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
fl ow across the junction from the semiconductor and fi ll
the free-energy states in the metal. This fl ow of electrons
creates a depletion or potential across the junction. The
diff erence 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 involving 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
potential 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 suffi -
cient thermal energy to cross the junction. The reverse
leakage current will be in the nanoampere to microam-
pere 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 eff ects are
present, Schottky diodes have carrier lifetimes of less
than 100 ps and are extremely fast switching semi-
conductors. Schottky diodes are used as rectifi ers at
frequencies of 50 GHz and higher.
Another signifi cant diff erence 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.
Figure 6.
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 resis-
tance, R
S
; breakdown voltage, V
BR
; and forward voltage,
V
F
,) can be optimized for specifi c applications. 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 saturation
current, I
S
. Yet, diff erent contact diameters and epitaxial-
layer thickness result in very diff erent values of junction
capacitance, C
J
and R
S
. This is portrayed by their SPICE
parameters in Table 1.
Table 1. HBAT-540x and HSMS-270x SPICE Parameters.
Parameter HBAT-540x HSMS-270x
BV 40 V 25 V
CJ0 3.0 pF 6.7 pF
EG 0.55 eV 0.55 eV
IBV 10E-4 A 10E-4 A
IS 1.0E-7 A 1.4E-7 A
N 1.0 1.04
RS 2.4 Ω 0.65 Ω
PB 0.6 V 0.6 V
PT 2 2
M 0.5 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 capabil-
ity. The trade-off is a higher value of junction capacitance.
The forward voltage and current plots illustrate the diff er-
ences in these two Schottky diodes, as shown in Figure
7.
PN
CURRENT
0.6V
+–
BIAS VOLTAGE
PN JUNCTION
CAPACITANCE
METAL
N
CURRENT
0.3V
+–
BIAS VOLTAGE
SCHOTTKY JUNCTION
CAPACITANCE
