7
Applications Information
Schottky Diode Fundamentals
The HSMS-270x 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
difference 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 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
(electrons). Because no minority-carrier (hole) charge
storage effects are present, Schottky diodes have carrier
lifetimes of less than 100 ps. This extremely fast switching
time makes the Schottky diode an ideal rectifier at fre-
quencies 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.
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, V
BR
; and forward voltage, V
F
,)
can be optimized for specific 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, different contact diameters and epitaxial-layer
thickness result in very different values of C
J
and R
S
. This
is seen by comparing their SPICE parameters in Table 1.
Table 1. HSMS-270x and HBAT-540x SPICE Parameters.
Parameter HSMS- 270x HBAT- 540x
BV 25 V 40 V
CJ0 6.7 pF 3.0 pF
EG 0.55 eV 0.55 eV
IBV 10E-4 A 10E-4 A
IS 1.4E-7 A 1.0E-7 A
N 1.04 1.0
RS 0.65 Ω 2.4 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
differences 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
I
F
– FORWARD CURRENT (mA)
V
F
– FORWARD VOLTAGE (V)
.01
10
1
.1
300
100
0 0.1 0.30.2 0.50.4 0.6
HSMS-270x
HBAT-540x
Figure 6.
Figure 7. Forward Current vs. Forward Voltage at 25°C.
