# Film bulk acoustic resonator
A film bulk acoustic resonator (FBAR) is a radio-frequency filter component in which a thin film of piezoelectric material, sandwiched between two metal electrodes, is suspended over a cavity so that the film can vibrate vertically in bulk, resonating at gigahertz frequencies[1](https://patentimages.storage.googleapis.com/e8/2a/ca/ed92d5369a2107/US11601113.pdf). FBAR filters are a form of bulk acoustic wave (BAW) filter, and they are used in the radio front ends of mobile phones, where they pass wanted frequencies and reject unwanted ones so that a phone's many receivers process only the intended signal[2](https://docs.broadcom.com/doc/AV02-3869EN).
| Key fact | Detail |
|---|---|
| Device class | A bulk acoustic wave (BAW) filter with steeper rejection curves than surface acoustic wave (SAW) filters[3](https://www.broadcom.com/products/wireless/fbar) |
| Structure | A film of piezoelectric material between top and bottom electrodes, suspended over a cavity that lets the film vibrate[1](https://patentimages.storage.googleapis.com/e8/2a/ca/ed92d5369a2107/US11601113.pdf) |
| Piezoelectric layer | Sputtered aluminum nitride film, 1 to 2 microns thick, on a 6 micron silicon substrate[4](https://bsac.berkeley.edu/news/bsac-seminar-fbar-technology-past-present-and-future) |
| Efficiency gain | 0.3 to 0.5 dB less insertion loss than SAW filters, resulting in up to 50 mA less current consumption and improved battery life and talk time[3](https://www.broadcom.com/products/wireless/fbar) |
| Commercialization | First successful commercialization of miniature bulk wave filtering to a broad customer base began with the PCS duplexer production ramp in 2001 under the Agilent name[5](https://www.microwavejournal.com/articles/7325-executive-interview-bryan-ingram-avago-technologies) |
| Production scale | By 2008, over 1 million FBAR filters shipped per day, with the one-billion mark passed that year; 80 to 90 percent of US CDMA phones carried an FBAR filter, duplexer, or quintplexer[4](https://bsac.berkeley.edu/news/bsac-seminar-fbar-technology-past-present-and-future) |
| Frequency range | BAW filters can be manufactured to 10 GHz or beyond[6](https://www.mouser.sg/applications/BAW-vs.-SAW-RF-Filters/) |
## How it works
The resonator is an acoustic cavity at film scale. A piezoelectric layer converts electrical energy into mechanical vibration; when the film's thickness matches a half-wavelength of the acoustic wave, it resonates, and this sharp mechanical resonance translates into a sharp electrical resonance used for filtering[1](https://patentimages.storage.googleapis.com/e8/2a/ca/ed92d5369a2107/US11601113.pdf). In the production device described by Rich Ruby, Director of Technology at Avago Technologies, the acoustic stack consists of metal electrodes and a sputtered piezoelectric aluminum nitride film forming a free-standing membrane 1 to 2 microns thick on a 6 micron silicon substrate, fabricated in a CMOS fab[4](https://bsac.berkeley.edu/news/bsac-seminar-fbar-technology-past-present-and-future).
The cavity beneath the film is what distinguishes the two main structural families. In an air gap-type FBAR, part of the substrate beneath the membrane is removed by etching to define a cavity, so acoustic energy in the piezoelectric layer is not transferred to the substrate[7](https://patents.google.com/patent/US7205702B2/en). In a Bragg reflector-type FBAR, the substrate is instead separated from the resonator by a reflection layer of alternating materials with a large difference in elastic impedance, which reflects the acoustic wave back into the resonator; this structure is mechanically firm but requires at least four layers of precisely controlled thickness, adding manufacturing time and cost[7](https://patents.google.com/patent/US7205702B2/en). The Bragg reflector approach corresponds to what the industry calls the solidly mounted resonator (SMR); as Avago executive Bryan Ingram noted, the Infineon process uses SMR technology while FBAR uses a suspended resonator membrane[5](https://www.microwavejournal.com/articles/7325-executive-interview-bryan-ingram-avago-technologies).
## History and commercialization
FBAR technology predates its commercial success. Its origins trace to the 1980s, though the technology's large-scale commercialization is closely associated with Avago Technologies, now Broadcom[8](https://www.newsonictech.cn/site/news-detail/370). Under the Agilent name, that company was the first to successfully commercialize miniature bulk wave filtering to a broad customer base, starting with its PCS duplexer production ramp in 2001[5](https://www.microwavejournal.com/articles/7325-executive-interview-bryan-ingram-avago-technologies). The company accumulated well over 200 patents on FBAR and BAW filter technology, including several fundamental patents dating into the 1990s[5](https://www.microwavejournal.com/articles/7325-executive-interview-bryan-ingram-avago-technologies).
The commercial driver was the evolution of carrier networks. As networks moved to CDMA and 3G, filter requirements for the US PCS band and a few other bands became challenging enough that SAW filters no longer sufficed, and FBAR technology was well suited to meet them[2](https://docs.broadcom.com/doc/AV02-3869EN). Narrow duplex spacing in bands such as PCS necessitates high-Q filtering, which made FBAR duplexers especially successful there[5](https://www.microwavejournal.com/articles/7325-executive-interview-bryan-ingram-avago-technologies). By 2008 the [Fort Collins, Colorado](https://www.edgepedia.org/fort-collins-colorado) fab shipped over 1 million filters per day, and between 80 and 90 percent of all US CDMA phones, for carriers including Sprint, Verizon, PCMetro, and AT&T Cingular, contained an FBAR filter, duplexer, or quintplexer[4](https://bsac.berkeley.edu/news/bsac-seminar-fbar-technology-past-present-and-future).
## Role in mobile phones
A modern phone runs multiple radios at once, including cell band, [Bluetooth](https://www.edgepedia.org/bluetooth), WiFi, and GPS, and must operate across different regions and carriers; each additional frequency band increases the demands on RF filtering[2](https://docs.broadcom.com/doc/AV02-3869EN). A duplexer, the component that separates a phone's transmit and receive paths, sits in front of the low-noise amplifier on the receive side, so its loss adds directly to the noise figure of the receiver[9](https://sharetechnote.com/html/RF_Duplexer.html). Small duplexers in handsets are typically SAW, BAW, or FBAR based[9](https://sharetechnote.com/html/RF_Duplexer.html).
**Coexistence filtering.** FBAR's steep rejection curves address a specific design problem: letting several radios operate simultaneously without interfering. In 2010 Avago announced two FBAR bandpass filters for 4G WiMAX/LTE handsets, the ACPF-7024, a 2400 to 2482 MHz WiFi/Bluetooth bandpass filter with over 40 dB of attenuation to interfering signals, and the ACPF-7025, a 2496 to 2690 MHz WiMAX filter with 60 dB typical cell band rejection, 43 dB typical PCS band rejection, and 43 dB typical WiFi/WLAN/Bluetooth rejection. Using Microcap bonded-wafer chip-scale packaging, the two filters measured 2.5 x 2.5 mm and 2.0 x 1.6 mm respectively[10](https://investors.broadcom.com/news-releases/news-release-details/avago-technologies-fbar-filter-technology-speeds-design-4g).
## Performance compared with SAW
Against [surface acoustic wave filters](https://www.edgepedia.org/surface-acoustic-wave-filter), FBAR offers superior performance with steeper rejection curves, 0.3 to 0.5 dB less insertion loss, and up to 50 mA less current consumption, which improves battery life and talk time[3](https://www.broadcom.com/products/wireless/fbar). FBAR filters also provide small size, low insertion loss, and high rejection in high power and temperature conditions[3](https://www.broadcom.com/products/wireless/fbar). A further practical difference is frequency reach: while SAW filters serve lower bands, BAW filters can be manufactured to 10 GHz or even beyond[6](https://www.mouser.sg/applications/BAW-vs.-SAW-RF-Filters/).
The competitive landscape is band-dependent. In the mid band from 1.4 to 2.7 GHz, two technologies compete for high-performance filters: guided wave surface acoustic wave devices (GWSAW, also called piezo on insulator or POI) and free-standing bulk-wave devices, that is, FBAR[11](https://ieee-ius.org/2026/the-golden-age-of-acoustic-resonators-filters). Above 4 GHz, described as the new frontier for acoustic filters, new device types including XBAR are emerging[11](https://ieee-ius.org/2026/the-golden-age-of-acoustic-resonators-filters). The economics behind this competition are substantial: in a smartphone, the RF front-end modules are among the most expensive components, and the 30 to 50 filters inside each module define its cost, size, and performance[11](https://ieee-ius.org/2026/the-golden-age-of-acoustic-resonators-filters).
## Beyond filters
FBAR resonators have been applied outside filtering. An FBAR circulator demonstrated non-reciprocity with a resonator quality factor of 1250 and a piezoelectric coupling coefficient \( k_{t}^{2} \) of 3 percent, achieving a modulation-to-RF frequency ratio better than 1:800 (3 MHz against 2.5 GHz)[12](https://arxiv.org/abs/1710.05003). At the high-frequency frontier, sputtered scandium aluminum nitride (ScAlN) thin-film bulk acoustic resonators have reached 12.5 GHz with an electromechanical coupling \( k^{2} \) of 9.5 percent and a quality factor of 208, a figure of merit \( Q \cdot k^{2} \) of 19.8, using a 90 nm thick film with 20 percent scandium doping on a 38 nm floating platinum bottom electrode for centimeter-wave operation[13](https://arxiv.org/abs/2504.20014).
## References
1. [US11601113B2, Film bulk acoustic wave resonators (patent PDF)](https://patentimages.storage.googleapis.com/e8/2a/ca/ed92d5369a2107/US11601113.pdf)
2. [Dennis Moy, Avago Technologies FBAR Filter Technology White Paper, Wireless Semiconductor Division](https://docs.broadcom.com/doc/AV02-3869EN)
3. [FBAR Devices, Broadcom product page](https://www.broadcom.com/products/wireless/fbar)
4. [BSAC Seminar: FBAR Technology: Past, Present, and Future, Dr. Rich Ruby, Avago, Berkeley Sensor & Actuator Center](https://bsac.berkeley.edu/news/bsac-seminar-fbar-technology-past-present-and-future)
5. [Executive Interview: Bryan Ingram, Avago Technologies, Microwave Journal](https://www.microwavejournal.com/articles/7325-executive-interview-bryan-ingram-avago-technologies)
6. [BAW vs. SAW RF Filters, Mouser Electronics](https://www.mouser.sg/applications/BAW-vs.-SAW-RF-Filters/)
7. [US7205702B2, Film bulk acoustic resonator and method for manufacturing the same, Google Patents](https://patents.google.com/patent/US7205702B2/en)
8. [FBAR是如何诞生的？, 新声半导体 (Newsonic Semiconductor)](https://www.newsonictech.cn/site/news-detail/370)
9. [RF Duplexer, ShareTechnote](https://sharetechnote.com/html/RF_Duplexer.html)
10. [Avago Technologies FBAR Filter Technology Speeds Design of 4G WiMAX/LTE Handsets, Broadcom press release, August 12, 2010](https://investors.broadcom.com/news-releases/news-release-details/avago-technologies-fbar-filter-technology-speeds-design-4g)
11. [The Golden Age of Acoustic Resonators & Filters, IEEE International Ultrasonics Symposium](https://ieee-ius.org/2026/the-golden-age-of-acoustic-resonators-filters)
12. [An FBAR Circulator, arXiv](https://arxiv.org/abs/1710.05003)
13. [Thin-film scandium aluminum nitride bulk acoustic resonator with high Q of 208 and K2 of 9.5% at 12.5 GHz, arXiv](https://arxiv.org/abs/2504.20014)
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*Topic: Encyclopedia › Technology and the built world › Computing and digital systems › Computer hardware › Semiconductor devices & fabrication › Power semiconductors, MEMS, and semiconductor sensors*
*Initially written Oct 15, 2026 · Edited: Oct 25, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license