How does heat change the characteristics of a diode? (Temperature Characteristic)

The forward voltage, VF, of a switching diode lowers as temperature rises. The reverse leakage current, IR, increases as temperature rises.

The electrical characteristics of semiconductor devices are generally sensitive to ambient and operating junction temperatures. The characteristics of Si diodes generally change as follows in the operating range.

  1. The forward voltage, VF, of a switching diode lowers as temperature rises.
  2. The reverse leakage current, IR, increases as temperature rises.
Fig. 1 Si pn-junction diode I<sub>F</sub>-V<sub>F</sub> characteristic (Forward temperature characteristics)
Fig. 1 Si pn-junction diode IF-VF characteristic (Forward temperature characteristics)
Fig. 2 Si pn-junction diode I<sub>R</sub>-V<sub>R</sub> characteristic (Reverse temperature characteristics)
Fig. 2 Si pn-junction diode IR-VR characteristic (Reverse temperature characteristics)

This is true of diodes mainly for the following reasons:

  1. Change in resistance: Heat intensifies the lattice vibrations of atoms, hindering the diffusion of electrons.
  2. Change in the number of conduction electrons: Heat increases the energy of donor electrons, making it easier to excite them into conductors.

Because of a strong bond, silicon carbide (SiC), a wide-bandgap semiconductor, is affected more greatly by lattice vibration than silicon.  And donor electrons are less likely to be excited in SiC than in silicon owing to a wide bandgap. Therefore, silicon and SiC diodes have different temperature characteristics in the current range in which they are normally used.
The following shows the IF–VF curves of silicon and SiC Schottky barrier diodes (SiC SBDs).
In the case of silicon SBDs, forward voltage (VF) decreases as temperature increases. Conversely, in the case of SiC SBDs, VF increases as temperature increases. At higher IF, the forward voltage (VF) of both silicon and SiC SBDs increases as temperature increases.

Fig. 3 temperature characteristic of SBD
Fig. 3 temperature characteristic of SBD

I explained the Schottky barrier diode as an example, but as shown in Fig. 1, pn junction diodes show the same tendency. Because of the temperature characteristics described above, silicon diodes are more susceptible to thermal runaway than SiC diodes.
In circuit design, it is necessary to take temperature dependence of electrical characteristics into consideration.

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