advantage of pn junction diode as a switch over conventional switch
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P-N junction’s conduction is through minority carrier diffusion. The minority carriers once injected into a region is difficult to remove. These carriers sometimes are also called stored charge. It is the same mechanism of stored charge in a saturated BJT. This is the reason why BJT can be turned on fast….but once in saturation, it takes a long time to turn it off.
Schottky is more like a degenerated ohmic contact. It has a lower turn on voltage. Its conduction is through majority carrier. Therefore, there is no stored charge as in P-N junction. In fact, in the old days, people use Schottky diode to clamp (in parallel with) the collector base junction of BJT (generally referred to as Schottky clamped transistor) to make the BJT switching significantly faster. The logic family using this technique is called Schottky TTL.
There are many different kinds of Schottky made with different metal contacting typically n-type silicon. Aluminum, Platinum, Tungsten, Titanium, Titanium Tungsten, etc, along with all sorts of silicide could be the contacting metal and produce different Schottky with different electrical characteristic. The forward voltage ranges from .2V to .6V, with the lower the forward, the higher the reverse leakage.
The reasons to use Schottky are typically for speed or lower forward voltage. These characteristics could translate into different benefits. In switching regulator as an example, the lower forward may mean higher efficiency. However, there are drawbacks associated with Schottky. Not every process has a Schottky. Adding a Schottky adds complexity to the process. A Schottky is significantly bigger in silicon area compared to a P-N diode for the same current. These contribute to additional cost to use Schottky. Since reverse leakage is a function of the area, the bigger area makes the already high reverse leakage even higher.
Schottky is more like a degenerated ohmic contact. It has a lower turn on voltage. Its conduction is through majority carrier. Therefore, there is no stored charge as in P-N junction. In fact, in the old days, people use Schottky diode to clamp (in parallel with) the collector base junction of BJT (generally referred to as Schottky clamped transistor) to make the BJT switching significantly faster. The logic family using this technique is called Schottky TTL.
There are many different kinds of Schottky made with different metal contacting typically n-type silicon. Aluminum, Platinum, Tungsten, Titanium, Titanium Tungsten, etc, along with all sorts of silicide could be the contacting metal and produce different Schottky with different electrical characteristic. The forward voltage ranges from .2V to .6V, with the lower the forward, the higher the reverse leakage.
The reasons to use Schottky are typically for speed or lower forward voltage. These characteristics could translate into different benefits. In switching regulator as an example, the lower forward may mean higher efficiency. However, there are drawbacks associated with Schottky. Not every process has a Schottky. Adding a Schottky adds complexity to the process. A Schottky is significantly bigger in silicon area compared to a P-N diode for the same current. These contribute to additional cost to use Schottky. Since reverse leakage is a function of the area, the bigger area makes the already high reverse leakage even higher.
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