11 THz Figure-of-Merit Phase-change RF Switches for
Reconfigurable Wireless Front-ends
Jeong-Sun Moon, Hwa-Chang Seo, Dustin Le, Helen Fung, Adele Schmitz, Thomas Oh, Samuel Kim,
Kyung-Ah Son, Daniel Zehnder, and Baohua Yang
HRL Laboratories, Malibu, CA, 90265, USA
Abstract — We report on GeTe-based, phase-change RF
switches in a series configuration with an embedded microheater for thermal switching. With heater parasitics reduced,
these GeTe RF switches show on-state resistance of 0.12
ohm*mm and off-state capacitance of 0.12 pF/mm. The RF
switch figure-of-merit is estimated to be 11 THz, which is
about 15 times better than state-of-the-art silicon-oninsulator switches. With 50-µm-wide GeTe switches, RF
insertion loss was 0.25 dB and isolation was 24 dB at 20 GHz.
Harmonic powers were suppressed >90 dBc at 35 dBm,
meeting wireless requirements. The GeTe switches were
characterized under W-CDMA signals without spectral
regrowth up to 25 dBm.
Index Terms—RF switches, phase-change material,
wireless communications, power handling, insertion loss.
I. INTRODUCTION
Wireless systems are evolving into complex hardware
architectures with carrier aggregation and multiple-inputmultiple-output (MIMO) antennas to support higher peak
data rates and spectral efficiencies under International
Mobile Telecommunications-Advanced guidelines. For
instance, a 4G cellular RF front-end needs to support >16
bands, 60 RF ports and 30 RF switches per RF port [1].
The increased complexity of RF front-ends poses severe
design and layout challenges. Tunable antenna and
possibly reconfigurable RF front-ends are being
considered to maintain T/R performance [2]. Low-loss,
linear RF switches are key parts of these tunable RF
systems.
Currently, commercial RF switches include FET
switches with SOI [3-4], SOS [5], GaN [6] technologies
and RF MEMS [7]. Key features of RF switches include
low insertion loss, high isolation, excellent linearity,
power handling, easy integration with conventional
semiconductor technologies, and reliability and
packaging. For comparison, typical RF switch RonCoff
values are 230-300 femtosecond for RF SOI switches, 448
for RF SOS switches, 453 for GaN FET switches, and ~4
for RF MEMS switches. RF MEMS switches offer the
best FOM with excellent linearity >70 dBm. For high RF
power handling, GaN FET switches showed 40 W
continuous-wave RF power handling with <0.3 dB of
compression [6].
More recently, GeTe-based phase-change materials
(PCMs) are being evaluated for implementation into RF
switches [8]. GeTe RF switches are distinguished by
resistance change between their amorphous (high
resistance) and crystalline (low resistance), leveraging
resistance phase-change memory (RPCM) development.
The static resistance ratio is on the order of ~106. With
GeTe switches fabricated in vertical via configurations,
RF insertion loss of 0.66 dB has been reported at 10 GHz
with its third-order intercept point (IIP3) of 37 dBm [9].
Very recently, several lateral GeTe RF switches have been
reported with excellent RF switch FOM, 1/(2*Ron*Coff),
of >100 THz [10] as an intrinsic switch FOM. On the
other hand, with a micro-heater embedded for thermal
actuation, switch FOM varies due to parasitics, ranging
from 1 THz [11], to >4 THz [12], to 7.3 THz [13]. RF
power handling of these GeTe RF switches both at onstate and off-state has not been fully investigated. So far,
the maximum on-state RF power handling ranges from
>0.6 W [11], to >20 dBm [12], to >2 W [14]. Yet, many
aspects of GeTe phase-change RF switches have not been
addressed, including spectral regrowth under wireless
waveforms such as 3G and 4G with and without
interferers, operating temperatures, RF stability, hotswitching,
and
single-pole-multi-throw
switch
performance.
In this talk, we report on GeTe RF switches on silicon
substrate with state-of-the-art switch FOM of 11 THz.
This was accomplished using an embedded refractory
micro-heater with reduced parasitics. The spectral
responses of the GeTe-based RF switches were tested for
the first time under W-CDMA signals. With a 15 dBm
interferer, we did not see spectral regrowth of the
switches. Under single tone, the harmonic powers were at
90 dBc at 35 dBm with GeTe width of 150 µm. While at a
very early development stage, we report that GeTe PCM
RF switches are a promising technology upon improved
reliability for future wireless RF front-ends.
II. PCM RF SWITCHES
A. RF Characteristics and Modeling
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
(a)
was modeled with Coff of 6 fF (0.12 pF/mm). Figure 2(b)
shows an equivalent circuit model with lumped elements
of Ron, L, and Coff. The parasitic pad capacitance and
inductors were extracted with standard ‘open’ and ‘short’
Inserion Loss (dB)
R = 2.4 ohm, L = 83 pH
-10
-0.5
-20
-1
Coff = 6 fF
-30
Isolation (dB)
(b)
0
0
(a)
-1.5
-40
Switch FOM = 11 THz
-2
0
10
20
30
-50
50
40
Frequency (GHz)
(b)
Figure 1. (a) An optical photograph of fabricated GeTe RF
switch in a series configuration, (b) Measured isolation of
various GeTe switches to reduce heater’s capacitive
coupling
Figure 2. (a) Measured and modeled s-parameters of
GeTe RF series switches with switch-on and switch-off
states, (b) An equivalent circuit with lumped elements of
Ron, Coff, and inductor L, used to model the s-parameter
data with excellent agreement.
devices on the same wafer. The measured and modeled
data (in Black) are in excellent agreement up to 50 GHz.
With Ron of 2.4 ohm and Coff of 6 fF, the RF switch
FOM is 11 THz, showing the potential to enable RF
switch to millimeter wave frequencies.
B. RF Power handling, Harmonics and Linearity
Previously [ref], we reported RF power handling of
GeTe switches at 2 GHz both for GeTe on and off states.
Frequency =2 GHz
W = 150 um
40
1f
20
50
0
90 dBc
1f
Harmonic Power (dBm)
Harmonic Power: f, 2f, 3f (dBm)
Figure 1(a) shows a fabricated GeTe phase-change RF
switch in a series configuration with embedded refractory
micro-heater for robust heater operation in thermal
switching of the GeTe channel. The GeTe PCM material
is deposited on SiO2/Si wafers. As deposited, the GeTe is
amorphous with sheet resistance >1 M/sq. With the
GeTe material patterned by ICP dry etching for isolation,
Ti-based ohmic contact electrodes are formed. The ohmic
contact resistivity is measured by the standard
transmission-line method (TLM), which yields a record
low lateral contact resistance of ~15 µm. Sheet
resistance was 17 ohm/sq after annealing above 200ºC.
The on-state resistivity of GeTe material used here was
3.4 x 10-4 cm [12]. Figure 1(b) shows measured
isolation of GeTe switches with different layouts, overlap
to underlap, to study the parasitic coupling of the
embedded heater. With underlap layout, isolation was ~25 dB at 67 GHz.
Figure 2 plots the measured and modeled S-parameters
of GeTe series switches up to 50 GHz for both GeTe with
on-state and off-state. The channel width was 50 µm with
a gap of 2 µm. The measurements were done using
standard SOLT calibration on CS-5 impedance standard.
For GeTe on-state, the insertion loss was 0.25 dB at 20
GHz. The S21 was modeled with an Ron of 2.4 ohm and
an inductor L of 83 nH, as shown in Figure 2(b). With
total channel width of 0.05 mm, Ron was 0.12 mm. For
GeTe off-state, isolation was 24 dB at 20 GHz. The S21
-20
-40
0
90 dBc
Frequency =2 GHz
W = 50 um
-50
2f
-100
2f
-60
3f
-150
-5
0
5
10
15
20
25
30
35
3f
CW RF Power_incident (dBm)
-80
-100
20
25
30
35
40
CW RF Power_incident (dBm)
Figure 3. Measured harmonic power of GeTe RF
switches at on-states as a function of input RF power
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
spectral regrowth measurements under the W-CDMA at
the Watt level.
-10
(a)
GeTe sw: W= 50 µm
W-CDMA = 25 dBm
W-CDMA Spectrum (dB)
-20
GeTe_25 dBm
Thru_25 dBm
TABLE 1: SURVEY OF RF SWITCHES
-30
Technology
-40
SOI
SOS
pHEMT
GaN
Phase-change
Metal Thru line
-50
-60
GeTe sw
-70
Lds
(um)
0.32
0.25
0.5
2
2
Ron
(mm)
0.8
1.6
1.5
1.7
0.12
Ron*Coff
(fsec)
250
448
360
453
15
Reference
[3]
[5]
[6]
This work
Due to SSPA in the test setup
-80
1.29
1.295
(b)
1.3
1.305
1.31
Frequency (GHz)
W-CDMA Spectrum (dB)
15 dBm interferer
W-CDMA =
20 dBm
-20
VI. CONCLUSION
-40
-60
Metal Thru line
-80
GeTe sw: W= 50 µm
-100
1.285
1.29
1.295
1.3
1.305
1.31
We optimized RF designs of GeTe RF switches and
demonstrated small-signal RF performance with RF
switch FOM of 11 THz. We also characterized the switch
linearity in terms of harmonic powers and under a WCDMA signal up to 25 dBm and found no spectral
regrowth. We will present GeTe switch testing under the
Watt level of W-CDMA signals. The overall RF
performance is state-of-the-art, making these nascent
PCM RF switches highly promising upon improved
reliability.
1.315
Frequency (GHz)
Figure 4. Measured spectra of GeTe PCM RF switches
and metal thru lines (a) under W-CDMA signals and (b)
with 15 dBm blocker
A 50 µm GeTe RF switch can handle input RF power up
to 2.8 W, and a 100 µm GeTe RF switch shows input RF
power handling >5.6 W. When the GeTe is off-state, a 50
µm GeTe RF switch can handle input RF power up to 10
W, showing a 1 dB compression point (P1dB) of >10 W.
With a GeTe PCM RF switch width of 50 µm, the offstate P1dB is >80 W/mm.
Figure 3 shows single-tone harmonic power of two
different GeTe switches with 50-µm and 150-µm width,
respectively. These GeTe switches are fully on-state. The
harmonic powers (2f, 3f) are greatly suppressed below 90
dBc at 35 dBm in the case of the 150-µm-wide GeTe
switch. The two-tone intermodulation product data were
reported as a function of input RF power up to 30 dBm
RF input power previously.
Figure 4 compares measured spectra of the GeTe switch
and metal thru line under a W-CDMA signal at 25 dBm;
there is no difference in the spectra found. We also
introduced a 15-dBm interferer to see if there would be
spectral regrowth. At 20 dBm, under a W-CDMA and a
15-dBm interferer signal, we found no difference between
the GeTe switch and a metal thru line. We will discuss the
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978-1-4799-8275-2/15/$31.00 ©2015 IEEE