• Diode voltage specifications vary with temperature.Stockbyte/Stockbyte/Getty Images
  
  The first diode functioned by heating a cathode in a vacuum to cause electrons to boil off and collect on an anode. Later, semiconductor diodes need no heating to function, but they are still affected by changes in temperature. In fact, the temperature characteristics of diodes add greatly to their usefulness as engineers put them to work as measuring devices, as well as rectifiers.
  
  

Thermionic Diodes

• The principle behind the thermionic diode was discovered during the 19th century by Frederick Guthrie. This is a vacuum tube diode, in which the cathode is heated to encourage electron flow. As the temperature of the cathode rises, the forward voltage drop decreases.
  
  

Effect of Temperature on PIV

• The peak inverse voltage, which is the maximum reverse bias voltage a diode can withstand, rises with temperature increase. This cannot normally be measured because exceeding PIV usually results in destruction of the diode.
  
  

Effect of Temperature on Forward Drop

• The forward-biased voltage drop of a diode decreases with an increase in temperature. This is a linear and predictable relationship.
  
  

Using Diodes for Temperature Measurement

• Because of the linear relationship between forward voltage drop and temperature, circuit designers use diodes as temperature measuring devices.