Power MOSFET Basics
Vrej Barkhordarian,International Rectifier,El Segundo,Ca.
Discrete power MOSFETs
employ semiconductor
processing techniques that are
similar to those of today's VLSI
circuits,although the device
geometry,voltage and current
levels are significantly different
from the design used in VLSI
devices,The metal oxide
semiconductor field effect
transistor (MOSFET) is based
on the original field-effect
transistor introduced in the
70s,Figure 1 shows the
device schematic,transfer
characteristics and device
symbol for a MOSFET,The
invention of the power
MOSFET was partly driven by
the limitations of bipolar power
junction transistors (BJTs)
which,until recently,was the
device of choice in power
electronics applications.
Although it is not possible to
define absolutely the operating
boundaries of a power device,
we will loosely refer to the
power device as any device
that can switch at least 1A.
The bipolar power transistor is
a current controlled device,A
large base drive current as
high as one-fifth of the
collector current is required to
keep the device in the ON
state.
Also,higher reverse base drive
currents are required to obtain
fast turn-off,Despite the very advanced state of manufacturability and lower costs of BJTs,these
limitations have made the base drive circuit design more complicated and hence more expensive than the
power MOSFET.
Source
Contact
Field
Oxide
Gate
Oxide
Gate
Metallization
Drain
Contact
n* Drain
p-Substrate
Channel
n* Source
t
ox
l
V
GS
V
T
0
0
I
D
(a)
(b)
I
D
D
SB
(Channel or Substrate)
S
G
(c)
Figure 1,Power MOSFET (a) Schematic,(b) Transfer Characteristics,(c)
Device Symbol.
Another BJT limitation is that both electrons and holes
contribute to conduction,Presence of holes with their higher
carrier lifetime causes the switching speed to be several orders of
magnitude slower than for a power MOSFET of similar size and
voltage rating,Also,BJTs suffer from thermal runaway,Their
forward voltage drop decreases with increasing temperature
causing diversion of current to a single device when several
devices are paralleled,Power MOSFETs,on the other hand,are
majority carrier devices with no minority carrier injection,They
are superior to the BJTs in high frequency applications where
switching power losses are important,Plus,they can withstand
simultaneous application of high current and voltage without
undergoing destructive failure due to second breakdown,Power
MOSFETs can also be paralleled easily because the forward
voltage drop increases with increasing temperature,ensuring an even distribution of current among all
components.
However,at high breakdown voltages (>200V) the on-state voltage drop of the power MOSFET becomes
higher than that of a similar size bipolar device with similar voltage rating,This makes it more attractive
to use the bipolar power transistor at the expense of worse high frequency performance,Figure 2 shows
the present current-voltage limitations of power MOSFETs and BJTs,Over time,new materials,
structures and processing techniques are expected to raise these limits.
2000
1500
1000
500
0
1 10 100 1000
Maximum Current (A)
Holdoff Voltage (V)
Bipolar
Transistors
MOS
Figure 2,Current-Voltage
Limitations of MOSFETs and BJTs.
Drain
Metallization
Drain
n
+
Substrate
(100)
n
-
Epi Layer
Channels
n
+
pn
+
p
+
Body Region p
+
Drift Region
G
S
D
Source
Gate
Oxide
Polysilicon
Gate
Source
Metallization
Figure 3,Schematic Diagram for an n-Channel Power MOSFET and the Device.
Figure 3 shows schematic diagram and Figure 4 shows the physical origin of the parasitic components in
an n-channel power MOSFET,The parasitic JFET appearing between the two body implants restricts
current flow when the depletion widths of the two adjacent body diodes extend into the drift region with
increasing drain voltage,The parasitic BJT can make the device susceptible to unwanted device turn-on
and premature breakdown,The base resistance RB must be minimized through careful design of the
doping and distance under the source region,There are several parasitic capacitances associated with
the power MOSFET as shown in Figure 3.
C
GS
is the capacitance due to the overlap of the source and the channel regions by the polysilicon gate
and is independent of applied voltage,C
GD
consists of two parts,the first is the capacitance associated
with the overlap of the polysilicon gate and the silicon underneath in the JFET region,The second part is
the capacitance associated with the depletion region immediately under the gate,C
GD
is a nonlinear
function of voltage,Finally,C
DS
,the capacitance associated with the body-drift diode,varies inversely
with the square root of the drain-source bias,There are currently two designs of power MOSFETs,usually
referred to as the planar and the trench designs,The planar design has already been introduced in the
schematic of Figure 3,Two variations of the trench power MOSFET are shown Figure 5,The trench
technology has the advantage of higher cell density but is more difficult to manufacture than the planar
device.
Metal
C
GS2
C
gsm
LTO
C
GD
R
Ch
C
GS1
R
B
BJT
n
-
p
-
C
DS
JFET
R
EPI
n
-
n
-
Epi Layer
n
-
Substrate
Figure 4,Power MOSFET Parasitic Components.
BREAKDOWN VOLTAGE
Breakdown voltage,
BV
DSS
,is the voltage at
which the reverse-biased
body-drift diode breaks
down and significant
current starts to flow
between the source and
drain by the avalanche
multiplication process,
while the gate and
source are shorted
together,Current-voltage
characteristics of a
power MOSFET are
shown in Figure 6.
BVDSS is normally
measured at 250μA drain
current,For drain
voltages below BV
DSS
and with no bias on the
gate,no channel is
formed under the gate at
the surface and the drain
voltage is entirely
supported by the
reverse-biased body-drift
p-n junction,Two related
phenomena can occur in
poorly designed and
processed devices:
punch-through and
reach-through,Punch-
through is observed
when the depletion
region on the source side
of the body-drift p-n
junction reaches the
source region at drain
voltages below the rated
avalanche voltage of the
device,This provides a
current path between
source and drain and
causes a soft breakdown
characteristics as shown
in Figure 7,The leakage
current flowing between
source and drain is denoted by I
DSS
,There are tradeoffs to be made between R
DS(on)
that requires shorter
channel lengths and punch-through avoidance that requires longer channel lengths.
The reach-through phenomenon occurs when the depletion region on the drift side of the body-drift p-n
junction reaches the epilayer-substrate interface before avalanching takes place in the epi,Once the
depletion edge enters the high carrier concentration substrate,a further increase in drain voltage will
cause the electric field to quickly reach the critical value of 2x10
5
V/cm where avalanching begins.
Source
Gate
Source
Gate
Oxide
Channel
Oxide
n
-
Epi Layer
n
+
Substrate
(100)
Drain
(b)
G SS
Electron Flow
D
(a)
Figure 5,Trench MOSFET (a) Current Crowding in V-Groove Trench MOSFET,
(b) Truncated V-Groove MOSFET
ON-RESISTANCE
The on-state resistance of a power MOSFET is made up of several components as shown in Figure 8:
(1)
where:
R
source
= Source diffusion resistance
R
ch
= Channel resistance
R
A
= Accumulation resistance
R
J
= "JFET" component-resistance of the
region between the two body regions
R
D
= Drift region resistance
R
sub
= Substrate resistance
Wafers with substrate resistivities of up to
20m?-cm are used for high voltage
devices and less than 5m?-cm for low
voltage devices.
R
wcml
= Sum of Bond Wire resistance,the
Contact resistance between the source
and drain Metallization and the silicon,
metallization and Leadframe
contributions,These are normally
negligible in high voltage devices but can
become significant in low voltage devices.
Figure 9 shows the relative importance of
each of the components to R
DS(on)
over the
voltage spectrum,As can be seen,at high
voltages the R
DS(on)
is dominated by epi
resistance and JFET component,This
component is higher in high voltage
devices due to the higher resistivity or
lower background carrier concentration in
the epi,At lower voltages,the R
DS(on)
is
dominated by the channel resistance and
the contributions from the metal to
semiconductor contact,metallization,
bond wires and leadframe,The substrate contribution becomes more significant for lower breakdown
voltage devices.
TRANSCONDUCTANCE
Transconductance,gfs,is a measure of the sensitivity of drain current to changes in gate-source bias.
This parameter is normally quoted for a V
gs
that gives a drain current equal to about one half of the
maximum current rating value and for a VDS that ensures operation in the constant current region.
Transconductance is influenced by gate width,which increases in proportion to the active area as cell
density increases,Cell density has increased over the years from around half a million per square inch in
1980 to around eight million for planar MOSFETs and around 12 million for the trench technology,The
limiting factor for even higher cell densities is the photolithography process control and resolution that
allows contacts to be made to the source metallization in the center of the cells.
RRRRRRRR
DS(on source ch A J D sub wcml)
=++++++
Gate
Voltage
7
6
5
4
I
DS
VS V
DS
LOCUS
3
2
1
0 5 10 15
0
5
10
15
20
25
(Saturation
Region)Linear Region
Normalized Drain Current
Drain Voltage (Volts)
Figure 6,Current-Voltage Characteristics of Power MOSFET
Channel length also affects transconductance,Reduced
channel length is beneficial to both gfs and on-resistance,
with punch-through as a tradeoff,The lower limit of this
length is set by the ability to control the double-diffusion
process and is around 1-2mm today,Finally the lower the
gate oxide thickness the higher gfs.
THRESHOLD VOLTAGE
Threshold voltage,V
th
,is defined as the minimum gate
electrode bias required to strongly invert the surface
under the poly and form a conducting channel between
the source and the drain regions,V
th
is usually measured
at a drain-source current of 250μA,Common values are
2-4V for high voltage devices with thicker gate oxides,and
1-2V for lower voltage,logic-compatible devices with
thinner gate oxides,With power MOSFETs finding increasing use in portable electronics and wireless
communications where battery power is at a premium,the trend is toward lower values of RDS(on) and
Vth.
DIODE FORWARD VOLTAGE
The diode forward voltage,VF,is the
guaranteed maximum forward drop of
the body-drain diode at a specified
value of source current,Figure 10
shows a typical I-V characteristics for
this diode at two temperatures,P-
channel devices have a higher VF due
to the higher contact resistance
between metal and p-silicon
compared with n-type silicon.
Maximum values of 1.6V for high
voltage devices (>100V) and 1.0V for
low voltage devices (<100V) are
common.
POWER DISSIPATION
The maximum allowable power
dissipation that will raise the die
temperature to the maximum
allowable when the case temperature
is held at 25
0
C is important,It is give
by Pd where:
T
jmax
= Maximum allowable temperature of the p-n junction in the device (normally 150
0
C or 175
0
C) R
thJC
= Junction-to-case thermal impedance of the device.
DYNAMIC CHARACTERISTICS
Sharp
Soft
I
D
BV
DSS
V
DS
Figure 7,Power MOSFET Breakdown
Characteristics
N+
P-BASER
SOURCE
R
CH
R
A
R
J
R
D
R
SUB
N+ SUBSTRATE
SOURCE
GATE
DRAIN
Figure 8,Origin of Internal Resistance in a Power MOSFET.
P
d
T
j
R
thJC
=
-max 25
(2)
When the MOSFET is used as a switch,its basic function is to control the drain current by the gate
voltage,Figure 11(a) shows the transfer characteristics and Figure 11(b) is an equivalent circuit model
often used for the analysis of MOSFET switching performance.
The switching performance of a device is determined by the time required to establish voltage changes
across capacitances,R
G
is the distributed resistance of the gate and is approximately inversely
proportional to active area,L
S
and L
D
are source and drain lead inductances and are around a few tens of
nH,Typical values of input (C
iss
),output (C
oss
) and reverse transfer (C
rss
) capacitances given in the data
sheets are used by circuit designers as a starting point in determining circuit component values,The data
sheet capacitances are defined in terms of the equivalent circuit capacitances as:
50V 100V 500VVoltage Rating:
Packaging
Metallization
Source
Channel
JFET
Region
Expitaxial
Layer
Substrate
R
EPI
R
CH
R
wcml
Figure 9,Relative Contributions to R
DS(on)
With Different Voltage Ratings.
C
iss
= C
GS
+ C
GD
,C
DS
shorted
C
rss
= C
GD
C
oss
= C
DS
+ C
GD
Gate-to-drain capacitance,C
GD
,is a
nonlinear function of voltage and is the most
important parameter because it provides a
feedback loop between the output and the
input of the circuit,C
GD
is also called the
Miller capacitance because it causes the total
dynamic input capacitance to become greater
than the sum of the static capacitances.
Figure 12 shows a typical switching time test
circuit,Also shown are the components of
the rise and fall times with reference to the
V
GS
and V
DS
waveforms.
Turn-on delay,t
d(on)
,is the time taken to
charge the input capacitance of the device
before drain current conduction can start.
Similarly,turn-off delay,t
d(off)
,is the time
taken to discharge the capacitance after the after is switched off.
0.0 0.5 1.0 1.5 2.0 2.5
0.1
1
10
100
T
J
= 150
0
C
T
J
= 25
0
C
VGS = 0V
V
SD
,Source-to-Drain Voltage (V)
I
SD
,Reverse Drain Current (A)
Figure 10,Typical Source-Drain (Body) Diode Forward
Voltage Characteristics.
I
D
V
GS
Slope = g
fs
G
R
G
C
GD
L
D
D
D'
S'
C
DS
L
S
S
C
GS
C I
D
Body-drain
Diode
(a) (b)
Figure 11,Power MOSFET (a) Transfer characteristics,(b) Equivalent Circuit Showing Components That
Have Greatest Effect on Switching
GATE CHARGE
Although input capacitance
values are useful,they do not
provide accurate results when
comparing the switching
performances of two devices
from different manufacturers.
Effects of device size and
transconductance make such
comparisons more difficult,A
more useful parameter from the
circuit design point of view is
the gate charge rather than
capacitance,Most
manufacturers include both
parameters on their data sheets.
Figure 13 shows a typical gate
charge waveform and the test
circuit,When the gate is
connected to the supply voltage,
V
GS
starts to increase until it
reaches V
th
,at which point the
drain current starts to flow and
the C
GS
starts to charge,During
the period t
1
to t
2
,C
GS
continues to charge,the gate
voltage continues to rise and
drain current rises
proportionally,At time t
2
,C
GS
is completely charged and the
drain current reaches the
predetermined current I
D
and
stays constant while the drain
voltage starts to fall,With
reference to the equivalent
circuit model of the MOSFET shown in Figure 13,it can be seen that with C
GS
fully charged at t
2
,V
GS
becomes constant and the drive current starts to charge the Miller capacitance,C
DG
,This continues
until time t
3
.
R
D
-
+
V
DD
V
DS
V
GS
R
G
D.U.T.
-10V
Pulse Width < 1μs
Duty Factor < 0.1%
(a)
Figure 12,Switching Time Test (a) Circuit,(b) VGS and VDS
Waveforms
td(on) tr td(off) tf
V
GS
100%
90%
V
DS
(b)
Charge time for the Miller capacitance is
larger than that for the gate to source
capacitance C
GS
due to the rapidly changing
drain voltage between t
2
and t
3
(current = C
dv/dt),Once both of the capacitances C
GS
and C
GD
are fully charged,gate voltage (V
GS
)
starts increasing again until it reaches the
supply voltage at time t
4
,The gate charge
(Q
GS
+ Q
GD
) corresponding to time t
3
is the
bare minimum charge required to switch
the device on,Good circuit design practice
dictates the use of a higher gate voltage
than the bare minimum required for
switching and therefore the gate charge
used in the calculations is Q
G
corresponding to t
4
.
The advantage of using gate charge is that
the designer can easily calculate the
amount of current required from the drive
circuit to switch the device on in a desired
length of time because Q = CV and I = C
dv/dt,the Q = Time x current,For
example,a device with a gate charge of
20nC can be turned on in 20μsec if 1ma is
supplied to the gate or it can turn on in
20nsec if the gate current is increased to
1A,These simple calculations would not
have been possible with input capacitance
values.
dv/dt CAPABILITY
Peak diode recovery is defined as the
maximum rate of rise of drain-source
voltage allowed,i.e.,dv/dt capability,If this
rate is exceeded then the voltage across the
gate-source terminals may become higher
than the threshold voltage of the device,
forcing the device into current conduction
mode,and under certain conditions a
catastrophic failure may occur,There are two possible mechanisms by which a dv/dt induced turn-on
may take place,Figure 14 shows the equivalent circuit model of a power MOSFET,including the
parasitic BJT,The first mechanism of dv/dt induced turn-on becomes active through the feedback action
of the gate-drain capacitance,CGD,When a voltage ramp appears across the drain and source terminal
of the device a current I
1
flows through the gate resistance,R
G
,by means of the gate-drain capacitance,
C
GD
,R
G
is the total gate resistance in the circuit and the voltage drop across it is given by:
(3)
When the gate voltage V
GS
exceeds the threshold voltage of the device V
th
,the device is forced into
conduction,The dv/dt capability for this mechanism is thus set by:
V
DD
DI
D
D
G
S
C
GS
C
DG
S
I
D
TEST CIRCUIT
(a)
O
GS
O
GD
GATE
VOLTAGE
V
G
V
G(TH)
t
0
t
1
t
2
t
3
t
4
t
DRAIN CURRENT
DRAIN
VOLTAGE
V
DD
I
D
WAVEFORM
(b)
Figure 13,Gate Charge Test (a) Circuit,(b) Resulting Gate
and Drain Waveforms.
VIRRC
dv
dt
GS G G GD
==
1
(4)
It is clear that low V
th
devices are more prone to
dv/dt turn-on,The
negative temperature
coefficient of V
th
is of
special importance in
applications where high
temperature environments
are present,Also gate
circuit impedance has to be
chooses carefully to avoid
this effect.
The second mechanism for
the dv/dt turn-on in
MOSFETs is through the
parasitic BJT as shown in
Figure 15,The capacitance
associated with the
depletion region of the body
diode extending into the
drift region is denoted as
C
DB
and appears between
the base of the BJT and the drain of the MOSFET,This capacitance gives rise to a current I
2
to flow
through the base resistance R
B
when a voltage ramp appears across the drain-source terminals,With
analogy to the first mechanism,the dv/dt capability of this mechanism is:
(5)
If the voltage that develops across R
B
is greater
than about 0.7V,then the base-emitter junction
is forward-biased and the parasitic BJT is
turned on,Under the conditions of high (dv/dt)
and large values of R
B
,the breakdown voltage of
the MOSFET will be limited to that of the open-
base breakdown voltage of the BJT,If the
applied drain voltage is greater than the open-
base breakdown voltage,then the MOSFET will
enter avalanche and may be destroyed if the
current is not limited externally.
Increasing (dv/dt) capability therefore requires
reducing the base resistance R
B
by increasing
the body region doping and reducing the
distance current I
2
has to flow laterally before it
is collected by the source metallization,As in
the first mode,the BJT related dv/dt capability
becomes worse at higher temperatures because
R
B
increases and V
BE
decreases with increasing
temperature.
dv
dt
V
RC
th
G
GD
=
DRAIN
APPLIED
RAMP
VOLTAGE
NPN
BIPOLAR
TRANSISTOR
C
DB
R
B
I
2
D
S
SOURCE
C
GS
R
G
G
C
GD
I
1
Figure 14,Equivalent Circuit of Power MOSFET Showing Two Possible
Mechanisms for dv/dt Induced Turn-on.
GATE
SOURCE
N+ A
L
N+
R
S
C
DS
DRAIN
Figure 15,Physical Origin of the Parasitic BJT
Components That May Cause dv/dt Induced Turn-on
dv
dt
V
RC
BE
B
DB
=
References:
"HEXFET Power MOSFET Designer's Manual - Application Notes and Reliability Data," International
Rectifier
"Modern Power Devices," B,Jayant Baliga
"Physics of Semiconductor Devices," S,M,Sze
"Power FETs and Their Applications," Edwin S,Oxner
"Power MOSFETs - Theory and Applications," Duncan A,Grant and John Gower
Vrej Barkhordarian,International Rectifier,El Segundo,Ca.
Discrete power MOSFETs
employ semiconductor
processing techniques that are
similar to those of today's VLSI
circuits,although the device
geometry,voltage and current
levels are significantly different
from the design used in VLSI
devices,The metal oxide
semiconductor field effect
transistor (MOSFET) is based
on the original field-effect
transistor introduced in the
70s,Figure 1 shows the
device schematic,transfer
characteristics and device
symbol for a MOSFET,The
invention of the power
MOSFET was partly driven by
the limitations of bipolar power
junction transistors (BJTs)
which,until recently,was the
device of choice in power
electronics applications.
Although it is not possible to
define absolutely the operating
boundaries of a power device,
we will loosely refer to the
power device as any device
that can switch at least 1A.
The bipolar power transistor is
a current controlled device,A
large base drive current as
high as one-fifth of the
collector current is required to
keep the device in the ON
state.
Also,higher reverse base drive
currents are required to obtain
fast turn-off,Despite the very advanced state of manufacturability and lower costs of BJTs,these
limitations have made the base drive circuit design more complicated and hence more expensive than the
power MOSFET.
Source
Contact
Field
Oxide
Gate
Oxide
Gate
Metallization
Drain
Contact
n* Drain
p-Substrate
Channel
n* Source
t
ox
l
V
GS
V
T
0
0
I
D
(a)
(b)
I
D
D
SB
(Channel or Substrate)
S
G
(c)
Figure 1,Power MOSFET (a) Schematic,(b) Transfer Characteristics,(c)
Device Symbol.
Another BJT limitation is that both electrons and holes
contribute to conduction,Presence of holes with their higher
carrier lifetime causes the switching speed to be several orders of
magnitude slower than for a power MOSFET of similar size and
voltage rating,Also,BJTs suffer from thermal runaway,Their
forward voltage drop decreases with increasing temperature
causing diversion of current to a single device when several
devices are paralleled,Power MOSFETs,on the other hand,are
majority carrier devices with no minority carrier injection,They
are superior to the BJTs in high frequency applications where
switching power losses are important,Plus,they can withstand
simultaneous application of high current and voltage without
undergoing destructive failure due to second breakdown,Power
MOSFETs can also be paralleled easily because the forward
voltage drop increases with increasing temperature,ensuring an even distribution of current among all
components.
However,at high breakdown voltages (>200V) the on-state voltage drop of the power MOSFET becomes
higher than that of a similar size bipolar device with similar voltage rating,This makes it more attractive
to use the bipolar power transistor at the expense of worse high frequency performance,Figure 2 shows
the present current-voltage limitations of power MOSFETs and BJTs,Over time,new materials,
structures and processing techniques are expected to raise these limits.
2000
1500
1000
500
0
1 10 100 1000
Maximum Current (A)
Holdoff Voltage (V)
Bipolar
Transistors
MOS
Figure 2,Current-Voltage
Limitations of MOSFETs and BJTs.
Drain
Metallization
Drain
n
+
Substrate
(100)
n
-
Epi Layer
Channels
n
+
pn
+
p
+
Body Region p
+
Drift Region
G
S
D
Source
Gate
Oxide
Polysilicon
Gate
Source
Metallization
Figure 3,Schematic Diagram for an n-Channel Power MOSFET and the Device.
Figure 3 shows schematic diagram and Figure 4 shows the physical origin of the parasitic components in
an n-channel power MOSFET,The parasitic JFET appearing between the two body implants restricts
current flow when the depletion widths of the two adjacent body diodes extend into the drift region with
increasing drain voltage,The parasitic BJT can make the device susceptible to unwanted device turn-on
and premature breakdown,The base resistance RB must be minimized through careful design of the
doping and distance under the source region,There are several parasitic capacitances associated with
the power MOSFET as shown in Figure 3.
C
GS
is the capacitance due to the overlap of the source and the channel regions by the polysilicon gate
and is independent of applied voltage,C
GD
consists of two parts,the first is the capacitance associated
with the overlap of the polysilicon gate and the silicon underneath in the JFET region,The second part is
the capacitance associated with the depletion region immediately under the gate,C
GD
is a nonlinear
function of voltage,Finally,C
DS
,the capacitance associated with the body-drift diode,varies inversely
with the square root of the drain-source bias,There are currently two designs of power MOSFETs,usually
referred to as the planar and the trench designs,The planar design has already been introduced in the
schematic of Figure 3,Two variations of the trench power MOSFET are shown Figure 5,The trench
technology has the advantage of higher cell density but is more difficult to manufacture than the planar
device.
Metal
C
GS2
C
gsm
LTO
C
GD
R
Ch
C
GS1
R
B
BJT
n
-
p
-
C
DS
JFET
R
EPI
n
-
n
-
Epi Layer
n
-
Substrate
Figure 4,Power MOSFET Parasitic Components.
BREAKDOWN VOLTAGE
Breakdown voltage,
BV
DSS
,is the voltage at
which the reverse-biased
body-drift diode breaks
down and significant
current starts to flow
between the source and
drain by the avalanche
multiplication process,
while the gate and
source are shorted
together,Current-voltage
characteristics of a
power MOSFET are
shown in Figure 6.
BVDSS is normally
measured at 250μA drain
current,For drain
voltages below BV
DSS
and with no bias on the
gate,no channel is
formed under the gate at
the surface and the drain
voltage is entirely
supported by the
reverse-biased body-drift
p-n junction,Two related
phenomena can occur in
poorly designed and
processed devices:
punch-through and
reach-through,Punch-
through is observed
when the depletion
region on the source side
of the body-drift p-n
junction reaches the
source region at drain
voltages below the rated
avalanche voltage of the
device,This provides a
current path between
source and drain and
causes a soft breakdown
characteristics as shown
in Figure 7,The leakage
current flowing between
source and drain is denoted by I
DSS
,There are tradeoffs to be made between R
DS(on)
that requires shorter
channel lengths and punch-through avoidance that requires longer channel lengths.
The reach-through phenomenon occurs when the depletion region on the drift side of the body-drift p-n
junction reaches the epilayer-substrate interface before avalanching takes place in the epi,Once the
depletion edge enters the high carrier concentration substrate,a further increase in drain voltage will
cause the electric field to quickly reach the critical value of 2x10
5
V/cm where avalanching begins.
Source
Gate
Source
Gate
Oxide
Channel
Oxide
n
-
Epi Layer
n
+
Substrate
(100)
Drain
(b)
G SS
Electron Flow
D
(a)
Figure 5,Trench MOSFET (a) Current Crowding in V-Groove Trench MOSFET,
(b) Truncated V-Groove MOSFET
ON-RESISTANCE
The on-state resistance of a power MOSFET is made up of several components as shown in Figure 8:
(1)
where:
R
source
= Source diffusion resistance
R
ch
= Channel resistance
R
A
= Accumulation resistance
R
J
= "JFET" component-resistance of the
region between the two body regions
R
D
= Drift region resistance
R
sub
= Substrate resistance
Wafers with substrate resistivities of up to
20m?-cm are used for high voltage
devices and less than 5m?-cm for low
voltage devices.
R
wcml
= Sum of Bond Wire resistance,the
Contact resistance between the source
and drain Metallization and the silicon,
metallization and Leadframe
contributions,These are normally
negligible in high voltage devices but can
become significant in low voltage devices.
Figure 9 shows the relative importance of
each of the components to R
DS(on)
over the
voltage spectrum,As can be seen,at high
voltages the R
DS(on)
is dominated by epi
resistance and JFET component,This
component is higher in high voltage
devices due to the higher resistivity or
lower background carrier concentration in
the epi,At lower voltages,the R
DS(on)
is
dominated by the channel resistance and
the contributions from the metal to
semiconductor contact,metallization,
bond wires and leadframe,The substrate contribution becomes more significant for lower breakdown
voltage devices.
TRANSCONDUCTANCE
Transconductance,gfs,is a measure of the sensitivity of drain current to changes in gate-source bias.
This parameter is normally quoted for a V
gs
that gives a drain current equal to about one half of the
maximum current rating value and for a VDS that ensures operation in the constant current region.
Transconductance is influenced by gate width,which increases in proportion to the active area as cell
density increases,Cell density has increased over the years from around half a million per square inch in
1980 to around eight million for planar MOSFETs and around 12 million for the trench technology,The
limiting factor for even higher cell densities is the photolithography process control and resolution that
allows contacts to be made to the source metallization in the center of the cells.
RRRRRRRR
DS(on source ch A J D sub wcml)
=++++++
Gate
Voltage
7
6
5
4
I
DS
VS V
DS
LOCUS
3
2
1
0 5 10 15
0
5
10
15
20
25
(Saturation
Region)Linear Region
Normalized Drain Current
Drain Voltage (Volts)
Figure 6,Current-Voltage Characteristics of Power MOSFET
Channel length also affects transconductance,Reduced
channel length is beneficial to both gfs and on-resistance,
with punch-through as a tradeoff,The lower limit of this
length is set by the ability to control the double-diffusion
process and is around 1-2mm today,Finally the lower the
gate oxide thickness the higher gfs.
THRESHOLD VOLTAGE
Threshold voltage,V
th
,is defined as the minimum gate
electrode bias required to strongly invert the surface
under the poly and form a conducting channel between
the source and the drain regions,V
th
is usually measured
at a drain-source current of 250μA,Common values are
2-4V for high voltage devices with thicker gate oxides,and
1-2V for lower voltage,logic-compatible devices with
thinner gate oxides,With power MOSFETs finding increasing use in portable electronics and wireless
communications where battery power is at a premium,the trend is toward lower values of RDS(on) and
Vth.
DIODE FORWARD VOLTAGE
The diode forward voltage,VF,is the
guaranteed maximum forward drop of
the body-drain diode at a specified
value of source current,Figure 10
shows a typical I-V characteristics for
this diode at two temperatures,P-
channel devices have a higher VF due
to the higher contact resistance
between metal and p-silicon
compared with n-type silicon.
Maximum values of 1.6V for high
voltage devices (>100V) and 1.0V for
low voltage devices (<100V) are
common.
POWER DISSIPATION
The maximum allowable power
dissipation that will raise the die
temperature to the maximum
allowable when the case temperature
is held at 25
0
C is important,It is give
by Pd where:
T
jmax
= Maximum allowable temperature of the p-n junction in the device (normally 150
0
C or 175
0
C) R
thJC
= Junction-to-case thermal impedance of the device.
DYNAMIC CHARACTERISTICS
Sharp
Soft
I
D
BV
DSS
V
DS
Figure 7,Power MOSFET Breakdown
Characteristics
N+
P-BASER
SOURCE
R
CH
R
A
R
J
R
D
R
SUB
N+ SUBSTRATE
SOURCE
GATE
DRAIN
Figure 8,Origin of Internal Resistance in a Power MOSFET.
P
d
T
j
R
thJC
=
-max 25
(2)
When the MOSFET is used as a switch,its basic function is to control the drain current by the gate
voltage,Figure 11(a) shows the transfer characteristics and Figure 11(b) is an equivalent circuit model
often used for the analysis of MOSFET switching performance.
The switching performance of a device is determined by the time required to establish voltage changes
across capacitances,R
G
is the distributed resistance of the gate and is approximately inversely
proportional to active area,L
S
and L
D
are source and drain lead inductances and are around a few tens of
nH,Typical values of input (C
iss
),output (C
oss
) and reverse transfer (C
rss
) capacitances given in the data
sheets are used by circuit designers as a starting point in determining circuit component values,The data
sheet capacitances are defined in terms of the equivalent circuit capacitances as:
50V 100V 500VVoltage Rating:
Packaging
Metallization
Source
Channel
JFET
Region
Expitaxial
Layer
Substrate
R
EPI
R
CH
R
wcml
Figure 9,Relative Contributions to R
DS(on)
With Different Voltage Ratings.
C
iss
= C
GS
+ C
GD
,C
DS
shorted
C
rss
= C
GD
C
oss
= C
DS
+ C
GD
Gate-to-drain capacitance,C
GD
,is a
nonlinear function of voltage and is the most
important parameter because it provides a
feedback loop between the output and the
input of the circuit,C
GD
is also called the
Miller capacitance because it causes the total
dynamic input capacitance to become greater
than the sum of the static capacitances.
Figure 12 shows a typical switching time test
circuit,Also shown are the components of
the rise and fall times with reference to the
V
GS
and V
DS
waveforms.
Turn-on delay,t
d(on)
,is the time taken to
charge the input capacitance of the device
before drain current conduction can start.
Similarly,turn-off delay,t
d(off)
,is the time
taken to discharge the capacitance after the after is switched off.
0.0 0.5 1.0 1.5 2.0 2.5
0.1
1
10
100
T
J
= 150
0
C
T
J
= 25
0
C
VGS = 0V
V
SD
,Source-to-Drain Voltage (V)
I
SD
,Reverse Drain Current (A)
Figure 10,Typical Source-Drain (Body) Diode Forward
Voltage Characteristics.
I
D
V
GS
Slope = g
fs
G
R
G
C
GD
L
D
D
D'
S'
C
DS
L
S
S
C
GS
C I
D
Body-drain
Diode
(a) (b)
Figure 11,Power MOSFET (a) Transfer characteristics,(b) Equivalent Circuit Showing Components That
Have Greatest Effect on Switching
GATE CHARGE
Although input capacitance
values are useful,they do not
provide accurate results when
comparing the switching
performances of two devices
from different manufacturers.
Effects of device size and
transconductance make such
comparisons more difficult,A
more useful parameter from the
circuit design point of view is
the gate charge rather than
capacitance,Most
manufacturers include both
parameters on their data sheets.
Figure 13 shows a typical gate
charge waveform and the test
circuit,When the gate is
connected to the supply voltage,
V
GS
starts to increase until it
reaches V
th
,at which point the
drain current starts to flow and
the C
GS
starts to charge,During
the period t
1
to t
2
,C
GS
continues to charge,the gate
voltage continues to rise and
drain current rises
proportionally,At time t
2
,C
GS
is completely charged and the
drain current reaches the
predetermined current I
D
and
stays constant while the drain
voltage starts to fall,With
reference to the equivalent
circuit model of the MOSFET shown in Figure 13,it can be seen that with C
GS
fully charged at t
2
,V
GS
becomes constant and the drive current starts to charge the Miller capacitance,C
DG
,This continues
until time t
3
.
R
D
-
+
V
DD
V
DS
V
GS
R
G
D.U.T.
-10V
Pulse Width < 1μs
Duty Factor < 0.1%
(a)
Figure 12,Switching Time Test (a) Circuit,(b) VGS and VDS
Waveforms
td(on) tr td(off) tf
V
GS
100%
90%
V
DS
(b)
Charge time for the Miller capacitance is
larger than that for the gate to source
capacitance C
GS
due to the rapidly changing
drain voltage between t
2
and t
3
(current = C
dv/dt),Once both of the capacitances C
GS
and C
GD
are fully charged,gate voltage (V
GS
)
starts increasing again until it reaches the
supply voltage at time t
4
,The gate charge
(Q
GS
+ Q
GD
) corresponding to time t
3
is the
bare minimum charge required to switch
the device on,Good circuit design practice
dictates the use of a higher gate voltage
than the bare minimum required for
switching and therefore the gate charge
used in the calculations is Q
G
corresponding to t
4
.
The advantage of using gate charge is that
the designer can easily calculate the
amount of current required from the drive
circuit to switch the device on in a desired
length of time because Q = CV and I = C
dv/dt,the Q = Time x current,For
example,a device with a gate charge of
20nC can be turned on in 20μsec if 1ma is
supplied to the gate or it can turn on in
20nsec if the gate current is increased to
1A,These simple calculations would not
have been possible with input capacitance
values.
dv/dt CAPABILITY
Peak diode recovery is defined as the
maximum rate of rise of drain-source
voltage allowed,i.e.,dv/dt capability,If this
rate is exceeded then the voltage across the
gate-source terminals may become higher
than the threshold voltage of the device,
forcing the device into current conduction
mode,and under certain conditions a
catastrophic failure may occur,There are two possible mechanisms by which a dv/dt induced turn-on
may take place,Figure 14 shows the equivalent circuit model of a power MOSFET,including the
parasitic BJT,The first mechanism of dv/dt induced turn-on becomes active through the feedback action
of the gate-drain capacitance,CGD,When a voltage ramp appears across the drain and source terminal
of the device a current I
1
flows through the gate resistance,R
G
,by means of the gate-drain capacitance,
C
GD
,R
G
is the total gate resistance in the circuit and the voltage drop across it is given by:
(3)
When the gate voltage V
GS
exceeds the threshold voltage of the device V
th
,the device is forced into
conduction,The dv/dt capability for this mechanism is thus set by:
V
DD
DI
D
D
G
S
C
GS
C
DG
S
I
D
TEST CIRCUIT
(a)
O
GS
O
GD
GATE
VOLTAGE
V
G
V
G(TH)
t
0
t
1
t
2
t
3
t
4
t
DRAIN CURRENT
DRAIN
VOLTAGE
V
DD
I
D
WAVEFORM
(b)
Figure 13,Gate Charge Test (a) Circuit,(b) Resulting Gate
and Drain Waveforms.
VIRRC
dv
dt
GS G G GD
==
1
(4)
It is clear that low V
th
devices are more prone to
dv/dt turn-on,The
negative temperature
coefficient of V
th
is of
special importance in
applications where high
temperature environments
are present,Also gate
circuit impedance has to be
chooses carefully to avoid
this effect.
The second mechanism for
the dv/dt turn-on in
MOSFETs is through the
parasitic BJT as shown in
Figure 15,The capacitance
associated with the
depletion region of the body
diode extending into the
drift region is denoted as
C
DB
and appears between
the base of the BJT and the drain of the MOSFET,This capacitance gives rise to a current I
2
to flow
through the base resistance R
B
when a voltage ramp appears across the drain-source terminals,With
analogy to the first mechanism,the dv/dt capability of this mechanism is:
(5)
If the voltage that develops across R
B
is greater
than about 0.7V,then the base-emitter junction
is forward-biased and the parasitic BJT is
turned on,Under the conditions of high (dv/dt)
and large values of R
B
,the breakdown voltage of
the MOSFET will be limited to that of the open-
base breakdown voltage of the BJT,If the
applied drain voltage is greater than the open-
base breakdown voltage,then the MOSFET will
enter avalanche and may be destroyed if the
current is not limited externally.
Increasing (dv/dt) capability therefore requires
reducing the base resistance R
B
by increasing
the body region doping and reducing the
distance current I
2
has to flow laterally before it
is collected by the source metallization,As in
the first mode,the BJT related dv/dt capability
becomes worse at higher temperatures because
R
B
increases and V
BE
decreases with increasing
temperature.
dv
dt
V
RC
th
G
GD
=
DRAIN
APPLIED
RAMP
VOLTAGE
NPN
BIPOLAR
TRANSISTOR
C
DB
R
B
I
2
D
S
SOURCE
C
GS
R
G
G
C
GD
I
1
Figure 14,Equivalent Circuit of Power MOSFET Showing Two Possible
Mechanisms for dv/dt Induced Turn-on.
GATE
SOURCE
N+ A
L
N+
R
S
C
DS
DRAIN
Figure 15,Physical Origin of the Parasitic BJT
Components That May Cause dv/dt Induced Turn-on
dv
dt
V
RC
BE
B
DB
=
References:
"HEXFET Power MOSFET Designer's Manual - Application Notes and Reliability Data," International
Rectifier
"Modern Power Devices," B,Jayant Baliga
"Physics of Semiconductor Devices," S,M,Sze
"Power FETs and Their Applications," Edwin S,Oxner
"Power MOSFETs - Theory and Applications," Duncan A,Grant and John Gower
