Sangyeop Lee

1.2–17.6 GHz Ring-Oscillator-Based Phase-Locked Loop with Injection Locking in 65 nm Complementary Metal Oxide Semiconductor

A wide-frequency-range phase-locked loop (PLL) with subharmonic injection locking is proposed. The PLL is equipped with a wide tunable ring-type voltage-controlled oscillator (ring VCO), frequency dividers, and a doubler in order to the widen injection-locked tuning range (ILTR). In addition, high-frequency injection signals are used to improve phase noise, which is supposed to be generated by a reference PLL. The proposed circuit is fabricated by using a 65 nm Si complementary metal oxide semiconductor (CMOS) process. The measured frequency tuning range is from 1.2 to 17.6 GHz with a frequency doubler and dividers. The phase noise at 14.4 GHz (=32×450 MHz) with injection locking was -109 dBc/Hz, which shows a 21-dB reduction compared with that in the case without injection locking.

Mapping of magnetic anisotropy in strained ferromagnetic semiconductor GaMnAs films

The effect of strain on the magnetic anisotropy of GaMnAs films has been systematically investigated using Hall effect measurements. The magnitude of the strain, which was caused by differences in the lattice constant between the GaMnAs film and buffer layer, was controlled by adjustment of the alloy composition in the GaInAs buffer layer. The in-plane and out-of-plane components of the magnetic anisotropy were obtained from the angular dependence of the planar Hall resistance and the anomalous Hall resistance, respectively. The anisotropy constants obtained allow us to construct a three-dimensional magnetic free energy surface, which provides a clear understanding of the transition behavior of the magnetization between the in-plane and out-of-plane direction in the GaMnAs films.

Tunneling magnetoresistance from non-collinear alignment of magnetization in Fe/GaAlAs/GaMnAs magnetic tunnel junctions

Tunneling magnetoresistance (TMR) observed in Fe/GaAlAs/GaMnAs magnetic tunnel junctions depends strongly on relative configuration of magnetizations in the GaMnAs and Fe layers. Using a series of field orientations, we show that non-collinear alignments of magnetization in this structure occur when the magnetic field is applied away from the easy axes of the magnetic layers; and we find that the values of TMR observed for non-collinear spin configurations are very different from those of collinear configurations. The observed behavior suggests the possibility of using this effect for multivalued magnetic memory devices.

A sub-1mw 5.5-GHz PLL with digitally-calibrated ILFD and linearized varactor for low supply voltage operation

This paper proposes an ultra-low-power 5.5GHz PLL which employs a divide-by-4 injection-locked frequency divider (ILFD) and linearity-compensated varactor for low supply voltage operation. The digital calibration circuit is introduced to control the ILFD frequency automatically. The proposed varactor, which applies a forward-body-bias (FBB) technique, is employed for linear-frequency-tuning under the power supply of 0.5 V. The proposed PLL was fabricated in 65 nm CMOS. With a 34.3-MHz reference, it shows a 1-MHz-offset phase noise of -106 dBc/Hz, a reference spur level lower than -65 dBc, and the total power consumption of 950μW at 5.5 GHz.

Temperature Behavior of Uniaxial Anisotropy along [100] Direction in GaMnAs Films

We have investigated a uniaxial anisotropy along the [100] direction of GaMnAs film grown on (001) GaAs. The hysteresis observed in the angular dependence of the planar Hall resistance shifted toward the [010] and [010] directions. This phenomenon was analyzed by introducing an additional uniaxial anisotropy field Hu2 along the [100] direction. The magnitude of Hu2 is much smaller than those of the cubic Hc and uniaxial Hu1 anisotropy fields for this material. The temperature behavior of Hu2 appears to be very similar to that of Hu1, suggesting the possibility that Hu1 and Hu2 have a common origin.

2.4--10 GHz Low-Noise Injection-Locked Ring Voltage Controlled Oscillator in 90 nm Complementary Metal Oxide Semiconductor

A wide-frequency-range low-phase-noise voltage controlled oscillator (VCO) with subharmonic injection locking is proposed and an implementation in 90 nm complementary metal oxide semiconductor (CMOS) is demonstrated. The result is a combination of the scalability and wide frequency tuning range naturally expected of a ring VCO and phase-noise performance close to that of an LC VCO using injection locking. The fabricated circuit is only 0.00054 mm2 in area, and its frequency tuning range is from 2.4 to 10 GHz. For a 500 MHz input reference signal and a 9.0 GHz (=18×500 MHz) output frequency, the 1-MHz-offset phase noise was -125 dBc/Hz. This is a 54 dB improvement over the free-running VCO phase noise of -71 dBc/Hz. The power consumption at 9.0 GHz is 9.5 mW.

A 0.5-V 5.5-GHz class-C-VCO-based PLL with ultra-low-power ILFO in 65 nm CMOS

In this paper, an ultra-low-power 5.5-GHz PLL is proposed which employs the new divide-by-4 injection-locked frequency divider (ILFD) and a class-C VCO for operation under a power supply of 0.5 V. A forward-body-biasing (FBB) technique can decrease threshold voltage of MOS transistors, which can improve operation frequency and can widen the lock range of the ILFD. The double-switch injection technique is also proposed to widen the lock range of the ILFD. The proposed PLL was fabricated in 65nm CMOS. The whole circuit consumes 1.6 mW under the power supply of 0.5V. With a 34.6-MHz reference, it shows a 1-MHz-offset phase noise of -105 dBc/Hz and a reference spur level lower than -65dBc at 5.5 GHz.

Asymmetry in the angular dependence of the switching field of GaMnAs film

Planar Hall effect measurements were carried out on two GaMnAs ferromagnetic films with different Mn concentrations (6.2% and 8.3% Mn). The switching fields of magnetization taken from field scans of the planar Hall effect showed significantly different angular dependences in the two samples. While the angular dependence of the switching field for the sample with 8.3% Mn had a symmetric rectangular shape in the polar plot, that of the other sample with 6.2% Mn exhibited a clearly asymmetric behavior, with large steps at the 〈110〉 crystallographic directions. This switching field behavior was analyzed by considering pinning fields for crossing the 〈110〉 directions. The fitting of step features appearing at the 〈110〉 directions revealed the presence of a new uniaxial anisotropy field Hu2 along the [100] direction, in addition to the commonly observed cubic Hc anisotropy field (along the 〈100〉 directions) and uniaxial anisotropy Hu1 fields (along either the [110] or the [11¯0] direction) in the GaMnAs film.

Influence of uniaxial anisotropy on the domain pinning fields of ferromagnetic Ga1−xMnxAs films

The domain pinning fields of ferromagnetic Ga1−xMnxAs films were investigated using the planar Hall effect (PHE). Two in-plane components of GaMnAs films’ anisotropy fields (H4∥ and HU∥), which determine the direction of magnetic easy axes in the (001) plane, were obtained from the PHE’s angular dependence fitted with magnetic free energy within the scheme of the Stoner–Wohlfarth model. The domain pinning fields were obtained both with and without consideration of the deviation angle, δ, of magnetic easy axes from the ⟨100⟩ crystallographic direction of each sample. The values of domain pinning fields are clearly different between the two methods of analysis and the discrepancy increases with δ. This indicates that the correct direction of the magnetic easy axis (i.e., the influence of uniaxial anisotropy) must be considered to obtain precise values of pinning fields in GaMnAs films.

Thickness dependence of uniaxial anisotropy fields in GaMnAs films

Our investigation of thin GaMnAs films with different thicknesses revealed that the magnetic properties of this material strongly depend on film thickness. For this study, a single GaMnAs film was selectively etched, and its properties were then investigated by planar Hall effect measurements. A particularly important conclusion from the results is the emergence of a uniaxial anisotropy field along the [100] crystalline direction, which increases rapidly with increasing film thickness. We argue that such thickness dependence of the [100] uniaxial anisotropy results from the crystal structure of the film, rather than from the effects of the interface between the GaMnAs and the substrate.