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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 38, NO. 1, JANUARY/FEBRUARY 2002
A New Punch-Through IGBT Having
a New n-Buffer Layer
Hideo Iwamoto, Hideki Haruguchi, Yoshifumi Tomomatsu, John F. Donlon, Senior Member, IEEE, and
Eric R. Motto, Member, IEEE
Abstract—Insulated gate bipolar transistors (IGBTs) based on
the non-punch-through (NPT) design approach exhibit excellent
safe operating area (SOA) and short-circuit endurance, a positive
temperature coefficient of on-state voltage over the operating
current range, and low silicon cost. These merits have supported
the development and commercialization of NPT IGBTs above
the 1200-V class. However, the need for quite thin silicon to
obtain competitive on-state losses at 1200-V and below classes
has hindered the use of the NPT approach in this area. A new
punch-through (PT) IGBT has been developed which exhibits
the merits of the NPT approach, rugged SOA and short-circuit
endurance, while also having a better tradeoff relation between
on-state voltage and turn-off loss than either existing NPT or
third-generation PT IGBTs.
past several years, special processing has permitted the use of
the PT or buffer layer approach to achieve high-voltage IGBTs
(2500 and 3300 V) which combine the best characteristics of
both PT and NPT approaches. This tact has now been taken
in the 1200-V design area to produce a new PT IGBT that not
only exhibits the best merits of the NPT IGBT, rugged SOA and
short-circuit endurance and positive temperature coefficient
of on-state voltage over the operating current range, but also
shows a better tradeoff relation between on-state voltage and
turn-off loss compared with current third-generation PT IGBTs.
Index Terms—Insulated gate bipolar transistor, power semiconductor.
A. Comparison Between PT and NPT IGBTs
I. INTRODUCTION
T
HE manufacture of power semiconductors has always
involved tradeoffs between conduction losses, switching
speed, and safe operating area (SOA). For insulated gate bipolar
transistors (IGBTs) these tradeoffs are further complicated by
such factors as silicon and processing limitations and costs.
Two approaches to IGBT design have evolved, punch-through
(PT) and non-punch-through (NPT). Each approach has characteristics that are beneficial or detrimental to the end user.
The PT approach has low on-state voltage, low leakage current,
good SOA, and a small but negative temperature coefficient of
on-state voltage. The NPT approach uses lower cost silicon, has
a very rugged SOA and short circuit endurance, high leakage
current, and a large but positive temperature coefficient of
on-state voltage. The NPT approach requires relatively thin
(when compared to the PT approach) silicon to achieve on-state
characteristics approaching those of the PT approach. This has
been practical for voltage designs above 2000 V but has resulted
in high processing cost for lower voltage designs. Over the
Paper IPCSD 01–076, presented at the 1999 Industry Applications Society
Annual Meeting, Phoenix, AZ, October 3–7, and approved for publication in
the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Power Electronics
Devices and Components Committee of the IEEE Industry Applications Society.
Manuscript submitted for review October 15, 1999 and released for publication
October 31, 2001.
H. Iwamoto and H. Haruguchi are with Mitsubishi Electric Corporation, Fukuoka 819-01, Japan (e-mail: iwamotoh@mail.oka.melco.co.jp;
haruguch@mail.oka.melco.co.jp).
Y. Tomomatsu is with Fukuryo Semiconductor Engineering Corporation,
Fukuoka 819-01, Japan (e-mail: tomomaty@mail.oka.melco.co.jp).
J. F. Donlon and E. R. Motto are with Powerex, Inc., Youngwood, PA
15697-1800 USA (e-mail: jdonlon@pwrx.com; emotto@pwrx.com).
Publisher Item Identifier S 0093-9994(02)00783-1.
II. DEVICE STRUCTURE
The comparison of characteristics of the PT IGBT and NPT
IGBT are shown in Table I [3]. As shown in this table, the
PT IGBT has the following disadvantages compared with the
NPT IGBT: 1) higher crossover point of on-state I–V curves (the
crossover point is the device current at which the forward characteristic of the device transitions from a negative temperature
coefficient to a positive temperature coefficient) and 2) slightly
lower destruction threshold, i.e., lower short-circuit endurance
and RBSOA. Described below are the structure and production
methods of the PT IGBT that were developed in order to achieve
better characteristics than the NPT IGBT.
B. Breakover Voltage
is equivalent to the
The IGBT’s breakover voltage
under an open base condition and it
p-n-p transistor’s
can be calculated from the following formula by using the collector-base breakdown voltage
(1)
is the p-n-p transistor’s current gain when voltage is
applied across the IGBT, collector-to-emitter (when the drift
layer is depleted). As shown in the above formula, the IGBT’s
is lower than the collector-base
breakover voltage
[equivalent to the diode’s breakbreakdown voltage
can be raised by increasing
down voltage]. However,
as much as possible. In the off-state, when
a high voltage is applied across the collector-to-emitter, the
is significantly higher
p-n-p transistor’s current gain
is given by the
than that of the on-state current gain.
following equation:
0093–9994/02$17.00 © 2002 IEEE
(2)
IWAMOTO et al.: A NEW PT IGBT
169
TABLE I
PT AND NPT IGBT CHARACTERISTICS
where
is the base transport factor, and
efficiency.
is calculated by
is the Injection
(3)
is the
drift layer width, and
is the diffusion length. In
buffer layer. Also,
general, the NPT IGBT does not have an
its injection efficiency is controlled by limiting the concentration of the collector layer and the NPT IGBT does not involve
lifetime control. In order to increase the breakover voltage of an
needs to be minimized by increasing the thickNPT IGBT,
. On the other hand, the PT IGBT’s
buffer layer
ness of
is short or thin. Sufficient voltage can be obtained even if the
is made thin because its
is low. In other words, the PT
IGBT’s breakover voltage can be improved and its leakage curlayer because the
rent can be reduced in spite of a thinner
buffer layer.
PT IGBT has an
(a)
C. On-State Voltage
The IGBT
tion [1]:
can be obtained from the following equa-
(4)
is the voltage drop caused by the channel current of
where
is the forward voltage drop
the MOSFET region, and
collector layer, the
of the diode region that consists of the
buffer layer, and the
drift layer. As aforementioned, the
drift layer needs to be thicker than that of the
NPT IGBT’s
PT IGBT in order to secure its breakdown voltage because the
buffer layer. This increases the
NPT IGBT does not have an
. Since
greatly
PNP transistor’s current gain
, the NPT IGBT’s
increases and,
depends on
becomes higher than that
therefore, the NPT IGBT’s
of the PT IGBT. Thus, the PT IGBT exhibits the advantage that
its on-state voltage is lower than that of the NPT IGBT.
D. Switching Characteristics
A tail current is generated in the switching process when exbase
cess carriers, which are stored in the p-n-p transistor’s
area, disappear by their recombination. It is reported that the tail
current can be calculated from the following equation [2]:
(5)
is the carrier lifetime of base area. This equation inwhere
needs to be reduced in order to decrease
dicates that
the tail current and the tail time. However, the NPT IGBT’s tail
(b)
(c)
Fig. 1. Cross-sectional views of 1200-V IGBTs. (a) PT IGBT with new buffer
layer. (b) PT IGBT. (c) NPT IGBT.
current is higher and its tail time is longer. This is due to the
that results because the NPT IGBT has neither lifetime
high
buffer layer. On the other hand, the PT IGBT,
control nor an
buffer
with the use of lifetime control, has a short lifetime
and tail current are low and
layer. Therefore, the PT IGBT’s
its tail time is short. Additionally, the use of local lifetime redrift layer can reduce the tail time while still
duction in the
controlling the saturation voltage.
E. Cross Point of On-State Characteristics
Described below are the reasons why the NPT IGBT’s cross
C and
C characpoint (the point where the
teristic curve of I versus V in on-state cross) is lower than that
of the PT IGBT.
1) Because the built-in potential decreases as the junction
temperature rises, saturation voltage in the low-current
region drops as the temperature rises.
2) The inclination or slope of the I–V curve’s straight-line
area is a function of mobility and lifetime.
3) Because the NPT IGBT does not use lifetime control, its
lifetime value is very long and it barely affects the saturation voltage. Its I–V characteristics are, therefore, influenced by the mobility’s temperature dependency alone.
The mobility drops as the temperature rises, and the inclination of the I–V curve becomes smaller as the temperature rises (i.e., on-state voltage significantly increases),
and the crossover point becomes lower as a result.
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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 38, NO. 1, JANUARY/FEBRUARY 2002
Fig. 4. Off-state characteristics of IGBTs whose relative impurity
concentration of the n layer is 1.0 shown in Fig. 3.
Fig. 2.
Impurity concentration profile of new PT IGBT.
Fig. 5. Electrical field distribution of new PT IGBT (simulation). V
1200 V.
Fig. 3. Simulation result of 1200-V IGBTs. Case A: without n-buffer layer;
case B: with n-buffer layer ( = ); case C: with n-buffer layer ( <
).
4) In the case of the standard PT IGBT, the whole IGBT is
lifetime controlled by electron beam irradiation and its
lifetime greatly affects the saturation voltage. The lifetime is dependent on temperature and it becomes longer
as the temperature rises. Therefore, the increase in lifetime offsets the drop in mobility and the difference as a
function of temperature between the I–V curve’s inclination becomes small. As a result, the crossover point rises.
As seen from the above descriptions, the crossover point of the
NPT IGBT is lower because of the lifetime control and not because of the structure. It means the crossover point of the PT
IGBT can be lowered by reducing the lifetime control. With
conventional PT IGBTs, the whole chip is treated for lifetime
control by electron beam irradiation. Therefore, its crossover
point can be lowered using local lifetime control [4].
=
F. Ruggedness
Destruction tolerance is greatly influenced by both
layer thickness and
. In order to secure the required
layer needs to
breakover voltage for an NPT IGBT, the
be thicker. Consequently, the destruction tolerance becomes
high. Therefore, the destruction tolerance of the PT IGBT
could be increased by optimizing the thickness of the
layer. However, this approach also increases the saturation
voltage and switching loss. One can reduce these drawbacks
by forming a local lifetime-controlled layer adjacent to the
layer and optimizing the doping density in the
layer. That
is, it is possible to ensure high destruction tolerance without
much effect on the saturation voltage and switching losses by
buffer layer in which the lifetime is controlled
adopting an
layer.
short and increasing the doping density slightly in the
G. How to Realize a Superior PT IGBT
The PT IGBT’s characteristics can be improved by the following.
IWAMOTO et al.: A NEW PT IGBT
171
Fig. 6. Off-state characteristic of new PT IGBT. (a) With proton irradiation.
(b) Without proton irradiation. (Horizontal: 200 V/div; vertical: 200 A/div.)
(a)
(b)
and switching loss tradeoff. Measurement conditions: T
= 15 V; I = 30 A; switching loss: V = 600 V;
= 15 V; and I = 30 A.
=
Fig. 8. Forward I–V characteristics at T = 25 C and 125 C (horizontal:
500 mV/div, vertical: 5 A/div). (a) New PT IGBT (30 A). (b) Conventional PT
IGBT (30 A).
1) The
drift layer should be made as thin as possible in
order to reduce the saturation voltage.
2) To reduce turn-off time, it is effective to reduce the lifedrift layer that is not depleted when the
time of the
off-state voltage is applied.
3) An -buffer layer needs to be constructed in order to secure a high breakover voltage for the IGBT that has a thin
drift layer.
4) The lifetime control should be minimized in order to
lower the crossover point of the on-state characteristic
I–V curves.
drift layer should be optimized
5) The thickness of the
to improve the destruction tolerance.
Fig. 1 compares schematic cross sections of the newly developed PT IGBT [Fig. 1(a)] with a conventional PT IGBT [Fig.
1(b)] and the NPT IGBT [Fig. 1(c)].
Fig. 1(a) is the new PT IGBT with the newly developed
-buffer layer. A collector region is formed with an approximately 100- m boron diffused layer over an -type single
layer is designed so
crystal wafer. The thickness of the
that the depletion layer extends to the collector region when
layer
rated voltage is applied at off state. That is, the
is some 10- m thicker than that of the PT IGBT shown in
Fig. 1(b). This -buffer layer is formed by first applying proton
irradiation near the border between the collector area and
layer and then it is annealed. The buffer layer having
the
substrate is formed by changing
lower resistivity than the
the proton irradiated area to a donor area. Also, the IGBT’s
is controlled by changing the anneal time to control
the local lifetime [5]. Fig. 2 shows the resulting measurement
of the newly developed PT IGBT’s impurity concentration
Fig. 7. V
125 C; V
6V
:V
H. New Structure for PT IGBT
Based on the aforementioned analysis, an -buffer layer was
constructed by newly developed methods, and a new PT IGBT
drift layer thickness has been developed.
with an optimized
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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 38, NO. 1, JANUARY/FEBRUARY 2002
(a)
(b)
Fig. 9. Turn-off switching waveforms. (a) New PT IGBT. (b) Conventional PT IGBT. V
(I : 5 A/div; V : 100 V/div; t: 500 ns/div).
= 600 V; 6V = 15 V; T = 125
C; R
= 11 ; L = 1 mH
profile after annealing. This curve confirms that the resistivity
drops when the proton irradiated area turns to a donor area and
the -buffer layer is formed.
Fig. 1(b) shows the structure of a conventional PT IGBT that
uses an epitaxial wafer, and Fig. 1(c) shows the structure of a
conventional NPT IGBT that uses an -type single crystal wafer
of 250- m thickness. The conventional PT IGBT uses electron
beam irradiation and the whole chip is lifetime controlled. The
is
NPT IGBT does not involve lifetime control and its
maintained within an appropriate value by limiting the collector concentration. All the IGBT chips that were used for the
new PT, the conventional PT, and the NPT fabricated models exA/cm ).
amined for this comparison are 1200 V, 30 A (
Fig. 10. Switching waveform of new PT IGBT under the condition of short
= 800 V; V = 15 V
circuit. Test conditions: T = 125 C; V
(V : 200 V/div; I : 50 A/div; t: 5 s/div).
6
III. DEVICE SIMULATION
Breakover voltage simulation results are shown for three
1200-V designs in Fig. 3. Case A is for an IGBT without
-buffer layer, Case B is for an IGBT with an -buffer layer
layer, and Case C is for an
having the same lifetime as the
IGBT with an -buffer layer having shorter lifetime than the
drift layer. The concentration of the -buffer layer comes
from the value obtained from Fig. 2. The -layer thickness was
kept constant (150 m) and its concentration varied.
The simulation confirms that, for Case A, the breakover
decreases when the impurity concentration
voltage
drops below 1.2. That is because the depletion layer extends to
collector at low
drift layer concentration. On the
the
other hand, for Case B, the breakover voltage increases as the
impurity concentration decreases. This is because the -buffer
layer prevents the depletion layer from extending to the
collector. As for Case C, the breakover voltage is much higher
of the p-n-p transistor,
than Case B. This is because the
which is composed of the IGBT’s base area, area, and
collector area, decreases.
Fig. 4 shows the off-state I–V characteristics of the three
layer concentration of 1.0 (relative
cases, A, B, and C, at an
value). From these figures, it is clear that the breakover voltage
drift layer concentration
of Case A peaks at a particular
value and Case C has the highest breakdown voltage.
needs to be reduced so that a
According to (1),
high breakover voltage can be attained. The new IGBT with the
-buffer layer that is formed by the aforementioned methods has
a short lifetime and low resistance. Therefore, higher breakover
drift layer is made
voltage can be attained even though the
thinner than that of an NPT IGBT.
Shown in Fig. 5 is the electrical field distribution inside the
chip when 1200 V is applied across the collector-to-emitter
when the impurity concentration is 1.0 in Fig. 3 Case B. Fig. 5
shows that the electric field suddenly drops near the -buffer
layer for Case B. That is, the depletion layer is blocked from
reaching through the collector layer by the -buffer layer. A
stable breakover voltage can be obtained even if slight variadrift layer thickness and
drift layer impurity
tions in
concentration are observed during production. Thus, this is one
of the merits of forming the -buffer layer.
Simulation was also carried out to assess the temperature dependence of the on-state I–V characteristic. It was confirmed
that, with the use of local lifetime control, the crossover point
C and
C was far below
of the two I–V curves at
rated current.
IV. EXPERIMENTAL RESULTS
A fabricated model of a PT IGBT, whose
drift layer thickness is 150 m and impurity concentration in the case of Fig. 3
is 1.0, has been made and evaluated. The evaluation results are
as follows.
IWAMOTO et al.: A NEW PT IGBT
173
Fig. 11. Switching waveform for RBSOA test of new PT IGBT. Measurement conditions:
(V : 200 V/div; I : 50 A/div; t: 500 ns/div).
A. Breakover Voltage
The new PT IGBT’s V–I characteristic at off-state is shown in
Fig. 6. Fig. 6(b) shows the characteristics before proton irradiation (equivalent to Case A of simulation results) and Fig. 6(a)
shows them after the proton irradiated area became a donor area
(equivalent to Case C of the simulation.) Fig. 6(a) shows that the
new PT IGBT has a sufficient breakover voltage for the 1200-V
class as the result of formation of an -buffer layer and reduction of the proton irradiated area’s lifetime. Also, the simulation
results shown in Fig. 4 very much agree with the measurement
results shown in Fig. 6; and those figures point out the trend of
the breakover voltage shift before and after the formation of the
-buffer layer match.
B. Tradeoff Between On-State Voltage and Switching Loss
and turn-off switching loss is
Tradeoff between
shown in Fig. 7. Generally, the turn-off loss is defined as a loss
generated at turn-off from when the collector voltage reaches
10% at its rise to when the collector current falls to 10%.
However, if the turn-off loss is measured according to this
definition, the turn-off losses of the new PT IGBT and NPT
IGBT turn out to be understated and inaccurately compared
since the current tail drags long into the area where current is
low. In order to make a fair comparison against the conventional
PT IGBT, the turn-off loss here is defined as the loss from when
the voltage reaches 10% at its rise to when the current falls to
2%. Fig. 7 is the measurement result based on this definition.
values of the chips used in the new PT IGBT are
of the
changed by altering the anneal time. Also, the
NPT IGBT is changed by altering the boron concentration of
drift layer of the new PT IGBT
the collector layer. The
can be made thinner than that of the NPT IGBT and, therefore,
the new PT IGBT has better characteristics. Also, the new PT
IGBT has better characteristics than the conventional PT IGBT
drift layer. This is because the new
despite it having a thicker
PT IGBT uses local lifetime control with proton irradiation.
C. On-State Characteristics
Shown in Fig. 8 are the output characteristics of the new PT
C and
IGBT and the conventional PT IGBT at
C. The crossover point of the new PT IGBT that uses local
T
= 125
C; V
= 1200 V; I
= 150 A;
6V
= 15 V
lifetime control is very low. Therefore, this IGBT has the merit
that current can be easily balanced when connected in parallel.
D. Switching Characteristics
Fig. 9 shows the current and voltage switching waveforms
of the new PT IGBT and conventional PT IGBT at load at
turn-off. These waveforms show that the turn-off loss of the new
PT IGBT does not increase much compared to the conventional
PT IGBT because of the small increase in the tail current even
though its temperature is high (125 C). This is because the new
PT IGBT uses local lifetime control by proton irradiation and is
little affected by lifetime temperature shift compared to the conventional PT IGBT whose whole wafer’s lifetime is controlled
by electron beam irradiation. However, the new PT IGBT’s tail
time is longer than that of the conventional PT IGBT because
drift layer is thicker than that of the
the new PT IGBT’s
conventional PT IGBT. Yet, the new PT IGBT’s turn-off loss is
lower since its tail current is lower and its length is shorter.
E. Ruggedness
Shown in Fig. 10 is the switching waveform of the new
PT-IGBT under short-circuit condition. As shown here, the new
PT IGBT is not damaged, even if the short-circuit duration is 20
s. Thus, it has more than sufficient short-circuit endurance for
actual application use. Fig. 11 shows the switching waveform
for the RBSOA test of the new PT IGBT. It was confirmed
that the new PT IGBT is capable of turning off 150 A (current
A/cm ) which is 5 the rated current under such
density
harsh conditions. Thus, the new PT IGBT has a larger turn-off
capability (RBSOA) than that of the conventional PT IGBT.
V. CONCLUSION
Experimental results demonstrated that heat treatment (annealing) reduced the resistivity in the proton irradiated -type
region of a PT IGBT. Application of this phenomenon allowed
development of a 1200-V PT IGBT having a new -buffer layer.
Experiments verified that the new PT IGBT has the merits of
an NPT IGBT, with the crossover point of the I–V curves at
C and
C far below the rated current and with
high short-circuit endurance. In addition, lower losses than with
existing third-generation PT IGBTs were achieved.
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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 38, NO. 1, JANUARY/FEBRUARY 2002
REFERENCES
[1] B. J. Baliga, Modern Power Devices. Melbourne, FL: Krieger, 1992,
p. 358.
[2] D. S. Kuo et al., “Modeling the turn-off characteristics of bipolar MOS
transistor,” IEEE Electron Device Lett., vol. EDL-6, pp. 211–214, June
1985.
[3] J. Yamashita, T. Yamada, S. Uchida, H. Yamaguchi, and S. Ishizawa, “A
relation between dynamic saturation characteristics and tail current of
nonpunchthrough IGBT,” in Conf. Rec. IEEE-IAS Annu. Meeting, vol.
3, 1996, pp. 1425–1432.
[4] K. Mochizuki, K. Ishii, M. Takeda, H. Hagino, and T. Yamada, “Examination of punch through IGBT for high voltage and high current applications,” in Proc. Int. Symp. Power Semiconductor Devices, 1997, pp.
237–240.
[5] W. Wondrak and D. Silber, “Buried recombination layers with enhanced
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pp. 322–326, 1985.
N
Hideo Iwamoto was born in Japan in 1943. He received the B.E. degree in electrical engineering from
Osaka University, Osaka, Japan, and the Doctor of
Engineering degree from Yamaguchi University, Yamaguchi, Japan, in 1967 and 2001, respectively.
He was with Mitsubishi Electric Corporation from
1967 to 1991. He was with Powerex, Inc., Youngwood, PA, from 1992 to 1998, prior to reurning to
Mitsubishi Electric Corporation, Fukuoka, Japan, in
1999. He has been engaged in the design and development of power semiconductor devices.
Mr. Iwamoto is a member of the Institute of Electrical Engineers of Japan.
Hideki Haruguchi was born in Kagoshima Prefecture, Japan, in 1968. He received the B.S. degree in
electrical engineering from Kumamoto University,
Kumamoto, Japan, in 1991.
Since 1991, he has been with the Engineering
Development and Design Section, Power Semiconductor Division, Mitsubishi Electric Corporation,
Fukuoka, Japan, where he has worked on the design
of power semiconductor devices, in particular, the
design of IGBT chips.
Yoshifumi Tomomatsu was born in 1967. He
received the B.S. degree in applied physics from
Fukuoka University, Fukuoka, Japan, in 1989.
In 1989, he joined Fukuryo Semiconductor Engineering Corporation, Fukuoka, Japan, a subsidiary
group of Mitsubishi Electric Corporation. He is
a Design and Development Engineer for power
semiconductors (IGBT and diode).
Mr. Tomomatsu is a member of the Institute of
Electrical Engineers of Japan.
John F. Donlon (S’64–M’66–SM’93) received the
B.S. degree with high honors from the University
of Lowell, Lowell, MA, and the M.S. degree from
Syracuse University, Syracuse, NY, in 1966 and
1970, respectively, both in electrical engineering.
While at Syracuse University, he studied under a
National Aeronautics and Space Administration
Traineeship.
He is Senior Application Engineer at Powerex,
Inc., Youngwood, PA, and has been involved in the
rating, evaluation, and application of power semiconductors for the past 31 years. He represents Powerex on the Electronic Industries Alliance JEDEC Standards Committee for Rectifiers and Thyristors and
chairs the Committee on Transistors. He has been active in the publication of
application notes and technical papers describing the characteristics and proper
application of power semiconductors.
Mr. Donlon is a senior member of the IEEE Industry Applications Society
and a member of the IEEE Power Electronics Society, Tau Epsilon Sigma, and
Eta Kappa Nu.
Eric R. Motto (M’90) received the B.S. degree in
electrical engineering from The Pennsylvania State
University, University Park, and the B.A. degree in
mathematics from Saint Vincent College, Latrobe,
PA, in 1987.
He is a Principal Application Engineer with Powerex, Inc., Youngwood, PA, and has been involved in
the evaluation and application of IGBT and intelligent power modules since 1990. He has written and
presented more than 20 conference technical papers
and authored numerous application notes and trade
magazine articles.