Temperature coefficient of resistors question.

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Dear all,

I have a question about temp. coefficient of resistors.

In case of diffusion resistors, temp. coeffcient of that is positive.

But in case of poly resistors, coefficient is negative.

Why?

Could you tell me the reason physically?

Thanks,
Regards.
 
Poly resistors can have positive and negative temp. coefficient. This depend on clusters size of polycrystal. If clusters are small then temp. coefficient is negative because contact resistance between clusters play dominant role and this resistance decrease when temperature increase. In case when clusters are big then resistance of crystalline silicon play the main role and this resistance increases with temperature.
 
For diffusion resistor, when temperature rises, the carrier's mobility will decrease, so the resistance will increase.
 

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