I can find lots of info on collector-base junction breakdown and the resulting collector current, but in a common-emitter circuit how is collector current affected by breakdown of the base-emitter junction?

 

I can find lots of info on collector-base junction breakdown and the resulting collector current, but in a common-emitter circuit how is collector current affected by breakdown of the base-emitter junction?

For the base-emitter junction to be in breakdown, it must be reverse-biased and therefore the transistor will be turned off; therefore, I should think the collector current will be essentially zero, not counting leakage.

I think.

 

With the damage caused by breakdown of the base-emitter junction I was wondering if the collector-base junction could be affected in any way so that the collector-emitter leakage might suddenly become an avalanche?

 

With the damage caused by breakdown of the base-emitter junction I was wondering if the collector-base junction could be affected in any way so that the collector-emitter leakage might suddenly become an avalanche?

I don't think there will necessarily be any significant damage to the BE junction unless you force so much current through it that it's destroyed by heat. My understanding is that reverse-biasing the BE junction to the point where it breaks over and conducts a modest amount of current (like a Zener diode) can somewhat affect the β of the transistor and/or increase its noise level; other than that I doubt the transistor would be any the worse for wear.

 

Here on page 23
https://books.google.pl/books?id=6a... Analog Integrated Circuits avalanche&f=false
We can read that base-emitter breakdown is damaging to the device. It can cause a large degradation in β_f.

 

I can find lots of info on collector-base junction breakdown and the resulting collector current, but in a common-emitter circuit how is collector current affected by breakdown of the base-emitter junction?

In a common-emitter BJT configuration, for say an NPN transistor, the collector current is affected by the voltage drop across the BE junction (knee). So if you have 12V on the collector to power something, the collector is only going to see 11.x volts at whatever current the load requires unless the BJT is being operated in the linear region and is limiting the current flow to something less than the load requires.

You can calculate the current. Ei = Bi+Ci

Voltage and current NEVER act independently of one another on their own.

 

Thanks, I'm aware of that, but with b-e breakdown the base current will be negative. Suppose the emitter is at 0V, the base is pulled down to -7V, the collector load is 1k. What wiil the collector current be then? I would guess ~0, but is that certain?

 

probably not. If working in the avalanche region, the transistor looks a lot like a neon bulb, but with the ability to pass a lot more current. If you send your transistor into avalance you have to must limit the peak current, and there the peak power going into the device.

 

Interesting pulser. Did you notice any long-term effects on the 2N3904 when driving it like that?

 

I have been told that the base-emitter junction of a transistor is small and fragile and is damaged by avalanche breakdown that concentrates heat on it. Therefore the hFE of the transistor is reduced permanently.

 

I only ran it for a few weeks. It continued to avalanche as long as I needed it to. The reduction in hFE is a new and interesting idea.

 

Here on page 23
https://books.google.pl/books?id=6aXvOF4coukC&pg=PA22&dq=Analysis+and+Design+of+Analog+Integrated+Circuits+avalanche&hl=pl&sa=X&ved=0ahUKEwjSrrzf8Y3oAhWs8qYKHTH0CPwQ6AEIKzAA#v=onepage&q=Analysis and Design of Analog Integrated Circuits avalanche&f=false
We can read that base-emitter breakdown is damaging to the device. It can cause a large degradation in β_f.

Hi,

I have actually experienced this firsthand.
Long while back i was working with a transmitter using a somewhat higher frequency transistor which of course had a high Beta at the higher frequencies. After connecting the transistor backwards where the collector and emitter were swapped, the emitter base of course got a reverse bias higher than 5v probably around 9 to 15 volts DC. This damaged the transistor to the point where it no longer worked at the high frequency as an oscillator. Using a new transistor, the circuit worked fine.
So my conclusion was that the Beta came down so far that it was not high enough to sustain oscillations after the damage.
I cant remember the number of the transistor after all these years but i do remember Radio Shack had an equivalent replacement and it worked ok too unless it was damaged.

 

In old-time CRT displays a bipolar transistor was often used to drive the horizontal deflection yoke and high voltage. When the transistor was turned of it was necessary the get the collector current low enough before the collector voltage got so high that the safe operating area was exceeded.

To get the transistors to turn off quickly enough it was necessary to apply the maximum reverse bias to the base-emitter junction, usually using some kind of inductor or transformer, and that resulted in avalanching the base-emitter junction. In one paper suggested that the initial turn-off current should be the same magnitude as the turn-on current just prior to turn-off. Motorola found that this turn-off drive caused gradual a reduction in low current hFE. The high current hFE was not degraded.

So maybe heating is the reason.

 

I used to work on TDR equipment which used a transistor (2N2369 I think) in avalanche to generate the pulse to sent along the cable. These worked long term - until the electricity company connected it to a live power cable!