Application Note

VISHAY GENERAL SEMICONDUCTOR
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Rectifiers
Application Note
The First Commercial 200 V TMBS® Rectifier
vs. Traditional Rectifiers in Telecom Applications
By Max Chen and Henry Kuo
ABSTRACT
2. DESIGN, SIMULATION AND FABRICATION
The design and manufacturing process for the first
commercial 200 V Trench MOS Barrier Schottky (TMBS®)
rectifier is described. Its electrical characteristics are
compared to traditional 200 V planar Schottky and 200 V
ultrafast recovery diodes, showing the strengths of the
TMBS rectifiers compared to both technologies. The 200 V
TMBS is subsequently evaluated in telecom switch mode
power supply (SMPS) and DC/DC converter applications,
where its impact on improving system efficiency is analyzed
and reported.
2.1 The 200 V TMBS Design
For the design of the 200 V TMBS rectifier shown in Fig. 1,
including the main cells and edge termination structure,
comprehensive numerical analysis using the SILVACO
two-dimensional device simulator was performed to show
the relationship between device’s electric characteristics
and critical structural parameters.
Active
W1
1. INTRODUCTION
Traditionally, the reverse blocking voltage of Schottky
barrier rectifiers has been limited to well below 200 V. This
is partly because, when the reverse blocking capability
approaches 200 V; the forward voltage drop (VF) of the
Schottky rectifier will approach that of a PIN rectifier, making
it less effective in the application. Another reason for this
limitation is the need to terminate the high reverse electric
field properly. A P-type guardring structure is usually built
into the N-type silicon, and this guard-ring structure will
induce a high degree of minority-carrier injection to the
N-type drift region under forward conduction mode. When
this happens, the Schottky rectifier will incur high switching
losses in the circuit, also making it a less attractive
component choice compared to ultrafast recovery diodes.
The edge termination of this 200 V TMBS device is
accomplished by a novel design fabricated together with the
main cells of the device, such as that described by Hsu et al
[1]. By properly selecting the critical device dimensions, a
reverse blocking capability meeting 200 V application
requirements is achieved using multi-trenches with
dimensions of only 2.0 μm to 2.5 μm deep and a thin gate
oxide layer of 0.30 μm or less. This design greatly facilitates
manufacturing throughput and process control while
eliminating the need for deep trench fabrication.
Active Edge to Contact
TEOS
Metal
H1
Metal Extension
Trench Termination Width
N- Epitaxy
≈
N+ Substrate
≈
Fig. 1 - The Schematic Cross Section of a TMBS Diode with the
Novel Trench Termination Design. Wt, Wm, and Ht Represent
Trench Width, Mesa Width, and Trench Depth Respectively
The structural parameters were recursively simulated based
on those relationships to meet our target specifications for
on-state voltage drop and reverse leakage at 125 °C. With
an eye towards manufacturing throughput, we found the
optimum structure has a trench depth of 2.1 μm and oxide
thickness of 0.27 μm. These dimensions are much smaller
than those previously published for 100 V trench rectifiers by
Shimizu et al. [2]; they are also smaller than the published
figures for 150 V GD-trench rectifiers with a device
simulation with trench depth of 8.0 μm and oxide thickness
of 0.7 μm presented by Mahalingam and Baliga [3]. This
shallow-trench thin-oxide TMBS (STTO-TMBS) structure
can greatly reduce the process cycle time required for
trench etching in fabrication and thus improve
manufacturing process throughput.
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Document Number: 89107
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APPLICATION NOTE
Addressing the weaknesses of high VF and high switching
losses in traditional 200 V Schottky rectifiers, the industry’s
first 200 V Trench MOS Barrier Schottky (TMBS) rectifier,
constructed with a shallow trench and thin gate oxide
structure, is proposed and presented in this paper.
Wm
Termination
Application Note
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Vishay General Semiconductor
The First Commercial 200 V TMBS® Rectifier
vs. Traditional Rectifiers in Telecom Applications
2.2 Simulation Results
The electric field distribution along the centerline of an active
cell was plotted against the depth into the N-type drift region
for the optimized design, under 200 V reverse bias voltage.
A similar simulation was carried out on a traditional 200 V
planar Schottky device and the electric field curve was
overlapped to the TMBS curve as depicted in Fig. 2.
material used and the design envelope of the trench
physical dimensions. It also paves the way for ensuring
excellent avalanche capability when reverse overstress
occurs.
2.5 x 105
E-Field (V/cm)
2.0 x 105
TMBS
1.5 x 105
Planar Schottky
1.0 x 105
0.5 x 105
Fig. 3 - The Simulated Electric Field Distributions and Equipotential
Lines of a TMBS Diode Under 200 V Reverse Bias
0
0
5
10
15
20
25
Drift Region Depth (µm)
Fig. 2 - The Simulated Electric Field Strength at a Reverse Voltage
of 200 V for TMBS and Traditional Planar Schottky
As described in Fig. 2, under 200 V reverse bias, we find that
the TMBS rectifier has an electric field distribution with
peaks at the depth of 2.8 μm and a value of 2.2 x 105 V/cm;
this field strength is 73 % of the critical electric field of the
silicon at 3.0 x 105 V/cm. On the other hand, the planar
Schottky rectifier’s electric field strength curve follows that
of a typical metal-semiconductor junction characteristic,
with a linear distribution. Due to the nonlinear electric field
strength behavior of TMBS rectifier, we have been able to
achieve the desired 200 V reverse blocking capability with a
higher concentration of silicon than that found in a planar
Schottky rectifier as indicated by gradients of the respective
electric field strength curves which follow the formula:
qN
dE(x)
------------- = - ----------Adx
S
Besides the active cells, the electric field distribution was
also analyzed for the edge termination regions. As shown in
Fig. 3, under 200 V reverse bias voltage, the maximum
electric field strength is uniform across the active cell trench
corners and the termination regions. With this result, we
have ensured the device performance is optimized per the
The fabrication process starts with trench etching in the
active and termination regions (Fig. 1). After gate oxidation,
an N+ polysilicon layer is deposited and an etching back
process forms the poly spacer in the termination region.
With the MOS structure formed in the trenches by the
polysilicon-oxide-silicon configuration, a passivation layer
is
deposited,
followed
by
definition
of
the
metal-semiconductor (M-S) contact region. After the
formation of the M-S Schottky contact with a selected
barrier metal, a conduction metal layer is deposited, and a
mask is used to define the metal region. The wafers are then
subjected to a back grinding and backside metalization
process to form the ohmic contact for the wafer backside.
For the critical Schottky contact in this process flow, and to
achieve specific device characteristics, a proprietary
sputtering process and salicide structure have been
developed to enable selection of Schottky barrier heights
from 0.70 eV to above 0.80 eV.
3. DEVICE CHARACTERIZATION
3.1 Parametric Measurement
Our 200 V TMBS wafers were sawn to chips and assembled
in TO-220 packages for parametric measurement. Their
static DC characteristics were checked by multifunction
production testers and high-power curve tracers
(Tektronix-371A). A constant temperature chamber was
used for specific-TJ measurement to simulate the actual
device working environment. An LEM tester with module
LEM Qrr 50 A was used for device switching performance
characterization, where several combinations of IF, dI/dt,
and VR conditions were measured.
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APPLICATION NOTE
Another TMBS advantage is the narrowness of the required
drift region, which is 13 μm for TMBS and much less than
the 19 μm found in a planar Schottky rectifier. Combining
the above two factors, the TMBS rectifiers provide much
less drift region conduction resistance, which is sufficient to
negate the area loss contributed by the non-conducting
MOS trench structures.
2.3 Fabrication
Application Note
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Vishay General Semiconductor
The First Commercial 200 V TMBS® Rectifier
vs. Traditional Rectifiers in Telecom Applications
to an UFRD having the same die size, the 200 V TMBS
showed a VF improvement of 16 % at a current density of
180 A/cm2 (at TJ = 125 °C) while its switching characteristics
are the same (see Table 1).
220
200
160
3.2 Benchmarking Test with Planar Schottky and
Ultrafast Rectifier
The electrical characteristics of packaged 200 V TMBS
rectifiers were compared to industry standard planar
Schottky barrier rectifiers with a 90 A datasheet rating and
to three types of 30 A rated ultrafast recovery diodes (UFRD)
from leading power semiconductor manufacturers. When
compared to planar Schottky rectifiers, the forward voltage
drop (VF) of 200 V TMBS device achieved a better than 13 %
improvement at a current density of 180 A/cm2 and junction
temperature of 125 °C (Fig. 4). In fact, the 200 V TMBS
provided various degrees of VF improvement over the whole
forward current range. On the other hand, when compared
Planar
Schottky
180
TMBS
JF (A/cm2)
The 200 V TMBS is designed to have optimized VF
performance with the current density near 180 A/cm2. As far
as switching performance is concerned, in high
forward-current and high-frequency switching applications,
200 V TMBS limits the minority carrier injection and quickly
recombines minority carriers through high-concentration
majority carriers. The device’s punch-through design, which
enhances this recombination effect, also contributes to the
excellent switching performance of the 200 V TMBS
rectifier.
140
120
100
UFRD Type 1
80
UFRD Type 2
60
UFRD Type 3
40
0.45
0.50
0.55
0.60
0.70
0.65
0.75
0.80
0.85
VF (V)
Fig. 4 - JF - VF Curves for 200 V Rectifiers Under Benchmark
(TJ = 125 °C)
TABLE 1 - ELECTRICAL CHARACTERISTICS COMPARISON OF 200 V RECTIFIERS
TMBS
PLANAR
SCHOTTKY
UFRD
TYPE 1
UFRD
TYPE 2
UFRD
TYPE 3
Die size
9.1 x 10-2
2.0 x 10-1
9.1 x 10-2
7.7 x 10-2
9.9 x 10-2
VR (V)
at 1 mA
210
222
269
266
287
IR (mA)
at 200 V/125 °C
8.3
1.1
0.1
1.8
0.1
VF (V)
at 15 A/125 °C
0.63
0.63
0.76
0.81
0.77
5.8
8.6
7.2
5.8
7.1
45
63
43
34
42
136
280
155
100
152
Die size (cm2)
Irr (A)
trr (ns)
Qrr (nC)
IF = 15 A, dI/dt = 200 A/μs,
VR = 200 V, 125 °C
2
UFRD Type 2
0
UFRD Type 3
-2
-4
-6
TMBS
UFRD Type 1
-8
Planar
Schottky
- 10
- 10
0
10
20
30
40
50
60
70
Time (ns)
Fig. 5 - Switching Performance Comparisons Between 200 V
Rectifiers (IF = 15 A, dI/dt = 200 A/μs, VR = 200 V, TJ = 125 °C)
Revision: 22-Oct-15
Document Number: 89107
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APPLICATION NOTE
One commercial UFRD (Type 2) was observed to have
better reverse-recovery performance compared to 200 V
TMBS under condition of IF = 15 A, dI/dt = 200 A/μs,
VR = 200 V, and TJ = 125 °C (Fig. 5), but its VF was the
highest observed also. According to data comparisons at
125 °C, this commercial UFRD was calculated to have a
19 % higher VF than the 200 V TMBS under the same
current density of 180 A/cm2.
Current (A)
200 V RECTIFIERS COMPARISON
Application Note
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The First Commercial 200 V TMBS® Rectifier
vs. Traditional Rectifiers in Telecom Applications
4. APPLICATION TEST
4.1 Efficiency Benchmark on SMPS
To assess the benefits of the device within an application,
the 200 V TMBS rectifiers with the die size specified in
Table 1 were used in a state-of-the-art AC/DC telecom
SMPS with a 1800 W maximum output. The SMPS is built
with a phase shift full-bridge ZVS (zero voltage switching)
topology. When measured at a 30 A, 51 VDC output current,
the 200 V TMBS solution provided a 0.44 % efficiency
improvement compared to a commercial 200 V planar
Schottky rectifier. It is worth noting that the die size of the
TMBS used in this evaluation occupies only 46 % of the area
of the planar Schottky. Four Schottky rectifiers were used in
this SMPS, each with a common-cathode dual-rectifier
configuration. The total silicon usage with the TMBS
approach is 0.73 cm2, compared to 1.60 cm2 for the
traditional planar Schottky technology in the 1800 W SMPS
evaluation. Following is the test result in a chamber with
constant 50 °C ambient temperature.
TABLE 2
PRODUCT
POWER LOADING
Pin (W)
Pout (W)
EFFICIENCY
573
515
89.9 %
1.13
574
515
89.7 %
0 (base point)
0.41
TMBS
Planar Schottky
~ 30 %
POWER SAVING (W)
UFRD Type 1
574
515
89.8 %
TMBS
1685
1551
92.1 %
7.42
1691
1549
91.6 %
0 (base point)
1688
1550
91.8 %
3.29
Planar Schottky
~ 90 %
UFRD Type 1
Note
• Efficiency comparisons between TMBS, planar Schottky, and UFRD used in a 1800 W rated SMPS at 50 °C ambient temperature; all devices
packaged in the TO-247. Die sizes as described in Table 1.
4.2 Efficiency Benchmark on DC/DC Converter
To understand the benefit of the TMBS device, we tested
one full-brick, 48 V input, and 32 V/18 A output isolated
telecom DC/DC converter. This DC/DC converter topology
has two forward converters in parallel with a 350 kHz
switching frequency and uses four 200 V-rated 30 A
rectifiers. Both of the UFRD Type 2 and UFRD Type 3
devices are ultrafast rectifiers with a typical trr of less than
17 ns.
The TMBS rectifier can deliver a 1 % efficiency improvement
over the ultrafast rectifier in this application at an ambient
temperature of 25 °C. Translated into power savings, this
means the TMBS can reduce power dissipation by more
than 5 W compared to ultrafast rectifiers in this DC/DC
converter application. This power reduction is very
important in DC/DC converter designs; and in particular
from a thermal management point of view. Based on our
comparisons to date with commercially available parts,
there are no available planar Schottky rectifiers that can
function within this 350 kHz application; when attempted,
the converter shut down immediately due to its overload
protection function.
In the next stage of testing, with the ambient temperature
increased to 35 °C, the TMBS rectifier has demonstrated
power savings of more than 9 W compared to the UFRD
Type 2 sample, while the UFRD Type 3 sample would cause
converter overload protection function to be triggered in this
35 °C ambient temperature test.
PRODUCT
POWER LOADING
Pin (W)
Pout (W)
EFFICIENCY
310
284
91.7 %
2.72
312
284
90.9 %
0 (base point)
UFRD Type 3
312
284
90.9 %
0.15
TMBS
621
569
91.6 %
7.14
627
569
90.7 %
1.71
628
568
90.5 %
0 (base point)
TMBS
UFRD Type 2
UFRD Type 2
UFRD Type 3
~ 50 %
~ 100 %
POWER SAVING (W)
Note
• Efficiency comparisons between TMBS and two types UFRD on 600 W rated DC/DC converter at 25 °C ambient temperature; all devices
packaged in the TO-263. Die sizes as described in Table 1.
Revision: 22-Oct-15
Document Number: 89107
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APPLICATION NOTE
TABLE 3
Application Note
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Vishay General Semiconductor
The First Commercial 200 V TMBS® Rectifier
vs. Traditional Rectifiers in Telecom Applications
5. SUMMARY
6. REFERENCES
The development of a new 200 V TMBS rectifier built on a
high-efficiency fabrication process and achieving improved
electrical characteristics has been reported. The device
parameters and specifications are carefully optimized to
maximize efficiency in the manufacturing process and
achieve target electrical characteristics. The forward voltage
drop of the fabricated 200 V TMBS rectifier is 13 % lower
than same-size planar Schottky rectifiers while exhibiting a
reverse recovery charge of only 49 % of the comparable
performance planar device value. The 200 V TMBS was
evaluated in an 1800 W SMPS as the output rectifier, where
the TMBS device required less than half the die size area to
achieve 0.3 % better efficiency than the traditional planar
Schottky rectifier. In the DC/DC converter evaluation, the
overall performance of the 200 V TMBS is far superior to
200 V ultrafast diodes. These results indicate that TMBS
rectifiers have high potential to become the preferred
rectifier choice for 200 V applications in the future.
[1] W.C.W. Hsu et al., “A novel trench termination design for
TMBS diode application”, IEEE Elec. Dev. Let, Vol. 22,
No. 11, pp. 551 to 552 (2001).
[2] T.Shimizu et al., “100 V Trench MOS Barrier Schottky
Rectifier Using Thick Oxide Layer (TO-TMBS)”, Proc .Int.
Symp. on Power Semiconductor Devices and Ics, pp.
243 to 246 (2001).
[3] S. Mahalingam and B.J. Baliga, “A Low Forward Drop
High Voltage Trench MOS Barrier Schottky Rectifier with
Linearly Graded Doping Profile”, Proc. Int. Symp. on
Power Semiconductor Devices and ICs, pp. 187 to 190
(1998).
APPLICATION NOTE
Revision: 22-Oct-15
Document Number: 89107
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