02 Negative Temperature Coefficient

02 Negative Temperature Coefficient#

Aim#

To show how the resistance of a semiconductor (P-Ge) depends on temperature.

Subjects#

  • 5D20 (Resistivity and Temperature)

Diagram#

../../../../../_images/figure_0196.png

Fig. 476 .#

Equipment#

  • Bar of P-Ge

  • Power supply

  • Current meter with large display

  • Gas flame

Presentation#

Set the Ammeter at a \(1\mathrm{~A}\)-scale. The voltage of the power supply is raised until a current of about \(0.05\mathrm{~A}\) flows in the circuit. The bar of \(\mathrm{P}-\mathrm{Ge}\) is heated by the gas flame and soon the current rises to a much higher value. After a short time of heating the gas flame can be removed and the current continues to rise, faster and faster, only limited by the power supply.

Explanation#

The resistance of a semiconductor drops with temperature because at a higher temperature there are more free charge-carriers in it.

The current flowing in the material heats it up: \(P_{e l}=\frac{V^{2}}{R}\) The heat leaving the piece of material is proportional to \(\Delta T: P_{\text {out }} \propto \Delta T\) (Newton cooling). When \(P_{\text {out }}=P_{e l}\) there will be thermal equilibrium and the temperature is constant. Reaching such an equilibrium takes some time.

In this demonstration \(R\) lowers due to a rise in temperature and so \(P_{e l}\), rises due to a rise in temperature. When this rise is faster than the rise of \(P_{\text {out }}\) an ever faster rising of \(\Delta T\) (like an avalanche) will result.

Contents