measure of the time it takes for the
charge stored in the I layer to
decay, when forward bias is
replaced with reverse bias, to some
predetermined value. This lifetime
can be short (35 to 200 nsec. for
epitaxial diodes) or it can be
relatively long (400 to 3000 nsec.
for bulk diodes). Lifetime has a
strong influence over a number of
PIN diode parameters, among
which are distortion and basic
diode behavior.
To study the effect of lifetime on
diode behavior, we first define a
cutoff frequency f
C
= 1/τ. For short
lifetime diodes, this cutoff fre-
quency can be as high as 30 MHz
while for our longer lifetime
diodes f
C
≅
400 KHz. At frequen-
cies which are ten times f
C
(or
more), a PIN diode does indeed
act like a current controlled
variable resistor. At frequencies
which are one tenth (or less) of f
C
,
a PIN diode acts like an ordinary
PN junction diode. Finally, at
0.1f
C
≤
f
≤
10f
C
, the behavior of the
diode is very complex. Suffice it to
mention that in this frequency
range, the diode can exhibit very
strong capacitive or inductive
reactance — it will not behave at
all like a resistor. However, at zero
bias or under heavy forward bias,
all PIN diodes demonstrate very
high or very low impedance
(respectively) no matter what
their lifetime is.
Diode Resistance vs. Forward Bias
If we look at the typical curves for
resistance vs. forward current for
bulk and epi diodes (see Figure
10), we see that they are very
different. Of course, these curves
apply only at frequencies > 10 f
C
.
One can see that the curve of
resistance vs. bias current for the
bulk diode is much higher than
that for the epi (switching) diode.
Thus, for a given current and
junction capacitance, the epi
diode will always have a lower
resistance than the bulk diode.
The thin epi diode, with its
physically small I region, can
easily be saturated (taken to the
point of minimum resistance) with
very little current compared to the
much larger bulk diode. While an
epi diode is well saturated at
currents around 10 mA, the bulk
diode may require upwards of
100 mA or more. Moreover, epi
diodes can achieve reasonable
values of resistance at currents of
1 mA or less, making them ideal
for battery operated applications.
Having compared the two basic
types of PIN diode, we will now
focus on the HMPP-3890 epi
diode.
Given a thin epitaxial I region, the
diode designer can trade off the
device’s total resistance (R
S
+ R
j
)
and junction capacitance (C
j
) by
varying the diameter of the
contact and I region. The
HMPP-3890 was designed with the
930 MHz cellular and RFID, the
1.8 GHz PCS and 2.45 GHz RFID
markets in mind. Combining the
low resistance shown in Figure 10
with a typical total capacitance of
0.27 pF, it forms the basis for high
performance, low cost switching
networks.
1000
HSMP-3880 Bulk PIN Diode
Linear Equivalent Circuit
In order to predict the perfor-
mance of the HMPP-3890 as a
switch, it is necessary to construct
a model which can then be used in
one of the several linear analysis
programs presently on the market.
Such a model is given in Figure 11,
where R
S
+ R
j
is given in Figure 1
and C
j
is provided in Figure 2.
Careful examination of Figure 11
will reveal the fact that the
package parasitics (inductance
and capacitance) are much lower
for the MiniPak than they are for
leaded plastic packages such as
the SOT-23, SOT-323 or others.
This will permit the HMPP-389x
family to be used at higher fre-
quencies than its conventional
leaded counterparts.
20 fF
3
30 fF
1.1 nH
2
1
4
30 fF
20 fF
Single diode package (HMPP-3890)
20 fF
0.05 nH
3
30 fF
0.05 nH
2
12 fF
0.5 nH
0.5 nH
30 fF
0.05 nH
1
0.5 nH
0.5 nH
0.05 nH
4
20 fF
Anti-parallel diode package (HMPP-3892)
20 fF
0.05 nH
3
30 fF
0.05 nH
2
12 fF
0.5 nH
0.5 nH
30 fF
0.05 nH
1
0.5 nH
0.5 nH
0.05 nH
4
RESISTANCE (Ω)
100
20 fF
Parallel diode package (HMPP-3895)
10
HMPP-389x
Epi PIN Diode
Figure 11. Linear Equivalent Circuit of the
MiniPak PIN Diode.
1
0.01
0.1
1
10
BIAS CURRENT (mA)
100
Figure 10. Resistance vs, Forward Bias.
5
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
