Power Bipolar Junction

Transistor (BJT)
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A junction transistor consists of a
semiconductor crystal in which a p type region
is sandwiched between two n type regions.
This is called an n-p-n transistor. Alternatively
an n type region may be placed in between two
p type regions to give a p-n-p transistor.
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Power Bipolar Junction Transistor (BJT) is the
first semiconductor device to allow full control
over its Turn on and Turn off operations.
BJT was the first semiconductor device to closely
approximate an ideal fully controlled Power
switch.
Subsequently, many other devices that can
broadly be classified as Transistors have been
developed
Many of them have superior performance
compared to the BJT in some respects. They
have, by now, almost completely replaced BJTs
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BJT is a bipolar current controlled,
unidirectional device.

The construction and operating characteristics
of a Power BJT differs significantly from its
signal level counterpart (due to the
requirement for a large blocking voltage in the
OFF state and a high current carrying
capacity in the ON state)
If no external biasing voltages are applied all
transistor currents must be zero

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Power BJT operates in saturation and cut off region

When both B-E & C-B junction of a BJT are reverse
biased it is said to be in the cut off region.

When both B-E & C-B junction of a BJT are forward
biased it is said to be in the saturation region.

Power transistors have design requirements of high off
state blocking voltage and high on state current density




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A power BJT has a vertically oriented alternating layers
of n type and p type semiconductor materials. The
vertical structure is preferred for power transistors
because it maximizes the cross sectional area through
which the on state current flows. Thus, on state resistance
and power loss is minimized.

Practical Power transistors have their emitters and bases
interleaved as narrow fingers. This is necessary to
prevent current crowding and consequent second
break down. In addition multiple emitter structure also
reduces parasitic ohmic resistance in the base current
path.


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Doping density of the emitter of a Power BJT is
several orders of magnitude higher than the base
doping density.
b) Collector drift region is introduced in a Power
BJT to block high reverse voltage.
c) Doping density of the base region in a power BJT
is moderate .
d) Power BJT has low DC current gain compared to
signal level transistors.
e) In a Power BJT multiple, narrow finger like
distributed emitter structure is used to avoid emitter
current crowding


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ADVANTAGES OF BJTs

Have high switching frequencies.
Turn-on losses are small.
Controlled turn-on & turn-off characteristics.
No commutation circuit required.

DISADVANTAGES

Drive circuit is complex.
Has the problem of charge storage.
Has the problem of second breakdown.
Cannot be used in parallel due to
problems of negative temperature
coefficients

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A transistor can be driven into saturation by increasing the
base current for a given collector current.

In saturation the V
CE
voltage drop of a transistor is very
low.

For power application normally, n-p-n type transistor in the
common emitter configuration with the base as the control
terminal is used.



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The safe operating area of a power transistor specifies the
safe-operating limits of collector current versus collector
emitter voltage.

For reliable operation of the power transistor, the collector
current and voltage must always lie within this area.
Operating restrictions applicable to a power transistor
under forward and reverse bias conditions are represented
compactly in Forward Biased Safe Operating Area
( FBSOA)& Reverse Biased Safe Operating Area (RBSOA)
diagrams respectively.
Switching of Power transistors from ON
(saturation) to OFF (cut-off) state involves
considerable redistribution of minority carriers.
Therefore, switching operation is not
instantaneous.

Switching characteristics of a power transistor is
greatly influenced by the external load circuit and
the base drive circuit.

Energy loss takes place during each switching
operation of a power transistor.

Energy loss taking place during ON condition of
the transistor is called the conduction loss.

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Switching power loss is proportional to the
switching frequency while the conduction power
loss is proportional to the duty cycle.

A Power transistor is suitable for large current
switching at low to moderate (a few kHZ)
frequency.

Switching aid circuits (snubbers) are used for
enhancing the capacity utilization of a power
transistor. They also reduce switching loss internal
to the device.


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Base Drive Circuit Requirements

The performance of a Power transistor depends largely on the
base drive design.

Turn on
The rate of rise of base current in the beginning of the turn on
process determines the turn on delay time.
The magnitude of the base current during turn on decides the
values of the voltage fall time, current rise time and V
CE
(sat) for
a given collector current.

Turn off
The negative base current during turn off determines the
storage time, voltage rise time and current fall time.
A negative bias at the base also enhances the voltage
withstanding capacity of a power transistor.
The base drive voltage source should be bipolar and the base
drive resistance should be different during turn on and turn off
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