"Safe" VBE breakdown current  - Page 1
Forum section: electronics / beginners

author: ealex
author_rank: Frequent Contributor
timestamp: November 21, 2017, 02:34:47 pm
content: HelloI'm thinking of building a "high voltage" diode tester plugin for one of my meters, like Dave's new meter will have ( or at least i think i head that somewhere )I'm planning to use this schematic :where M_H and M_L are the meter's positive and negative input, and the DUT will be connected where the diode symbol is now.The question I have : there is a chance of probing BJT's with this, and I've read that reverse biasing the VBE junction can kill the transistor. Is there a safe current level that will not do any damage ?Also, around 100uA will be enough to test a normal diode ? ( I've seen some red led's that at 500uA are still too bright to look straight-on )

author: Gyro
author_rank: Super Contributor
timestamp: November 21, 2017, 07:09:04 pm
content: As I understand it, there is no 'completely safe' threshold for Base-Emitter reverse zener/avalanche breakdown. I think even small currents can degrade the gain of the transistor. If there is a reasonably safe current it will vary from device to device. The breakdown will also be noisy. It's certainly a condition that is best avoided.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 21, 2017, 07:21:37 pm
content: Degradation takes whole coulombs of charge (I don't remember how much for what degree of effect). Testing a few seconds at a few mA won't be a problem.Even higher voltage testing is useful, as you can discover higher Veb on symmetrical-junction transistors, and Vceo/Vcbo.Tim

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2017, 09:05:25 pm
content: There is no safe reverse breakdown current and damage is logarithmicly proportional to the total charge so it is the initial breakdown current which has the most effect. Hot carriers cause dislocations in the silicon resulting in lower hfe due to shorter minority carrier lifetime. This is similar to radiation damage and can be treated through annealing; maybe some space based design intended to operate in a high radiation environment tried this.The damage is not as significant for junctions which are already doped to produce short minority carry lifetime like in RF and fast saturated switching transistors. The application matters also; it is unlikely to make a difference with power transistors operating at high currents where their hfe is already low or transistors used as switches with high forced beta.It makes a huge and crippling difference where high gain at low current is required like a differential input stage. Bipolar input operational amplifiers damaged like this display an order of magnitude or greater increase in input bias current, the noise to go with it, and high offset voltage. For this reason, they usually include input protection diodes to limit the applied differential voltage. Monolithic matched transistors like the LM394 include protection diodes across the base-emitter junctions to prevent damage but this was not universal; the MAT01 from Analog Devices is unprotected.So do not run your supply of high gain 2N5086, 2N5087, 2N5088, 2N5089, BC546, BC548, BC550, BC556, BC558, and BC560 transistors and Analog Devices matched transistors through a base-emitter breakdown test; it will ruin them for their intended applications.

author: ealex
author_rank: Frequent Contributor
timestamp: November 22, 2017, 04:20:53 am
content: thanks for your answers.so i'll have to be careful - no random testing of anything that might be a tranzistor with the thing in post 1.i'll use it only for leds and zeners diodes

author: danadak
author_rank: Super Contributor
timestamp: November 22, 2017, 12:44:23 pm
content: This papers tests were done at 60 mA, but my failing memory I think Pease did some experimentsin the uA region on OpAmps and found serious degradation of characteristics. I can't seem to findsupporting info.This paper shows a number of references at end, that may shed more light on the issue.file:///C:/Users/user/Downloads/053209.pdfRegards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: November 22, 2017, 01:14:31 pm
content: Quote from: danadak on November 22, 2017, 12:44:23 pmStop storing your files on my computer.