5
Diode Construction
At Avago Technologies, two basic methods of diode fabri-
cation are used. In the case of bulk diodes, a wafer of very
pure (intrinsic) silicon is heavily doped on the top and
bottom faces to form P and N regions. The result is a diode
with a very thick, very pure I region. The epitaxial layer (or
EPI) diode starts as a wafer of heavily doped silicon (the
P or N layer), onto which a thin I layer is grown. After the
epitaxial growth, diusion is used to add a heavily doped
(N or P) layer on the top of the epi, creating a diode with
a very thin I layer populated by a relatively large number
of imperfections.
These two different methods of design result in two
classes of diode with distinctly dierent characteristics,
as shown in Table 1.
Table 1. Bulk and EPI Diode Characteristics.
Characteristic EPI Diode Bulk Diode
Lifetime Short Long
Distortion High Low
Current Required Low High
I Region Thickness Very Thin Thick
As we shall see in the following paragraphs, the bulk diode
is almost always used for attenuator applications and
sometimes as a switch, while the epi diode (such as the
HMPP-3890) is generally used as a switching element.
Diode Lifetime and Its Implications
The resistance of a PIN diode is controlled by the conductiv-
ity (or resistivity) of the I layer. This conductivity is controlled
by the density of the cloud of carriers (charges) in the I layer
(which is, in turn, controlled by the DC bias). Minority car-
rier lifetime, indicated by the Greek symbol τ, is a 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
inuence over a number of PIN diode parameters, among
which are distortion and basic diode behavior.
To study the eect of lifetime on diode behavior, we rst
dene a cuto frequency f
C
= 1/τ. For short lifetime diodes,
this cuto frequency can be as high as 30 MHz while for
our longer lifetime diodes f
C
≅ 400 KHz. At frequencies
which are ten times f
C
(or more), a PIN diode does indeed
act like a current controlled variable resistor. At frequen-
cies 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. Suce 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 15), we see
that they are very dierent. 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 rea-
sonable 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 o 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 15 with a typical total capacitance of 0.27 pF, it
forms the basis for high performance, low cost switching
networks.
Figure 10. Resistance vs. Forward Bias.
1000
100
10
1
RESISTANCE ( )
BIAS CURRENT (mA)
0.01 0.1 1 10 100
HMPP-389x
Epi PIN Diode
HSMP-3880 Bulk PIN Diode
Figure 10. Resistance vs, Forward Bias.
