3-5. Reverse recovery time

When diodes and other semiconductor devices are on, current flows because of free electrons and holes (carriers). In addition, these carriers are accumulated as electric charge in the depletion region formed across a junction or in a parasitic capacitor and also accumulated in the lightly doped region as excess carriers. (This causes the above-mentioned conductivity modulation.) When these devices transition from the on state to the off state, they release electrons just like a capacitor. The resulting flow of electrons is observed as a current flow in the opposite direction.
The forward voltage (VF) is positive while the current (iF) is between zero and the peak reverse recovery current (Irr). During this period, the electric charge in the depletion region and the parasitic capacitor is mainly released, causing negative current to flow. Once iF reaches Irr, the excessive carriers (electrons and holes) that contributed to the conductivity modulation disappear because they recombine. Therefore, the time required for this recombination depends on the carrier lifetime.

Figure 3-8 Reverse recovery time
Figure 3-8 Reverse recovery time

As described above, conductivity modulation occurs in bipolar devices such as pn junction diodes whereas theoretically it does not occur in unipolar devices such as SBDs. Therefore, SBDs exhibit almost zero reverse recovery time.
Figure 3-9 compares the reverse recovery characteristics of two typical pn junction diodes—the CRG09A rectifier diode (VR=400 V, IF=1 A) and the CMF02A fast recovery diode (VR=600 V, IF=1 A)—with the CMS20I40A SBD (VR=40 V, IF=2 A). Although it is difficult to make simple comparisons because their maximum rated reverse voltages are different, the reverse recovery time of the fast recovery diode (FRD) is much smaller than that of the rectifier diode whereas the reverse recovery time of the SBD is nearly zero.

Figure 3-9 Comparison of the reverse recovery characteristics of a rectifier diode, an FRD, and an SBD
Figure 3-9 Comparison of the reverse recovery characteristics of a rectifier diode, an FRD, and an SBD

Chapter3 Basics of Schottky Barrier Diodes

3-1. Classification of diodes
3-2. Comparison between SBDs and pn junction diodes
3-3. Applications of SBDs
3-4. Forward voltage
3-6. Maximum rated reverse voltage (VR)
3-7. Leakage current

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