INSTRUMENTATION by Ralph H. Müller
Applications of the Hall Effect TVTEW
MATERIALS,
new
instrumental
- ^ techniques, and new concepts all contribute to the advancement of science and technology which, for some time, has been proceeding at an exponential rate. Occasionally, two or all three of those essential factors conspire to produce something extraordinary. An example which we wish to discuss, has done just that, and, in addition, has brought an eighty-four year old phenomenon to new and unexpected applications. In 1879, Ε. Η. HaL at Johns Hopkins, discussed the effect which bears his name. When current is flowing in a magnetic field, an electromotive force is generated which is at right angles both to the magnetic field and to the current, and is proportional to the product of the intensity of the cur rent, the magnetic field, and the sine of the angle between the directions of these quantities. These are small but readily demonstrable effects, but with the advent of improved semiconductors, new interest from both the theoretical and experimental points of view in the ITall effect arose. During the Los Angeles Meeting, we had an excellent opportunity to get caught up on some recent develop ments in this field. We made one of our frequent visits to Beckman Instru ments, Inc., where we received the customary red carpet treatment. This is no measure of our importance, it's just the way they do things out there. Through the courtesy of Brice James and the research and engineering staffs, we learned the results of some two years of intensive research and development in Hall effect phenomena and the many useful devices and instruments which have emerged from these studies. It is not appropriate here to go into the detailed physics of the Hall effect but better, perhaps, to outline these developments which have resulted in pre-eminently usefu' devices and in struments. Of the many materials which could be used, indium antimonide and indium arsenide are the materials most com monly used in commercial Hall gen erators. Indium antimonide with its high carrier mobility, is superior to all other materials for power transfer
efficiency, but has a high temperature dependence. Indium arsenide has a lower carrier mobility, but a much lower temperature dependence. One of the most important develop ments has been the "thin film" Hall generator which owes its high sensi tivity to the drastic reduction of the thickness parameter. This is achieved by Beckman by vacuum deposits of the semiconductor in films only a few microns thick. An additional im provement for high permeability con sists in depositing a thin film on a fer rite slab and covering the film with another ferrite piece resulting in an ef fective air gap in the magnetic field some 15 times smaller than conven tional Hall generators. The advent of "thin film" Hall gen erators at once made voltage drive of a Hall generator possible. Heretofore, the generator has traditionally been re garded as a current controlled element. One of the most important results of voltage drive is the improvement of temperature coefficients which can be as high as fivefold. Some typical Hall generators made by Beckman's Helipot Division and designated as Hallefex Voltage Gener ators include the Model 350, which is 7 8 " χ 7=" χ 0.025" with an indium antimonide film for maximum sensi tivity. Model 351 is the same size but has an indium arsenide element for op timum performance over a wide range. Models 335 (indium antimonide) and 336 (indium arsenide) measure only 7*" χ 7 ι " χ 0.040" and have an effective air gap of less than 0.002". A few Hall effect applications may be mentioned here. For power meas urements, if the control current through the strip is proportional to, and in phase with, the line voltage while the magnetizing current is in phase with the line current, the Hall voltage output is a DC term proportional to real power and an AC double frequency term proportional to volt-amperes. In a DC to AC converter, the "chop ping" signal can be applied to cither the magnetic circuit or the Hall ele ment, depending upon the desired fre quency. For example, if the magnet is excited by a current: i= = I sin wt. and the signal to be converted h is ap
plied to the Hall element, the output voltage will be V = Khiz = K'ii sin wt. Function generators of wide variety can be made by rotating the Hall ele ment in uniform or nonuniform mag netic fields. In the former, sin 0, cos 0, sin2 0 or cos2 0 can be generated. In the latter, exponential functions can be generated, such as: Ke", Ke"5-, and K(l-es). As a multiplier, if the current ap plied to the magnetizing coil is thought of as one signal and the element con trol current the second signal, the Hall voltage is proportional to the product. Note that one of these in puts is electrically isolated from the output. A "clip-on" DC or AC ammeter is achieved by exciting the Hall element with a constant current source and placing it in the magnetic field caused by the movement of charged particles. With this, one can measure current in a conventional conductor as well as the flow of ionized gases or liquids. As a gaussmeter, control current to f he Hall element is supplied from a constant current source and the Hall voltage is directly proportional to mag netic field strength. No relative motion between magnetic field and pickup ele ment is required. The direction of the field can be determined by rotating the element until the maximum Hall voltage is obtained. Polarity of the field can be found because the Hall voltage changes sign with a 180° change in field direction. Many other applications are possible and include modulation, mechanical positioning of systems to ±0.0004", re volvers, linear or rotary displacement transducers, gyrators, frequency multi plexing, field direction (compass), and wave analysis. That there is more to come makes these superb developments all the more exciting. There are, of course, the re lated thermal and inverse effects of Xernst and von Ettinghausen and then, too, the Leduc and Corbino effects. We are getting to that delightful stage where fundamental ad\'ances are completely dependent upon instru mentation and the latter is constantly improved or often revolutionized by new materials and new concepts. VOL. 35, NO. 8, JULY 1963
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