Characteristics of a 1200V PT IGBT
With Trench Gate and Local Life Time Control
Eric R. Motto*, John F. Donlon*, H. Takahashi**, M. Tabata**, H. Iwamoto*
* Powerex Inc., Youngwood, Pennsylvania, USA
** Mitsubishi Electric, Power Device Division, Fukuoka, Japan
Abstract - A new 1200V IGBT with a VCE(sat)
2
of 1.9V at 125C and 140A/cm has been developed
using a trench gate PT (punch-through) structure
and local life time control. Compared to state-ofthe-art third generation planar devices, this device
represents a 30% improvement of on-state losses
at almost twice the current density. This paper will
describe the structure and characteristics of this
new IGBT.
I. INTRODUCTION
The IGBT (Insulated Gate Bipolar Transistor)
has become the power semiconductor device of choice
for a wide range of industrial power conversion
applications. Its popularity is the direct result of
technology advances that have produced devices with
rugged switching characteristics, low losses, and simple
gate drive. Devices with VCES ratings of 1200V are of
particular interest due to their widespread use with AC
mains voltages of 360VAC to 480VAC. This paper will
present a new 1200V IGBT device that has been
optimized using the latest processing technologies to
substantially reduce losses in these applications.
II. 1200V TRENCH GATE IGBT - KEY
TECHNOLOGIES
Historically, improvements in the performance
of IGBTs have been attained primarily through finer
surface patterns and shallow diffusion processing
technologies. Unfortunately, this approach has reached
the point of severely diminishing returns due in part to
the so-called "parasitic JFET" resistance between
adjacent cells in the MOSFET portion of the device and
in part to the VF of the diode structure in the bipolar
portion of the device. The key to producing the
significantly improved 1200V IGBT described in this
paper was the simultaneous optimization of these
critical areas. The following subsections will present the
FIGURE 1: STRUCTURE OF THE TRENCH GATE IGBT CHIP
structure and process technologies employed to reduce
the losses of this new IGBT.
A. Trench gate structure
In order to reduce the RDS(on) of the MOSFET
portion of the IGBT, a trench gate structure has been
adopted. This structure, illustrated in Figure 1, has
already been adopted to reduce the on-state voltage
drop of low voltage MOSFETS and IGBT devices rated
at up to 600V [6] [7] [10]. This paper reports the result
of adopting this technology to 1200V IGBT devices. A
schematic cross section comparison of a conventional
IGBT cell to the trench gate cell is shown in Figure 2.
The trench structure differs from the conventional
planar structure in that the gate oxide and conductive
polysilicon gate electrode are formed in a deep narrow
trench below the chip surface. When voltage is applied
to the gate, a channel forms along the vertical wall of
the trench perpendicular to the surface of the chip. This
is very different from the conventional planar structure
where the channel forms under the gate parallel to the
surface of the chip. The vertical channel of the trench
structure provides two key advantages. First, the
vertical channel requires less chip area permitting a
substantial increase in cell density. The increased cell
Emitter
Gate
n+
RChannel
RJFET
p
Rn-
n-
trench structure. In particular, the non-uniform current
density in the "JFET" region of the planar structure can
result in SOA aberrations at high current densities. In
the case of the new 1200V PT IGBT the more uniform
current flow of the trench structure, combined with the
greater cell density, has resulted in an increase in the
controllable current density from about 800A/cm2 in
third generation planar devices to more than
1400A/cm2.
B. PT chip structure
n+
p+
Collector
Conventional Planar Gate IGBT Cell
Emitter
Gate
n+
p
RChannel
To optimize the VF of the diode structure within
the bipolar portion of the IGBT, a PT (Punch Through)
rather than NPT (Non Punch Through), chip structure
was selected. The n+ buffer layer of the PT structure
essentially makes the diode a PIN type. The
advantages of this structure for optimizing the VF and
recovery characteristics of power diodes are well
known. In particular, the resistivity and thickness of the
n- drift layer can be optimized for low VF without the
severe problems of high off-state leakage current and
the need to process extremely thin silicon wafers that
plague the NPT IGBT structure.
n-
C. Local Lifetime Control
Rnn+
p+
Region of local
lifetime control
Collector
New 1200V Trench Gate IGBT Cell
FIGURE 2: UNIT CELL COMPARISON
TABLE I: REDUCTION OF VCE(SAT)
Components of VCE(sat)
Reduction Technique
RChannel
Adopt trench gate surface
structure to increase cell density
and channel width per unit area
Eliminate by adopting trench gate
structure
Utilize local lifetime control to
increase
on-state
carrier
concentration
RJFET
Rn-
density produces a greater channel width per unit chip
area resulting in a reduction in the Rchannel portion of the
IGBT’s on-state voltage drop. The second advantage of
the trench structure is complete elimination of the
"JFET" resistance (RJFET) which results from the
constriction of current flow in the region between
adjacent cells in the planar structure. Additionally, there
are some less obvious secondary advantages of the
In order to reduce the VF of the pin diode
structure within the bipolar portion of the IGBT, it is
necessary to optimize the doping profile, geometry and
carrier lifetime of the n- drift region and n+ buffer layer.
The carrier lifetime in the n+ buffer layer must be
reduced to increase the rate of recombination and
speed up turn-off switching. In the third generation
planar IGBT, the lifetime was reduced using a uniform
lifetime killing process employing electron irradiation.
Unfortunately, the uniform lifetime control also reduces
the carrier lifetime in the n- drift region causing an
increase in VCE(sat). To avoid this problem, a local
lifetime control process utilizing heavy ion irradiation
was developed. This process allows the lifetime killing
to be applied to the n+ buffer layer, as shown in Figure
2, without reducing the lifetime of the n- drift layer. The
longer lifetime in the drift layer produces a greater
carrier concentration during conduction which in turn
reduces the Rn- component of VCE(sat).
D. The RTC (Real Time Control) Circuit
Industrial power electronic equipment is often
required to survive catastrophic events such as
miswiring and shorted loads. In these applications, the
IGBT is required to survive low impedance short
circuits long enough for protection circuits to respond.
III. ELECTRICAL CHARACTERISTICS
The most outstanding characteristic of the new
1200V IGBT is its extremely low VCE(sat). This
characteristic is obtained as a collective result of the
new processing technologies described in Section II
and outlined in Table I. A saturation voltage
characteristic curve comparing a third generation
planar IGBT with the new trench gate device is shown
in Figure 4. The third generation planar device in this
comparison is a 100A 1200V device with a nominal
current density at rated current of 95A/cm2. The trench
gate device is also rated 100A, 1200V and has a
nominal current density of 140A/cm2. This curve shows
that the trench gate device has a VCE(sat) 0.9V lower
than the third generation planar device at 1.4 times
greater current density. Figure 5 shows the inductive
load turn-on and turn-off switching waveforms for the
new trench gate IGBT at Tj=125C. The local lifetime
Main Emitter Area
Current Mirror Emitter
Gate
C
Trench Gate
IGBT
1200V
Trench Gate
1200V Third
Generation
Planar
G
RTC
Circuit
E
FIGURE 3: TRENCH GATE IGBT CHIP AND RTC
The trench gate IGBT, by itself, has very limited short
circuit withstanding capability due to its extremely high
short circuit saturation current. To reduce this current,
the trench IGBT chip is fabricated with a current mirror
emitter. The current mirror is connected to an RTC
(Real Time Control) circuit to provide active clamping
of short circuit current. The RTC and trench gate IGBT
chip are shown in Figure 3. The RTC circuit is activated
by the current from the current mirror emitter on the
trench IGBT chip. During a short circuit, the high
current from the current mirror activates the RTC which
reduces the gate driving voltage on the IGBT, thereby
clamping the short circuit current to a safe level. The
RTC circuit restores the short withstanding capability to
more than 10µs, comparable to the third generation
planar devices.
FIGURE 4: (VCE(SAT)) CHARACTERISTIC FOR 100A IGBT CHIP
control gives low turn-off losses with a desirable
reduction in dv/dt compared to the third generation
planar device. With equivalent series gate resistance,
the turn on of the trench gate device is faster than the
third generation planar device due to a reduction in
reverse transfer capacitance. A substantial reduction in
turn-on losses was made possible by a new free wheel
diode which will be described in section V. Table II
compares the basic electrical characteristics of a 100A,
1200V trench gate and a third generation planar
device.
TABLE II: CHARACTERISTICS OF 100A, 1200V TRENCH GATE VERSUS THIRD GENERATION
Characteristic
Symbol
Conditions
U-Series
Trench Gate
Collector cutoff current
Gate-emitter threshold
voltage
Gate leakage current
Collector to emitter
saturation voltage
Input Capacitance
Output Capacitance
Reverse Transfer
Capacitance
Thermal Resistance
IC
ICES
VGE(th)
VCE=1200V, VGE=0V
IC=10mA, VCE=10V
1.0mA Max
6V
1.0mA Max
6V
IGES
VCE(sat)
VGE=20V, VCE=0V
VGE=15V
Tj=25C
IC=100A
Tj=125C
0.5µA Max
2.9V
2.85V
15nF
5nF
3nF
20µA Max
1.8V
1.9V
36nF
0.99nF
0.93nF
0.19C/W
0.25C/W
Cies
Coes
Cres
VCE=10V, VGE=0V
RTH(j-c)
IGBT
Turn-Off Switching
Inductive Load, TJ=125C
IC:20A/div, VCE:200V/div
t:200ns/div
described in Section II permits less silicon area to be
used for a given current rating. The resulting smaller
chip is less expensive to manufacture and can be
adapted to smaller, lower cost packages. Figure 6
shows the trade-off relationship between various
technologies considering a 100A 1200V IGBT. From
Figure 6 it is obvious that the trench gate PT IGBT with
local lifetime control has the best EOFF versus VCE(sat)
trade-off characteristic. Table III gives a performance
4.0
VCE
A
3.5
C
3.0
Turn-On Switching
Inductive Load, TJ=125C
IC:20A/div, VCE:200V/div
t:200ns/div
VCE
VCE(sat)
(V)
Tj=125C
IC=100A
B
D
Powerex/Mitsubishi
U-Series
2.5
E
Powerex/Mitsubishi
H-Series
2.0
New Powerex/Mitsubishi
Trench gate IGBT
1.5
1.0
0
5
10
15
20
25
30
ESW(off) (mJ/pulse) Tj=125C, IC=100A, VCC=600V
2
IC
A: Conventional NPT, tN-=250µm, JC=100A/cm
2
B: PT planar gate uniform lifetime control, JC=100A/cm
2
C: Thin Drift Region NPT, tN-=150µm, JC=100A/cm
2
D: PT planar gate, uniform lifetime control, JC=75A/cm
2
E: PT trench gate, local lifetime control, JC=140A/cm
FIGURE 6: EOFF VERSUS VCE(SAT) TRADE-OFF
FIGURE 5: 1200V TRENCH GATE IGBT SWITCHING WAVEFORMS
IV. PERFORMANCE ANALYSIS
The low VCE(sat) of the trench gate IGBT
combined with the improved turn-off current density
comparison for the different device technologies shown
in Figure 6. The low losses of the trench gate chip
result in a reduced junction to case temperature rise
compared to other technologies even though its smaller
chip has a higher thermal impedance. The lower losses
also allow use of smaller heat sinks which helps to
reduce the cost and size of finished equipment. The
combination of reduced chip size and reduced losses
Parameter
TABLE III: IGBT PERFORMANCE COMPARISON
Third
Third
Thin drift
Generation
Generation
region
H-Series
U-Series
NPT
VCE(sat) (V) Tj=125C
2
@ Current Density (A/cm )
Switching Loss - Eon+EOFF
Total Sinusoidal Output Inverter Loss
fPWM=5KHz
Total Sinusoidal Output Inverter Loss
fPWM=15KHz
Thermal Impedance RTH(j-c)
Temperature Rise - TJ-C fPWM=5KHz
Temperature Rise - TJ-C fPWM=15KHz
IGBT Chip size
Trench
Gate
2.3
75
1.0
1.0
2.85
95
0.95
0.94
3.4
95
0.89
1.0
1.9
140
0.84
0.62
1.0
0.89
0.89
0.59
1.0
1.0
1.0
1.0
1.25
1.08
1.03
0.8
1.25
1.15
1.03
0.8
1.5
0.93
0.88
0.6
make the 1200V, PT, trench gate IGBT an effective
technology for high power applications.
V. MODULE DEVELOPMENT
To obtain maximum performance from the new
trench gate chips an optimized module package and a
new free wheel diode with improved fast but soft
recovery characteristics were developed.
The new IGBT module package is based on the
"U-Series" package developed by Powerex/Mitsubishi
in 1996 [11]. Figure 7 shows a cross section drawing of
the new package. The key innovation is its insert
molded case in which the power electrodes are molded
into the sides as opposed to inserted after the case is
molded. The main electrodes are then connected
directly to the power chips using large diameter
aluminum bonding wires rather than solder. A
photograph of a dual (half bridge) module is shown in
figure 8.
One of the main objectives in the design of the
new package was a reduction in internal inductance.
The most significant improvement was made possible
by molding wide electrodes into the sides of the case to
form parallel plate structures having considerably less
Main Terminal Electrode Silicone Gel Cover Insert Molded Case
Al Bond Wires
Cu Base Plate
Power Chips
AlN Substrate
FIGURE 7: U-PACKAGE CROSS SECTION
FIGURE 8: TRENCH GATE IGBT MODULE
inductance than conventional electrodes. In addition,
the strain relieving "S" bends that were needed in the
electrodes of conventional modules are not needed in
the U-Series package because the aluminum bond
wires perform the strain relieving function. Elimination
of these "S" bends helped to further reduce the
electrode inductance. Overall, as a result of these
inductance reducing features, the new package has
about one third the inductance of conventional
modules.
Another advantage of the new package is a
significant improvement in manufacturability. The
number of soldering steps required is reduced from two
to one. With the conventional module, the chip to
substrate and substrate to base plate soldering is done
first with high temperature solder. Then, the case is
attached to the base plate and a second low
temperature soldering step is used to connect the
power electrodes. In the new module, the second step
is eliminated because the connections to the power
electrodes are made using the aluminum bond wires.
The soldering temperature of the chip and substrate
attachment can be reduced helping to minimize the
effects of the mismatched coefficients of expansion
between the base plate and the AlN DBC substrate.
The result is a reduction in thermal stress during
manufacturing, improved baseplate flatness, and
increased reliability.
Another advantage of the new package is a
substantially smaller ceramic substrate. The ceramic
area needed for soldering the power electrodes in the
conventional module is not required. As a result, higher
performance AlN ceramic can be cost effectively
utilized to minimize the thermal impedance of the new
package.
VI. CONCLUSION
A new 1200V Trench gate IGBT with local
lifetime control has been presented. The electrical
characteristics include low VCE(sat), low switching losses,
and robust switching SOA. 10µs short circuit
withstanding capability was achieved by implementing
an active current clamping circuit. A performance
analysis was presented to show the advantages of this
device for industrial power conversion applications.
VII. REFERENCES
[1] G. Majumdar, et al. "A New Generation High
Speed Low Loss IGBT Module", ISPSD, May
1992
[2] J Yamashita, et al. "A Study on the Short Circuit
Destruction of IGBTs" , ISPSD, May 1993
[3] Powerex "IGBTMOD and IntellimodTM Application
and Technical Data Book" PUB# 9DB-100,
March 1998
[4] Majumdar, et. al., "Enhancing SOAs of IGBT
Modules for Hard Switching Applications",
PCIM 1990
[5] Yamada, et. al., "Next Generation Power Module",
ISPSD 1994
[6] M. Harada, et. al., "600v Trench IGBT in
Comparison with Planar IGBT", ISPSD May
31, 1994
[7] T. Iida, et. al., "Low VCE(sat) IGBT Module by New
Structure - Trench", PCIM Europe June 28,
1994
[8] E. Motto, et. al., "Evaluating the Dynamic
Performance of High Current IGBT Modules"
PCIM/PQ 1994
[9] E. Motto "Protecting High Current IGBT Modules
From Over Current and Short Circuits" HFPC
Conference May ,1995
[10] Eric R. Motto, et. al. “New Process Technologies
Improve IGBT Module Efficiency” IEEE IAS
Conference October 1995
[11] Eric R. Motto, “A New Low Inductance IGBT
Module
Package”
PCIM
Conference
September 1996