high temperature affects gain. Why?

Hi,
Can someone please explain how does high temperature affects RF gain (of a PA, for example)?
I know gain is reduced as temp goes high, but I want to know more thoroughly.
I also know thermal noise is increased as temp goes high (does it affect gain?)
Thx!
Tom

The transistor transconductance (gm) is affected by temperature, and this would affect the gain.

Affected how?
and why?

Vt has approx. -2mV/C slope and gm=Ic/Vt
So, gm is dependent to temp.

All semiconductor devices (and to some extent, vacuum tubes, too) are temperature dependent. Their gain and in principle junction parameters depend exponentially upon temperature. For example, operation point of a transistor amplifier stage must be designed to achieve a stable operation over a specified temperature range.
Another problem is power dissipation; all unused power is released as heat. If not treated, the excessive heat can damage or destroy a semiconductor component.

Read a basic textbook on semiconductor devices used in electrical circuits.

---------- Post added at 21:23 ---------- Previous post was at 21:21 ----------

And there is more to add: not only a"high" temperature but any temperature changes do affect amplifier gain.