Transistors and Diodes Leakage Current - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 23, 2015, 04:57:22 am
content: i'm considering a transistor as a switch but it came to my attention about leakage current consumed during off period, surprisingly i cant find the info in all transistor datasheet in my keeping. is it of no concern or nill? if not, whats the ballpark figure for transistor/diode leakage current? thanks.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 23, 2015, 06:34:39 am
content: Leakage is easily found in diode datasheets. I think the corresponding transistor parameter would be cutoff current?These vary widely, anywhere between nA and several tens of uA.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: April 23, 2015, 06:57:18 am
content: QuoteWhich application?As already sayed, the leakage current varies a lot, especially with respect to temperature and reverse voltage.The leakage current of BJTs and silicon diodes are in general couple of orders of magnitudes smaler then the leakage current of MOSFETs and Schottky diodes.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 23, 2015, 07:52:25 am
content: Specifically my application is using1443 pnp bjt as high side switch,but my question is generally applicable to anything bjt fet and diode.i thought last time i saw the word "leakage current" in a diode datasheet. But i dont for bjt. I dont think "cutoff current" is it. Or is it?

author: npelov
author_rank: Frequent Contributor
timestamp: April 23, 2015, 08:32:41 am
content: I think it should be "cutoff current" (I guess) what you are looking for. Because the transistor has 2 junctions it's given depending on where you apply the voltage - collector-emiter, collector-base, emiter-base, depending on how you connect the transistor. Think where current can flow on any of the terminals check it's value in datasheet. However the reverse leakage current is heavily dependant on temperature, so the value is not much of a use. It might be 10 nA at 25 deg. C and 100nA or more at 30 deg C. For example for BC556 collector-base cut-off (CE is not specified)current is-15nAmax for 25 deg.C and4000nAfor 150degC and it's probably not linear dependency. Also they often specify collector-emiter cut-off current for reverse voltage applied to emiter-base. For MMBT3906 it's -50nA max, but only if you apply -3V to emiter base (or you can say +3V to base-emiter). With floating base it will be higher and it'll probably change from static electricity in the air.Awild guessis that you can use collector-base cut-off current if collector-emiter is not specified because base-emiter junction is in forward bias.But be warned that I'm not an expert. You should wait until someone with better experience confirms what I said.

author: Zero999
author_rank: Super Contributor
timestamp: April 23, 2015, 09:16:13 am
content: The leakage current in small BJTs is tiny, often much lower than specified on the datasheet. It's also often better to use a BJT as a signal diode, rather than the 1N4148.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 23, 2015, 02:50:14 pm
content: Quote from: Hero999 on April 23, 2015, 09:16:13 amunder what name is it? "cutoff current?" as npelov said "i guess". i want to make sure. thanks for the advice.

author: Zero999
author_rank: Super Contributor
timestamp: April 23, 2015, 02:58:07 pm
content: Yes, it's referred to as the cut-off current. To give you an idea of how low it is, try connecting a multemeter with a 10M input impedance in series with the transistor and a power supply, say 10V or whatever you expect and the meter reading will be near 0V.

author: TimFox
author_rank: Super Contributor
timestamp: April 23, 2015, 04:45:58 pm
content: There are three common specifications for collector cut-off current:Iceo = leakage from collector to emitter with base open circuit. This is the worst-case leakage, since leakage current from collector to base is amplified by the current gain.Ices = leakage from collector to emitter with base connected to emitter. This is almost the same spec asIcbo = leakage from collector to base with emitter open circuit. Essentially, the leakage current of the collector-base diode (reverse bias).Note that these leakage currents increase exponentially with junction temperature, following the behavior of a PN junction (and the more slowly increasing current gain).

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 25, 2015, 03:03:31 am
content: ok thanks guys. i thought cut-off current is the "minimum" current to make it turned ON. below that its turned OFF. i was thinking leakage or idling or quiescent current during OFF period can be very much much less than cut-off current specification.