Page 51 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7

FUNDAMENTAL LIMITS OF HIGH-EFFICIENCY SILICON AND COMPOUND
SEMICONDUCTOR POWER AMPLIFIERS IN 100-300 GHz BANDS

James F. Buckwalter , Mark J. W. Rodwell , Kang Ning , Ahmed Ahmed , Andrea Arias-Purdue ,
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Jeff Chien , Everett O’Malley and Eythan Lam 1
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1 University of California – Santa Barbara, 1160 Harold Frank Hall, Santa Barbara, CA 93106
NOTE: Corresponding author: James Buckwalter, buckwalter@ucsb.edu
Abstract – This paper reviews the requirements for future digital arrays in terms of power amplifier
requirements for output power and efficiency and the device technologies that will realize future energy-
efficient communication and sensing electronics for the upper millimeter-wave bands (100-300 GHz).
Fundamental device technologies are reviewed to compare the needs for compound semiconductors and
silicon processes. Power amplifier circuit design above 100 GHz is reviewed based on load line and matching
element losses. We present recently presented class-A and class-B PAs based on a InP HBT process that have
demonstrated record efficiency and power around 140 GHz while discussing circuit techniques that can be
applied in a variety of integrated circuits.

Keywords – Digital array, high-efficiency, millimeter-wave, power amplifier

1. INTRODUCTION factor, controlled with independent digitally-
controlled Baseband (BB) and Intermediate
Frequencies between 100-300 GHz, known as Frequency (IF), and this poses a large-scale
Upper millimeter-Wave (UmmW) bands, offer an integration challenge that must be solved with
opportunity for convergence of communication and unified design that includes IC, packaging, and
sensing systems to support future high-throughput device technologies.
backhaul and radar applications [1]. In particular,
frequency bands located at 140 and 220 GHz The large number of array elements in the UmmW
feature O2 and H2O absorption windows for low array suggests design architecture based on digital
propagation loss in outdoor channel array techniques rather than traditional RF beam-
environments [2]. Digital array applications in forming approaches that leverage signal processing
UmmW bands require mature electronic and techniques based on massive MIMO (mMIMO) for
packaging technologies and previously Integrated higher spatial resolution than conventional MIMO
Circuits (IC) demonstrated poor power efficiency systems [3]. Reusing time-frequency resources
and higher package costs when compared to lower across multiple users can ultimately support higher
millimeter-wave (LmmW) bands (28/39/60 GHz). spectral efficiency across a network and with the
While other bands, including the 60 GHz bands offer available bandwidth in UmmW devices link capacity
substantial bandwidth, the high absorption at might approach 1 Terabit/second [4]. Moreover, a
60 GHz prohibits energy efficient operation over large number of antennas will focus energy into
more than a kilometer and UmmW offers small regions in the space. Thus, in theory, the
opportunity for high-bandwidth. transmit power can be reduced while maintaining a
Moreover, the UmmW bands offer shorter high Signal-to-Noise Ratio (SNR), resulting in higher
wavelength relative to LmmW and this feature spatial energy efficiency. Of course, there is a circuit
allows more Transmit (TX) and Receive (RX) overhead to generate the RF signals across the
elements within a given aperture area. The array mMIMO array which scales linearly with the
spacing at 140 GHz would be approximately 1 mm number of elements. At some point, a larger array
and, therefore, a 1cm x 1cm array could host incurs substantial power consumption penalties.
around 100 elements while a 28 GHz array might Early demonstrations of mMIMO in sub-6 GHz
have only 4 elements in the same aperture area. based on commercially-available software-defined
Consequently, the UmmW beam-former array will radios require kilowatts in signal processing [5].
contain a relatively large number of steerable Early work on line-of-sight MIMO in millimeter-
elements that might be packed into the small form wave bands was demonstrated nearly a decade

© International Telecommunication Union, 2021 39