Q. What is meant by Thermal Runaway? Explain with reference to a transistor.How is it avoided in circuits?
The collector current of a transistor increases with increasing temperature.The characteristics of a transistor are thus temperature dependent.At high temperatures(150-200 degree Celsius),the semiconductor material degenerates and hence their performance is unpredictable.The following characteristics of transistor are temperature dependent:emitter junction resistance,beta,base-emitter voltage and leakage current.
The leakage current,an important transistor parameter,increases exponentially with temperarture and can set a limit to the operating temperature of a transistor.Increasing leakage current results in increasing collector current,and also increasing collector junction temperature,with a still further increase in leakage current.Thus a vicious cycle is set up,which if not kept under check,will result in the transistor being permanently damaged.It is referred to as'Thermal Runaway'.
Even in absence of emitter current,a minute collector current flows through the transistor.Denoted by Icbo,this current represents the current between collector and base with an open emitter.Icbo doubles for every 10 degree Celsius rise in temperature.At room temperature,the leakage current in silicon transistors is far less than that in germanium transistors.It is part of the total collector current and is also unpredictable.An increase in leakage current brings about a corresponding increase in the collector current,resulting in a shift of the operating point.Special biasing techniques keep the shift in the operating point stabilized.
Even in the absence of base current,a minute current flows through the collector of a transistor.Denoted by Iceo,this current represents the current between collector and emitter with an open base.Iceo is also temperature dependent.Special biasing techniques are employed to swamp the changes in Iceo.
T = Tj - Ta= θPd.
where,
T = temperature rise in collector junction.
Tj= junction temperature.
Ta= ambient temperature.
Theta = constant of proportionality called thermal resistance which depends on size of transistor.