# 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**

I_{s} = CT^{3}e^{-(Eg/kT)}

Where E_{g}
is the forbidden energy, T the temperature of the diode, K Boltzman’s constant
and C is a constant.

Taking
logarithm,

log_{e }I_{s}
= log_{e} (CT^{3}) – (E_{g}/k) x (1/T)

In the
operating range of the diodes, the temperature dependence of I_{s }is
mainly determined by the second term of the above equation. Hence, a plot of
log_{e }I_{s} against (1/T) is approximately linear. The slope
of the graph gives – (E_{g}/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 I_{s} 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 I_{s}, are property
tabulated.

A graph is
drawn with log_{e }I_{s} 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 I_{s} is measure keeping
the diode in the bath. From the calibration curve, the temperature of the bath
can be found.

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