6
Applications Information
Introduction
Agilent’s HSMS-286x family of
Schottky detector diodes has been
developed specifically for low
cost, high volume designs in two
kinds of applications. In small
signal detector applications
(Pin < -20 dBm), this diode is used
with DC bias at frequencies above
1.5 GHz. At lower frequencies, the
zero bias HSMS-285x family
should be considered.
In large signal power or gain
control applications
(Pin > -20 dBm), this family is used
without bias at frequencies above
4 GHz. At lower frequencies, the
HSMS-282x family is preferred.
Schottky Barrier Diode
Characteristics
Stripped of its package, a
Schottky barrier diode chip
consists of a metal-semiconductor
barrier formed by deposition of a
metal layer on a semiconductor.
The most common of several
different types, the passivated
diode, is shown in Figure 7, along
with its equivalent circuit.
Figure 7. Schottky Diode Chip.
RS is the parasitic series
resistance of the diode, the sum of
the bondwire and leadframe
resistance, the resistance of the
bulk layer of silicon, etc. RF
energy coupled into RS is lost as
heat —it does not contribute to
the rectified output of the diode.
CJ is parasitic junction capaci-
tance of the diode, controlled by
the thickness of the epitaxial layer
and the diameter of the Schottky
contact. R j is the junction
resistance of the diode, a function
of the total current flowing
through it.
8.33 X 10-5 nT
Rj = –––––––––––– = RV – Rs
IS + Ib
0.026
= ––––– at 25°C
IS + Ib
where
n = ideality factor (see table of
SPICE parameters)
T = temperature in °K
IS = saturation current (see
table of SPICE parameters)
Ib = externally applied bias
current in amps
IS is a function of diode barrier
height, and can range from
picoamps for high barrier diodes
to as much as 5 µA for very low
barrier diodes.
The Height of the Schottky
Barrier
The current-voltage characteristic
of a Schottky barrier diode at
room temperature is described by
the following equation:
V - IRS
I = IS (exp (––––––) - 1)
0.026
On a semi-log plot (as shown in
the Agilent catalog) the current
graph will be a straight line with
inverse slope 2.3 X 0.026 = 0.060
volts per cycle (until the effect of
RS is seen in a curve that droops
at high current). All Schottky
diode curves have the same slope,
but not necessarily the same value
of current for a given voltage. This
is determined by the saturation
current, IS, and is related to the
barrier height of the diode.
Through the choice of p-type or
n-type silicon, and the selection of
metal, one can tailor the
characteristics of a Schottky
diode. Barrier height will be
altered, and at the same time CJ
and RS will be changed. In
general, very low barrier height
diodes (with high values of IS,
suitable for zero bias applica-
tions) are realized on p-type
silicon. Such diodes suffer from
higher values of RS than do the
n-type. Thus, p-type diodes are
generally reserved for small signal
detector applications (where very
high values of RV swamp out high
RS) and n-type diodes are used for
mixer applications (where high
L.O. drive levels keep RV low) and
DC biased detectors.
Measuring Diode Linear
Parameters
The measurement of the many
elements which make up the
equivalent circuit for a packaged
Schottky diode is a complex task.
Various techniques are used for
each element. The task begins
with the elements of the diode
chip itself. (See Figure 8).
R
S
R
V
C
j
Figure 8. Equivalent Circuit of a
Schottky Diode Chip.
RS is perhaps the easiest to
measure accurately. The V-I curve
is measured for the diode under
forward bias, and the slope of the
curve is taken at some relatively
R
S
R
j
C
j
METAL
SCHOTTKY JUNCTION
PASSIVATION PASSIVATION
N-TYPE OR P-TYPE EPI LAYER
N-TYPE OR P-TYPE SILICON SUBSTRATE
CROSS-SECTION OF SCHOTTKY
BARRIER DIODE CHIP
EQUIVALENT
CIRCUIT