Donald R. Getzin Deportment of Chemical Engineering and Chemistry The Newark College of Engineering Newark, New Jersey 07102
An Inexpensive Experiment on Semiconductors
M a n y colleges and universities are now including chemical applications and materials chemistry in their elementary chemistry courses in an attempt to make these courses more relevant to our present technology and simultaneously to satisfy the special needs of engineering students. The laboratory portion of such courses has lagged behind these developments primarily because experiments either are unavailable or are too expensive. One topic currently being studied is semiconductors' and their properties. Some modern elementary chemistry textbooks already include information on semiconductors. Perhaps this is because concepts developed in chapters on crystal structure, atomic structure, covalent and metallic bonding, and metallic and electrolytic conductivity can readily be extended to explain the properties of these interesting materials. A suitable laboratory experiment on semiconductors is available2 and, with modifications, can be incorporated into an elementary chemistry program. The experiment discussed here is quite flexible and costs ten dollars or less depending on how much equip-
' A suitable program for the instructor to learn about semieonduotors is to study the following sources in the listed order: (a) DICKERSON, R. L., GRAY,H. B., AND HAIGHT,G. P., JR., "Chemical Principles," W. A. Benjamin, Inc., New York, 1970, Chapter 13. (b) VANVLACK,L. H., "Elements of Materials Science," (2nd ed.), Addison-Wesley Publishing Company, Reading, Mass., 1967, Chapter 5. (0) A D L ~ RR. , B., SMITH, A. C., AND LENGINI,R. L., "Introduction to Semiconductor Physics," J. Wiley and Sons, Inc., New York, 1966. ADLER,R. B., SMITH,A. C., AND LENGINI,R. L., o p . &. a This estimate includes the cost of a simple ohmmeter but not the cost of ordinary laboratory equipment such as beakers. If portable multimeters are initially available, the cost of the experiment is only about three dollars. All estimates do include the cost of a -10" to 200°C thermometer. ' These may be purchased from Semimetals, Inc., 172 Spruce St.. Westburv. L. I.. N. Y. A set. called the AAPT set. includes two doped g&mani;m bars (one 'N-type and one ~ 4 y p e and ) one N-type wafer. The set sells for $1. The sample should be immersed in boiling distilled water after soldering to remove excess flux. ' Teflon tape or Teflon tape dope is recommended since these do not soften a t the high temperatures used in one part of the experiment. Teflon tape dope ( I / * in.) is usually available from plumbing supply houses. After wrapping the tape dope around the sample and the thermometer bulb, the end may be sealed using a hot soldering iron. ' Explanations for this behavior and for the other semieonduetor properties referred to in this article may be obtained from the sources listed in footnote 1. The mathematical relationships needed to calculate the quantities mentioned subsequently may also be found there. 8 ADLER,R. B., SMITH,A. C., AND LENQINI, R.L., ~ p cit. . At 27°C the carrier mobility in the P-type and N-type samples are respectively 1900 (for "holes") and 3900 cm2/V.sec (for free electrons).
ment is initially a ~ a i l a b l e . ~Small germanium bars (0.045 X 0.045 X 0.50 in.) doped with impurities a t concentrations within an order of magnitude of lot5 atoms/cma can be purchased for less than $1 apiece.' Teflon-coated wire leads may be soldered to the ends with 60 Ph/40 Sn coreless solder using concentrated aqueous zinc chloride as the flux5 The bars can then be taped to the bulb of a therm~meter.~A simple ohmmeter can be constructed to measure the resistance of the germanium sample. By selecting from among the following experimental variations, an appropriate laboratory experiment can be constructed. For example, the first three variations alone constitute a suitable experiment for a typical freshman laboratory. 1) The appearance of the sample may be described and cornpared to the appearance of a, typicel metal. The germanium is gray in color and appears to he metallic. However it is quite brittle. Crystal structure models may be constructed with styrofoam halls and the lattice compared to typical body-centered and face-centered cubic metallic lattices. Germanium bas the diamond face-centered cubic structure. 2) The resistance of the semiconductor samole mav be measurwl a r mom temperhtwe. Fnrm the dimctr.ii,ns of the~nmple its conhcriviry or renislivity imy be c.,lculated and wmpurcd to typical values for metals and insulators.. 3) The resistance of the semiwnductor may be measured a t 5' i n t e m l s as the sample cools from 1 7 5 T down to 100°C. An air bath with a Bunsen burner is adequate for attaining these temperatures. This experiment may be repeated using tungsten wire for comparison. The resistance of the semiconductor, unlike the metal, is lower a t higher temperatures. For the semiconductor, a straight line results when the logarithm of the resistance is plotted against the recipmcal of the absolute temperature.' 4) If the hand theory of semiconductors is discussed in a sufficiently rigorous manner, the students are able to use the slope of the line in 3 to calculate the siee of the germanium energy gap, En. 5) If the effect of impurities on semiwnductor conductivity is dso discussed, the students may quickly and easily determine whether the sample is N-type and P-type. One multimeter (per laboratory class) with a 2.5-V scale is required however. With the multimeter attacbed to the ssmple leads, touching one end of the ssmple with a. hot glass rod causes the multimeter needle to momentarily move left or right depending upon whether the sample is N-type or P-type. Briefly, this happens because the charge carriers (free electrons or "holes") diffuseaway due to the thermal gradient. This causes the ends of the sample to acquire opposite charges for a few moments until thermal equilibrium is restored. The charges cause a potential difference between the two ends and the sign of this difference is determined by the charge on the mobile charge carriers. 6) Carrier concentration may he estimated by knowing the conductivity type ( P or N), the resistance of the sample a t mom temperature, the dimensions of the stlmple and the mobility of the charge carriers? 7) In the range fmm -10' to 30°C, the resistance of the doped semiconductor increases as lattice vibrations pmgressively interfere with the carrier current. This behavior is similar t o Volume 48, Number 8, August 1971
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that observed of metals. The phenomenon may be observed as the sample w a r m from ice-salt bath temperatures. The sample mmt be enclosed in test tube before immersio,, in the bath order to eliminate the conductivity of the salt solution fmm the measurement. I n this case, a. straight line results when the logarithm of the resistance is plotted agaimt the logarithm of the absolute temperature.
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Journal of Chemical Educafion
8 ) The data in 7 may be used to evaluate the parametas in the following phenomenologiwll equations: R = CTb, p = BT". The quant,ity p is the carrier mobility, R is 16e resistance and T is the absolute temperature.
This experiment, with most of the variations included, is being performed by 600 freshmen in the second semester of an elementary course.
