Magnetoresistor

Abstract: A magnetoresistor (10) includes a thin film (14) of a degenerately-doped semi-conductor material extending between conductive contacts (18,20) at opposite edges of the film (14). The film (14) has a plurality of openings (22) therethrough which are arranged in spaced parallel rows with the openings (22) in each row being between two openings (22) in the adjacent rows. This forms the film (14) into two set of paths (24,26) which extend around the openings (22). The openings (22) are selected to be of a size such that, when a magnetic field is applied perpendicular to the film (14), the electrical resistance of the magnetoresistor (10) increases because electron flow through the film (14) between the contacts (18,20) must be in a substantially circular path around the openings (22).

Description: The present invention relates to a thin film magnetoresistor as specified in the preamble of claim 1, for example as disclosed in SU-A-920596. Magnetoresistors are resistors whose resistance is varied by applying a magnetic field across the resistor. Magnetoresistors can be used, in conjunction with a permanent magnet, to sense the distance between the magnet and the magnetoresistors. This leads to their potential application in position sensing devices, such as are used in the automotive industry. There are essentially two classes of magnetoresistors. One class of these devices, known as magnetodiodes, operates on the basis of selective re-combination of electrons and holes. The other class operates on the deflection of the current paths in a rectangular or circular resistor. The performance of the first class of devices depends strongly on the electron-hole re-combination rate, and the performance of the second class of devices depends on the carrier mobilities. Both the electron-hole re-combination rate and the carrier mobilities are temperature-dependent quantities. Thus, to date, most field sensors using these magnetoresistors have large temperature coefficients. In the automotive industry, where there are wide variations in the temperature to which these sensors are subjected, it would be desirable to have a magnetoresistor which not only has a large variation in resistance, but also has a low temperature coefficient. A magnetoresistor according to the present invention is chacterised by the features specified in the characterising portion of claim 1. The present invention is directed to a thin film magnetoresistor in which the path of the resistance material is such that, when the device is subjected to a magnetic field, electrons flow in substantially a circular path so as to increase the resistance of the device. More particularly, the magnetoresistor comprises a thin film of a degenerately-doped semi-conductor material having a pair of conductive contacts at opposite ends of the film. The film is in the form of a series of paths which extend at least partially in a substantially circular pattern. Thus, with a magnetic field applied perpendicular to the film, electrons flow between the contacts in a substantially circular pattern and thereby increase the resistance between the contacts. The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a magnetoresistor made in accordance with the present invention; and Figure 2 is a top plan view of another magnetoresistor in accordance with the invention. The drawings are not necessarily to scale. Referring to Figure 1, there is shown a perspective view of a magnetoresistor 10 in accordance with the present invention. The magnetoresistor 10 comprises a substrate 12 of an insulating material, such as iron-doped gallium arsenide, having a thin film 14 of a degenerately-doped semi-conductor material, such as indium arsenide or indium antimonide, on a surface 16 thereof. The semi-conductor film 14, which is the active layer of the magnetoresistor, is of a thickness of no greater than about 5 micrometres, and preferably has a thickness no greater than about 3 micrometres. The semi-conductor film 14 is substantially rectangular, and separate conductive contacts 18 and 20 are on the substrate surface 16 at opposite edges of the semi-conductor film 14. The contacts 18 and 20 are in ohmic contact with the semi-conductor film 14. The semi-conductor film 14 has a plurality of openings 22 therethrough in the form of circular holes which are arranged in spaced parallel rows extending across the film 14 parallel to the contacts 18 and 20. The holes 22 in alternate rows are arranged between the holes 22 in the adjacent rows. This forms the film 14 into a plurality of spaced paths 24 which extend across the film 14 parallel to the contacts 18 and 20, and a plurality of spaced paths 26 which extend between the paths 24 substantially perpendicular to the contacts 18 and 20. The spacing between the holes 22, i.e., the width of the paths 24 and 26, should be no greater than the mean free path of electrons in the semi-conductor film 14. The holes 22 should have a radius such that the perimeter (circumference) of the hole is no greater than the mean free path of electrons in semi-conductor material of film 14. For example, in gallium arsenide or indium arsenide at room temperature, the mean free path is about 300 nanometres (3000 Angstroms). In the presence of a magnetic field applied along an axis perpendicular to the surface of the semi-conductor film 14, the in-plane trajectory of the electrons through the semi-conductor film 14 and along the paths 24 and 26 is deflected by the Lorentz law of force to form an arc of a circle in the plane of the semi-conductor film 14 with the radius of the circle, R H , the cyclotron radius, being: R H = mv/eB where m is the electron effective mass, v is its total velocity, e is the elemental electron charge, and B is the applied magnetic field. When the magnetic field B is such that the radius R of each hole 22 equals the cyclotron radius R H , each hole 22 acts as a minuscule cyclotron, and electrons flow along the paths 24 and 26 around the holes 22 as indicated by the arrows 28. The electrons so flow until they either re-combine or scatter. This results in an increase in the resistance of the magnetoresistor 10. The field at which this occurs is only a function of the geometry. This is so because the electron effective mass, and its Fermi velocity are parameters which are quite temperature-independent in degenerate semi-conductors. Thus, the magnetoresistor 10 has a high resistance and a low temperature coefficient. Referring to Figure 2, there is shown a top plan view of another magnetoresistor 30 in accordance with the present invention. The magnetoresistor 30 comprises a thin film 32 of degenerated semi-conductor material on the surface of an insulating substrate 34, and conductive contacts 36 and 38 on the substrate at opposite ends of the semi-conductor film 32. The semi-conductor film 32 comprises a plurality of spaced, parallel first paths 40 extending parallel to the contacts 36 and 38, and a plurality of spaced second paths 42 extending between the first paths 40 substantially perpendicular to the contacts 36 and 38. The second paths 42 between two adjacent first paths 40 are not aligned with the second paths 42 between the next adjacent pairs of first paths 40 so as to form openings 43 which are arranged in a substantially staggered brick-work pattern. The spacing N between adjacent first paths 40 is no greater than one-third of the mean free path of electrons in the semi-conductor film 32, and is preferably smaller than one-third of the mean free path. The spacing 2N between adjacent second paths 42 is about twice the spacing between adjacent first paths 40. The thickness of the paths 40 and 42 is about 2/5N. In the operation of the magnetoresistor 30, a voltage applied between the contacts 36 and 38 causes a flow of current along the paths 40 and 42. When a magnetic field is applied perpendicular to the surface of the semi-conductor film 32, at each intersection of a first path 40 and a second path 42, the charge carriers are preferentially steered in one direction in a substantially circular path around a corner or opening 43 by the Lorentz force as indicated by the arrows 44. This blocks the flow of electrons along the semi-conductor film 32 so as to achieve an increase in the resistance. Thus, the magnetic field applied to the magnetoresistor 30 results in an increase in the magnetoresistance. For the reasons stated with regard to the magnetoresistor 10, the magnetoresistor 30 also has a low temperature coefficient. The magnetoresistors 10 and 30 may be made by depositing a film of the semi-conductor material on the surface of a substrate using any suitable chemical vapor-deposition process for the particular material used. Metal contacts are then deposited on the substrate surface at the ends of the semi-conductor film either by evaporation in a vacuum or by sputtering. The holes 22 in the semi-conductor film of the magnetoresistor 10 and the openings between the paths 40 and 42 in the semi-conductor film of the magnetoresistor 30 are formed by electron-beam lithography. Thus, there is provided by the present invention a magnetoresistor in which the paths of the semi-conductor film are designed so that, when a magnetic field is applied to the semi-conductor film, the electrons will flow in a circular path and thereby increase the resistance of the magnetoresistor. The factors which affect this increase in resistance are temperature-independent so that the temperature coefficient of the magnetoresistor is low.

Claims: A magnetoresistor (10;30) comprising: a thin film (14;32) of a semi-conductor material and a pair of conductive contacts (18,20;36,38) at opposite ends of the film (14;32), said film having openings (22;43) formed therethrough such that, when a magnetic field is applied perpendicular to the semi-conductor film (14;32), the electrical resistance between the conductive contacts (18,20;36,38) increases, characterised in that said semi-conductor material is a degenerately-doped semi-conductor material which is of a thickness no greater than 5 micrometres and which provides a given mean free path of electrons in said thin film (14;32); said openings (22:43) in said semi-conductor film (14;32) form parallel rows of uniformly-shaped openings (22;43) therein, said openings having dimensions small enough to provide an opening perimeter that is not greater than the mean free path of electrons in said semi-conductor film (14;32); said openings (22;43) are spaced apart from one another by a predetermined spacing that is no greater than the mean free path of electrons in the semi-conductor film (14;32); said rows of openings (22;43) provide laterally-spaced paths (26;42) in said semi-conductor film (14;32) which are substantially perpendicular to the conductive contacts (18,20;36,38); said rows of openings (22;43) provide longitudinally-spaced paths (24;40) in said semi-conductive film (14;32) which are substantially parallel to the conductive contacts (18,20;36,38); said spaced paths each have a width not greater than the mean free path of electrons in said semi-conductor film (14;32); and said parallel and perpendicular paths (24,26;40,42) intersect with one another at locations adjacent said openings (22;43) to provide substantially annular paths around said openings (22;43) such that, when said magnetic field is applied perpendicular to the semi-conductor film (14;32), electrons flowing through the semi-conductor film (14;32) between the conductive contacts (18,20;36,38) are caused to flow in said substantially annular paths (28;44) around at least some of said openings (22;43) to thereby increase the electrical resistance between the conductive contacts (18,20;36,38). A magnetoresistor (10;30) according to claim 1, in which the openings (22;43) are arranged in rows substantially parallel to the conductive contacts (18,20;36,38) with the openings (22;43) in each row being laterally disposed from and spaced between the openings (22;43) in the adjacent rows. A magnetoresistor (10) according to claim 1 or 2, in which the openings are circular holes (22). A magnetoresistor (30) according to claim 1 or 2, in which the openings (43) are rectangular in shape, each rectangular opening (43) having a dimension N in the direction perpendicular to the contacts (36,38) that is no greater than one-third the mean free path of electrons through the film (32), and having a dimension parallel to the contacts (36,38) which is equivalent to 2N. A magnetoresistor (30) according to claim 4, in which the paths (40,42) of the film (32) have spaced parallel first portions (40) extending parallel to the contacts (36,38) and spaced parallel second portions (42) extending between the first portions (40) to form the rectangular openings (43).

Inventors: HEREMANS JOSEPH PIERRE, OLK CHARLES HOWARD, HEREMANS, JOSEPH PIERRE, OLK, CHARLES HOWARD
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1991
Main classification: H01L43
IPC codes: H01L43