At constant current (I) the forward bias potential (V) of a "pn" junction diode may be considered to vary linearly with temperature (t) within a temperature range. Based on this property we have constructed diode thermometer using germanium (Ge) and silicon (Si) diodes. The experimental "V-t" data at constant forward current have been fitted to a straight line for calibration of the diode thermometers using the least square method. Furthermore, the fitting parameters of the straight line have also been used to determine the band gap energy of germanium and silicon and experimental values of band gap for both the semiconductor materials are found to agree well with accepted values.