Ruonan Han

A 280-GHz Schottky Diode Detector in 130-nm Digital CMOS

A 2×2 array of Schottky-barrier diode detectors with an on-chip patch antenna and a preamplifier is fabricated in a 130-nm logic CMOS process. Each detector cell can detect the 25-kHz modulated 280-GHz radiation signal with a measured responsivity and noise equivalent power (NEP) of 21kV/W and 360pW/ √Hz, respectively. At 4-MHz modulation frequency, NEP should be about 40pW/ √Hz. At supply voltage of 1.2V, the detector consumes 1.6mW. By utilizing the detector, a millimeter-wave image is constructed, demonstrating its potential application in millimeter-wave and THz imaging.

Progress and Challenges Towards Terahertz CMOS Integrated Circuits

Key components of systems operating at high millimeter wave and sub-millimeter wave/terahertz frequencies, a 140-GHz fundamental mode voltage controlled oscillator (VCO) in 90-nm CMOS, a 410-GHz push-push VCO with an on-chip patch antenna in 45-nm CMOS, and a 125-GHz Schottky diode frequency doubler, a 50-GHz phase-locked loop with a frequency doubled output at 100 GHz, a 180-GHz Schottky diode detector and a 700-GHz plasma wave detector in 130-nm CMOS are demonstrated. Based on these, and the performance trends of nMOS transistors and Schottky diodes fabricated in CMOS, paths to terahertz CMOS circuits and systems including key challenges that must be addressed are suggested. The terahertz CMOS is a new opportunity for the silicon integrated circuits community.

Active Terahertz Imaging Using Schottky Diodes in CMOS: Array and 860-GHz Pixel

Schottky-barrier diodes (SBDs) fabricated in CMOS without process modification are shown to be suitable for active THz imaging applications. Using a compact passive-pixel array architecture, a fully-integrated 280-GHz 4 × 4 imager is demonstrated. At 1-MHz input modulation frequency, the measured peak responsivity is 5.1 kV/W with ±20% variation among the pixels. The measured minimum NEP is 29 pW/Hz1/2. Additionally, an 860-GHz SBD detector is implemented by reducing the number of unit cells in the diode, and by exploiting the efficiency improvement of patch antenna with frequency. The measured NEP is 42 pW/Hz1/2 at 1-MHz modulation frequency. This is competitive to the best reported performance of MOSFET-based pixel measured without attaching an external silicon lens (66 pW/Hz1/2 at 1 THz and 40 pW/Hz1/2 at 650 GHz). Given that incorporating the 280-GHz detector into an array increased the NEP by ~ 20%, the 860-GHz imager array should also have the similar NEP as that for an individual detector. The circuits were utilized in a setup that requires neither mirrors nor lenses to form THz images. These suggest that an affordable and portable fully-integrated CMOS THz imager is possible.

A CMOS High-Power Broadband 260-GHz Radiator Array for Spectroscopy

A high-power broadband 260-GHz radiation source using 65-nm bulk CMOS technology is reported. The source is an array of eight harmonic oscillators with mutual coupling through four 130-GHz quadrature oscillators. Based on a novel self-feeding structure, the harmonic oscillator simultaneously achieves the optimum conditions for the fundamental oscillation and the 2nd-harmonic generation. The signals at 260 GHz radiate through eight on-chip slot antennas, and are in-phase combined inside a hemispheric silicon lens attached at the backside of the chip. Similar to the laser pulse-driven photoconductive emitter in many THz spectrometers, the radiation of this source can also be modulated by narrow pulses generated on chip, which achieves broad radiation bandwidth. Without modulation, the chip achieves a measured continuous-wave radiated power of 1.1 mW, and an EIRP of 15.7 dBm. Under modulation, the measured bandwidth of the source is 24.7 GHz. This radiator array consumes 0.8-W DC power from a 1.2-V supply.

Towards terahertz operation of CMOS

The electromagnetic spectrum between 300GHz and 3THz is broadly referred as terahertz [1]. The utility of this portion of spectrum for detection of chemicals and bio agents, for imaging of concealed weapons, cancer cells and manufacturing defects [1, 2], and for studying chemical species using electron paramagnetic resonance, as well as, in short range radars and secured high data rate communications has been demonstrated. However, high cost and low level of integration for III–V devices needed for the systems have limited their wide use. The improvements in the high frequency capability of CMOS have made it possible to consider CMOS as a lower cost alternative for realizing the systems that can greatly expand the use of this spectrum range.

A 260GHz broadband source with 1.1mW continuous-wave radiated power and EIRP of 15.7dBm in 65nm CMOS

Terahertz spectroscopy using silicon technology is gaining attraction for future portable and affordable material identification equipment. To do this, a broadband THz radiation source is critical. Unfortunately, the bandwidth of the prior CMOS works is not sufficient. In [1], the 300GHz signal source achieves 4.5% tuning range by changing the coupling among multiple oscillators. In [2], the DAR array has 3% tuning range with radiation capability. Alternative to the continuous device-tuning method, THz time-domain spectroscopy utilizing the broadband spectrum of picosecond pulses is widely used in the optics community [3]. In this paper, a high-power pulse-based sub-millimeter-Wave radiation source using 65nm bulk CMOS technology is reported. The architecture of this transmitter is shown in Fig. 8.2.1, where four differential core oscillator pairs are mutually coupled through four quadrature oscillators. Each core oscillator pair generates 2nd-harmonic signals at 260GHz that are power-combined after radiating through eight on-chip antennas. Four shunt switches, controlled by narrow pulses (width≈45ps) modulate the radiation. The pulses are generated by local digital circuit blocks with programmable repetition rate up to 5GHz. This way, the broadband spectrum of the pulses is upconverted to the carrier frequency of 260GHz. Without modulation, the chip achieves a continuous-wave radiated power of 1.1mW. Under modulation, the measured bandwidth of the source is 24.7GHz, which makes it suitable for many FTIR-based THz spectrometers. In addition, if the switches are modulated by digital data, this chip can also be used as a transmitter for sub-millimeter/THz wireless communications.

280-GHz schottky diode detector in 130-nm digital CMOS

A 2×2 array of 280-GHz Schottky-barrier diode detectors with an on-chip patch antenna (255 × 250 μm2) is fabricated in a 130-nm logic CMOS process. The series resistance of diode is minimized using poly-gate separation (PGS), and exhibits a cut-off frequency of 2 THz. Each detector unit can detect an incident carrier with 100-Hz ~ 2-MHz amplitude modulation. At 1-MHz modulation frequency, the estimated voltage responsivity and noise equivalent power (NEP) of the detector unit are 250 V/W and 33 pW/Hz1/2, respectively. An integrated low-noise amplifier further boosts the responsivity to 80 kV/W. At supply voltage of 1.2 V, the entire chip consumes 1.6 mW. The array occupies 1.5 × 0.8 mm2. A set of millimeter-wave images with a signal-noise ratio of 48 dB is formed using the detector. These suggest potential utility of Schottky diode detectors fabricated in CMOS for millimeter wave and sub-millimeter wave imaging.

280GHz and 860GHz image sensors using Schottky-barrier diodes in 0.13μm digital CMOS

Millimeter and sub-millimeter-wave imaging using solid-state circuits is gaining attention for security and medical applications. To lower cost and increase integration, MOSFETs in CMOS are being investigated for implementing broadband detectors [1-3]. However, neither measured noise-equivalent power (NEP) nor noise floor of the imager was given in [1]. Although NEP of 17pW/Hz1/2 was achieved at 650GHz in [2], an external lens was attached to the 65nm SOI CMOS chip. In [3], an NEP of 66pW/Hz1/2 was measured at 1.05THz using 65nm CMOS without a lens attached to the chip. Additionally, although the efforts reported in [1-3] realized an array, none demonstrated the image-array function. As an alternative, polysilicon-gate-separation (PGS) Schottky-barrier diodes (SBD) with cut-off frequency of ~2THz were fabricated in CMOS without process modifications [4] and were used to demonstrate a 280GHz detector with NEP of 30pW/Hz1/2 [5,6]. To significantly enhance the scanning speed, a 16-pixel 280GHz SBD imager is fabricated and its array function is reported in this paper. The imager including baseband amplifiers achieves responsivity of 5.1kV/W and NEP of 29pW/Hz1/2. More importantly, its operation was demonstrated in a setup that requires no mirror or lens that is bulky and costly. Next, an 860GHz SBD detector is demonstrated with a measured non-amplified responsivity of 355V/W and NEP of 32pW/Hz1/2. This NEP is ~2X lower than the best reported work in CMOS [3]. Both chips are fabricated in a 0.13μm logic CMOS. The results suggest a path for high performance, compact and affordable sub-millimeter-wave and terahertz CMOS imagers using SBDs.

A High-Power Broadband Passive Terahertz Frequency Doubler in CMOS

To realize a high-efficiency terahertz signal source, a varactor-based frequency-doubler topology is proposed. The structure is based on a compact partially coupled ring that simultaneously achieves isolation, matching, and harmonic filtering for both input and output signals at f0 and 2f0. The optimum varactor pumping/loading conditions for the highest conversion efficiency are also presented analytically along with intuitive circuit representations. Using the proposed circuit, a passive 480-GHz frequency doubler with a measured minimum conversion loss of 14.3 dB and an unsaturated output power of 0.23 mW is reported. Within 20-GHz range, the fluctuation of the measured output power is less than 1.5 dB, and the simulated 3-dB output bandwidth is 70 GHz (14.6%). The doubler is fabricated using 65-nm low-power bulk CMOS technology and consumes near zero dc power.

25.5 A 320GHz phase-locked transmitter with 3.3mW radiated power and 22.5dBm EIRP for heterodyne THz imaging systems

Non-ionizing terahertz imaging using solid-state integrated electronics has been gaining increasing attention over the past few years. However, there are currently several factors that deter the implementations of fully-integrated imaging systems. Due to the lack of low-noise amplification above fmax, the sensitivity of THz pixels on silicon cannot match that of its mm-Wave or light-wave counterparts. This, combined with the focal-plane array configuration adopted by previous sensors, requires exceedingly large power for the illumination sources. Previous works on silicon have demonstrated 1mW radiation [1,3]; but higher power, as well as energy efficiency, are needed for a practical imaging system. In addition, heterodyne imaging scheme was demonstrated to be very effective in enhancing detection sensitivity [4]. Due to the preservation of phase information, it also enables digital beam forming with a small number of receiver units. This however requires phase locking between the THz source and receiver LO with a small frequency offset (IF<;1GHz). In [5], a 300GHz PLL is reported with probed output. In this paper, a 320GHz transmitter using SiGe HBTs is presented (Fig. 25.5.1). Combining 16 coherent radiators, this work achieves 3.3mW radiated power with 0.54% DC-RF efficiency, which are the highest among state-of-the-art silicon THz radiators shown in the comparison table in Fig. 25.5.6. Meanwhile, the output beam is phase-locked by a fully-integrated PLL, which enables high-performance heterodyne imaging systems.