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GIREP95 Physics Dept. Udine University GIREP

Workshops



Workshop 5A: TEACHING ELECTRICAL TRANSPORT PROPERTIES IN SOLIDS: AN EXPERIMENTAL APPROACH

Leaders:
A. Sconza, G.Torzo, G. Delfitto, Department of Physics, University of Padova (Italy)

For the general description of the contents of this workshop see the abstract of GT15.

room E
W5Aa: ELECTRICAL CONDUCTIVITY MEASURED AS A FUNCTION OF TEMPERATURE
A. Sconza, G. Torzo, Department of Physics, University of Padova (Italy)
W5Ab: THE HALL VOLTAGE MEASURED IN METALS AT ROOM TEMPERATURE, AND IN Ge AS A FUNCTION OF TEMPERATURE
A. Sconza, G. Torzo, Department of Physics, University of Padova (Italy)
M. Michelini, Department of Physics, University of Udine (Italy)
W5Ac: SPECTROSCOPIC MEASUREMENT OF THE ENERGY GAP
A. Sconza, G. Torzo, G. Delfitto, Department of Physics, University of Padova (Italy)
W5Ad: THE OPTICAL-INJECTION HAYNES-SHOCKLEY EXPERIMENT
A. Sconza, G. Torzo, Department of Physics, University of Padova (Italy)
W5Ae: THE PHYSICS OF THE PN JUNCTION: FORWARD CHARACTERISTICS
A. Sconza and G. Torzo, Department of Physics, University of Padova (Italy)
W5Af: THE PHYSICS OF THE PN JUNCTION: FORWARD VOLTAGE VS TEMPERATURE
A. Sconza and G. Torzo, Department of Physics, University of Padova (Italy)

W5Aa
ELECTRICAL CONDUCTIVITY MEASURED AS A FUNCTION OF TEMPERATURE*
A. Sconza, G. Torzo, Department of Physics, University of Padova (Italy)
* A. Sconza e G. Torzo, Eur. J. Phys.19, 123-126 (1989)

We first cool down the sample (kept inside a copper cylindrical holder) by coupling the holder with a cryogenic bath (liquid N2); then we slowly heat it up by driving a heater, wound around the cylinder, with a current ramp: so we may measure the sample resistance in quasi-static conditions (the temperature changes with a rate of nearly 0.1 degree per second) in the range -100 to +250deg.C.
The temperature is measured using a thermocouple (or a Pt thermometer) as sensor: in any case we may get from the sensor a voltage signal that is related to temperature by a known (nearly linear) relation.
The voltage signal across the sample, biased at constant current, is proportional to the resistance so that, by recording these two signals with a Personal Computer and an A/D interface board, we may get the temperature dependence of resistivity in real time on the PC monitor.

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W5Ab
THE HALL VOLTAGE MEASURED IN METALS AT ROOM TEMPERATURE, AND IN Ge AS A FUNCTION OF TEMPERATURE
A. Sconza, G. Torzo, Department of Physics, University of Padova (Italy)
M. Michelini, Department of Physics, University of Udine (Italy)

To measure the Hall voltage VH in any conducting sample we have to bias it with a current I and to apply a transverse magnetic field B: the resulting signal, inversely proportional to the thickness S and directly proportional to I and B, (VH = RH I B / S) is usually weak, so that it must be properly amplified.

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The misalignement of the lateral contacts can be compensated at B=0 by properly adjusting an "offset" potentiometer P.
A large B value may be produced by an electromagnet (for measurements at room temperature), or by rare-earth permanent magnets that can be easily inserted into the thermoregulator in order to perform Hall measurements at different temperatures.

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W5Ac
SPECTROSCOPIC MEASUREMENT OF THE ENERGY GAP
A. Sconza, G. Torzo, G. Delfitto, Department of Physics, University of Padova (Italy)

By using a home-made monochromator [1] we vary the wavelength in a light beam aimed at a thin semiconductor sample, and we measure both the transmitted light I(h) and the conductivity of the sample () vs the photon energy h=hc/.

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The plots of (h) and I(h) show a threshold energy value ht below which the sample is transparent and does not exhibit photoconductive effect. Above ht a sudden increase of light absorption and of the conductivity suggest that a minimum photon energy Eg= Eg=ht=hc/t is required to excite electrons into the conduction band.

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[1] A. Sconza and G. Torzo Am. J. Phys.62, 732-737 (1994)


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W5Ad
THE OPTICAL-INJECTION HAYNES-SHOCKLEY EXPERIMENT
A. Sconza, G. Torzo, Department of Physics, University of Padova (Italy)

A short light-pulse, generated by a laser diode, is periodically injected through a thin optical fiber into a bar-shaped sample, that is biased by a constant electric field E =Vs/L along the x axis. The injected excess charge carriers, drifting along the sample, are detected by a point-contact at a position z = d. The time-delay t between the detected pulse and the injection pulse may be measured on the screen of an oscilloscope, and the drift velocity is therefore calculated as vd = d/t.

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The minority carrier mobility is then calculated as vd/E.

By measuring the collected pulse area for different values of the distance d one can also calculate [2] the excess carrier lifetime .
The behaviour of positive charge carriers (holes) and negative ones (electrons) is investigated by using n-doped and p-doped samples, and by changing the sign of the sweep pulse.

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[2] A. Sconza and G. Torzo: EUR. J. Phys.8, 34-40 (1987)


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W5Ae
THE PHYSICS OF THE PN JUNCTION: FORWARD CHARACTERISTICS
A. Sconza and G. Torzo, Department of Physics, University of Padova (Italy)

As PN junction we use a "transdiode" (a transistor with base and collector short circuited). The forward current I is measured, at constant temperature, as a function of the biasing voltage V by a simple current-to-voltage converter. At each temperature the value of the inverse current I0 is extrapolated from the linear fit loge (I) = loge (I0) + eV/(kT).

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The slope of the plot of loge (I0) versus 1/kT gives [3] the value of the energy gap Eg.

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[3] A. Sconza, G. Torzo and G. Viola: Am. J.Phys. 62, 66-70 (1994)


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W5Af
THE PHYSICS OF THE PN JUNCTION: FORWARD VOLTAGE VS TEMPERATURE
A. Sconza and G. Torzo, Department of Physics, University of Padova (Italy)

The forward voltage Vf measured at constant current decreases linearly with temperature (diode thermometer): e Vf=E g0 + const T.
Therefore a plot of eVf versus T is essentially a straight line whose intercept at 0 K gives the energy gap Eg0.

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GIREP95 Workshops