Low on-resistance bipolar transistor - does it exist?

John Popelish wrote: Here is another impressive bit of silicon from Zetex:

This doesn't sound like a problem with the transistor, but with the circuit design. Could you send a schematic?

I can't say whether that is the method, or an incremental method is used (small change in collector current causes a small change in collector voltage drop, Ron= delta V / delta I ).

Yes. The base current leakage into the collector produces a minimum collector voltage drop for a given base current, even if the external collector current goes to zero. I think that for any given collector current, there is an optimum base drive, for lowest collector to emitter drop. More is not necessarily better.

Hey all;

A transistor thread in this group got me wondering about transistor characteristics. I have seen low (and very low) on resistance MOSFETS (2 ohms and less) but haven't seen a similar spec in a standard BJT. In fact, sifting through the Fairchild site, one can sort the MOSFETS based on Ron, but the same isn't true for the BJTs. Are some BJTs lower Ron than others, or is low Ron something that BJTs simply are not capable of? It's my understanding that a MOSFET is more susceptible to damage from static discharge than a BJT, and so it would seem to limit it's applications as a switch if you weren't supposed to have any voltage at the drain before things were powered up.

Thanks

Some of the mythology about MOSFETs is based on not thinking things through. They have very high input impedance, which means that unloaded, high voltages can remain high voltages when applied to the gate. There's nothing to dissipate the high voltage. A bipolar sees the same voltage on its base from static discharge, but the relative low impedance of the input means the "high voltage source" gets loaded down significantly, and isn't high enough to do damage. The voltage doesn't change, the "voltage sensitivity of the input" doesn't change, the impedance of the input of the device changes.

Hence, MOSFETs are more prone to static damage when lying loose. It was a real problem in the early days (or at least every time a MOSFET was mentioned, they mentioned being very very careful about protecting the device), though it was relatively soon after that the manufacturers started putting protection diodes on the gates to help protect against this (the protective diodes would not conduct normally, but would conduct when the voltage on the gate went above a certain level, providing a path to discharge the high static voltage on the gate).

But the worry about static damage was with the MOSFET lying loose. Once it was installed in a circuit, you could remove the jumper around the leads (some devices in the early days came with such a jumper, and if it didn't all the articles warned about putting your own jumper on the device before you started soldering). The circuit itself would generally protect the device, because there was a very real path to ground from the gate in the form of a resistor or something else that would provide a path to ground (such as a coil in the case of radio circuitry). The circuits could still be relatively high impedance, but even with relatively high value resistors they loaded things down enough to dissipate the static, if it ever got to that point in the circuit.

The real exceptions would be circuitry where a MOSFET gate is somehow open to the world, and there's nothing to protect that input. So something like an electrometer that depends on a really high input impedance to do it's work and needs the gate exposed to the world might suffer. That MOSFET as an "active antenna" in that shortwave receiver (where the device is more to transform a really high impedance point of the whip antenna to a lower impedance that the rest of the receiver can use) might be vulnerable, if the manufacturer didn't put protective diodes in the circuit. Plug in boards that have a MOSFET input connected to the connector might be vulnerable, if there is no pullup or pulldown resistor on the gate that provides a discharge path for static electricity on the pin.

Michael

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Oh, one other thing - is this how the Ron is calculated, i.e., Collector to emitter drop divided by collector current? If this were the case, my low current would increase Ron.

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Thanks for the reply John. I'm switching 12v at the collector with 5v at the base. The 12v is coming through a 100K resistor from the previous device, so the current is pretty small. I've found that using a standard (2N2222 type) transistor doesn't quite switch things all the way off ( there is an LED indicator that still glows faintly).

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Thanks for the input Michael. I actually have a MAX4662 analog switch

John, I have a schematic but unfortunately I don't have a scanner, so I'll try to take a picture and post it on ABSE. The schematic is from my guitar amp, and it is a control signal for channel switching and volume boost that I'm trying to switch remotely.

you probably want to look at Vces with BJTs it serves a somewhat equivalent purpose

Bye. Jasen