Transistor Temperature Stability
As the temperature of a transistor increases, the collector current will increase because
An approximate relationship for the collector current change is:
An increase in temperature produces an increase in the minority carrier current, but a negative change in VBE, so both effects lead to an increase in collector current with temperature. Since the emitter resistance shows up in the denominator of both terms, this shows that a large value of RE is desirable for temperature stability.
The presence of RE provides negative feedback which stabilizes the circuit against changes in temperature, supply voltage, etc., but it also decreases the voltage gain.
Sec 5.5, 5.6, p218