the strength of the magnetic field. The opposite directions of the magnetic field that is perpendicular to the plane of the well  104 , shown in  FIGS. 7 and 8 , causes current paths to elongate in one case and to shorten in the other case. 
         [0049]    While the direction of the magnetic field, i.e., coming out of the plane or going into the well  104 , can result in lengthening or shortening of the mean current path  120 , reversing the direction of the current can also result in lengthening or shortening of the mean current path  120 . By way of example, the magnetoresistance device  100  is depicted in  FIG. 9  with an into-plane magnetic field and with a voltage source  162  that has a reversed direction as compared to the voltage source  152  shown in  FIG. 8 . The reversed voltage source  162  results in a mean current path  134  that is substantially equivalent to the mean current path  124  of  FIG. 7 , except for the direction of the mean current path as indicated by the reference numeral  115 . 
         [0050]    The current flowing between the ohmic contacts  108  and  106 , for the configuration depicted in  FIG. 9 , can be used to determine the magnetic field intensity. This current is identified as I 134 . The current I 134  is compared to the measured current flowing through the ohmic contacts  106  and  108  for the configuration shown in  FIG. 8 . This current is identified as I 128 . The difference between the two current measurements is proportional to the intensity of the magnetic field crossing the well  104 . That is, the quantity (I 128 −I 134 ) is proportional to B. 
         [0051]    While the intensity of the magnetic field can be determined by measuring the change in voltage across the ohmic contacts  106  and  108 , the direction of the magnetic field can also be ascertained. Referring to  FIGS. 7 and 8 , the direction of the magnetic field can be ascertained by determining if the voltage across the ohmic contacts  106  and  108  increases or decreases when a magnetic field is applied. An increase in the voltage drop across the ohmic contact  106  and  108  indicates the magnetic field is coming out of the plane. 
         [0052]    Although single ended current-based or voltage-based measurements are relatively simple, these techniques suffer from electrical offsets present in the well  104 . For example, an offset in the well  104  causing excessive lengthening or shortening of the mean current path can result in inaccurate measurements. Also, the resistances involved in measuring magnetic fields are several orders of magnitude larger than any change in the resistance. Therefore, measuring a first resistance using a first single ended measurement and comparing that measurement to a second measurement using a second single ended measurement in order to determine the change between the first and the second resistances can be difficult. A differential measurement scheme can remedy the above shortcoming of the single ended measurement techniques. Referring to  FIG. 10 , a top plan view of an exemplary embodiment of a pair of magnetoresistance devices  200  and  201  is depicted with a voltage-based differential measurement scheme. Wells or diffusion areas  218  and  219  are positioned within a substrate  220 . The wells  218  and  219  form the pair of magnetoresistance devices  200  and  201 , respectively. The extension of the wells  218  and  219  into the substrate  220  can be minimal. Two pairs of ohmic contacts  204 ,  205 ,  206 , and  208  are used to connect the pair of magnetoresistive devices  200  and  201 . The position of these contacts with respect to each well  218  and  219  is asymmetrical. That is, all four contacts are at the top edge  210  of the wells  218  and  219 . The ohmic contacts  204 ,  205 ,  206 , and  208  could have been placed asymmetrically on another edge of the well  218  or  219 , e.g., bottom edge  211 . It is also envisioned that the ohmic contacts  204 ,  205 ,  206 , and  208  can be placed on different edges. For example, in one embodiment, the ohmic contact  204  and  206  can be placed on the left edge  213  of the well  218  while the ohmic contacts  205  and  208  can be placed on the top edge  210  of the well  219 . The ohmic contacts  206  and  205  are coupled to current sources  202  and  212 , respectively. The ohmic contacts  204  and  208  are coupled to electrical ground. 
         [0053]    Connections between the ohmic contact  206  and the current source  202 , the ohmic contact  205  and the current source  212 , and the ohmic contacts  204  and  208  and the electrical ground establish curved mean current paths  214  and  216 . The curved mean current paths  214  and  216  shown in  FIG. 10  are mainly on the upper portion of the wells  218  and  219  and do not significantly extend into the wells  218  and  219 . Due to the connectivity orientation of these ohmic contacts with the current sources  202  and  212  and the electrical ground, the curved mean current paths  214  and  216  have opposite directions as indicated by the arrows. Placing the pair of magnetoresistive device  200  and  201  in a magnetic field with magnetic lines perpendicular to the plane of the wells  218  and  219  results in lengthening or shortening of the mean current paths  214  and  216 , depending on the direction of the magnetic field lines. The change in the mean current paths  214  and  216  is such that while one lengthens the other shortens. 
         [0054]    To measure the intensity of the magnetic field, a differential voltage dV 11-12  can be measured with respect to the ohmic contacts  206  and  205 . In particular, V 11  is the voltage drop between the ohmic contacts  206  and  204 . V 11  constitutes the voltage across the first magnetoresistive device  200 . Similarly, V 12  is the voltage drop between the ohmic contact  205  and  208 . V 12  constitutes the voltage across the second magnetoresistive device  201 . dV 11-12  is the difference between V 11  and V 12 , i.e., V 11 −V 12 . Once the differential voltage dV 11-12  is measured, the current sources  202  and  212  are reversed and the differential voltage measurement repeated to produce dV 21-22 , i.e., V 21 −V 22 . The sum of dV 11-12  and dV 21-22  is proportional to the intensity of the magnetic field. 
         [0055]    As explained above, the differential measurement scheme presented in  FIG. 10  splits the magnetoresistive device into two devices  200  and  201 . Offsets present in both of these halves cancel out via the differential measurements. Therefore, while the differential measurement scheme is more complex than the single ended scheme, it improves inaccuracies associated with offsets that can be present in the single ended measurement schemes. 
         [0056]    While the differential scheme of  FIG. 10  is an improvement over the single ended measurement scheme, further improvement can be achieved by matching the pair of magnetoresistive devices  200  and  201  used in the differential scheme. Referring to  FIG. 11 , one embodiment of matching a pair of magnetoresistive devices  252  and  254  is shown. Each member of the pair is split into two halves, i.e.,  252 A,  252 B,  254 A, and  254 B. These sets of halves are oriented about the substrate  270  such that a centroid  250  is formed. A current source  272  is applied to a path  260  which is coupled to the halves  252 A and  252 B. A current source  274  is applied to a path  256  which is coupled to the halves  254 A and  254 B. Electrical ground is coupled to a path  258 . 
         [0057]    The differential measurement is performed as follows. V 11  represents a voltage on the path  260  with respect to the electrical ground, resulting in current passing through the halves  252 A and  252 B. V 12  represents a voltage on the path  256  with respect to the electrical ground, resulting in current passing through the halves  254 A and  254 B. The first differential voltage, i.e., dV 11-12  is the difference between V 11  and V 12 , i.e., V 11 −V 12 . Once the voltage dV 11-12  is measured, the bias currents  272  and  274  are reversed and the differential voltage measurement repeated to produce dV 2   21-22 , i.e., V 21 −V 22 . The sum of dV 11-12  and dV 11-12  is proportional to the intensity of the magnetic field. 
         [0058]      FIGS. 12-13  depict differential measurement techniques involving voltage sources. Referring to  FIG. 12 , a pair of magnetoresistive devices  300  and  301  is shown. Wells or diffusion areas  318  and  319  are positioned within a substrate  320 . Each of the two magnetoresistive devices  300  and  301  has a corresponding well  318  and  319  (as indicated by the dashed boxes). The extension of the wells  318  and  319  into the substrate  320  can be minimal. Four ohmic contacts  304 ,  307 ,  306 , and  308  are used to connect the pair of magnetoresistive devices  300  and  301 . The position of these contacts with respect to the wells  318  and  319  are asymmetrical. That is, all four contacts are at the top edge  310  of the wells  318  and  319 . The ohmic contacts  304 ,  307 ,  306 , and  308  could have been placed asymmetrically on other edges of the wells  318  and  319 , e.g., bottom edge  311 . It is also envisioned that the ohmic contacts  304 ,  307 ,  306 , and  308  can be placed on different edges. For example, in one embodiment, the ohmic contact  304  and  306  can be placed on the left edge  313  while the ohmic contacts  307  and  308  can be placed on the top edge  310  of the wells  318  and  319 , respectively. The ohmic contacts  306  and  307  are coupled to sense resistors  303  and  305 , respectively. The ohmic contacts  304  and  308  are coupled to electrical ground. Sense resistors  303  and  305  are connected to the voltage source  302 . 
         [0059]    Connections between the ohmic contact  306  and the sense resistor  303 , the ohmic contact  308  and the sense resistor  305 , and the ohmic contacts  304  and  307  and the electrical ground establish curved mean current paths  314  and  316  which run in opposite directions. The curved mean current paths  314  and  316  shown in  FIG. 12  are mainly on the upper portion of the well  318  and do not significantly extend into the well  318 . Placing the pair of magnetoresistive device  300  and  301  in a magnetic field with magnetic lines perpendicular to the plane of the well  318  results in lengthening or shortening of the mean current paths  314  and  316 , depending on the direction of the magnetic field lines. The change in the mean current paths  314  and  316  is such that while one lengthens the other shortens. 
         [0060]    The two sense resistors  303  and  305  are provided for measuring the current going through magnetoresistive devices  300  and  301 . By measuring the voltage drop across the sense resistors  303  and  305  and dividing the voltage drops by the resistances of the sense resistors  303  and  305 , the currents through the corresponding magnetoresistive device can be ascertained. 
         [0061]    To measure the intensity of the magnetic field, a differential current dI 11-12  can be measured with respect to the ohmic contacts  306  and  308 . The differential measurement scheme, described below, splits the magnetoresistive devices into two magnetoresistive devices  300  and  301 . Offsets present in both of these halves will be cancelled out via the differential measurements. In particular, I 11  is the current flowing through the ohmic contact  306  which constitutes the current flowing through the first magnetoresistive device  300 . I 11  is measured by dividing the voltage drop across the sense resistor  303  by the resistance of the sense resistor  303 . Similarly, I 12  is the current flowing through the ohmic contact  307  which constitutes the current flowing through the second magnetoresistive device  301 . I 12  is measured by dividing the voltage drop across the sense resistor  305  by the resistance of the sense resistor  305 . dI 11-12  is the current flowing through the ohmic contact  306  minus current flowing through the ohmic contact  307 , i.e., I 11 −I 12 . Once the differential current dI 11-12  is measured, the bias voltage  302  is reversed and the differential current measurement repeated to produce dI 21-22 , i.e., I 21 −I 22 . The sum of dI 11-12  and dI 21-22  is proportional to the intensity of the magnetic field. While the differential measurement scheme is more complex than the single ended scheme, it improves inaccuracies associated with offsets that can be present in the single ended measurement schemes. 
         [0062]    Furthermore, while the differential scheme is an improvement over the single ended measurement scheme, further improvement can be achieved by matching the pair of magnetoresistive devices used in the differential scheme. Referring to  FIG. 13 , one embodiment of matching a pair of magnetoresistive devices  352  and  354  is shown. Each member of the pair, i.e.,  352  and  354 , is split into two halves, i.e.,  352 A,  352 B,  354 A, and  354 B. The sets of halves are oriented about the substrate  370  such that a centroid  350  is formed. A voltage source  302  applies a current to a path  360 , which is coupled to the halves  352 A and  352 B, and a current to a path  356 , which is coupled to the halves  354 A and  354 B. The electrical ground is coupled to a path  358 . Two sense resistors  303  and  305  are provided for measuring the current going through each magnetoresistive device. By measuring the voltage drop across the sense resistors  303  and  305 , the currents through the corresponding magnetoresistive device can be ascertained. A differential measurement is performed as is described below. I 11  represents the current flowing through the halves  352 A and  352 B. I 12  represents the current flowing through the halves  354 A and  354 B. The first differential current, i.e., dI 11-12  is the difference between I 11  and I 12 , i.e., I 11 −I 12 . Once the differential current dI 11-12  is measured, the bias voltage  302  is reversed and the differential current measurement repeated to produce dI 21-22 , i.e., I 21 −I 22 . The sum of dI 11-12  and dI 21-22  is proportional to the intensity of the magnetic field. 
         [0063]    Measurement accuracy can further be improved by differentially measuring the mean current paths in magnetoresistive devices placed in a full bridge. Referring to  FIG. 14 , four magnetoresistive devices  402 ,  408 ,  414 , and  420  are placed in a bridge  400 . A person skilled in the art appreciates that the depicted bridge connection is only for schematic purposes and does not convey physical lay-out characteristics of the devices. The devices can have the same orientation as that depicted in  FIGS. 11 and 13 , however, the connections depicted in  FIG. 14  are configured to form a full bridge. A current source  430  is connected to the magnetoresistive devices  402  and  420  at the ohmic contacts  424  and  418 , respectively. Ohmic contacts  422  and  416  of the magnetoresistive devices  402  and  420  are coupled to ohmic contacts  404  and  412  of the magnetoresistive devices  408  and  414 , respectively. Ohmic Contacts  406  and  410  of the magnetoresistive devices  408  and  414  are coupled to the electrical ground. A current path develops between each pair of contacts located on a corresponding magnetoresistive device (current paths are not shown). A differential voltage dV 11-12  is measured between ohmic contacts  422  and  416 . This differential voltage includes the desired components that can lead to an accurate measurement of the intensity of the magnetic field as well as undesired components which are due to mismatches between the magnetoresistive devices. To cancel out the undesired components, the current source  430  is reversed and the differential voltage is measured again, i.e., dV 21-22 . Summing the two differential voltages removes the undesired components of the magnetoresistive devices (due to mismatch). Therefore, the sum of these differential voltages is proportional to the intensity of the magnetic field. 
         [0064]    The forgoing embodiments relate to measuring magnetic field intensity in the presence of a field that is perpendicular to the plane of the well or diffusion area. For these embodiments, the depth of the well is not critical. For magnetic fields that are parallel to the plane, a different structure is utilized. Referring to  FIG. 15 , in reference to one embodiment, a top plan view of a magnetoresistive device  500  for in-plane magnetic fields is shown. The magnetoresistive device  500  has a substrate  502  with a well or diffusion area  504 . The substrate can be a positively or a negatively doped substrate. The well  504  can be an N-well, a P-well, an N-diffusion, or a P-diffusion area. The well  504 , however, is of an opposite doping type than the substrate  502 . 
         [0065]    Ohmic contacts  506  and  508  are also placed over the well  504 . Ohmic contacts  506  and  508  run the width of the well  504 . A current source  522  is connected to the ohmic contact  506  while the ohmic contact  508  is connected to the electrical ground. 
         [0066]    The connections between the ohmic contact  506  and the current source  522  and the ohmic contact  508  and electrical ground establish a mean current path  520  (shown in  FIG. 16 ). The curved mean current path  520  is depicted in the absence of a magnetic field. Referring to  FIG. 16 , a cross-sectional view of the magnetoresistive device for in-plane magnetic field measurements is shown. As shown in  FIG. 16 , the curved mean current path  520  extends into the well  504 . In the embodiments of  FIGS. 15-16  no magnetic field is present. Referring to  FIG. 17 , an in-plane magnetic field having lines  524  is shown. Some of the lines  524  are shown in solid lines indicating these lines are at the top surface of the well  504 . Some of the lines  524  are shown in dashed lines indicating these lines are below the surface of the well  504 . The in-plane magnetic field causes the curved mean current path  520  to further dip into the well  504  producing deep mean current path  526 . In  FIG. 18 , the deep mean current path  526  is shown as a solid curved path while the curved mean current path  520 , referring to the mean current path with zero magnetic field, is shown as a phantom mean current path. If the in-plane magnetic field direction is in an opposite direction as compared to the magnetic field shown in  FIG. 17 , then the curved mean current path would be shallower than the curved mean current path  520 . This combination is shown in  FIGS. 19 and 20 . In  FIG. 19 , the in-plane magnetic field having lines  528  causes the curved mean current path  520  to become a shallow mean current path  530  (shown in  FIG. 20 ). In  FIG. 20 , the shallow mean current path  530  is shown as a solid curved path while curved mean current path  520 , referring to the mean current path with zero magnetic field, is shown as a phantom mean current path. Reversing the current source  522  has a similar effect as reversing the in-plane magnetic field direction, which is depicted in  FIG. 21 . The reversed current source  532  causes the shallow mean current path to become the deep mean current path  534 . 
         [0067]    The embodiments described above with respect to measuring magnetic fields that are perpendicular to the plane of the well or diffusion area are also applicable to the in-plane magnetic field embodiment. In particular, embodiments dealing with measuring magnetic fields that are perpendicular to the plane of the well or diffusion area using current-based single ended measurements, voltage-based single ended measurements, current-based differential measurements (including the common centroid embodiment for improved accuracy), voltage-based differential measurements (including the common centroid embodiment for improved accuracy), and differential measurement across a full bridge can also be used for the in-plane magnetic fields. 
         [0068]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.