Schottky diode leakage

Schottky diodes can have almost lOx the leakage current of a good Schottky. In this case, that was what was ultimately confusing the switcher IC to break into chaos every now and then. We replaced it with a quality diode and everything was fine thereafter. 

Incidentally, Schottky diodes do not have any reverse recovery current issues, but they do have some body capacitance, which can produce similar effects. However, that fear has always proven to be exaggerated. I have personally not seen any application having a performance issue explicitly related to a bad Schottky. The only exception was a case where the leakage current of the Schottky was so high, it was prematurely tripping the current comparators inside the switcher IC. And there was also a reliability issue once, concerning the dV/df rating of a commercial Schottky. Both these issues are discussed elsewhere in this book. 

The Schottky barriers were excellent diodes for films annealed at 600 °C, with turn on voltages of 0.6-0.8V and minimal reverse bias leakage.48 However, many of the contacts on the as-deposited films gave large reverse bias currents and nearly ohmic responses. This behavior is indicative of degeneracy of the semiconductor because of a high carrier density resulting from native defects. The improvement in the diode behavior of the annealed films is attributed to enhanced crystallinity and reduction of defects. 

We realize that the way to reduce conduction losses is by lowering the forward-drops across the diode and switch. So we look for diodes with a low drop — like the Schottky diode. Similarly, we look for mosfets with a low on-resistance Rds. However, there are compromises involved here. The leakage current in a Schottky diode can become significant as we try to choose diodes with very low drops. We can also run into significant body... 

When sufficient reverse bias is applied to the Schottky diode, the potential barrier f/(x) depletes the region near the interface of free carriers to the point that the carrier concentration arises predominantly either from the leakage over the barrier from the metal contact or from the intrinsic thermalization of electrons and holes from the valence and conduction bands. This leads to the formation of a quasi-Fermi level in the deep-depletion region. For the... 

Fig. 24. Capacitance-temperature characteristics of Pt Schottky diodes of undoped a-Si H samples deposited at substrate temperatures T = 450°C (-I-), 400 C (O), and 350°C ( ). Solid line curves were taken at 1 Hz the dashed curve was taken at 30 Hz. The high temperature decrease in the 30 Hz data is due to leakage over the barrier. [From Jousse and Deleonibus (1983).]...

Figure8.5 Band diagram ofa Schottky diode interface illustrating specific defect-assisted tunneling processes proposed for explaining the large leakage at low-bias voltages.

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