HBAT-5400 Datasheet, PDF (6/8 Pages) Agilent(Hewlett-Packard) – High Performance Schottky Diode for Transient Suppression

6
Applications Information
Schottky Diode Fundamentals
The HBAT-540x series of clipping/
clamping diodes are Schottky
devices. A Schottky device is a
rectifying, metal-semiconductor
contact formed between a metal
and an n-doped or a p-doped
semiconductor. When a metal-
semiconductor junction is formed,
free electrons flow across the
junction from the semiconductor
and fill the free-energy states in
the metal. This flow of electrons
creates a depletion or potential
across the junction. The differ-
ence in energy levels between
semiconductor and metal is called
a Schottky barrier.
P-doped, Schottky-barrier diodes
excel at applications requiring
ultra low turn-on voltage (such as
zero-biased RF detectors). But
their very low, breakdown-voltage
and high series-resistance make
them unsuitable for the clipping
and clamping applications involv-
ing high forward currents and high
reverse voltages. Therefore, this
discussion will focus entirely on
n-doped Schottky diodes.
Under a forward bias (metal
connected to positive in an
n-doped Schottky), or forward
voltage, V_F, there are many
electrons with enough thermal
energy to cross the barrier poten-
tial into the metal. Once the
applied bias exceeds the built-in
potential of the junction, the
forward current, I_F, will increase
rapidly as V_F increases.
When the Schottky diode is
reverse biased, the potential
barrier for electrons becomes
large; hence, there is a small
probability that an electron will
have sufficient thermal energy to
cross the junction. The reverse
leakage current will be in the
nanoampere to microampere
range, depending upon the diode
type, the reverse voltage, and the
temperature.
In contrast to a conventional p-n
junction, current in the Schottky
diode is carried only by majority
carriers. Because no minority
carrier charge storage effects are
present, Schottky diodes have
carrier lifetimes of less than
100 ps and are extremely fast
switching semiconductors.
Schottky diodes are used as
rectifiers at frequencies of 50 GHz
and higher.
Another significant difference
between Schottky and p-n diodes
is the forward voltage drop.
Schottky diodes have a threshold
of typically 0.3 V in comparison to
that of 0.6 V in p-n junction
diodes. See Figure 6.
PN
METAL N
CAPACITANCE
CURRENT
CAPACITANCE
CURRENT
0.6 V
0.3 V
–
+
BIAS VOLTAGE
PN JUNCTION
Figure 6.
–
+
BIAS VOLTAGE
SCHOTTKY JUNCTION
Through the careful manipulation
of the diameter of the Schottky
contact and the choice of metal
deposited on the n-doped silicon,
the important characteristics of
the diode (junction capacitance,
C_J; parasitic series resistance, R_S;
breakdown voltage, VBR; and
forward voltage, V_F,) can be
optimized for specific applica-
tions. The HSMS-270x series and
HBAT-540x series of diodes are a
case in point.
Both diodes have similar barrier
heights; and this is indicated by
corresponding values of satura-
tion current, I_S. Yet, different
contact diameters and epitaxial-
layer thickness result in very
different values of junction
capacitance, C_J and R_S. This is
portrayed by their SPICE param-
eters in Table 1.
Table 1. HBAT-540x and
HSMS-270x SPICE Parameters.
Parameter
BV
CJ0
EG
IBV
IS
N
RS
PB
PT
M
HBAT-
540x
40 V
3.0 pF
0.55 eV
10E-4 A
1.0E-7 A
1.0
2.4 Ω
0.6 V
2
0.5
HSMS-
270x
25 V
6.7 pF
0.55 eV
10E-4 A
1.4E-7 A
1.04
0.65 Ω
0.6 V
2
0.5
At low values of I_F ≤ 1 mA, the
forward voltages of the two
diodes are nearly identical.
However, as current rises above
10 mA, the lower series resistance
of the HSMS-270x allows for a
much lower forward voltage. This
gives the HSMS-270x a much
higher current handling capability.
The trade-off is a higher value of
junction capacitance. The forward
voltage and current plots illustrate
the differences in these two
Schottky diodes, as shown in
Figure 7.