What type of diode is leakiest? - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: February 19, 2021, 04:04:50 pm
content: Generally, I've noticed that Schottkys seem to exhibit more leakage than silicon. However it is not generally easy to compare as the DS give different conditions.1.0) BAT85. Vrev = -25V, T=25C, I = 2uA2.1) 1N4148. Vrev = -20V, T=25C, I = 2nA2.2) 1N4148. Vrev = -20V, T=150C, I = 50uA2.3) 1N4148. Vrev = -75V, T=25C, I = 5uAIf LTSpice models this well, maybe I could try that, but it is not certain which models include temperature.Vishay Document Number 85607 shows that the reverse leakage (at 25C) for very low Zener voltages at say 30% (very roughly) of breakdown is 150uA. For higher breakdown, this falls to 0.5uA, even at a higher fraction of breakdown.Then again, for TVS at the reverse stand off voltage (at 25C), Littelfuse show that P6KE6.8A has 1000uA, P6KE10A is down to 10uA, and much beyond that, all leakages are given as 1uA.

author: tszaboo
author_rank: Super Contributor
timestamp: February 19, 2021, 04:08:28 pm
content: Different manufacturers define or measure the leakage differently. It also changes with voltage applied and temperature and the size of the silicon. I gues the leakiest diode ever will be some old 1960 diode from a train engine used to rectify like a thousand amps.

author: 741
author_rank: Frequent Contributor
timestamp: February 19, 2021, 04:16:05 pm
content: Yes, for a specific outlying case - but is is fair to say (for example) a small signal, "currently used in designs/typical" Schottky leaks more?I think "yes". Compare BAT54, BAT 85 with 1N4148, 1N914 at room temperature/typical operating conditions.(If) I am "mostly right", I'd guess that comparing 'rating for rating' (eg 1A diode, same voltage range etc) this would apply generally, and the bottom line /rule of thumb (subject to reasonable caveats) would then be that "Schottkys are leakier than silicons".

author: Siwastaja
author_rank: Super Contributor
timestamp: February 19, 2021, 04:28:46 pm
content: Those modern schottky types with incredibly low Vf. It comes with a price.If sorting by leakage doesn't work due to inconsistent leakage reporting, sort by Vf and look for the lowest Vf diodes.Note that when designing a switch mode converter or a motor controller for example, lowest Vf may initially sound like having the best efficiency, but always calculate the total loss including leakage loss: (1-D) * Id * Vf + D * Ileak * Vsupply. It may happen the best diode doesn't have the lowest Vf. Remember the temperature sensitivity: you may need to calculate iteratively to see if you have a thermal runaway. For example, first calculate by total loss * Rth to obtain Tj, then recalculate the loss with the calculated new Tj, iterate.

author: madires
author_rank: Super Contributor
timestamp: February 19, 2021, 04:56:09 pm
content: Germanium diodes can be quite leaky, albeit some are low leakage types.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 19, 2021, 05:03:42 pm
content: Quote from: 741 on February 19, 2021, 04:16:05 pmYes. Definitely. And you'd probably want to see the BAT85 leakage in the two conditions where the 1N4148 leaks more. Actually, 2uA is already pretty good for a regular Schottky diode...

author: TimFox
author_rank: Super Contributor
timestamp: February 19, 2021, 05:15:52 pm
content: The generalizations above for different diode types are correct.However, when it comes to manufacturers’ guaranteed specifications for low current values, these must take into account the practical problems with the speed of automated measurement of low currents in production. Typical performance values can be measured with lab test equipment, but not quickly enough for production rates of low-priced components.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2021, 10:39:10 pm
content: Mind, leakage can always be worse -- 1N914/1N4148 tend to be particularly poor, not sure if it's something like edge effects (they're planar aren't they, not like a raw cleaved PN junction edge?), or process, or ambient light or what. (Though I have good reason to believe it's not ambient; photocurrent is only ~uA even in direct sunlight, IIRC. And N/A for 1N4148W and the like, obviously.)Oh also, they're gold doped, which increases leakage current while reducing breakdown voltage and recovery time. So that's a factor.Schottky are naturally leaky; the schottky junction has different physics. I forget offhand what exactly leakage current is; is it just higher because there's more charge carriers around? Or is the doping higher in general? Or is it something to do with the contact metal diffusing in (usually something like PtSi, IIRC; Pt and Au have similar effects as dopants in Si)? Bah, I should review my old textbook...So, from a practical standpoint, there's always a tradeoff between current rating (bigger junction leaks more, but handles more current), voltage rating, recovery time, and forward voltage drop. Just choose accordingly.Note also, by the way: manufacturers tend to avoid measurements in the sub-uA range. It takes time to measure currents that small, so testing has an outsized impact on production of jellybeans like diodes and transistors. BJT collector leakage might be ~nA; MOSFET gate leakage might be pA; but they'll never tell you that about a common commercial part. (Usually, a device gets split off under a different part number, whichistested in this way -- and, along with the smaller demand, the price is accordingly much higher. You pay for quality!)So, incidentally -- transistors come pretty small, at times. RF BJTs can have quite small junctions, with modest voltage ratings -- giving modest to very fast recovery, and very low leakage. Good luck getting guaranteed ratings on these, though; you'll have to do your own qualification and receiving testing. You pay for quality...JFETs are a traditional go-to, and with D+S tied, you simply get a diode from the gate. Mind, Rds(on) is still in series, more or less, so this tends to make rather poor diodes -- high internal resistance.PAD1is apparently made in exactly this way; note the high Vf even at rather low If.Tim

author: Shadowfire
author_rank: Contributor
timestamp: February 20, 2021, 08:37:28 pm
content: Depending on the circuit, you might be able to eliminate leakage by biasing it so that there is no voltage difference across the diode when it isn't actually conducting. It works quite well with the perfect rectifier/peak and hold type circuits; use a center tap to hold the diode voltage on both sides constant during the non-charging portion of the cycle, and suddenly the leakage current of the diode doesn't matter at all. Pair it with a Jfet input opamp, and suddenly your biggest threat to leakage is that someone did rework after the boards were washed and didn't re-clean the board.

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2021, 04:32:57 pm
content: Quote from: T3sl4co1l on February 19, 2021, 10:39:10 pmThe barrier height depends on whether N or P silicon is used, and what metal is used for the junction. Low barrier schottky diodes have a lower forward voltage drop but higher leakage, while the reverse is true for high barrier schottky diodes. There are a lot of choices for the metal.HP/Avago organized their schottky diodes by barrier height which was convenient for designers but I don't think anybody else does.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: February 23, 2021, 03:24:56 am
content: For leakage, I imagine selenium rectifiers are not the best.

author: AndersJ
author_rank: Frequent Contributor
timestamp: February 23, 2021, 05:34:45 am
content: Quote from: Shadowfire on February 20, 2021, 08:37:28 pmSchematic example?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2021, 07:14:48 am
content: Hmm, hard to find an example offhand actually. I'll just draw it...Note this assumes the amp inputs can safely be pushed somewhat beyond the supplies; if not, some variation should be used to account for that. For example, you could make sub-rails to clamp to, using zener shunt regulators, so the diodes clamp somewhat inside of +/-V. Also, bypass caps not shown.The operating mechanism is, since Vout (right) follows Vin (left), V(D2) = 0 so I(D2) = 0 -- the leakage is bootstrapped away. When driven suddenly, and hard, the amp's output is overpowered (or R3, or both) and D2 conducts into D3.Tim

author: Shadowfire
author_rank: Contributor
timestamp: February 24, 2021, 03:47:16 am
content: This kind of circuit here. Using not-really-ideal 1N4148 diodes results in a leakage current of around 1nA; the Jfet opamp leakage is measured in picoamps. Control your decay time via R2/C1.

author: Ian.M
author_rank: Super Contributor
timestamp: February 24, 2021, 04:26:49 am
content: Quote from: basinstreetdesign on February 23, 2021, 03:24:56 am*ANY* solid state rectifier (with the possible exception of a badly adjusted 'cat's whisker' detector) is highly likely to be less leaky than anElectrolytic rectifier, which will definitely leak significant reverse current and may also leak electrolyte!