BAT54 has different versions what they can be used for.

[sairfan1]

Please refer to attached image BAT54 inside is shown, looks like device marked A can be used a Diode but what other versions can be used for?

[ataradov]

They can be used as a pair of diodes. This is a really strange question. There are tons of schematics where diode arrangements like this are used.

BAT54C is a typical diode-OR circuit. BAT54S is a common clamping circuit.

[Zero999]

The BAT54 is a Schottky diode. The different part numbers indicate whether it's a single diode, or two, in various combinations.

The other versions can be used in any application two diodes are required, so long as they're joined together. For example two of the BAT54S can be used to make a bridge rectifier.

Fortunately the part numbers are easy to remember: A means common anode, C common cathode and  S series.

[newbrain]

Another typical application for the series is creating a diode ring for a double balanced RF mixer.

[thm_w]

BAT54C can be used to OR two power supplies together, eg USB and battery, into one output.
You can also always use one of the diodes and just ignore the unused part.

They can be used as a pair of diodes. This is a really strange question. There are tons of schematics where diode arrangements like this are used.

This is Beginners section, so its not really a strange question for someone with little experience.

[sairfan1]

these packages made me curious and I wanted to know what could be the reason. All those comments are really helpful. Thanks

[ataradov]

And if you arrange them by the use rate, it would be BAT54S, BAT54C, BAT54A, BAT54. And there is a huge gap between the first two and the last two.

The single version makes no sense to me. It is also available in a smaller 2-pin package, so there is no reason to use this package. And if you just need one diode, but already have any other type in the design, you might as well use that and eliminate a BOM item. The price difference between them is insignificant unless you need millions. But then smaller package is even cheaper.

[SiliconWizard]

The single version makes no sense to me. It is also available in a smaller 2-pin package, so there is no reason to use this package. And if you just need one diode, but already have any other type in the design, you might as well use that and eliminate a BOM item. The price difference between them is insignificant unless you need millions. But then smaller package is even cheaper.

While the rationale may look puzzling at first sight, one use case would be specifically if, for some design you'd have 2 variants, populated differently on the same footprint, either as a C, D or the single one. Not that I have a specific circuit to suggest right now, but I guess that could be. Admittedly, for this particular need, if this is what was targeted, I'm wondering how the vendor could have determined there was a sizable market for that. Oh well.

[amyk]

The single version makes no sense to me. It is also available in a smaller 2-pin package, so there is no reason to use this package. And if you just need one diode, but already have any other type in the design, you might as well use that and eliminate a BOM item. The price difference between them is insignificant unless you need millions. But then smaller package is even cheaper.

While the rationale may look puzzling at first sight, one use case would be specifically if, for some design you'd have 2 variants, populated differently on the same footprint, either as a C, D or the single one. Not that I have a specific circuit to suggest right now, but I guess that could be. Admittedly, for this particular need, if this is what was targeted, I'm wondering how the vendor could have determined there was a sizable market for that. Oh well.

There is the possibility that the single variant contains one good diode and one bad diode on the same die.

[ataradov]

There is the possibility that the single variant contains one good diode and one bad diode on the same die.

There is almost no chance of that. It is seriously not worth the time and logistics to keep track which diodes are half bad. And then you will have to bond to the same pin a diode on a good side, which complicates bonding process.

If you are not getting 99% yields on two diodes, you are doing semiconductors wrong. So, no matter what you will have to substitute with good ones.

[sleemanj]

It is also available in a smaller 2-pin package, so there is no reason to use this package.

Maybe better thermal performace. 

Does seem a bit weird though, if they were binning failures you'd expect to see all 4 possible single diode combinations.

[Shell Albert]

the two diodes pair is used under circumstances like this.
it's called diode gate logic.