Micronote 401 - Introduction to Schottky Rectifiers (78.46 kB)

MicroNote
Series 401
by Kent Walters and Bob Werner
Introduction
to Schottky
Rectifiers
Schottky rectifiers have been
used for over 25 years in the
power supply industry. The
primary advantages are very
low forward voltage drop and
switching speeds that
approach zero time making
them ideal for output stages of
switching power supplies. This
latter feature has also
stimulated their additional use
in very high frequency
applications including very low
power involving signal and
switching diode requirements
of less than 100 picoseconds.
These require small Schottky
devices with low capacitance.
The reverse recovery time of
Schottky diodes are extremely
fast (but soft) recovery
characteristics. What little
reverse recovery time they
may exhibit is primarily
dictated by their capacitance
rather than minority carrier
recombination as in
conventional pn junction
rectifiers. This characteristic
provides very little reverse
current overshoot when
switching the Schottky from
the forward conducting mode
to the reverse blocking state.
The combination of very “fastsoft switching properties” of a
Schottky can also eliminate
the need for snubber circuits in
many applications that may
otherwise be required with fast
or ultrafast rectifiers displaying
abrupt recovery
characteristics. These
features make schottky
rectifiers a very attractive
choice for low parasitic
switching losses.
Design considerations with
Schottky devices are limited in
some applications compared
to pn junction rectifiers
because their reverse leakage
currents are many times
higher. Also Schottky rectifiers
have maximum rated junction
temperatures typically in the
range of 125°C to 175°C,
compared to the typical 200°C
for conventional pn junctions
which further influences
leakage current behavior.
For some applications,
Schottky devices are limited in
available reverse blocking
voltage ratings compared to
conventional pn junction
rectifiers. Nevertheless with
judicious selection, many
applications are optimized with
Schottky rectifiers and their
unique operating
characteristics. Schottky
rectifiers seldom exceed 100
volts in their working peak
reverse voltage (VRWM ), since
devices moderately above this
rating level will result in forward
voltages equal to or greater
than equivalent pn junction
rectifiers.
The Schottky rectifier
properties described above are
primarily determined by the
metal energy barrier height of
material deposited on the
silicon by the manufacturer. A
metal with a low energy barrier
height will minimize forward
voltage, but will also be
restricted in its high
temperature operating
capability and have very high
reverse leakage currents. A
high barrier metal height
selection will minimize
temperature and leakage
current sensitivity but will
increase the forward voltage.
Depending on the application
requirements, these design
features can be used as a
tradeoff in proper choice when
selecting a schottky rectifier
using different barrier metals
from a manufacturer.
Microsemi Corporation offers a
variety of barrier metal options
on an n-epitaxial layer over a
low resistivity substrate for
optimizing parametric
performance in addition to a
protective guardring and
passivation. This configuration
is shown in Figure 1. A reliable
schottky junction is designed
with a pn junction guard ring to
Schottky Barrier
Metal
Front Metal
Series 401
SiO2 Passivation
Schottky Rectifiers
terminate the perimeter with a
diffused p region. This also
serves as a transient voltage
suppressor for reverse energy
absorption and over-voltage
protection in close proximity to
the Schottky junction. This
perimeter region effectively is
driven into avalanche
breakdown before the Schottky
is damaged by excessive
amounts of reverse current
flow and energy during
transient events.
The voltage-current device
protective relation is illustrated
in Figure 2 along with the
typical electrical parameters
specified for Schottky
rectifiers. In some cases,
Schottky rectifiers are also
specified with a reverse
avalanche energy test. This
will help ensure a safe level of
operation in very fast switched
applications resulting in high
Ldi/dt inductive (such as
transformer) voltage kicks or
other sources of over-voltage
transients that can briefly drive
the Schottky beyond its
maximum rated VRWM . In such
applications, these special
reverse energy requirements
should be requested for
additional screening by the
user when not otherwise
specified.
Microsemi Corporate Applications
Kent Walters
(602) 941-6300
n- epi layer Guardring
Back Metal
n+ substrate
Figure 1
Vertical structure of Schottky Rectifier Die
PARAMETERS SPECIFIED AT 25°C
VF
IF
VRWM
IFSM
IR
VBR
Forward Voltage at I F
Forward Current
Working Peak Reverse Voltage
Forward Surge Current Maximum
(Specified non-repetitive 8.3ms, 1/2 sine)
Maximum Leakage Current at VRWM
Not Specified
(Typically 20% to 30% higher than VRWM)
Forward
Current
IF
Reverse
Voltage
Schottky
without
guardring
VBR
VRWM
VF at IF
Forward
Voltage
IR at VRWM
25°C
Schottky
Junction
25°C
IR at VRWM
100°C to 150°C
Avalanche
of pn junction
100 to 150°C
Reverse
Current
Figure 2
Schottky Voltage-Current Characteristics and Typical Parameters