# Dynamic resistance

1. Aug 9, 2016

### Grim Arrow

I'm studying semiconductors and i might need a little help with some questions:

First the dynamic resistance is the internal resistance of the diodes , but it's called dynamic because it isn't constant, right?

Does all semiconductors, pure and doped, have internal/dynamic resistance? The reason i claim this is because diodes and transistors have such a resistance and they are made of doped semiconductors. But some pure semiconductor devices like the photoresistor also have a resistance that changes with temperature or light.

Is that internal resistance caused by the collision of the flowing electrons with the atoms of the semiconductive materials just like in ordinary resistors? If so, this explains why temperature and light can decreace it.

In diodes and especialy the zener diodes is the internal resistance caused by the flowing electrons colliding with the semiconductor atoms, or by the depletion layer, or both?

Thanks.

2. Aug 9, 2016

### LvW

For each device with a non-linear voltage-current chracteristic we can define two resistances:
* The static resistance R is simply the DC voltage-to-current ratio at a certain operating point, and
* the dynamic resistance depends on the slope of this voltage-to-current curve and is defined as r=d(V)/d(I).
Hence, the dynamic resiatnce is defined using the tangent at the corresponding operating point. This dynamic resistance is always a small-signal resistance only.
* Example: The exponential I=f(V) curve for a pn diode always has a static resistance R that is larger than the dynamic resistance r (R>r).

Last edited: Aug 9, 2016
3. Aug 9, 2016

### Grim Arrow

But both static and dynamic resistances are physicaly the same thing - the electrons trying to make their way through the semiconductor material, colliding with it's atoms and slowing down. Right? That's the internal resistance. And in diode there is this resistance and the resistance wich the pn junction causes to the current flow( the usualy 0,7 volts that you need to apply to it in order to turn it on). Is that right?

4. Aug 10, 2016

### LvW

Of course, both static and dynamic resistances are "internal" resistances.

Quote: it's called dynamic because it isn't constant, right?

No - that`s not correct. As I have mentioned - for a non-linear V-I characteristic both (static and dynamic) are not constant.
The dynamic resistance r applies to (small) signal changes only !