Download Ka Band High Power AlGaAs PIN Diode Switches

Ka Band High Power AlGaAs PIN Diode Switches
Andrzej Rozbicki, James Brogle, Nitin Jain*, Timothy Boles, David Hoag
MA-COM Technology Solutions – a Cobham Company, Lowell, MA, 01851, USA
* Anokiwave Inc., San Diego, CA, 92130, USA
SP3T switch manufactured in GaAs and AlGaAs shows
difference of 0.12 dB at 20GHz. Further, it has been proven
that AlGaAs PIN diode switches demonstrate improvements
in terms of reduced insertion loss with no degradation of
isolation. Also for many years MA-COM PIN based switch
circuits fabricated from this material have been tested and
have demonstrated broadband RF performance from 50 MHz
to greater than 77GHz.
A photograph of a shunt PIN diode and series PIN diodes
used in these MMIC circuits are depicted in Fig 1.
Abstract — In this paper we present the design and
performance of millimeter wave MMIC switches in the patented
MA-COM AlGaAs heterojunction PIN Diode process that allow
us to produce high power and low insertion loss devices. The
design process from a reflective SPDT switch to a non-reflective
version of the switch, with intense use of HFSS and ADS
software, is presented. These switches were designed to meet
demanding requirements: low insertion loss less than 0.8 dB,
40dBm peak power and 37dBm CW power, and 30dB isolation.
Index Terms — AlGaAs, PIN diode, monolithic microwave
integrated circuit; switch, Ka Band.
I. INTRODUCTION
Monolithic microwave integrated circuits (MMICs) based
on high power PIN diodes are increasingly used for many
transmit/receive systems in advanced defense electronics and
telecommunications applications. Examples of such systems
include radar, half-duplex data links, Internet-protocol-based
(IP-based) wireless LAN’s, and millimeter-wave imagers. For
such applications, switches are required to have high power
handling (several watts), low insertion loss, good matching,
fast switching (several nanoseconds) and for non-reflective
switches good return loss in off state is required. Also these
devices have to be small in size to support tight-fitting and
portable applications.
In the design process intense modeling including ADS and
HFSS software was implemented to ensure first-pass success.
The switches presented here use AlGaAs/GaAs based heterojunction technology [1] that provides PIN diodes with reduced
series resistance.
The design and performance of the reflective SPDT switch
and of the non-reflective switch are described. The process of
simulation and matching with measured results are presented.
Air Bridge
Series Diodes
Air Bridge
Fig. 1.
diodes
Cathode connection
Photographs of a shunt PIN diode and two series PIN
The shunt diode anode diameter, used in this application, is
46um and has Rs = 1.12 Ohm @ 1 GHz with a bias current of
10mA, and Rs = 0.88 Ohm @ 1 GHz with a bias current of
20mA, and Coff = 0.12pF. These parameters allowed us to
design a low insertion loss switch with high isolation. The
series diode used in the non-reflective version of the SPDT
switch has an anode diameter of 20um. This diode has Rs=4.7
Ohm @ 1 GHz with a bias current of 20mA, and Coff = 0.08
pF.
In order to correctly predict the performance of the circuit,
first an ADS equivalent circuit of the PIN diode of previously
manufactured diodes was created. For the shunt diode a
lumped-element equivalent circuit model was used. The model
consist of tree main elements in series: 1) a diode with
junction parameters in parallel Rj, Cj, and diffusion
capacitance Cd; 2) Rs that represents the resistance of the
bulk semiconductor regions plus the resistance of the contacts;
and 3) i-region parameters Ri and Ci in parallel. Also the via
inductance Lvia and a distributed parasitic capacitance Cpar
were included in this model.
Detailed PIN diode equivalent circuits parameters were
derived from S-parameters measured on existing PIN diodes.
Then S-parameters were transformed to Z-parameters, a
resistance versus frequency was extracted for forward biased
II. DISCUSSION
The MA-COM patented technology used to manufacture
AlGaAs hetero-junction PIN diodes use the advantage of the
P+ AlGaAs junction over comparable P+ GaAs to I region
junction. As Hoag et. al describes “It was found that the
recombination rate for electrons at the P+ GaAs - I-region
junction is sizable in comparison to the P+ AlGaAs
heterojunction. The combination of lower recombination rates
and higher carrier injection will yield a greater number of
carriers thus lowering the effective resistivity of the I-region’’
[1]. Indeed average measured insertion loss response for a
978-1-4244-2804-5/09/$25.00 © 2009 IEEE
Anode
Shunt
Diode
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IMS 2009
diode and shunt capacitance versus frequency for reverse
biased diode.
The size of the shunt diode was selected so that the diode is
capable to handle the maximum peak power of 10W through
the switch. Simulated power dissipation (Fig. 2) for diodes in
Cathode
connection
Anode
1000
Pdiss_diode
Pdiss_on_diode
Pdiss_off_diode
mW
Fig. 4.
Total Diss Power
800
The series diode was not used in the reflective switch design
because the topology chosen was the typical shunt connected
PIN diode switches with quarter wavelength elements. This
shunt diode topology offers relatively high isolation and
lowest insertion loss as well as it is capable of handling more
RF power then a diode in a series type switch because the
shunt diode is electrically and thermally grounded at one
electrode.
The insertion loss of a shunt PIN diode switch is a function
of diode off-capacitance CT and defined by:
600
Diode ON
400
200
Diode OFF
0
24
26
28
30
32
34
36
38
40
42
44
46
Frequency (GHz)
Fig. 2.
Power
IL = 10Log[1+( fCTZ0)2] (1)
Power dissipation in shunt diodes with 10W Incident
As previously specified the shunt diode used in this design has
very low capacitance CT = 0.12pF in Off state. The shunt
diodes designed with the MA-COM process are rated at 78V
breakdown voltage that guaranties high linearity at high power
applications. The breakdown voltage was measured at 10uA.
A simple thermal analysis was included to ensure that the
diode selected would handle high power conditions.
Combinations of silicon nitride layers of 4000Å thickness
have been applied as the MIM capacitors dielectric layer and
the diode passivation layers. The silicon nitride layer forms
capacitors with a breakdown voltage higher than 250V [2].
Also it was proven that the incorporation of thicker silicon
nitride films improve the switch performance over the 30 GHz
to 40Ghz frequency range.
Circuit insertion losses were optimized by properly
selecting the elements for a low ripple Chebyshev response.
Lower insertion loss of a system with a shunt diode could be
achieved by lowering the system impedance. At 50 ohm, for a
given shunt diode with Ct of 0.12pf, the theoretical insertion
loss is 1.5dB. If the system impedance would be 40 ohm the
insertion loss would be lowered to 1.0dB, and at 30ohm would
be 0.63dB. Such an approach was applied in this design. The
quarter wavelength section of each arm was carefully
simulated and the impedance was lowered so that at 35GHz
the impedance is 38 Ohms. In order to maintain relatively high
return loss and low VSWR the common arm was designed
also as a quarter wavelength section with lower impedance in
range of 36ohms. To compensate the lower impedance of the
input there is a small section of transmition line connecting the
quarter wavelength section with the input test pad that
represents high impedance. Each part of these transmission
lines were modeled using HFSS and simulation of the whole
system was done in ADS software. Examples of the HFSS
models are presented in Fig. 3, 4, 5, and 8.
the OFF state should not exceed 300mW and for both diodes
in the circuit at any given state should not exceed 550mW at
40GHz.
Such equivalent circuits were used in further design process
to determine the accurate size of the diode. As the size and
parameters of the diode were established, then an HFSS
model, Fig. 3, of the shunt diode was designed and used in
further simulations.
ON
OFF
Anode
Ground Via
Fig. 3.
HFSS model of AlGaAs series diode
HFSS Model of the shunt PIN diode in ON and OFF state.
In case of series diodes, existing diode S-parameter
measurements as well as an HFSS model of the series diode
Fig. 4 were used for simulations. The small series diode has
relatively low isolation 9dB @ 35GHz and during the process
of design it became evident that there was a need for higher
isolation in order to maintain lower insertion loss and VSWR
of the outputs. The isolation of two diodes in series was
increased to 20dB @ 35GHz. Circuit simulations indicated
that adding one additional diode into the circuit did not have a
significant influence on the return loss of the termination
circuit. There was a need to adjust the L-C balance of the
circuit that was changed.
454
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0.0
Isolation [dB]
Air bridges
-10.0
Insertion Loss
-20.0
-0.2
-0.4
-0.6
-30.0
-40.0
30.0
Fig.5.
High impedance section with a blocking capacitor, HFSS
model (left) and on-wafer photograph (right)
0.0
Isolation
32.0
34.0
36.0
38.0
-0.8
-1.0
40.0
Insertion Loss [dB]
Capacitor
Freq [GHz]
Fig. 7.
III. REFLECTIVE SPDT SWITCH
In result of this process, a reflective SPDT switch was
designed and manufactured. Fig. 6 shows a photograph of the
single chip.
BIAS
RF
Low Z
IV. NON REFLECTIVE CIRCUIT
The second phase was required to redesign this basic
configuration of the SPDT switch into multi-operational
switch that partially included the SPDT design with the
requirements of providing non reflective outputs. Because this
switch was solely a custom design it is not possible to present
details of the switch configuration. But for purposes of this
publication it is sufficient to present and discuss a part of the
circuit that is in scope of this paper. The following describes
the termination circuit as seen in Fig. 8 and its performance.
BIAS
High Z
RF
RF
DC Ctrl
Bias
GND Via
Shunt Diode
Fig. 6.
Measured insertion loss and isolation
Photograph of a Die of the reflective SPDT switch
RF output
To achieve optimal results and small dimensions of 2.2 mm
x 1.2 mm the switch represents a typical T with bias circuits
between the common arm and output arm. The trace
connecting the bias pad and its capacitor to the arm represents
a high pass filter and works very well in the desired range of
30GHz to 40 GHz frequency. Each arm was designed as a low
impedance quarter-wave transmission line which ends in a
25um width air-bridge (see Fig. 1) connected to the shunt
diode anode. The width of the bridge was chosen so that it can
handle high RF current density and is in balance with the Rs
and Cj of the diode. To lower the diode’s connection
inductance to ground as well as to balance the diode’s
symmetry the cathode is connected to the ground over two
vias symmetrically placed on each side.
The RF outputs are connected with the shunt diode over a
high impedance transmition line that includes a blocking
capacitor (see Fig. 5). The line was analyzed in HFSS for best
performance. The typical insertion loss of 0.55dB was
achieved at 35GHz but not more than 0.67 dB from 30 to
40GHz. Typical isolation of 27 to 30 dB in this frequency
range is shown in Fig. 7, in addition to insertion loss.
Series
Diodes
Terminatio
n Resistor
Fig. 8. RF Output with the termination circuit, HFSS model (left)
and on-wafer photograph (right)
As previously described the two series diodes were
implemented as switch elements to the RF output in order to
control the termination resistor. The HFSS model and the
actual layout of these two series diodes with the termination
resistor are presented in Fig. 8.
Because of space limitation the connection between the
shunt diode and the RF output pad was achieved through two
chamfered 90 degree bends including a DC blocking
capacitor. Also the bias pads were relocated to opposite side
of the arms and this change was included in the HFSS
simulation, but it is not presented in this paper. Intensive
analysis using HFSS and ADS resulted in achieving good
performance of the new design. The typical insertion loss of
one arm in ON state increased from 0.55dB to 0.7dB at
35GHz but not more than 0.8dB from 30 to 40GHz. This
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remarkable insertion loss is the best that is known for this type
of switch [3]. As presented in Fig. 9, the measured insertion
loss is similar to HFSS/ADS simulation results.
Samples of this chip underwent some power tests. The tests
demonstrated an insertion loss of 0.78dB at 35GHz with a
10% duty cycle pulse with 0.87W input power. The insertion
loss increased to 0.92dB at an input level of 8.41W
(39.2dBm). The minimal amount of 0.14dB compression
indicates that this circuit can likely handle 10W (40dBm) peak
power.
dB(S(6,5))
dB(S(4,3))
0.0
-0.2
Measured
-0.4
HFSS/ADS
V. CONCLUSIONS
-0.6
The two Ka band PIN diode switches described here have
demonstrated excellent RF characteristics and high power
capabilities. The reflective switch has an insertion loss of
0.65dB at 35GHz and good flatness between 30 and 40 GHz.
Isolation is better than 27dB from 30 to 40 GHz. The switch is
capable of handling 40dBm peak power and 37dBm CW.
The non-reflective switch has typical insertion loss of
0.75dB at 35GHz that does not exceed 0.8dB between 30 and
40 GHz. The isolation is better than 30dB from 30 to 40GHz.
The switch is capable of handling 40dBm peak pulsed power
and 37dBm under CW conditions.
The performance was achieved due to two main factors:
MA-COM’s unique manufacturing process and intensive
involvement of high quality simulation tools like HFSS and
ADS. The design process shown in this paper proves that use
of available tools like HFSS and ADS make it possible to
design and achieve complicated high frequency MMIC circuit
and be successful in the first run. The authors believe that
some adjustments are needed to the simulation process
including better correlation of the simulation tool with the real
product. In case of insertion loss and isolation the achieved
results are very good, only return loss of the terminated port
should be carefully analyzed for possible improvement.
-0.8
-1.0
30
32
34
36
38
40
freq, GHz
Fig. 9.
Measured vs. simulated insertion loss
VSWR does not exceed 1.5:1 including a 3x1mil ribbon
connection at ON state. The return loss (Fig 10) of the
terminated output is 10dB at 34 GHz and 13dB at 39 GHz
including the ribbon connection.
dB(S(6,6))
dB(S(4,4))
0
Measured
-5
HFSS/ADS
-10
-15
-20
30
32
34
36
38
40
REFERENCES
freq, GHz
[1] D. Hoag, J. Brogle, T. Boles, D. Curcio, and D. Russell,
“Heterojunction PIN diode switch,” 2003 IEEE MTT-S
International Microwave Symposium Digest, vol. 1, pp. 255258, 8-13 June 2003.
[2] D. Hoag, D. Curcio, T. Boles, “Development of High
Voltage mmW GaAs PIN Diode Switch”, GaAs MANTECH,
2001
[3] O. Levy, A. Madjar, D. Kryger, S. Matarasso, “Fully
terminated Ka band high isolation, high power MMIC SPDT
switch in GaAs PIN technology,” IEEE MTT-S Int. Microwave
Symp. Dig., vol. 3, pp. 2019-2022, June 2003.
Fig. 10. Measured vs. simulated return loss of the terminated port
The return loss of a terminated port below 34GHz is lower
then 10dB and it is required to perform additional analysis in
order to get better performance. The isolation is 32dB at
35GHz but not less then 30dB from 30Ghz to 40GHz. The
expected isolation was around 35dB in HFSS/ADS
simulations.
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