Recent Advances in Telecommunications and Circuits
Effects of Charge Imbalance on Super Junction Power MOSFET
CHUN KEUN KIM, JONG MIN GEUM, and YONG TAE KIM
Semiconductor Materials and Devices Laboratory
Korea Institute of Science and Technology
P.O.Box 131, Cheongryang, Seoul 130-760
Republic of Korea
ytkim@kist.re.kr http://www.kist.re.kr
Abstract: We have optimized charge imbalance and designed 100-600V super junction Power MOSFET. The
breakdown voltage becomes higher and the on resistance is also reduced while the charge imbalance goes to
zero. But, the optimum charge imbalance shifts to -3 and -23 % corresponding to 600 and 100 V super junction
MOSFET. For the case of 100V, the on resistance is 1.33 mΩcm2, which is reduced by 40 % comparing that
of the Planar MOSFET at the same breakdown voltage.
Key-Words: Power device, Charge imbalance, Pillar Area, Super Junction, MOSFET
voltage and on-resistance for 100-600V Planar
MOSFET and Super Junction MOSFET.
1 Introduction
Low on-resistance of power MOSFET can be
minimizing the loss of supply of electric power;
thereby power efficiency can be greatly improved.
And also, there is strength of a simplification of the
driver circuit because it has excellent characteristics
of switching and high input on-resistance[1].
However, there is a problem in case of Power
MOSFET for high voltage, that the resistivity and
the thickness of the drift region might be increased
with increasing the breakdown voltage for
improvement of high voltage characteristic, and
therefore the characteristic of on-resistance is
rapidly increased according to the increase of
Breakdown Voltage. Accordingly, there is a drift
towards switching to Super Junction Power
MOSFET, which has as lower on-resistance as a
third of the on-resistance of the conventional planar
power MOSFET [2]. In this paper, we have
optimized charge imbalance in the trench filling of
Super Junction MOSFET based on the ratio between
N and P pillar area. So far, the charge balance has
been calculated with trench tilt angle, resulting in
low product yield since charge balance could not be
uniformly maintained during the fabrication[3,4].
Planar MOSFET, which has the same unit cell size
of the Super Junction MOSFET, is designed at first
before designing the Super Junction Power
MOSFET because the upper section structure of unit
cell of the Super Junction MOSFET for high voltage
is almost the same as the gate structure of
conventional planar MOSFET. The modified charge
imbalance is comparatively analyzed to get the
improvement of characteristics of breakdown
ISBN: 978-960-474-308-7
2 Design of 600V Super Junction
MOSFET
Super Junction MOSFET is normally consisted of
P-pillar under P-Body of Planar Power MOSFET.
The depletion region will be generated between p/n
pillars when the voltage is impressed into drain and
source, and this region will be working as the role of
a dielectric, which has no electric charge at the
voltage of about 50V. In other words, the impressed
voltage will be offset by the electric charge of p/n
pillar and completely removed, and thereby the
condition of drift region is changed like a dielectric,
which has no electric charge. Then, overall electric
field level is increased with maintaining flat electric
field distribution when the drain voltage is further
increased, so an avalanche occurs at threshold
voltage. The Super Junction MOSFET, which has a
squared form of electric field distribution, has
higher breakdown voltage than the Planar
MOSFET, which has a triangle form, because
integral of electric field is the voltage.[5,6,7]
The cross section structures of Power MOSFET
are shown in Fig. 1, the on-resistance can be divided
into four sections as a Channel, JFET, N-drift region
and substrate region. The breakdown voltage of the
device for general high voltage is decided by
depletion layer, which is built between the Body (Pwell) and N-drift region owing to the impressed
voltage at drain when the gate voltage is same with
the source voltage as shown in the Figure 1(left). It
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Recent Advances in Telecommunications and Circuits
Thereby, the charge on the electron of two regions is
same, and the regions of N and P are completely
depleted, so the vertical electric field is consistent
with no electric charge. This is so called ‘charge
imbalance”. In this work, we have suggested a new
charge balance equation to improve the mismatch of
conventional one based on the different trench angle
with the same pillar depth. However, the new one is
calculated with the pillar area ratio as shown below.
Fig. 1 Structure of Planar power MOSFET and
Super junction power MOSFET, Electric field when
device have Breakdown Voltage.
From this equation, we can get the charge
imbalance corresponding to the difference in the
pillar area.
is impossible to have below a certain constituent
of resistance due to a relation with breakdown
voltage, because over a certain thickness and
concentrations are required to maintain breakdown
voltage in N-drift region. Therefore, the onresistance in N-drift region accounts for 90% of all
on-resistance, and it is more severely affecting a
trend like this with higher breakdown voltage. To
overcome the Trade-Off relation between
breakdown voltage and the characteristics of onresistance, super junction MOSFETstructure was
proposed by using deep trench filling technology as
shown in Fig. 1. However, as shown in Fig. 2, the
electric charge in P-pillar is the same as that in the
N-pillar, the depletion layer sis expanded
horizontally and vertically.
.
Fig. 3 Breakdown voltage of 600V super
junction MOSFET according to different trench
angle
Fig. 4 Voltage drop of 600V super junction
MOSFET according to different trench angle
Fig. 2 Design of super junction MOSFET
ISBN: 978-960-474-308-7
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Recent Advances in Telecommunications and Circuits
with the smaller cell size. Then, we have tried to
optimize the charge imbalance for the case of 100 V
super junction MOSFET. From the simulation result,
the depth of pillar of 100 V is reduced by a seventh
of 600 V pillar depth.
3 Design of 100 V Super Junction
MOSFET
Electrical characteristics of 100V super junction
MOSFET show the same trend as 600 V case. The
breakdown voltage increases with the smaller half
cell pitch size, in contrast, the on resistance goes up
with the smaller cell size.
Fig. 7 shows that the charge imbalance shifts to 23% and the breakdown voltage is 155 V. At the
same charge imbalance, the on resistance is 1.33
mΩcm2 as shown in Fig. 8. The on resistance is as
low as 40 % of the Planar Power MOSFET.
Fig.5 Different cell sizes (from left, the half cell
pitch size is 8.5, 6.5, and 4.5, respectively) of
600V super junction MOSFET
Fig. 6 Breakdown voltage of 600 V super junction
MOSFET according to the half cell pitch size
previous designed Planar Power MOSFET applied
Deep Trench Filling technology.
The breakdown voltage of Super Junction
MOSFET increases while the pillar trench angle
closes to 90o as shown in Fig. 3. But, Fig. 4 shows
that the Pillar built by vertical angle has the smaller
on-resistance.
To evaluate the impact of half cell pitch size
performance characteristic, the p/n pillar change
resistivity, breakdown voltage (BV) and the electric
characteristics are simulated with N-pillar and Ppillar concentrations. The simulation results show
that the electrical performance is excellent when the
doping concentration of N-pillar is 3.74 x 1015 and
the P-pillar concentration is 7.02x1015.
Fig 5 shows the change of cell size corresponding
to half cell pitch size and Fig. 6 is the simulation
result of breakdown voltage of 600V super junction
MOSFET. This simulation result indicates that the
smaller cell size has the higher breakdown voltage
and the charge imbalance is shifted from the zero
point to -3%. However, the on resistance increases
ISBN: 978-960-474-308-7
Fig. 7 Breakdown voltage of 100 V super junction
MOSFET according to the charge imbalance
Fig. 8 Voltage drop of 100 V super junction
MOSFET according to the charge imbalance
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Recent Advances in Telecommunications and Circuits
4 Conclusion
We have designed 100-600V Planar and Super
junction MOSFETs and compared their electrical
performances. Optimizing the charge imbalance and
the half cell pitch size in the super junction
structure, the breakdown voltage and the onresistance are obtained. As a result, the charge
imbalance is optimized depending on the breakdown
voltage and the on resistance of 100-600V super
junction MOSFET.
References:
[1] E. G. Kang and M. Y. Sung, J. KIEEME, 15,
2002, pp.758
[2] T. J. Nam, H. S. Chung and E. G. Kang,
Optimal Design of GaN Power MOSFET
Using Al2O3 Gate, J. KIEEME, Vol. 24, No. 9,
2011, pp.713
[3] Malvino, Albert Paul, Bates, David J.,
Electronic Principles, McGraw-Hill College,
2006
[4] Gates, Earl D.i,Introduction to Electronics 4/E,
Delmar , 2001
[5] S. S. Kyoung, J. H. Seo, Y. H. Kim, J. S. Lee,
E. G. Kang, and M. Y. Sung, J. KIEEME, 22,
2009, p. 12
[6] H. S. Lee, E. G. Kang, A. R. Shin, H. H. Shin,
and M. Y. Sung, KIEE, 2006, pp. 7.
[7] William H.Hayt, Jr. Engineer Ingelect
Romagnetics-7/E, McGrawhill, 2005
ISBN: 978-960-474-308-7
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