Terahertz CMOS circuit design and applications for ultra-high data rate (>100Gbps) communication

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Wedged between traditional microwave and optics, the term "Terahertz" was first named by Fleming to designate EM spectrum range from 300GHz to 3THz. It remains one of the least tapped spectra over time. Terahertz image and spectroscopic systems have drawn substantial attention recently due to their unique capabilities in detecting and/or analyzing concealed objects through fog and fabrics. The generation of practical Terahertz signals is nevertheless nontrivial and can only be accomplished by using Free-Electron Radiation, Optical Lasers, Gunn Diodes and sometimes oscillators made of III-V compound based HBT/HEMTs. However, these prior arts suffer major disadvantages of size, cost, complexity, efficiency and/or even cryogenic cooling to produce meaningful signals in the notorious "Terahertz Gap". In the past, commercial CMOS technology has never been seriously considered for ultra-high frequency (>100GHz) circuit/system applications due to its limited IT (current gain cut-off frequency) and fmax (maximum frequency of oscillation). However, continuous device scaling may shift the paradigm and enable further scaled CMOS to reach Terahertz in cut-off frequencies. According to 2007 ITRS, both IT and fmax of 32nm CMOS will approach the lower bound of the Terahertz (600-800GHz, respectively). Under such circumstances, can CMOS possibly play significant roles in the coming Terahertz era, especially for needed portable systems? This question may be answered by assessing some recent circuit innovations devised for Terahertz signal generation and detection. Among mm-Wave signal generation techniques, differential push-push oscillator has been widely adopted. To attain sufficient gain for circuit oscillation, designers often select fundamental oscillation frequency f0 near fmax/2. This, however, renders the achievable oscillation frequency equivalent or lower than that of fmax. In order to alleviate such limitation, we have recently developed a Linear Superposition (LS) technique to synthesize signals at 4 times (N=4) of f0 with unprecedentedly high fundamental-to-4th-harmonic conversion ratio of 0.17 (or -15.4dB) and without extra filtering [1]. We also demonstrated a prototype for the first time to produce Terahertz signal at 324GHz in 90nm CMOS, which is about twice of the device fmax (∼160GHz for large gate width RF devices). Moreover, we can also extend the LS algorithm to any even number (such as N:=2k (k=1, 2, 3, 4... n). For example, choosing LS at N>4 can produce Nf0 signals beyond 4f0 but with worse DC-to-RF conversion efficiency. Further study also showed that higher power conversion efficiency (1.7%) and higher output power (up to -1.4dBm) may be achievable by using a novel "LS VCO + Class-B Power Amplifier" cascaded circuit scheme in 32nmCMOS. As for the mm-Wave signal detection, it may be difficult using active CMOS as receiver front-end in the near term due to gain constraints. However, one may still use passive detectors with frequency multipliers as exploited for present day sub-millimeter and THz systems. In particular, Schottky diodes have been widely used for this purpose. It turns out possible to implement THz Schottky diodes in CMOS without any process modifications. Schottky contacts can be formed between the CoSi2 and lightly doped N-well region without additional source/drain implants. On the other hand, Ohmic contacts can be formed on heavily doped n+ implanted region as well. The Schottky contact areas must be minimized in order to maximize diode's cut-off frequencies. A diode formed using 16 0.32μm x 0.32μm cells connected in parallel (Rs=13 Ω and C=8 fF) has measured IT of 1.5 THz. With further optimization, it would be possible to increase the fT to 2THz. With such diodes, it should be possible to build detectors operating above 500 GHz [2]. Such integrated CMOS signal generator and detector may open doors for various Terahertz system developments, such as that of potable Terahertz imagers for anti-terrorist and transportation or Homeland security applications and for short distance communications at ultrahigh data rate, including the multiband RF-interconnect for core-to-core communications at 100Gb/sec/link for Chip-MultiProcessors (CMP) [3]; high-speed and low power vertical links for layer-to-Iayer communications in 3DIC; on-board or board-to-board data links for automotive/aviation wireless harness; and finally for space-borne communications with long distances, with data rates comparable to that of fiber optical communications, such as OC768 (40Gbps) and beyond.

Original languageEnglish
Title of host publicationASICON 2009 - Proceedings, 2009 8th IEEE International Conference on ASIC
Number of pages2
StatePublished - 1 Dec 2009
Event2009 8th IEEE International Conference on ASIC, ASICON 2009 - Changsha, China
Duration: 20 Oct 200923 Oct 2009

Publication series

NameASICON 2009 - Proceedings 2009 8th IEEE International Conference on ASIC


Conference2009 8th IEEE International Conference on ASIC, ASICON 2009

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    Chang, M-C. (2009). Terahertz CMOS circuit design and applications for ultra-high data rate (>100Gbps) communication. In ASICON 2009 - Proceedings, 2009 8th IEEE International Conference on ASIC (pp. 1-2). [5351288] (ASICON 2009 - Proceedings 2009 8th IEEE International Conference on ASIC). https://doi.org/10.1109/ASICON.2009.5351288