Application Note

VISHAY GENERAL SEMICONDUCTOR
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Rectifiers
Application Note
TMBS®, Trench MOS Barrier Schottky Rectifiers Address
Weaknesses of Traditional Planar Schottky Devices
By Max Chen, Henry Kuo, and Sweetman Kim
Until recently, silicon-based Schottky barrier rectifiers were
limited to operating voltages below 100 V for most
applications. However, the trend in high-frequency
applications has been towards greater power consumption,
requiring higher reverse bias voltages (100 V and above) for
higher-output adapters. In consumer electronics, there has
been a rapid increase in the power consumption of
computers, game consoles, and LCD TVs. A similar
development has occurred in the design of high-power,
high-efficiency telecom base station power supplies. Higher
voltage spikes and / or higher switching frequencies are the
inevitable outcome of this trend.
Consequently, Schottky rectifiers are now being designed
with higher operating voltage ranges. To meet the
requirements of the consumer electronics and telecom
applications, 100 V Schottky rectifiers have become more
common, with new designs moving toward reverse bias
voltage ratings of 120 V and 150 V, or even 200 V. However,
as higher operating voltages have become more common,
the performance limitations of Schottky devices have
become more obvious.
As shown in Fig. 1, planar structures typically implement
a carefully designed P-type guard-ring structure, a
high-resistivity silicon epitaxial layer, and a high Schottky
barrier height to achieve breakdown voltages of 100 V and
above with an acceptable reverse leakage current. In this
structure, the P-type guard-ring injects minority carriers into
the semiconductor drift region; these carriers slow down the
Schottky rectifiers under switching conditions.
As the operating voltage moves to 100 V and above, the
high-resistivity silicon and high Schottky barrier height
become factors in significantly increasing on-state voltage
drop and slowing switching speeds. These limitations can
be alleviated by using an innovative device built on a trench
metal oxide (MOS) technology.
Oxide
Metal
(Poly-Si)
Semiconductor
(Drift Region)
Fig. 2 - SEM Photograph of a Single TMBS Sub-Micron Cell
Poly Silicon
Contact Metal
Schottky Metal
Oxide
N- Epi
P+ Guard-Ring
Metal
Oxide
Metal
(Replaced by
Poly Silicon)
Silicon
N+ Substrate
N- Epi
N+ Substrate
Metal
Fig. 1 - Planar Structures Achieve Breakdown Voltages of 100 V and
Above by Utilizing a P-type Guard-Ring Structure, a
High-Resistivity Silicon Epitaxial Layer, and a High Schottky
Barrier Height
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APPLICATION NOTE
Fig. 3 - Multi-Cell Structure of a TMBS
The increased on-state voltage drop and slower switching
speeds of traditional planar Schottky rectifiers can be
overcome by a new series of 100 V rectifiers that apply the
Trench MOS Barrier Schottky (TMBS®) structure. A single
TMBS sub-micron cell is shown in the SEM photograph of
Fig. 2 and the multi-cell structure of the device is illustrated
in Fig. 3. The parameters that affect TMBS performance
include the trench depth, mesa width, trench oxide
thickness, doping of the epitaxial layer, and electric field
termination. These parameters are related to stress, charge
coupling, optimized forward voltage drop (VF), and reverse
current (IR).
Application Note
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TMBS®, Trench MOS Barrier Schottky Rectifiers Address
Weaknesses of Traditional Planar Schottky Devices
As shown in Fig. 4, depletion regions occur when the TMBS
device is in reverse bias, and the MOS couples the charge
along its sidewall. The depletion regions of two adjacent
MOS transistors will overlap as the reverse bias increases,
resulting in “pinch-off” (an overlapped depletion region). The
Schottky diode’s interface leakage current (IR) is reduced by
this pinch-off electric field.
Schottky Depletion Region
region) will be obtained even with much lower resistivity
silicon, as indicated by the sharp gradient of TMBS
electric-field curve.
By altering the electric field distribution, the TMBS structure
moves the stronger electric field away from the Schottky
metal-silicon interface to the silicon bulk. As indicated in
Fig. 5, the surface electric field (E-field) of the TMBS device
is much lower than the planar Schottky devices. The
reduced surface field will suppress the barrier-lowering
effect, which significantly reduces the leakage current for a
given Schottky barrier height. This allows lower Schottky
barrier heights to be used without sacrificing reverse
leakage performance, which in turn results in a lower
forward on-state voltage drop.
N- Epi
N+ Substrate
MOS Depletion Range
In the Trench MOS Barrier structure, stored charges are
minimized under switching conditions as depletion regions
diminish minority carrier injections to the drift region. The
switching speed is much improved, especially under high
working temperature and high conduction current
conditions.
As depicted in Fig. 5, the Trench MOS structure provides
charge-coupling effects in the drift region that will change
the shape of the electric field distribution from linear to
non-linear, where same reverse breakdown voltages (which
is the integration of electric field along the distance of drift
E-field (V/cm)
TMBS
Fig. 4 - The Depletion Regions of two Adjacent MOS Structures
will Overlap as the Reverse Bias Increases, Resulting in an
Overlapped Depletion Region
Planar
Schottky
Position (µm)
Fig. 5 - A Comparison of the Electric Field Curves of the TMBS and
Planar Schottky Along the Semiconductor Drift Region Shows the
Surface Electric Field of the TMBS is Much Lower than the
Planar Schottky
TMBS AND PLANAR SCHOTTKY PERFORMANCE AND ELECTRICAL COMPARISON
When compared to conventional planar Schottky rectifiers
with the same chip sizes and barrier heights, a 100 V TMBS
device shows remarkable forward voltage drop
performance (Fig. 6). The switching performance of the
TMBS is also better, as shown in Fig. 7.
VF 25 °C 8 SKY 100 V
20
VF 125 °C 8 SKY 100 V
10
9
16
8
Trench MOS
14
7
Trench MOS
6
IF (A)
IF (A)
12
10
Material B
5
8
4
6
3
4
2
2
1
0
0.000
0.200
0.400
0.600
0.800
1.000
0
0.000
Material B
0.100
0.200
0.300
0.400
0.500
0.600
Fig. 6 - VF Comparison of TMBS and Conventional Planar Schottky Rectifiers
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APPLICATION NOTE
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Application Note
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TMBS®, Trench MOS Barrier Schottky Rectifiers Address
Weaknesses of Traditional Planar Schottky Devices
application. In the first test, we used a 350 W switch mode
power supply (SMPS) as a test vehicle. TMBS devices rated
at 40 A and 100 V were compared to industry-standard
Schottky rectifiers rated at 20 A and 100 V (MXXXXH100CT)
and two such devices with ratings of 60 A and 100 V
(XXCTQ100 and STXXXXH100CT). All devices in this
evaluation are packaged in the standard TO-220.
100 V TMBS Rectifier
0V
ΔV
Δt
100 V Conventional
Planar Schottky Diode
ΔV = 200 mV
Δt = 25.0 ns
Fig. 7 - trr Curves of TMBS and Planar Schottky Rectifiers Show
TMBS has Better Switching Performance
We performed a series of experiments in which TMBS
rectifiers were tested against benchmark planar Schottky
rectifiers to compare the two structures in an actual
The results show that at a 67 % output (about 235 W) to full
rated power levels, the 20 A rated device has the lowest total
power supply efficiency of 78.3 % (Table 1). When the 60 A
rated devices were substituted for the 20 A devices in the
same power supply slot, efficiency improvements of 0.8 %
and 0.9 % were observed. These improvements translate
into a savings of 2.35 W and 2.77 W, respectively, for the
power supply. Even when the 40 A TMBS rectifier is used to
replace a traditional planar device rated at 60 A, we see an
improvement in efficiency of 0.4 %. Compared to a baseline
20 A planar Schottky rectifier, the efficiency improvement is
1.3 %, for a power savings of 3.72 W.
TABLE 1
PART NUMBER
INPUT
OUTPUT
POWER (W)
POWER (W)
EFFICIENCY
POWER SAVING
0 (Base)
Industry 20 A planar Schottky (MXXXXH100CT)
299
234
78.3 %
Industry 60 A planar Schottky (XXCTQ100)
298
236
79.1 %
2.35 W
Industry 60 A planar Schottky (SXXXXXH100CT)
297
235
79.2 %
2.77 W
Industry 40 A TMBS (VTS40100CT)
297
236
79.6 %
3.72 W
Note
• Efficiency evaluation on a 350 W SMPS (switch mode power supply), comparing TMBS with industry-standard planar Schottky products.
TMBS provides efficiency improvement of 1.3 %, for a power saving of 3.72 W
By evaluating TMBS rectifiers with ratings of 40 A and 100 V
(VTS40100CT) in a 120 W adapter, we were able to further
demonstrate the capabilities of the new TMBS devices. The
typical solution for this application is a synchronous
rectification approach implemented with two 40 A, 100 V
MOSFETs and a matching driver IC. From test data as
described in Table 2, and under full-rated 120 W output
conditions, the pair of TMBS rectifiers provide the same
total adapter efficiency of 87 % as the more complicated,
more costly, and less robust synchronous rectification
solution.
ORIGINAL 40 A/100 V SR SOLUTION
INPUT VOLTAGE
CHANGE TO 40 A/100 V TMBS
90 VAC
100 VAC
90 VAC
100 VAC
Iin (A)
1.56
1.39
Iin (A)
1.56
1.40
Pin (W)
140
139
Pin (W)
140
139
V0 (V)
20.2
20.2
V0 (V)
20.2
20.2
INPUT VOLTAGE
I0 (A)
6.0
6.0
I0 (A)
6.0
6.0
P0 (W)
121
121
P0 (W)
121
121
Efficiency %
86.4
87.1
Efficiency %
86.2
87.0
Note
• Efficiency evaluation on 120 W adapter. The TMBS rectifier pair provides the same efficiency as the more complicated, more costly, and
less robust synchronous rectification solution
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APPLICATION NOTE
TABLE 2
Application Note
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Vishay General Semiconductor
TMBS®, Trench MOS Barrier Schottky Rectifiers Address
Weaknesses of Traditional Planar Schottky Devices
The ability to withstand higher energy transients during
reverse bias is another advantage of the TMBS structure.
The strongest electric field of a conventional planar Schottky
rectifier is at the surface of the device, which will limit heat
dissipation and avalanche energy absorption. The strongest
electrical field of a TMBS device, by contrast, distributes at
the bottom of each trench well. The silicon bulk can thus
absorb and dissipate more avalanche energy than at the
surface.
For high ESD or rectification applications such as OR-ing
diodes in hot-plug systems and diodes in SMPS, the ability
to enable high reverse avalanche energy makes the TMBS
rectifier the more suitable structure. The average of 8/20 μs
reverse surge energy of the 40 A, 100 V VTS40100CT is
about 170 mJ. This is about twice the conventional planar
Schottky diode’s reverse surge energy under the same test
condition.
At a low reverse bias, TMBS devices have a much higher
capacitance (CJ) than planar Schottky diodes. This high
capacitance is caused by the trench sidewall and bottom
capacitance being in parallel with the original Schottky
barrier capacitance. When reverse bias increases, however,
the TMBS capacitance falls significantly. Depletion regions
will completely overlap when reverse biases are high
enough.
TMBS and planar Schottky diodes have a similar electrical
field distribution. Fig. 8 shows the capacitance of Vishay
TMBS and a planar Schottky having the same chip size. We
can see that the CJ of the 100 V TMBS is close to that of the
planar Schottky rectifier for reverse bias voltages over 30 V.
However, since TMBS has a higher current density, a
smaller chip with lower capacitance may be used for a
particular application.
Planar Schottky
TMBS
10 000
TJ = 25 °C
f = 1.0 MHz
Vsig = 50 mVp-p
Junction Capacitance (pF)
Junction Capacitance (pF)
10 000
1000
100
10
TJ = 25 °C
f = 1.0 MHz
Vsig = 50 mVp-p
1000
100
0.1
1
10
100
Reverse Voltage (V)
0.1
1
10
100
Reverse Voltage (V)
Fig. 8 - Capacitance Comparison Between TMBS and Planar Schottky Diodes Shows that TMBS is Close to That of the Planar Schottky Rectifier
for Reverse Bias Voltages Over 30 V
200 V devices are the next stage for TMBS rectifiers. The
devices are currently under development, and the
preliminary test results as described in Table 3 have been
very encouraging, showing that the TMBS rectifier provides
lowest on-state voltage drop compared to the
industry-benchmark 200 V planar Schottky rectifiers and
200 V ultra-fast p-n junction diodes. The comparison
underscores the switching performance advantages of the
TMBS rectifier.
In addition to offering half the stored charge of
industry-benchmark devices, the peak switching reverse
recovery current (Irr) of TMBS rectifiers is also 34 % lower by
comparison. This low Irr will contribute to the lower
switching losses of the transistor switches and improve the
power conversion efficiency of the complete circuit,
particularly in designs with switching frequencies above
300 kHz. The first 200 V rated TMBS rectifier has been
released in Q4, 2006.
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APPLICATION NOTE
So far, benchmark and application tests have focused on
TMBS products with a 100 V reverse bias voltage. However,
120 V products have been developed and released for
applications that see higher voltage spikes and where a
100 V rated product might provide inadequate headroom to
ensure long-term reliability. These 120 V products offer
current ratings from 15 A to 60 A and are offered in the
ITO-220, TO-220, TO-262, TO-263, and TO-247 packages.
Application Note
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TMBS®, Trench MOS Barrier Schottky Rectifiers Address
Weaknesses of Traditional Planar Schottky Devices
TABLE 3 - ADVANTAGES OF TMBS OVER PLANAR SCHOTTKY IN RECTIFICATION APPLICATIONS
PRODUCT TYPE
INDUSTRY 200 V PLANAR SCHOTTKY
(MXXXX200CT)
INDUSTRY 200 V ULTRAFAST DIODE
(SXXX3002CG)
VISHAY 200 V TMBS
(EXPERIMENTAL)
VR (V)
at 1 mA/25 °C
259
296
210
IR (mA)
at 200 V/125 °C
0.10
0.01
6.70
VF (V)
at 5 A/125 °C
0.57
0.64
0.53
VF (V)
at 15 A/125 °C
0.69
0.80
0.65
5 A, 300 A/μs,
100 V, 10 %,
125 °C
7.2
7.0
4.7
33
29
25
122
104
60
Irr (A)
trr (ns)
Qrr (nC)
Note
• Comparison of electrical characteristics for the experimental 200 V TMBS rectifier and 200 V planar Schottky rectifier and ultrafast diode.
As demonstrated in Table 1, the lower forward voltage drop
and faster switching speed of the TMBS allow switch mode
power supplies to achieve higher efficiency operation. The
performance comparisons above (see Table 2) also show
that TMBS achieves the same efficiency as a MOSFET
synchronous rectifier in a 120 W adapter application, but at
a lower cost. This application will be discussed further
below, as well as other applications for the TMBS.
Due to the higher efficiency and better thermal performance
of low on-resistance MOSFETs, synchronous rectification
circuits have used them instead of planar Schottky rectifiers.
However, as we have shown, TMBS is a cost-effective
alternative to MOSFET synchronous rectifiers. Furthermore,
TMBS offers the better performance in low output current
devices such as adapters and open-frame power supplies.
Synchronous MOSFET rectifiers may indeed provide good
performance in high-current rectification applications, but
the percent of MOSFET switching losses grows higher in the
total power loss at output rectification in small current
applications.
In addition, by requiring complex circuitry to control it, the
MOSFET creates additional failure risks that can reduce the
reliability of power supply systems. Therefore, low-VF TMBS
rectifiers are more suitable for lower-current adapters and
open frame power supply designs. The performance of
TMBS can compete with MOSFET in lower-current
rectification applications and enjoys the advantages of
shortened design cycles and lower production costs by
requiring no additional control circuits. A block diagram
showing a MOSFET implementation of synchronous
rectification is shown in Fig. 9, and an alternate circuit using
TMBS is shown in Fig.10. As illustrated in these two figures,
a single TMBS diode may replace the MOSFETs and drive
circuit, thus simplifying the circuit significantly.
Vin
Vout
Load
Vin
Vout
Load
Fig. 10 - Alternate Circuit Using TMBS
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Fig. 9 - MOSFET Implementation of Synchronous Rectification
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TMBS®, Trench MOS Barrier Schottky Rectifiers Address
Weaknesses of Traditional Planar Schottky Devices
Diodes for redundant power supplies used in high-reliability
power systems (Fig. 11) are called OR-ing diodes, which are
widely used in computer servers and telecommunication
systems. The OR-ing diode at SMPS 2 must have a reverse
breakdown voltage greater than the output voltage of
SMPS 1 in order to protect SMPS 1. It must also have a low
reverse leakage current. However, if SMPS 1 malfunctions
and shuts down and SMPS 2 turns on, it is important that
the OR-ing diode have a low forward voltage drop to
minimize power losses from SMPS 2.
SMPS 1
System
SMPS 2
Fig. 11 - Redundant Power Supply Used in a High-Reliability Power System with OR-ing Diodes
In high-reliability power systems, the system is protected
from a shutdown resulting from malfunctions in one of the
power supplies by having redundant power supplies share
electrical loads. The design aims to isolate the failed SMPS
from the common bus if an SMPS failure occurs, and thus
requires a switch to isolate the failed SMPS. This switch
must be highly sensitive and able to react with high interrupt
speed when an SMPS fails; but it must also have low
conductivity for big transient loads.
For OR-ing functions in isolated redundant power supplies,
TMBS diodes offer significant improvements over
conventional diodes. Low-R(DS)on MOSFETs (OR-ing FETs)
are sometimes used in these applications. However, OR-ing
FETs have a lower turn speed than OR-ing diodes, which
may cause undesirable 0.8 V to 1.5 V voltage drops by the
body diode of the OR-ing FET after it is tripped by
high-transient voltage changes from high loading to low
loading. The more reliable redundant-power-system design
is to use one OR-ing FET in parallel with one low-VF
Schottky diode. The 100 V TMBS Schottky diode offers an
outstanding design solution in telecom redundant power
systems.
Another advantage of TMBS diodes is a higher reverse
energy capability, allowing them to better withstand
transients when power supplies switch on. The unique
trench well structure and silicon bulk in the TMBS absorb
and dissipate more avalanche energy than the plain
interface structure of conventional planar Schottky
rectifiers. This ability to enable higher reverse avalanche
energy makes the TMBS more suitable for use in high-ESD
or rectification applications than the average 8 μs by 20 μs
reverse surge energy of the VTS40100CT is about 170 mJ,
which is about two times that of conventional planar
Schottky diodes under the same testing conditions.
APPLICATION NOTE
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