Applications Information
Product Selection
Avago’s family of surface mount Schottky diodes provide
unique solutions to many design problems. Each is opti‑
mized for certain applications.
The first step in choosing the right product is to select
the diode type. All of the products in the HSMS‑282x fam‑
ily use the same diode chip–they differ only in package
configuration. The same is true of the HSMS‑280x, ‑281x,
285x, ‑286x and ‑270x families. Each family has a different
set of characteristics, which can be compared most easily
by consulting the SPICE parameters given on each data
sheet.
The HSMS‑282x family has been optimized for use in RF
applications, such as
•
DC biased small signal detectors to 1.5 GHz.
•
Biased or unbiased large signal detectors (AGC or
power monitors) to 4 GHz.
•
Mixers and frequencymultipliers to 6 GHz.
The other feature of the HSMS‑282x family is its unit‑to‑unit
and lot‑to‑lot consistency. The silicon chip used in this
series has been designed to use the fewest possible pro‑
cessing steps to minimize variations in diode characteris‑
tics. Statistical data on the consistency of this product, in
terms of SPICE parameters, is available from Avago.
For those applications requiring very high breakdown
voltage, use the HSMS‑280x family of diodes. Turn to the
HSMS‑281x when you need very low flicker noise. The
HSMS‑285x is a family of zero bias detector diodes for small
signal applications. For high frequency detector or mixer
applications, use the HSMS‑286x family. The HSMS‑270x
is a series of specialty diodes for ultra high speed clipping
and clamping in digital circuits.
8.33 X 10
-5
nT
R
j
= –––––––––––– = R
V
– R
I
S
+I
b
0.026
≈ ––––– at 25 °C
I
S
+I
b
V - IR
S
where
–––––
I = I
S
(e
0.026
– 1)
n = ideality factor (see table of SPICE parameters)
s
T = temperature in °K
I
S
= saturation current (see table of SPICE parameters)
I
b
= externally applied bias current in amps
R
v
= sum of junction and series resistance, the slope of the
V‑I curve
I
S
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
8.33 X 10
-5
nT
barrier diodes.
R
j
= –––––––––––– = R
V
– R
s
The Height of
I
the Schottky Barrier
S
+I
b
The current‑voltage characteristic of a Schottky barrier
diode at
0.026
temperature is described by the following
≈
room
at 25 °C
–––––
I
equation:
S
+ I
b
I = I
S
(e
S
–––––
V - IR
0.026
– 1)
Schottky Barrier Diode Characteristics
Stripped of its package, a Schottky barrier diode chip
consists of a metal‑semiconductor barrier formed by de‑
position of a metal layer on a semiconductor. The most
common of several different types, the passivated diode,
is shown in Figure 10, along with its equivalent circuit.
R
S
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 R
S
is lost as heat—it does not contribute to the rectified out‑
put of the diode. C
J
is parasitic junction capacitance of the
diode, controlled by the thick‑ness of the epitaxial layer
and the diameter of the Schottky contact. R
j
is the junc‑
tion resistance of the diode, a function of the total current
flowing through it.
On a semi‑log plot (as shown in the Avago 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 R
S
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, I
S
, 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 applications) are realized on p‑type silicon. Such
diodes suffer from higher values of RS than do the n‑type.
R
S
PASSIVATION
LAYER
METAL
PASSIVATION
N-TYPE OR P-TYPE EPI
SCHOTTKY JUNCTION
N-TYPE OR P-TYPE SILICON SUBSTRATE
C
j
R
j
CROSS-SECTION OF SCHOTTKY
BARRIER DIODE CHIP
EQUIVALENT
CIRCUIT
Figure 10. Schottky Diode Chip.
5
HSMS-285A/6A fig 9
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
