LEARNING TOOLS FOR DIODE-BASED CIRCUITS
FOR MICROWAVE AND RF APPLICATIONS
Mario Guerra, Almudena Suárez
guerraso@teleline.es
almu@dicom.unican.es
Dpto. Ingeniería de Comunicaciones. University of Cantabria.
Abstract
In this paper, some JAVA applets are
presented, to help the understanding of RF
diodes and illustrate their applications. One
of them allows the study of a microwave
detector, based on a Schottky diode. The
other two show PIN diode applications, one
of them in the design of a variable phase
shifter and the other, in a matched variable
attenuator. Very good results have been
obtained when using these tools in a
Telecommunication-Engineering
subject,
dealing with high frequency circuits.
I. Introduction
The good knowledge of semiconductor
devices, their operation principles and
applications, is crucial for the design of
nonlinear circuits. Thus, it is essential for
students pursuing a radiocommunication
speciality. Due to their complex physics and
intrinsic nonlinearity, the understanding of
these devices is often difficult for the
student.
The purpose here has been to develop
several JAVA applets, showing the
principles, models and main applications of
the diodes most commonly employed at RF
and microwave frequencies. The aim is to
help the students differentiate the diodes
(from the point of view of their principles and
applications) and to provide tools enabling
the good understanding of circuits based on
them.
In this paper three JAVA applets are
presented, illustrating the applications of
Schottky and PIN diodes. One of the applets
shows a Schottky-based microwave-signal
detector, to be used in a slotted line. The
two other applets show PIN-diode
applications. One of them is a PIN-based
phase-shifter, employing a circulator, and
the other one is a PIN-based variable
attenuator, using a 3 dB-coupler.
II. Schottky-diode applications.
Microwave detector
The Schottky diode junction is a metalsemiconductor hetero-junction. The junction
is formed between an N-type semiconductor
and a metal. The working principles of the
Schottky diode are similar to those of a P-N
junction,
but
without
the
charge
accumulation problems of the latter [1-2],
since the conduction is only due to majority
carriers. They can be used for the design of
12th EAEEIE conference, Nancy, 2001
microwave detectors. Such detectors make
use of the nonlinear characteristic of the
Schottky's
equivalent
current
source
characteristic, given by i( v)  I s (e v  1) .
Provided the signal amplitude V is small, it is
possible to develop i(v) as a Taylor series
around the bias point. After low pass
filtering, a (quasi) DC output may be
obtained.
Fig. 1 shows the input screen of the
JAVA applet. The Schottky diode is
employed to detect the amplitude of the
standing wave in a slotted line. An
interesting aspect of the applet is the retracing of the standing wave when the user
modifies the load impedance. This standing
wave is often badly understood by the
student and the dynamical representation
(carried out in this applet), under load
impedance variations, has proven very
helpful for teaching.
By moving the cursor along the line, the
detected DC current, at the selected
position, is provided. Results are shown on
the text fields, with their corresponding
labels. Units for all the magnitudes are
shown in tooltips. In a different screen, the
variations in the detected current, versus the
microwave amplitude, are also traced,
showing the possibility of saturated
behavior.
III. PIN-diode applications. Phase
shifter and variable attenuator.
A PIN diode is obtained by connecting
together a highly doped P+ layer of
semiconductor, a long intrinsic (I) layer and
a highly doped N+ layer [1-2]. The presence
of a wide intrinsic section increases the
breakdown voltage of the device, thus
allowing high reverse voltages. It is also
responsible for an almost constant value of
reverse bias capacitance. This capacitance
is very small due to the wide intrinsic
section, so the diode reverse impedance is
very high.
2
On the other hand, the intrinsic
semiconductor exhibits a variable resistance
RI as a function of forward bias. This
enables use of PIN diodes for the design of
variable attenuators. For sufficient forward
current, the forward impedance becomes
very small, which, together with the high
reverse impedance, enables switching
applications.
The two JAVA applets dealing with
PIN diode applications at microwaves are
going to be presented in the following. One
of them consists of a phase shifter and the
other is a variable attenuator.
III.1 Phase shifter
The low impedance value of the PIN diode
in forward bias, together with the high value
in reverse bias, makes it very well suited for
switching applications. This ability can be
exploited in phase-shifter design.
Fig. 2 shows the input screen of the
second JAVA applet. A number of PIN
diodes [2] are connected in parallel across
the transmission line in port 2. Phase
shifting is achieved between the input port 1
and the output port 3.
Phase shifting is obtained through signal
reflection at the first conducting diode. The
line length, up to this first conducting diode,
determines the phase shift. The applet
displays the phase-shift variation versus the
line length (between diodes) and versus the
number N of consecutive "off" diodes. The
equation providing the phase shift is the
following:
Phase (l)    2 l X  2 NDoff ·l (1)
where lx is the length of the first line section
and l, the length of the following sections.
NDoff is the number of consecutive diodes in
reverse bias, prior to the first one in forward
bias.
12th EAEEIE conference, Nancy, 2001
3
III.2 Variable attenuator
Another important application of PIN diodes
exploits the variable intrinsic resistance, RI,
which depends on the forward-bias current.
This is used in the design of variable
attenuators. The intrinsic resistance is
inversely proportional to the forward current
If. It also depends on the width of the
intrinsic region W I and the effective mobility
 and lifetime :
WI2
Ri 
(2)
2   If
(a)
Figure 3. PIN-based phase shifter.
(b)
Figure 1. Schottky-based detector.
Application in a slotted line. (a) Load ZL.
(b) Short-circuit. Load ZL = 0.
It is considered that the diodes in reverse
bias nearly behave like as open circuits,
while the diode in forward bias is nearly a
short circuit. The circulator is considered to
be an ideal one, so the reflected wave is
transferred to the matched load of port 3,
without loss.
The third JAVA applet (Fig. 3) analyzes a
variable-attenuator configuration, based on
the use of a 3-dB coupler, which enables a
matched design [2]. The input signal (at port
1) is split between ports 2 and 3. For a very
low value of RI, the signal is almost entirely
reflected at these two ports, obtaining very
low attenuation. For very high value, the
signal is almost entirely delivered to the 50Ohm loads, so there is a very low reflection,
resulting in a very high attenuation value.
The same variable attenuation principle
can be used for switching purposes, when
the diode current is modified from a zero
value (high attenuation, ideally an open
12th EAEEIE conference, Nancy, 2001
circuit) to a sufficiently large value (nearly a
short circuit). Through this applet it is
possible to calculate insertion loss (for
forward biasing of the diode) and isolation
(for reverse biasing), due to the diode nonideality.
4
The applet displays, in one of the output
charts, the reflection coefficient versus the
bias current If. With the aid of the mouse, it
is possible to select a particular current
value Ifo. For this particular value, the
second chart provides the attenuation
versus the input frequency. As in the
previous applets units are shown in tooltips.
Conclusions
(a)
In this work some tools for the better
understanding of diode-based circuits for RF
and microwave applications have been
presented. One of the tools shows the use
of a Schottky detector in a microwave
slotted line. Two other tools are devoted to
PIN diode applications. One of them is a
phase-shifter, employing several diodes and
a circulator, and the other is a variable
attenuator, employing a 3-dB hybrid coupler.
Very good results have been obtained when
using these tools for illustrating the RFdevice applications in a subject dealing with
high frequency circuits.
Appendix
The Applets were developed using JAVA
1.3, so to run them it is necessary to install
the Java 1.3 plug-in which, if not installed,
will be downloaded automatically from Sun’s
ftp when clicking the html file that launches
the applet. These are JAR files, so they will
uncompress in memory prior to the
execution.
References
(b)
Figure 4. PIN-based variable attenuator. The
upper chart shows the variation of the reflection
coefficient versus the forward current. The lower
chart shows the variation of the attenuation
versus the generator frequency. The current
value is selected with the mouse in the upper
chart. (a) For low If. (b) For higher If.
[1] Sze, S.M, Physics of semiconductor devices,
John Wiley & Sons, New York, 1981.
[2] Combes, P.F., Graffeuil, J. and Sautereau,
J.F.: Microwave components, devices and active
circuits, John Wiley & Sons, New York, 1988.