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

The experiment is performed by contacting a semiconductor wafer with a "hot" probe such as a heated soldering iron and a "cold" probe. Both probes are wired to a sensitive current meter.

The hot probe is connected 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:

When applying the probes to n-type material one obtains a positive current reading on the meter, while p-type material yields a negative current.

A simple explanation for this experiment is that the carriers move within the semiconductor from the hot probe to the cold probe.

The heat source will cause charge carriers (electrons in an n-type, electron holes in a p-type) to move away from the lead.

The heat from the probe creates an increased number of higher energy carriers which then diffuse away from the contact point. This will cause a current/voltage difference.

For example, if the heat source is placed on the positive lead of a voltmeter attached to an n-type semiconductor, a positive voltage reading will result as the area around the heat source/positive lead becomes positively charged.

If the material were of p-type, positively charged holes will be majority carriers, and polarity of induced voltage would be reversed. The direction of current would also be reversed if they were shorted with a wire.

Thus a measurement of either the short circuit current or the open circuit voltage tells us the type of material.