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hanks to their low
conduction and switching
energy loss, IGBTs arewidely used
in high power applications such
as power supplies, motor driving
inverters, electrical vehicles,
etc. The requirement for more
state of the art power devices for
power applications has triggered
efforts around novel silicon based
development, as well as wide
band gap material development,
to achieve characteristics that
stretchthe ideal limitof silicon. The
theoretical silicon limit for IGBTs
was investigated by A. Nakagawa,
[1] and in order to realize optimal
silicon characteristics, various
injection methods to enhance
IGBT structures were proposed,
such as CSTBT, IEGT, and narrow
mesa IGBT [2-4].
In order to push IGBT silicon to the
limit, extremely high electron injection
efficiency from the MOS gate is required,
while the hole carrier injection should
be restricted to the level of contribution
only for the conductivity modulation.
[1] For Fairchild’s 4th generation FS
IGBTs, electron injection was enhanced
by a very fine cell pitch design and hole
carrier injection was restricted by a new
buffer structure, achieving remarkably
better trade off performance as well as
strong latch up immunity. To realize the
narrow mesa or high-density cathode
design of the trench IGBT, a self-aligned
contact process was applied. This
proved to be very effective in optimizing
the critical dimension of active cell
design for the enhanced on-state
performance, as well as to maximize the
latch up current capability. In addition,
multiple buffer layers were adopted for
the anode side of the IGBT in order to
not only effectively control the minority
carrier injection during the on state,
but also to completely block the electric
field during the off state [5].
The vertical structures of the proposed
IGBT are illustrated in Fig.1 for the
cathode and anode side. Figs.1(a) and
1(c) showthat thehighdensity cell design
with submicron narrow mesa width is
successfully realized by employing a self-
aligned contact process, without any
photo misalignment. The higher density
active pattern shown in this figure is
beneficial for extremely enhanced
electron injection from the cathode
side and, as a result, the higher
electron current density. The new
buffer structure with multiple layers,
as shown in Fig.1(b), is very helpful
for the ideal carrier distribution during
IGBT operation. Generally, a single
buffer layer with 1~5e15cm–3 is used
for both hole injection control and
electric field blocking efficiently. In this
experiment, a thin buffer layer with
a much higher doping concentration
was additionally embedded for
better trade-off performance. In
other words, the higher doping
concentration in the double buffer
layer is even more effective for the
electric field blocking and hole carrier
injection control by the first FS layer
(L1). The lower doping concentration
for the second buffer layer (L2) is
preferred for forming a lightly doped
p-type collector for high-speed
switching performance without any
lifetime killing process. In addition,
the device switching waveform can
be effectively improved by varying the
doping concentration and thickness
of the double buffer layers, due to
proper carrier distribution control
during switching ON/OFF operation.
T
4th Generation Field Stop (FS) IGBT with High
Performance and Enhanced Latch Up Immunity
Kyuhyun Lee, Sungmin Yang, Sekyeong Lee, Jiyong Lim and Youngchul Choi - Fairchild Semiconductor
New-Tech Magazine Europe l 40