Transistor Thermal Runaway

As the ambient temperature increases, the power rating of the transistor decreases. This is referred to as power derating. Figure below shows the typical power derating curve of a silicon transistor with change in transistor case temperature.

Typical power derating curve of a transistor

Transistor in operation dissipates power and therefore its junction temperature rises. This causes the collector current to increase. This may lead to more power dissipation and further increase in temperature and subsequently an increase in collector current. If this cycle continues, it may result in permanently damaging the transistor. This phenomenon is referred to as thermal runaway

Proper design of the transistor circuit with a suitable operating point is necessary to prevent the phenomenon of thermal runaway. If the transistor circuit is not designed properly, it may lead to thermal runaway.

The steady-state temperature rise at the transistor collector junction is given by

Where,
T_J is the transistor junction temperature TA the ambient temperature P_D the power dissipated in the transistor Θ the thermal resistance (^oC/W)

Thermal resistance is defined as the ratio of the rise in transistor junction temperature to the amount of power dissipated.

The inverse of the thermal resistance gives the temperature rate at which the power is dissipated under steady-state conditions:

The value of thermal resistance depends upon the transistor size, the size of heat sink used and other cooling methods used such as forced-air cooling, etc.

The required condition for thermal stability is that the rate at which heat is produced at the collector junction should not exceed the rate at which it can be dissipated by the transistor under steady-state conditions. The position of the operating point of the transistor amplifier determines whether the transistor is inherently stable against thermal runaway or not. There are two conditions for thermal stability of transistors.
• For operating point with collector-to-emitter voltage (V_CEQ) less than V_CC/2, the circuit is thermally stable as increase in collector current results in reduced power generation. • In the case of operating points having V_CEQ > V_CC/2, the circuit is not inherently thermally stable. Refer to emitter-bias circuit in figure below. If V_CEQ > V_CC/2, then for such circuits to be thermally stable they should satisfy the following condition

Where,
V_CC is the supply voltage I_C the collector current R_E the emitter resistor R_C the collector resistor S_ICO the stability factor I_CO the leakage current Θ the thermal resistance of the transistor

Emitter-bias circuit