2.4.7.9 The "hot-probe" experimentTable of Contents - 1 2 3 4 5 6 7 8 9 R S

The "hot-probe" experiment provides a very simple way to distinghuish between n-type and p-typesemiconductors using a soldering iron and a standard multimeter.

The experiment is performed by contacting a semiconductor wafer with a "hot" probe such as a heatedsoldering iron and a "cold" probe. Both probes are wired to a sensitive current meter. The hot probe isconnected to the positive terminal of the meter while the cold probe is connected to the negative terminal.The experimental set-up is shown in the figure below:

hotprob1.gif

Fig.1 Experimental set-up of the "hot-probe" experiment.When applying the probes to n-type material one obtains a positive current reading on the meter, while p-typematerial yields a negative current.

A simple explanation for this experiment is that the carriers move within the semiconductor from the hotprobe to the cold probe. While diffusion seems to be a plausible mechanism to cause the carrier flow it isactually not the most important mechanism since the material is uniformely doped. However, as will bediscussed below there is a substancial electric field in the semiconductor so that the current is dominated bythe drift current.

Starting from the assumption that the current meter has zero resistance, and ignoring the (small)thermoelectric effect in the metal wires one can justify that the Fermi energy does not vary throughout thematerial. A possible corresponding energy band diagram is shown below:

2.4.7.9 The "hot-probe" experiment http://ecee.colorado.edu/~bart/book/hotprobe.htm

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hotprobe.gif

Fig.2 Energy band diagram corresponding to the "hot-probe" experiment illustrated by Fig.1.

This energy band diagram illustrates the specific case in which the temperature variation causes a linearchange of the conduction band energy as measured relative to the Fermi energy, and also illustates the trendin the general case. As the effective density of states decreases with decreasing temperature, one finds thatthe conduction band energy decreases with decreasing temperature yielding an electric field which causes theelectrons to flow from the high to the low temperature. The same reasoning reveals that holes in a p-typesemiconductor will also flow from the higher to the lower temperature.

The current can be calculated from the general expression

(eqt18.gif)

where

(eqt19.gif)

The current will therefore increase with doping and with the applied temperature gradient as long as thesemiconductor does not become degenerate or intrinsic within the applied temperature range.

2.4.7.8 2.4.7

Bart J. Van Zeghbroeck, 1997

2.4.7.9 The "hot-probe" experiment http://ecee.colorado.edu/~bart/book/hotprobe.htm

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