Contains research objectives and reports on two research projects

Ahilea, E.

Berkowitz, D. A.

Campbell, J. F., Jr.

Dunavan, D. S.

Nottingham, W. B.

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I. PHYSICAL ELECTRONICSProf. W. B. Nottingham D. A. Berkowitz D. S. DunavanE. Ahilea J. F. Campbell, Jr. L. E. SpragueRESEARCH OBJECTIVESPresent objectives lie in three separate areas of physical electronics. The firstrelates to the properties of semiconductors, the second to thermionic emission problems,and the third to the improvement of instrumentation.The electrical properties of p-n junctions are being investigated as a function of thetemperature and the applied voltage. The observations include a careful determinationof the current flow as a function of the voltage and also of the capacity of the junction asa function of the voltage. Analysis of such observations yields interesting data, theinterpretation of which will serve to measure the applicability of present theories to theelectrical properties of junctions.In most applications of semiconductor materials, it is desirable to make electricconnections with them by means of "ohmic" contacts. The techniques for making ohmiccontacts vary considerably and the results are not always successful. Very little dataare available on the electrical properties of "ohmic" contacts as a function of currentflow, current direction, and temperature. Exploratory studies have been undertakento investigate these contacts.Other conduction studies include an investigation of the properties of zinc sulfideand of photoconductive properties of germanium as they are related to surface condi-tions.Basic to a better understanding of dispenser cathodes is the development of a methodfor evaluating them. Preliminary experiments and theory indicate that the best methodinvolves the use of the cathode as a receiver of electrons from a similar cathode usedas an emitter. Evaluations by this method are being undertaken. The most importantparameter in thermionic studies is temperature. The electron-energy distributionshould serve as one of the most accurate methods of determining the effective temper-ature of an emitter. Instruments and methods are being developed to make this meansof temperature determination reliable and accurate.Not only is it of interest to study the properties of semiconductors but we considerit a worth-while objective to use semiconducting devices in our instrumentation. Tworecent developments are: First, the development of a micro-microammeter capableof measuring currents with an accuracy of 1 per cent, or better, over the entire range-14from 1 ma down to 10 - . This device uses a combination of vacuum tubes and solid-state rectifiers and transistors. Our development is aimed, specifically, towardperfecting the stability of the unit with respect to power-line variations and room-temperature changes. The second instrumental development relates to transistorizedpower supplies; specifically, to one that will take the place of storage batteries in theoperation of potentiometers. Present research indicates that a power supply with aconstancy in the fourth or fifth figure can be obtained. As this objective is met, otherapplications of importance will undoubtedly be investigated and attempts will be madeto solve associated problems.W. B. NottinghamA. PHYSICAL ELECTRONICS IN THE SOLID STATE1. Conduction in Zinc Sulfide Single CrystalsThis is the final report on the experiments performed on the needle-shaped singlecrystals of zinc sulfide (ZnS). The details of the experiment and the analysis of results(I. PHYSICAL ELECTRONICS)will be found in the author's thesis (1). The techniques of preparing the ZnS crystalsand details of the experiment were described in Quarterly Progress Report No. 51,pages 1-2. The needle-shaped crystals have current leads permanently attached toeither end. Two other ZnS crystals are used as potential probes part way along theneedle length. The potential can be measured with only 200 electrons per secondflowing across the crystal-to-crystal contact.Three ZnS:C1 crystals, and two ZnS:Cu,C1 crystals were studied. In ZnS:C1, the4.5dark conductivity is dependent upon the voltage: in the forward direction, i = kf V425in the reverse direction, i = k V . The rectification is not a contact phenomenon, butra distributed property of the crystal. The exponents in the empirical current-voltagerelationship vary from crystal to crystal, and in any one crystal they change after expo-sure to light. As the temperature is increased from room temperature to 1500 C, theproportionality factors kf and k r increase, but above 1500 C there is no increase. Atthe first application of the voltage, a permanent high-resistance region builds up, overseveral hours, at the cathode end of the crystal. The crystals are hexagonal, and thec-axis is parallel to the needle axis. The build-up of the high-resistance region proceedsfaster in one direction along the c-axis than it does in the opposite direction.The conduction in ZnS:Cu,C1 crystals obeys Ohm's law at all temperaturesbetween 500 C and 2500 C and is the same in both directions. No high-resistanceregions and no saturation were observed.When the crystals were imbedded in a medium of the same index of refraction,examination with a petrographic microscope indicated narrow regions of nonuniformityspaced 20 to 100 4 apart along the needle length. These regions are taken to be regionsof extremely high impurity concentration which contribute many electron trapping levelsto the forbidden band. The presence of substitutional chlorine in the bulk ZnS latticecreates acceptor levels. In the condition of thermodynamic equilibrium, the acceptorlevels are occupied by the donor electrons associated with the chlorine impurity, andthe localized trapping levels remain empty. If the acceptor levels outnumber thedonors, the conductivity is expected to be p-type. The number of holes in the valenceband is calculated as a function of temperature, with the assumption that a fraction (1-6)of the acceptor levels is occupied by electrons at 0O K. The hole mobility is estimatedon the basis of optical mode scattering. By fitting the resulting expression for theconductivity to the data, the energy of the acceptor level extrapolated to 0' K is foundto be 1.24 ± 0.02 ev above the top of the valence band.D. A. BerkowitzReferences1. D. A. Berkowitz, Electrical conduction in zinc sulfide single crystals, Ph.D.Thesis, Department of Physics, M.I.T., December 10, 1958.(I. PHYSICAL ELECTRONICS)2. Characteristics of Semiconductor JunctionsVoltage-current and capacitance measurements on a silicon diffused junction(kindly furnished by the Bell Telephone Laboratories, Incorporated) were made atseveral temperatures. It has been previously noted (1, 2, 3) that the reverse char-acteristic of these junctions at lower temperatures shows a pronounced lack ofsaturation, which has been attributed to recombination within the transition region.At medium reverse voltages, the current thus produced should be proportionalto the volume of the transition region, which, in turn, is proportional to the widthof the transition region. The capacitance of the junction because of the presenceof the carrier-free transition region will be inversely proportional to the widthof the region, so that the product of the capacitance and that part of the reversecurrent resulting from barrier recombination should be a constant as long as thevoltage is large compared with the built-in voltage and as long as carrier multi-plication is negligible.The capacitance measurements show that the small-signal capacitance variesas the inverse cube root of the applied voltage, hence the graded junction approxi-mation should be good. If we assume this model, then, from the voltage-currentand voltage-capacitance data, we should be able to calculate a number of param-eters of the junction (built-in potential, area of junction, width of the transitionregion as a function of voltage, carrier lifetime, energy level of recombinationcenters, and so forth). An analysis of data at one temperature (1000 C) indicatesthat such determinations can, indeed, be made; the values of the various param-eters are entirely reasonable and in good agreement with other studies (1, 3), andshow complete internal consistency. However, a comparison with data obtainedat other temperatures indicates that the agreement is not as consistent as could behoped; in particular, to get agreement with capacitance measurements, the dielectricconstant of silicon would have to vary 20 per cent in the range from 300 C to 1300 C.This would seem to be a rather large temperature variation; but, since no dataseem to be available on the variation of the dielectric constant of silicon, theaccuracy of the present model cannot be determined.Obviously, this work is far from complete, and no complete analysis can bepresented.J. F. Campbell, Jr.References1. C. T. Sah, R. N. Noyce, and W. Shockley, Proc. IRE 45, 1228 (1957).2. E. M. Pell and G. M. Roe, J. Appl. Phys. 26, 658 (1955).3. E. M. Pell and G. M. Roe, J. Appl. Phys. 27, 768 (1956).(I. PHYSICAL ELECTRONICS)3. Surface States on SemiconductorsAs we mentioned in Quarterly Progress Report No. 51, page 2, we attempted tocreate trapping on the surface of an n-type germanium sample by bombarding the gasessurrounding the sample with a Tesla coil. This was tried with the residual gas in theevacuated tube and with oxygen introduced from a flask. Most of these attempts in thepast had failed.More recently, we found that a short bombardment of ambient residual gas underlow-vacuum conditions (2 X 10 - 4 mm Hg and higher) increased the photoconductiveresponse - sometimes as much as 20 times. The response was usually linear withlight intensity, and sometimes increased more than linearly with light intensity. Thetransient response was usually immediate, and sometimes with only a very slightrounding of the oscilloscope trace of the response. The increase in response diminishedin time after the bombardment. Apparently, the bombardment cleans away some adsorbedlayer and thus improves carrier lifetime in the sample.Bombardment with oxygen resulted in similar changes in the response but createda good amount of rounding of the response trace. This roundness was accompanied bya response that rose less than linearly with light intensity except for the highest illumin-ation levels obtained in the apparatus. Spreading the incident light on a larger sectionof the sample increased this effect. However, the rise of the transient response wasusually slower (and rounder) than the decay, contrary to Gebbie's findings (QuarterlyProgress Report, Research Laboratory of Electronics, M.I. T., July 15, 1955, pp. 3-4).When the oxygen was pumped out, the roundness disappeared within a few minutes, andthe response again became linear and usually somewhat weaker. This suggests that theroundness is caused by relatively slow charging and somewhat faster discharging (bothwithin milliseconds) of surface states across a high-resistance layer created by bom-bardment in oxygen, possibly by the adsorption of ozone.E. AhileaB. EXPERIMENTAL TECHNIQUES1. A Vacuum-Tube-Transistorized Electrometer Millivoltmeter CircuitA new electrometer circuit has been designed and constructed and is being tested.The circuit consists of three sectionalized units: a basic amplifier unit with an outputmeter; a detachable input circuit that incorporates the input resistors and feedbacknetwork; and a power supply. The detachable input circuit allows several differentinput and feedback arrangements to be used with one basic amplifier.-3 -12Where used as an electrometer the unit will measure 10 to 10 amp, full-scale,direct-reading. Currents of less than 1012 amp can be measured by observing therate of charging of a small condenser.(I. PHYSICAL ELECTRONICS)When it is used as a millivoltmeter, potentials from 10 to 1000 mv, full-scale,can be measured in very high impedances.The circuit and other details of this unit will be published at a later date.W. B. Nottingham, D. S. Dunavan2. Transistor-Regulated Power SupplyFurther work has been done on the low-voltage-regulated power supply describedin Quarterly Progress Report No. 51, page 5. The temperature compensation circuitshown there was difficult to adjust for optimum performance. It has been redesignedand is shown in Fig. I-1. The values given for the resistances provide a fixed outputTHERMISTORVECO51AI OR 50K D-351AI SVI3960 KT-42N408SO0.025 4fd 24 K5.1KFig. I-1. Temperature compensation circuit for regulated power supply.of 25 volts. This will vary slightly from one circuit to another because of variationsin the Zener breakdown voltages. Tests were made on a model that incorporates thiscircuit at an output current of 200 ma. Over a period of 5 days the room temperaturewas varied between 200 C and 300 C several times. The maximum output variationswere ±5 my from an average value of 25.1700 volts. Variation with changing linevoltage amounts to less than ±1.5 my for +15-volt changes from the normal 115 volts.D. S. Dunavan

Physical Electronics

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