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Power Transistors
Lecture Note 5
The switching speed of modern transistors is much higher than that of Thyristors and are used extensively in dcdc and dc-ac converters. However, their voltage and current ratings are lower than those of thyristors and are
therefore used in low to medium power applications.
These devices are used a switching devices and are operated in the saturation region resulting in low
on-state voltage drop.
The transistor remains on so long as the control signal is present.
power BJT
Low power BJT
power MOSFET
power BJT
power BJT Vs low Power BJT
vertically oriented fourlayer structure of
alternating p-type and ntype.
maximising the cross-section area results in
increase current rating of BJT
minimize the on-state resistance
reduce the power losses
power BJT
power BJT Vs low Power BJT
doping of emitter layer
and collector layer is
quite large typically
A special layer called the collector drift region
(n-) has a light doping level
The thickness of the drift region determines the breakdown
voltage of the transistor
Steady State Characteristics
similar to signal level transistors
except that the V-I c/cs has
Cutoff region, Active region, quasi saturation and hard saturation.
Cutoff Region:
The C–B and B–E junctions
are reverse-biased
if VBE<0.7V IB=0A
IC= 0
Steady State Characteristics
if VBE>0.7V
Since the potential difference between C and E is zero
both junctions are forward-biased
Steady State Characteristics
if VBE>0.7V
Since the potential at C > at B
the collector–base junction is reversed-biased and the
base–emitter junction is forward-biased.
is the maximum collector to emitter voltage that can be
sustained when BJT is carrying substantial collector current.
is the collector base breakdown voltage when the emitter
is open circuited
is the maximum collector to emitter breakdown voltage that
can be sustained when base current is zero
The transistor is used as a switch therefore it
is used only between saturation and cutoff.
These values are
adjusted in
away to make
the BJT works as
a switch
→ IC↑ and VCE falls below VBE.
This continues until Collector-Base junction is forward biased
and the BJT goes into saturation region.
A high value of ODF cannot reduce the CE voltage significantly.
VBE increases due to increase base current resulting in increased power loss.
Once the transistor is saturated, the CE voltage is not reduced in relation to increase in base current.
If the transistor is under driven IB to IBS it may operate in active region, VCE increases resulting in increased power loss.
Diode Provide an alternate path a
current through the device cannot rise fast to the saturating
level of ICS since the inductive nature of the coil opposes any
change in current through it.
For full ON of BJT, Ic reach s
to steady state value the coil
acts as a short circuit since
VL=0, where VL=Ldi/dt
What are the states of the diode during Pulses applied?
once input pulse drops to
zero, the current IC does not
fall to zero immediately since
inductor will now act as a
current source.
• Due to internal capacitances the collector current does
not respond immediately.
• The delay is due to the time required to charge up the BEJ
to the forward bias voltage VBE(0.7V).
The base current is normally more than that required to saturate the transistor.
the higher the ODF, the greater is the amount of extra charge stored in the base.
When the input voltage is reversed from V1 to -V2, the
reverse current –IB2 helps to discharge the base.
Without –IB2 the saturating charge has to be removed entirely
due to the storage time ts would be longer.
Once the extra charge is removed, BEJ charges to the
input voltage –V2 and the base current falls to zero.
tf depends on the time constant which is
determined by the reverse biased BEJ capacitance.
the BJT is a positive temperature coefficient device. As
they get warm, hFe increases, causing more current
flow. This can lead to thermal runaway. That is why
most class A common emitter configurations use an
emitter resistor to place limits on the hFe demand,
eliminating thermal drift and runaway.
A slab of p-type
material is formed
and two n-regions
are formed in the
The source and drain
terminals are connected
through metallic contacts to
n-doped regions, but the
absence of a channel
between the doped nregions.
The source and drain are connected n+ regions.
These regions are heavily doped.
The gate is not
directly connected to
the P-type region.
There is insulating
oxide (Si02) layer
between gate metal
and p-type layer.
The p-type body
region forms the
channel between
drain and source.
What happens if we increase VDS while VGS=0V in MOSFET?
Effect of VGS in the formation of Inversion layer in MOSFET
Increase VDS by keeping VGS as constant
Increase VGS; keep VDS constant
For little VDS, the drain current increases linearly
with VDS, this is called ohmic or linear region.
•Additional increase of VDS causes the pinch off point to
shift towards the source, dropping the effective length of
channel. This effect is called channel length modulation.
Volt-Ampere Characteristics of Power MOSFET
Alternative Solution
Insulated Gate Bipolar Transistor (IGBT)
 The Insulated Gate Bipolar Transistor (IGBT) is a minority-carrier device with
high input impedance and large bipolar current-carrying capability.
 Many designers view IGBT as a device with MOS input characteristics and
bipolar output characteristic that is a voltage-controlled bipolar device.
 It combines the best attributes of both Power MOSFET and BJT devices to
achieve optimal device characteristics.