Power BJT is the power level counterpart of the normal signal level BJT. Power BJT, as already mentioned in the earlier articles, is only used as the Switch in power electronics circuits.
Construction of power BJT
Power BJT is the four-layer structure which differs from the signal level BJT which normally has three layers Emitter, Base and Collector (as seen in the previous article).
- As seen in the above figure power BJT has n+pn–n+ layers
- Two n+ layers act as Emitter and Collector Regions
- Layer p acts as Base and
- The extra n– layer is the Drift Region of the BJT. This drift region exactly performs the same function as in case of power diode.
Symbol for Power BJT
The symbol for Power BJT is same as we use for the normal signal-level BJT.
Rating of the power BJT
- voltage rating – upto 1200V
- current rating – upto 400A
- frequency rating – upto 10KHz
We use the Power BJT mainly for the switch on and switch off i.e. for the switching process.
Operation of Power BJT
Power BJT operates in the same fashion as the normal BJT as given below
For the Active Region:
- Emitter supplies the majority carriers.
- These majority carriers when passing through the lightly doped base then some of the carriers are neutralized.
- The remaining carriers are the collected by the reversed biased collector (actually collector-base junction).
This is the operation of BJT in active region which is used for the amplification purpose. But we do not the power BJT in the Active Region because we only need to use it as a Switch hence it should be operated in the Saturation and the Cut-off Region.
Steady State Characteristics of Power BJT: Output V-I Characteristics
Steady-state Characteristics or the output V-I characteristics of a power BJT is the graph representing the relationship between the output collector-emitter voltage i.e. VCE and the output collector current i.e. ic (Note that here we assumed the Common Emitter configuration of the BJT).
Following important facts about the power BJT can be deduced from the above output characteristics;
- First of all, we should remember that only the Saturation Region (as shown in the above characteristics) is important for us.
- Since, the BJT is a current controlled device hence the Base current IB controls the output voltage VCE hence the graph is plotted for different values of I
- What is meant by BVSUS, BVCEO and BVCBO in the steady state characteristics?
- BVSUS is the maximum value of the collector-emitter voltage (VCE) at which the breakdown of the BJT occurs.
- BVCEO is the collector-emitter breakdown voltage when the base is kept open circuited. BVCEO is defined for the Common Emitter configuration.
- BVCBO is the collector-base breakdown voltage when the emitter is kept open circuited. BVCBO is defined for the Common Emitter configuration.
Saturation Region in the Characteristics
If we compare the output V-I characteristics of the normal BJT with the above characteristics of the power BJT, we notice that the only difference between the two characteristics is the Quasi Saturation Region.
- When both the junctions of a BJT are forward biased then the BJT enters into the Saturation Region.
- Assume that the BJT is in Active Region initially then it enters into the Saturation. And when the Collector-Base Junction becomes forward region (due to the saturation) then holes from the base (majority carriers in base) diffuse into the Drift Region because the collector-base junction is now forward biased.
- Due to this, a familiar phenomenon occurs which is known as Double Injection into the Drift Region (refer to the article on Power Diode for the cause of Double Injection).
- Because of the Double Injection excess charge carriers get stored in the Drift Region which causes the hinderance to the fast Saturation hence we get a region known as Quasi saturation region. Quasi means ‘apparent’ i.e. the process occurs so slowly that we do not observe the saturation to take place.
Finally, the power BJT gets turned-on and finished with the important physical processes that place when a BJT is about to turn-on.