mmbt transistors - Page 1
Forum section: electronics / general

author: Dajgoro
author_rank: Frequent Contributor
timestamp: November 30, 2014, 06:07:15 am
content: Looking at new devices, most of the general propose transistors are from more modern series such as mmbt, and not like the old 2Nxxxx or BC/BDxxx series.Except from the smd package, what benefit do transistors such as the mmbt3904 have over a classic 2N2222?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2014, 06:17:19 am
content: Well, 3904 in any prefix is better than 2N2222 by an awful lot of ways... why anyone continues to design 2N/PN2222s into new designs, I have no clue. It's a number best left to history, along with its complement 2907 of course, and 2N3055.If you don't know why, check the datasheets. Get a sense the 2N2222 is somehow...lacking?Though when you need the current capacity, 2N4401/3 is a direct replacement. But in almost all the places people generically use 2222/2907, they mean 3904/6. Or any of a thousand other generics (BCsomething, 2SC945, ...).MMBT is just one company's prefix for a SOT-23 version (I'm guessing On Semi), which everyone else makes of course so it's genericized.Tim

author: SeanB
author_rank: Super Contributor
timestamp: November 30, 2014, 06:25:38 am
content: Only good thing about the venerable 2N2222 is the 800mA current capability, versus the 100mA of the 3904. Other than that you will find that almost every transistor under 100mA is interchangeable at low frequency, aside from some specialist applications which require the specified transistor, where you need either high gain, controlled gain, switching speed, capacitance or transition frequency.

author: Andreas
author_rank: Super Contributor
timestamp: November 30, 2014, 06:35:13 am
content: Quote from: T3sl4co1l on November 30, 2014, 06:17:19 amIf you carefully look at the datasheet:The 2N2222 is optimized for switching applications of up to around 150 mA (500mA peak) with low saturation voltage and fast switching times.Whereas the 2N3904 is best fitted for analog amplifiers with up to around 10-30 mA.With best regardsAndreas

author: Dajgoro
author_rank: Frequent Contributor
timestamp: November 30, 2014, 06:45:55 am
content: Quote from: T3sl4co1l on November 30, 2014, 06:17:19 amFor generic application, both seem to have simmilar specifications.Can you specify a generic case where a mmbt would leave a 2N in the dust?I mostly use such transistors in simple applications like switching something that requires a bit more power, pulling down lines, simple amplifiers, etc...

author: dannyf
author_rank: Super Contributor
timestamp: November 30, 2014, 12:58:57 pm
content: QuoteAs transistors, they are fairly comparable. However, 3904 is more geared towards RF analog work (amplification for example) where 2222 is for switching.But a lot of cross-overs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2014, 04:34:56 pm
content: Quote from: Andreas on November 30, 2014, 06:35:13 am3904 is smaller, and therefore does better on common small loads: inverter, switching, current sinking, etc. Why spec 500mA when all you need is 10?It's perfectly fine for switching of course; I've done ~10ns tr/tf before in saturated operation.As for MMBT vs. 2N, keep in mind all 2N part numbers (don't know about PN- though?) are JEDEC registered. Meaning, the JEDEC spec defines the part and any dies that conform to it can be packaged, labeled and sold as such. The spec includes package, so there is no such thing as "2N2222 in SOT-23" -- TO-18 is part of the spec. Which is something else a lot of people don't appreciate: they're probably used to seeing plastic TO-92 transistors, then when they go to buy the 'famed' 2N2222, it's expensive and in a metal can!Most likely, when you buy 2N4401 or anything like that, you're just getting the exact same silicon that's in a '2222. Except the '2222 might not be the same from every manufacturer, every time. They can put in whatever, as long as it meets or exceeds the spec. The 'exceeds' part is when you get oscillation and stuff,There are SMTs in 2N's. Compare 2N7000 and 2N7002 for instance.As far as I know, MMBTs are not registered and are at whichever manufacturer's discretion. Mimicking familiar names affords quick identification though.Tim

author: lewis
author_rank: Frequent Contributor
timestamp: December 05, 2014, 11:26:48 pm
content: Quote from: Dajgoro on November 30, 2014, 06:07:15 amCost. I use BC817-40 an awful lot, £0.0056 vs £15.316 for this beast:http://uk.farnell.com/on-semiconductor/jantx2n2222a/transistor-npn-50vdc-0-8a-to-206aa/dp/2366960

author: nuno
author_rank: Frequent Contributor
timestamp: December 06, 2014, 03:00:21 am
content: Quote from: T3sl4co1l on November 30, 2014, 04:34:56 pmYou mean you let it saturate and then pulled it out of there in 10ns?

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2014, 03:08:54 am
content: Quote from: nuno on December 06, 2014, 03:00:21 amI was testing different transistors for risetime a while ago, and found similar results with many PN2222s - not all, but many (interestingly, the general trend was "the older the better"). You have to drive themhardon the desaturating edge.

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2014, 03:23:21 am
content: I was driving the base a volt or so under the emitter, just because that was easier at the time - I don't see why a low impedance short to the emitter wouldn't work, though. The stored charge will get out when it gets out, you just have to make sure that it has a place to go really quickly.Lowest I found was IIRC around 3ns, 2N2369 (waybetter than the datasheet worst-case). RF transistors were nothing amazing in saturation, though better than a 2222; I seem to recall the neighborhood of 4-6ns for at least an MPSH10 and a BFS17W - maybe more parts, but I don't remember.I didn't haveequipmentthat could resolve under 1ns at the time, but nothing came against the limits I had anyway.Interestingly, the BFS17WSPICE modelI have has no problem doing it in 330ps in the perfect environment of the simulator. I wonder if they just didn't bother to properly model charge storage because it's not meant to be saturated...

author: nuno
author_rank: Frequent Contributor
timestamp: December 06, 2014, 03:48:03 am
content: Quote from: c4757p on December 06, 2014, 03:23:21 amI have BC548/558 SPICE models (can't remember where they came from) and they do not model charge storage; so it was quite a surprise when I built the circuit and saw this few us "turn-off latency" (I wasn't aware of the charge storage thing at the time).

author: SeanB
author_rank: Super Contributor
timestamp: December 06, 2014, 05:06:35 am
content: Quote from: lewis on December 05, 2014, 11:26:48 pmnot exactly comparing apples with apples. The Farnell one is a transistor that has been tested to a military spec, and the parameters like leakage, gain, lifetime and case integrity have been tested, so that these will work over the full -55 to +150C military temperature range, and will, if you get the certificate with them, be usable and certifiable for use in space based applications. At that price and with 84 in stock yes they are a lot more expensive than the same transistor that was only tested during manufacture that it was not short circuit, would not leak more than 1mA at 30V and had a gain more than 50 at 1mA of colector current. All done with an ultra short test using 3 ADC readings with a single relay operation during manufacture.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2014, 05:12:50 am
content: Yeah, SPICE is notoriously bad at charge storage. It's modeled as an exponential capacitor -- a battery, in essence, and the analogy is unsurprising.A semiconductor junction tends to separate charges across the junction, so you get an excess of N on one side and an excess of P on the other. If there are charges just floating around (say from spontaneous pair production -- shining light on a solar cell for instance), they migrate across the junction and create a voltage. The V-Q characteristic is comparable to a battery, where it has a near-constant voltage, until charge is all but depleted, at which point it drops exponentially. The charge is just microscopic compared to an electrochemical battery. And, like a battery, it will self-discharge, usually on the order of 10s of us for silicon of average voltage ratings without special treatment.A continuous supply of charge, say from continuous exposure to light, gives a V-I curve with a similar profile, for a similar reason; current is limited to the amount of pair production going on.When it comes to BJTs, you need to slam the base around, which is all the more reason why you should (properly) think of them as voltage controlled devices: collector current is exponential with Vbe, and it takes current to change Vbe (pushing charge around). The only gross difference to a MOSFET is, the base conducts forward current, which is handy in some cases and annoying in others.Another way to enhance switching speed is to avoid saturation completely. The latter is exemplified with ECL and CML circuits: the transistors go between 'off' and 'linear', but do not saturate. Schottky TTL works by clamping the last ~0.4V of saturation with a schottky junction (Baker clamp), shunting base current and preventing stored charge.It's even more advantageous to avoid turn-off, because the circuit itself becomes very slow when insufficient current is handy: dynamic resistances all increase (e.g., r_e = Vth / Ic), bandwidth drops, and slew rate drops (I = C * dV/dt). The best analog circuitsHere's an example of the dependence of doping on recovery:2N3904, B-C junction (E open), square wave input, series diode (DUT), 50 ohm load, 1:1 scale I think.B-E junction (C open): higher doping, somewhat smaller junction.In contrast, B-E with C tied to B (so collector current flows as the base is forward biased) gives almost nonexistent recovery.Recombination: by adding impurities to trap free charges and effectively short-circuit them from within the semiconductor itself, you get a semiconductor (diode, transistor, whatever) that works more poorly (higher leakage, lower breakdown voltage, lower hFE), but runs much faster, especially in saturation speed. TTL chips and 2N2369 are examples of gold doped transistors -- keep in mind, TTL ran at 5V when almost all bipolar chips of the day handled 30V with ease (e.g., uA741). Today, you will occasionally see reference to platinum doping or electron irradiation applied to very high speed rectifiers -- same idea, applied very carefully so as not to impair the breakdown voltage (say for 600 and 1200V diodes!), while increasing forward voltage drop and reverse leakage slightly.Among top shelf RF parts, supposedly, PHEMTs aren't so great in pulsed applications, but GaN FETs are okay. None of them are ever qualified or documented for conventional parameters, pulsed or time-domain operation, or sometimes even basic DC parameters.On a related note, I would love if LDMOS devices became just a little bit more powerful (higher Idss, lower Rds(on)) and way cheaper (IRF510 is cents, while your average LDMOS of similar size is $15 and up) so I can use them for MHz+ converters. Super-Junction MOSFETs have made great strides in the 600V+ market, but the Cdss is still astronomical at low voltages -- it manifests as reverse recovery, even in the forward direction.Tim

author: lewis
author_rank: Frequent Contributor
timestamp: December 06, 2014, 10:58:25 am
content: Quote from: SeanB on December 06, 2014, 05:06:35 amI know! My post was tongue-in-cheek.