What is thermal instability in transistors and its contributing factors?
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The main factor affecting thermal stability in bipolar transistors is the effect of temperature on the saturation current (Is) of the base-emitter diode.
The current-voltage characteristic of this junction can be represented by the Shottky equation -
Ibe = Is{exp(qV/kT) - 1}
where q is the electronic charge, k is Boltzmann's constant, V is the base-emitter voltage and T is the absolute temperature.
Since Is is strongly dependent on temperature, and increases with temp, Ibe increases when the temperature increases even with constant V. If the transistor is biased from a constant-voltage supply Ibe and therefore Ic will increase with temperature. This is the primary mechanism of thermal instability.
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