IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 19, NO. 12, DECEMBER 2009
807
On Estimating and Canceling Parasitic Capacitance
in Submillimeter-Wave Planar Schottky Diodes
Haiyong Xu, Member, IEEE, Gerhard S. Schoenthal, Member, IEEE, Lei Liu, Member, IEEE,
Qun Xiao, Member, IEEE, Jeffrey L. Hesler, Member, IEEE, and Robert M. Weikle, II, Senior Member, IEEE
Abstract—The equivalent circuit model of a submillimeter
planar diode is investigated. In particular, the parasitic finger to
pad capacitance that shunts the diode junction severely degrades
the performance of frequency multiplier. Therefore, precise estimation of its value is crucial to circuit design in the submillimeter
wave range. In this letter, a parallel diode structure is analyzed,
and symmetry is utilized to remove parasitic elements external
to the finger loop. This analytical approach is verified through
measurement on a 200 GHz planar diode circuit. To increase the
diode junction capacitance modulation ratio, one possible solution
is also suggested.
Index Terms—Frequency multiplication, parasitic capacitance,
planar Schottky diodes, submillimeter wave diodes.
I. INTRODUCTION
HE planar surface-channel Schottky diode, proposed in
the late 1980’s, has been developed and used for more
than a decade for submillimeter applications [1], [2]. With the
advancement of planar diode processing technology, device performance has continued improving mixers based on discrete
planar diodes, achieving noise temperatures within a factor of
1.5 of the best whisker-contacted Schottky diode mixers [3].
Precise alignment of the diode chip to the surrounding circuit
however limits the use of this approach. This has led to a wafer
bonding process that allows integration of high quality GaAs devices onto quartz substrates [4]. In this way, precision alignment
is obtained and repeatability is improved through lithographic
definition. For integrated circuit design using this approach, an
accurate estimation of finger inductance and finger to pad capacitance is crucial [5]. Several papers are presented to address
these problems [6]–[8].
Unfortunately obtaining the finger to pad capacitance through
conventional modelling or measurement of single diode structures is difficult due to the large pad to pad capacitance that
shunts the diode. In this letter, an analysis technique that utilizes a parallel diode structure is described. Due to symmetry,
only the internal loop circuit comprising the two fingers needs
be considered. This eliminates the effects of circuit elements
T
Manuscript received April 28, 2009; revised August 28, 2009. Current version published December 04, 2009. This work was supported by the U.S. Army
National Ground Intelligence Center on Contract W911W5-06-R-0001.
H. Xu, and R. M. Weikle, II are with the School of Engineering and Applied
Science, University of Virginia, Charlottesville, VA 22904-4714 USA (e-mail:
hx4g@virginia.edu).
L. Liu is with the Department of Electrical Engineering, University of Notre
Dame, Notre Dame, IN 46556 USA.
G. Schoenthal and J. Hesler are with Virginia Diodes, Inc., Charlottesville,
VA 22904 USA.
Q. Xiao is Tyco Electronic Corporation, Lowell, MA 01853 USA.
Digital Object Identifier 10.1109/LMWC.2009.2033518
Fig. 1. (a) Schematic drawing of a single diode structure, including contact
pad, finger and anode junction. (b) Equivalent circuit model of a single diode
structure, including parasitic capacitors and a finger inductor.
outside the loop, allowing the internal parasitics of the diode to
be determined.
II. MODEL
The standard surface-channel diode structure, shown in
Fig. 1(a), includes two contact pads, finger, anode and GaAs
substrate. The finger-to-pad capacitance
is a critical
circuit parameter for planar diodes because it directly shunts
the anode junction capacitance.
The equivalent circuit used to model a submillimeter-wave
diode structure is shown in Fig. 1(b). The circuit model in, a pad-to-pad capacitor
,
cludes a finger inductor
a finger-to-pad capacitor
and two fringing capacitors
. The entire structure can be simulated with numerical
electromagnetic solvers such as Ansoft’s High Frequency
Structure Simulator (HFSS). From this, equivalent circuit parameter values can be extracted by fitting or analytic modelling.
However, it is difficult to obtain accurate values because this
is internal to the device and
finger-to-pad capacitance
its response masked by the finger inductance and pad-to-pad
capacitance, which is significantly larger. The pad-to-pad
capacitance can in principle be tuned by an external matching
circuit, while the finger-to-pad capacitance cannot due to its
proximity to the anode.
To obtain accurate values for the finger-to-pad capacitance, a
parallel diode structure is examined. The advantage of this approach is that symmetry can be exploited to remove all external
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808
IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 19, NO. 12, DECEMBER 2009
Fig. 4. Equivalent circuit model for the ADS simulation.
Fig. 2. (a) Schematic drawing of a parallel diode structure with two diode finger
separated by a fixed distance, D. (b) Equivalent circuit model of a parallel diode
structure with considering of mutual inductance between fingers.
Fig. 5. HFSS and ADS simulated phases of the loop parameter,
Fig. 3. Simplified one port equivalent circuit model for the parallel diode.
parasitics and impedances from consideration. A schematic
drawing of a parallel diode structure, along with equivalent
circuit, is shown in Fig. 2. Each anode port is in parallel with
and in series with
. It should be added that the finger
mutual inductance, , will influence the overall inductance,
and this effect is taken into account in the simulation.
Initially, the entire structure is simulated for a typical diode,
the separation (D) is 10 m, the finger length (L) is 8 m, and
the anode diameter of 1 m. The circuit is symmetric through
the center. By absorbing ports 1, 2 into the network, the two port
-parameter matrix is
(1)
where the port numbering remains the same for clarity.
For an odd mode excitation, the incident waves are out of
phase
(2)
and the symmetry results in a virtual short along the central
plane bisecting the structure. Thus, the equivalent loop circuit
is simplified to a one port circuit, as shown in Fig. 3. In this
model, the mutual inductance is included as,
.
The odd mode reflection coefficient seen at port 3 is
(3)
where
and
were obtained from the HFSS simulation.
0
.
Based on the equivalent circuit, the reflection coefficient at
port 3 is calculated from simple circuit analysis.
and
are found by
The two unknown parameters,
solving for the -parameters at two different frequencies. The
pH and
fF. This
calculated results give
inductance includes the coupling effect between two fingers. To
assess the validity of the lumped circuit model, that model is
simulated in Agilent’s Advanced Design System (ADS) to predict -parameters of the loop circuit, as shown in Fig. 4. The
model consists of finger inductors, finger-to-pad capacitors, two
ports and a load. Using the above calculated values, the simu, is shown in Fig. 5.
lated phase of the loop parameter,
The HFSS simulated result is also shown in the same figure. The
phase difference between them is less than 0.5 over a frequency
rang from 1.4 THz to 1.9 THz. Both simulated magnitude values
are 1.0.
Another important consideration is the mutual inductance
between two fingers. A coupled-line model is used to determine
the transmission line parameters as described by Mongia [9].
The even and odd modes for a coupled-line circuit are simulated
in HFSS, and the distributed parameters are found in terms of
the characteristic impedances and propagation constants of the
even and odd modes. For the present case, the self inductance
of coupled line is found to be, 0.88 pH/ m, while the mutual
inductance is calculated to be
pH/ m. This gives an inductance for the structure of
pH,
which is in agreement with the simulated value. The self and
mutual capacitance for the coupled-line are calculated at the
fF/ m, which can be neglected in the circuit
level of
simulation.
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XU et al.: ON ESTIMATING AND CANCELING PARASITIC CAPACITANCE IN SUBMILLIMETER-WAVE PLANAR SCHOTTKY DIODES
809
However, this suggested tuning idea is not a wide band solution
since the stub inductance is frequency dependent
(4)
V. CONCLUSION
Fig. 6. Scanning electron micrograph of a planar diode circuit without anode.
III. MEASUREMENT AT 200 GHZ
To verify the theory, parasitic capacitances were measured
for 200 GHz circuits. A scanning electron micrograph of a
planar diode circuit without anode is shown in Fig. 6. The
finger is 2.5 m wide by 2 m thick. There is no SiO layer
between finger and GaAs mesa in order to reduce the parasitic
finger-to-pad capacitance, while the distance between finger
to epi layer is 3 m. The anode is 3 m away from the edge
of the mesa. The capacitance is measured with a HP 4275A
multi-frequency LCR meter. The measurement frequency is
set at 10 MHz. The average measured pad to pad capacitance
without finger is 7.6 fF. The average measured pad-to-pad
capacitance with finger (no anode) is 9.42 fF. The difference is
1.82 fF, which is the finger-to-pad capacitance.
On the other hand, the finger-to-pad capacitance was calculated through parallel diode method as stated in the letter. The
value is approximate 1.6 fF. The difference, 0.22 fF, maybe due
to the mismatch between fabrication and simulation structure,
and measurement accuracy.
IV. PARASITIC CAPACITANCE CANCELLING
To cancel the finger-to-pad capacitance and improve the
diode modulation ratio, one solution is to extend the diode
finger beyond the anode. This finger stub will act as an inductor
parallel to the finger-to-pad capacitor. At the design frequency,
, the inductance,
, is set to be resonant with the capacitance,
as shown in (4). This LC circuit will become open
circuit. Thus, the parasitic finger-to-pad capacitance will have
no effect on diode junction capacitance at the center frequency.
The equivalent circuit parameters of a planar diode at terahertz frequency range have been investigated in this letter. Accurate estimation of the finger-to-pad capacitance is achieved
using a parallel diode structure and is verified by the simulations
and measurement over a broad frequency range. This method
will be a valuable tool in the circuit design to yield more precise
designs for integrated submillimter wave diode circuits. Furthermore, one solution is proposed to cancel the finger-to-pad
capacitance, which will increase the diode junction capacitance
modulation ratio at the design frequency.
ACKNOWLEDGMENT
The authors wish to thank Dr. Z. Liu, Autoliv, Inc., for discussions and suggestions.
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