diode and transistor testing

[afesheir]

hi men and ladies ...
when testing a diode or a transistor, we adjust the multimeter on the "diode" function and test the diode in both the forward and reverse biasing ... my question is: what does the value displayed on the multimeter denote ...? it is a voltage or a resistance ...

When measuring a transistor, this value is greater between emmiter and base than between collector and base ... when i asked why, i was told that this is due to the higher emmiter doping concentration ... but how can this high concentration make this value greater ...?? plz help and thx

 

[Nigel Goodwin]

It's the forward voltage drop across the diode (at that particular current and temperature).

 

[ronsimpson]

My meter (in diode mode) outputs 1mA/3 volts. The meter reeds volts. It the voltage is above 2 volts it reeds “over load”. For a good diode it will reed OL in one direction and the forward voltage drop (at 1mA) in the other direction. You can tell what type of diode by the voltage rating. Silicon? Schottky? Germanium? A 1000 volt diode will read different than a 50 volt diode.

For testing a transistor you are really only testing for diodes from B-E and B-C. Just like high voltage silicon is different than low voltage silicon, in some cases you might see a difference between B-E and B-C silicon.

This is not a great test but it is simple and fast. Generally when a diode or transistor “dies” it will flunk the simple forward voltage drop test.

 

[afesheir]

Nigel Goodwin ...
ronsimpson ...
Thank you for ur help ...

My meter (in diode mode) outputs 1mA/3 volts. The meter reeds volts. It the voltage is above 2 volts it reeds “over load”. For a good diode it will reed OL in one direction and the forward voltage drop (at 1mA) in the other direction. You can tell what type of diode by the voltage rating. Silicon? Schottky? Germanium? A 1000 volt diode will read different than a 50 volt diode.

For testing a transistor you are really only testing for diodes from B-E and B-C. Just like high voltage silicon is different than low voltage silicon, in some cases you might see a difference between B-E and B-C silicon.

This is not a great test but it is simple and fast. Generally when a diode or transistor “dies” it will flunk the simple forward voltage drop test.

do u mean that the reading of the meter indicates the maximum forward voltage ...??

also i didn't get how the different doping concentration could result in different (voltage drops) across BC junction and EB junction

 

[ronsimpson]

Yes the voltage is the forward voltage under this temperature and that one current. It probably is OK to test a signal diode at 1mA but a 100 amp power diode should be tested at much more current. (600mV not resistance)

Who cares how a transistor works. Be thankful that it works. You will probably never meet an engineer who decide how to dope silicon. It is taught in school but only 1 in a million engineers will actually dope silicon. I think you can understand that, what and how much impurities are added to silicon changes how it works. Pure silicon acts very different than doped silicon. What “junk” that is added makes a big difference.

 

[Nigel Goodwin]

do u mean that the reading of the meter indicates the maximum forward voltage ...??

No, as we both said, it's the forward voltage drop under the exact test conditions, it will vary under other conditions.

also i didn't get how the different doping concentration could result in different (voltage drops) across BC junction and EB junction

No idea, and why would you want to know?.

 

[whiz115]

also i didn't get how the different doping concentration could result in different (voltage drops) across BC junction and EB junction

hi!

"an increase in doping concentration affords an increase in conductivity due to the higher concentration of carriers available for conduction"

https://en.wikipedia.org/wiki/Semiconductor

Nigel and ronsimpson are right, although i understand your interest to learn such things (i'm newbie too...) try not to waste your time searching so deep, unless you want to become some kind of engineer...

 

[afesheir]

You will probably never meet an engineer who decide how to dope silicon. It is taught in school but only 1 in a million engineers will actually dope silicon

yes ronsimpson ... i understand but it is my curiosity ...

No idea, and why would you want to know?.

because i want to understand what i am studying ... what lets me accept what i am studing except the scientific proof ???!!

try not to waste your time searching so deep

hehehe ... i hope, but if i do, i will sleeeeep and will never ask about anything and then i may loose my engineering nature ..!!

THANKS TO EVERYBODY WHO CONTRIBUTED ANSWERING MY QUESTION

 

[Centretek]

When a piece of N doped silicon is fused to a piece of P doped silicon, there is a region in the middle where the dopings cancel out. This makes for a high resistance zone called the depletion layer. When a biasing voltage is applied to the junction in one direction the 'depletion zone' gets bigger and so the resistance gets higher. If the voltage is reversed the 'depletion zone' gets smaller and the resistance decreases.
The first voltage is a 'Reverse bias' and the second is a 'Forward bias'.