# PN Junction Thermometer

Investigatory Project on PN Junction Thermometer

Aim: To construct a thermometer using semiconductor diode, calibrate it and to measure the unknown temperature of a bath.

Materials and Apparatus

(i) A germanium diode (OA 79, DR 25 or DS 10)

(ii) High resistance rheostat

(iii) 0 – 100 microammeter

(iv) Dry cell (9V)

(v) Oil bath (a tumbler and oil)

(vi) Thermometer etc.

Principle

The reverse saturation current in a diode is given by,

Is = CT3e-(Eg/kT)

Where Eg is the forbidden energy, T the temperature of the diode, K Boltzman’s constant and C is a constant.

Taking logarithm,

loge Isloge (CT3) – (Eg/k) x (1/T)

In the operating range of the diodes, the temperature dependence of Is is mainly determined by the second term of the above equation. Hence, a plot of loge Is against (1/T) is approximately linear. The slope of the graph gives – (Eg/k).

Procedure

The circuit is wired as shown in the figure. The diode is fixed to the cap of tumbler. Two holes are provided on the cap and a thermometer is inserted in one and a wire stirrer in the other. The tumbler is filled with oil heated to about 150°C. The cap is fixed and the oil is stirred well. At any particular temperature T, the current is measures as a function of the reverse bias voltage V. The reverse voltage is measured with a digital multimeter. The reverse current is measured by the microammeter. From this reverse saturation current Is i.e., the steady reverse current, is noted. The cap of the tumbler is opened and the temperature of the oil is allowed to fall by, say, 20°C. The cap is fixed and the reverse saturated current is determined as before. The temperature T and the saturated current Is, are property tabulated.

A graph is drawn with loge Is and (1/T). The graph is a straight line. It is the calibration curve of the diode thermometer. To find the unknown temperature of a bath the saturation current Is is measure keeping the diode in the bath. From the calibration curve, the temperature of the bath can be found.