Theory - 35 :- Thermal runaway. Static and Dynamic characteristics.

 In a transistor since there are two PN junctions there are three voltage parameters VBE , VBC,  VCE,  three current parameters IB, IE, IC   shown in Fig 2.

Any change in any one parameter causes changes in all the other parameters. Hence it is not very easy to correlate the effect of one parameter  with the others. To have a clear understanding of their relationship a minimum of two characteristics graphs should be plotted for any transistor. They are,

• lnput characteristics
• Output characteristics

lnput characteristics                

For simplicity in emitter amplifier. shown in fig. understanding, consider a common emitter amplifier Circuit The two characteristics graphs are

• The output voltage VCE is maintained constant and the input voltage VBE is set at severalv convenient levels .For each level of input voltage, the input current IB is recorded.  
• IB is then plotted versus VBE to give the common‐emitter   input characteristics..vThe graph shows the relationship between the input voltage VBE and input current IB  for different values
  
  
  

Output characteristics  

• The Base current IB is held constant at each of several fixed levels. For each fixed value ofv IB ,  the output voltage VCE is adjusted in convenient steps and the corresponding levels of collector current IC are recorded.
• For each fixed value of IB,   IC level is Recorded at each VCE  step.For each IB  level, IC isv plotted versus VCE to give a family of characteristics.

Behavior of IC   for different values of VCE is explained 

When VcE is 0, the collector‐base diode is not reverse biased. Therefore, the collector current is negligibly small For V,, between 0.7V and 1 V, the collector diode gets reverse‐biased. 

Once reverse biased, the collector gathers all the electrons that reach its depletion layer. Hence the collector current rises sharply and then becomes almost constant

Above the knee voltage and below the break down voltage, the collector current does not rise steeply or the current is almost constant even if the value of VCE is increased. Thus the transistor works like a controlled constant current source in this region.

Thermal Runaway and the use of Heat sink in BJT                  

The maximum average power in which a transistor can dissipate depends upon the construction of transistor and lie in the range of few milliwatts and 200W. The maximum power is limited by the temperature that the collector Base junction can withstand. The maximum power dissipation is usually specified for the transistor enclosure is 25 degree Celsius. The junction temperature may increase either because of rise in ambient temperature or because of self heating. The problem of self heating arises due to dissipation of power at the collector junction.  

The leakage current Icbo is extremely temperature dependent and increases with the rise in temperature of collector‐base junction. With the increase in collector current Ic, collector power dissipation increases which raises the junction temperature that leads to further increase in collector current Ic. The process is cumulative and may lead to the eventual destruction of transistor. This phenomenon is known as THERMAL RUNAWAY of transistor. In practice the Thermal Runaway can be prevented by a well‐designed circuit called as STABILIZATION Circuitry.

When the power   dissipation at the collector‐base junction of a transistor is small, as in case of a small signal transistor, the surface area of the transistor  case is normally large enough to allow all of the heat to escape. But for the large power dissipation that can occur in high power transistor the transistor surface area is not enough and junction temperature may rise to a dangerous level. However, power handling capacity of a transistor can be increased by making suitable provision for rapid conduction of heat away from the transistor junction. This is achieved by selecting a sheet of metal called the HEAT SINK which increases the area of contact

Heat sink for transistors

The temperature sensitivity of transistors is its great disadvantage. If the temperature of the transistor Increases, the transistor no more behaves in the normal way. Hence heat‐sinks are provided for medium power and high power transistors to avoid the transistor's case temperature rising high. The job of a heat sink is to radiate the excessive heat of transistor case into air thereby holding the case temperature at ambient (room temperature).

 Fig   A few popular types of heat‐sink for transistors are shown