Power BJT and Power MOSFET

Power Bipolar Transistors
& Power MOSFETS
Dr.S.Periyanayagi,
Professor & Head,
Department of ECE,
Ramco Institute of Technology.

Power BJT
• Power transistors - various transistor limitations.
• The limitations involve
– maximum rated current (on the order of amperes)
– maximum rated voltage (on the order of 100 V)
– maximum rated power (on the order of watts or tens of watts)

Vertical Power Transistor Structure
Picture Taken from the ebook ‘Semiconductor Physics and Devices ‘, Donald A Neamen and Dhrubes Biswas

• Figure shows the structure of a vertical NPN power transistor.
• However, with small switching devices, the collector terminal is
still formed at the surface.
• In the vertical configuration for the power bipolar transistor, the
collector terminal is at the “bottom” of the device.
• This configuration is preferred, since it maximizes the cross-
sectional area through which current is flowing in the device.
• The doping concentrations and dimensions are not the same as in
small switching transistors.
• The primary collector region has a low-doped impurity
concentration so that a large base–collector voltage can be
applied without initiating breakdown.
• Another n region, with a higher doping concentration, reduces
collector resistance and makes contact with the external collector
terminal.

• The base region is also much wider than normally encountered in
small devices.
• A large base–collector voltage implies a relatively large space
charge width being induced in both the collector and base
regions.
• A relatively large base width is required to prevent punch-
through breakdown.
• Power transistors must also be large-area devices in order to
handle large currents.
• The interdigitated structure is shown in Figure 15.11. Relatively
small emitter widths are required to prevent the emitter current
crowding effects.

Power Transistor Characteristics
• The relatively wide base width implies a much smaller current gain
for power transistors compared to small switching transistors
• large area device implies a larger junction capacitance and hence
lower cutoff frequency for a power transistor compared to a small
switching transistor, general-purpose small-signal BJT to those of
two power BJTs.
• The current gain is generally smaller in the power transistors,
typically in the range of 20 to 100, and may be a strong function of
collector current and temperature.
• Figure 15.12 shows typical current gain versus collector current
characteristics for the 2N3055 power BJT at various temperatures

• The maximum rated collector current IC,max may be related to the
maximum current that the wires connecting the semiconductor to the
external terminals can handle, the collector current at which the
current gain falls below a minimum specified value, or the current
that leads to the maximum power dissipation when the transistor is
biased in saturation.
• The maximum rated voltage in a BJT is generally associated with
avalanche breakdown in the reverse-biased base–collector junction. In
the common-emitter configuration, the breakdown voltage
mechanism also involves the transistor gain, as well as the breakdown
phenomenon in the PN junction.

• When the transistor is biased in the forward-active mode, the
collector current begins to increase significantly before the actual
breakdown voltage is reached.
• All the curves tend to merge to the same collector–emitter voltage
once breakdown has occurred.
• This voltage, VCE, sus, is the minimum voltage necessary to sustain
the transistor in breakdown.
• Another breakdown effect is called second breakdown, which occurs
in a BJT operating at high voltage and high current.
• Slight non-uniformities in current density produce local regions of
increased heating that increases the minority carrier concentrations in
the semiconductor material, which in turn increases the current in
these regions.

• This effect results in positive feedback, and the current continues to
increase, producing a further increase in temperature, until the
semiconductor material may actually melt, creating a short circuit
between the collector and emitter.
• The average power dissipated in a BJT must be kept below a
specified maximum value, to ensure that the temperature of the device
remains below a maximum value.
• If we assume the collector current and collector–emitter voltage are
dc values, then at the maximum rated power PT for the transistor,
cCET IVP 

• The maximum current, voltage, and power limitations can be
illustrated on the
• IC versus VCE characteristics as shown in Figure 15.14. The average
power limitation, PT, is a hyperbola described by Equation (15.6).
• The region where the transistor can be operated safely is known as the
safe operating area (SOA) and is bounded by IC,max, V CE, sus, PT, and
the transistor’s second breakdown characteristic curve.
• Figure 15.14a shows the safe operating area using linear current and
voltage scales.
• Figure 15.14b shows the same characteristics using log scales.

As mentioned, the base width of a power BJT is relatively wide so that
the current
gain is then relatively small. One method that is used to increase the
effective current
gain is to use a Darlington pair such as shown in Figure 15.17

Power MOSFETs
• Basic operation of the power MOSFET is the same as that of any
MOSFET.
• The current handling capability of these devices is usually in the
ampere range, and the drain-to-source blocking voltage may be in
the range of 50 to 100 volts or even higher.
• One big advantage that a power MOSFET has over a bipolar power
device is that the control signal is applied to the gate whose input
impedance is extremely large.
• Even during switching between on and off states, the gate current is
small,
• so large currents can be switched with very small control currents.
• Large currents can be obtained in a MOSFET with a very large
channel width. To achieve a large channel width device with good

• There are two basic power MOSFET structures.
• The first is called a DMOS device and is shown in Figure 15.20. The
DMOS device uses a double diffusion process:
• The p-base or the p-substrate region and the n source contact are
diffused through a common window defined by the edge of the gate.
• The p-base region is diffused deeper than the n source, and the
difference in the lateral diffusion distance between the p-base and the
n source defines the surface channel length.
• Electrons enter the source terminal and flow laterally through the
inversion layer under the gate to the n-drift region.

• The electrons then flow vertically through then-drift region to the
drain terminal. The conventional current direction is from the drain to
the source.
• The n-drift region must be moderately doped so that the drain
breakdown voltage is sufficiently large. However, the thickness of the
n-drift region should also be as thin as possible to minimize drain
resistance.
• The second power MOSFET structure, shown in Figure 15.21, is a
VMOS structure.
• The vertical channel or VMOS power device is a nonplanar structure
that requires a different type of fabrication process. In this case, a p-
base or p-“substrate” diffusion is performed over the entire surface
followed by the n+ source diffusion.

• A V-shaped groove is then formed, extending through the n-drift
region.
• It has been found that certain chemical solutions etch the (111) planes
in silicon at a much slower rate than the other planes.
• If (100) oriented silicon is etched through a window at the surface,
these chemical etches will create a V-shaped groove.
• A gate oxide is then grown in the V-shaped groove and the metal gate
material is deposited.
• An electron inversion layer is formed in the base or substrate so that
current is again essentially a vertical current between the source and
the drain.
• The relatively low-doped n-drift region supports the drain voltage
since the depletion region extends mainly into this low-doped region.

• Many individual MOSFET cells are connected in parallel to fabricate
a power MOSFET with the proper width-to-length ratio.
• Figure 15.22 shows a HEXFET structure. Each cell is a DMOS
device with an n polysilicon gate.
• The HEXFET has a very high packing density-it may be on the order
of 105 cells per cm2 .
• In the VMOS structure, the anisotropic etching of the grooves must
be along the [110] direction on the (100) surface.
• This constraint limits the design options available for this type of
device.

A HEXFET structure

Power Mosfet Characteristics
• The drain currents are in the ampere range and the breakdown
voltages are in the hundreds of volts range.
• An important parameter of a power MOSFET is the on resistance
• where RS is the resistance associated with the source contact, RCH is
the channel resistance, and RD is the resistance associated with the
drain contact.
• The RS and RD resistance values are not necessarily negligible in
power MOSFETs since small resistances and high currents can
produce considerable power dissipation

Picture Taken from the ebook ‘Semiconductor Physics
and Devices ‘, Donald A Neamen and Dhrubes Biswas

• We have noted in previous chapters that mobility decreases with
increasing temperature.
• The threshold voltage varies only slightly with temperature so that, as
current in a device increases and produces additional power
dissipation, the temperature of the device increases, the carrier
mobility decreases, and RCH increases, which inherently limits the
channel current.
• The resistances RS and RD are proportional to semiconductor
resistivity and so are also inversely proportional to mobility and have
the same temperature characteristics as RCH.
• Figure 15.23 shows a typical “on-resistance” characteristic as a
function of drain current.

• The increase in resistance with temperature provides stability for the
power MOSFET.
• If the current in any particular cell begins to increase, the resulting
temperature rise will increase the resistance, thus limiting the current.
• With this particular characteristic, the total current in a power MOSFET
tends to be evenly distributed among the parallel cells, not concentrated
in any single cell, a condition that can cause burnout.

• Power MOSFETs differ from bipolar power transistors in both operating
principles and performance.
• The superior performance characteristics of power MOSFETs are faster
switching times, no second breakdown, and stable gain and response time
over a wide temperature range.
• Figure 15.24a shows the transconductance of the 2N6757 versus
temperature.
• The variation with temperature of the MOSFET trans conductance is less
than the variation in the BJT current gain that is shown in Figure 15.12.
• Figure 15.24b is a plot of drain current versus gate-to-source voltage at
three different temperatures.
• At high current, the current decreases with temperature at a constant gate-
to-source voltage, providing the stability that has been discussed.

• Power MOSFETs must operate in a SOA. As with power BJTs, the
SOA is defined by three factors: the maximum drain current, ID,max,
rated breakdown voltage, BVDSS, and the maximum power dissipation
given by
PT =VDS ID
• The SOA is shown in Figure 15.25a in which the current and voltage
are plotted on linear scales.
• The same SOA curve is shown in Figure 15.25b in which the current
and voltage are plotted on log scales.

Reference
• Donald A Neamen,‘Semiconductor Physics and Devices’, Basic
Principles , Fourth Edition.

Power BJT and Power MOSFET

More Related Content

What's hot (20)

Similar to Power BJT and Power MOSFET (20)

More from PeriyanayagiS (7)

Recently uploaded (20)

Power BJT and Power MOSFET