Power Spectral Density Of Thermal Noise Research Articles

Stiffness calibration of qPlus sensors at low temperature through thermal noise measurements

Non-contact atomic force microscopy (nc-AFM) offers a unique experimental framework for topographical imaging of surfaces with atomic and/or sub-molecular resolution. The technique also permits to perform frequency shift spectroscopy to quantitatively evaluate the tip–sample interaction forces and potentials above individual atoms or molecules. The stiffness of the probe, k, is then required to perform the frequency shift-to-force conversion. However, this quantity is generally known with little precision. An accurate stiffness calibration is therefore mandatory if accurate force measurements are targeted. In nc-AFM, the probe may either be a silicon cantilever, a quartz tuning fork (QTF), or a length extensional resonator (LER). When used in ultrahigh vacuum (UHV) and at low temperature, the technique mostly employs QTFs, based on the so-called qPlus design, which actually covers different types of sensors in terms of size and design of the electrodes. They all have in common a QTF featuring a metallic tip glued at the free end of one of its prongs. In this study, we report the stiffness calibration of a particular type of qPlus sensor in UHV and at 9.8 K by means of thermal noise measurements. The stiffness calibration of such high-k sensors, featuring high quality factors (Q) as well, requires to master both the acquisition parameters and the data post-processing. Our approach relies both on numerical simulations and experimental results. A thorough analysis of the thermal noise power spectral density of the qPlus fluctuations leads to an estimated stiffness of the first flexural eigenmode of ≃2000 N/m, with a maximum uncertainty of 10%, whereas the static stiffness of the sensor without tip is expected to be ≃3300 N/m. The former value must not be considered as being representative of a generic value for any qPlus, as our study stresses the influence of the tip on the estimated stiffness and points towards the need for the individual calibration of these probes. Although the framework focuses on a particular kind of sensor, it may be adapted to any high-k, high-Q nc-AFM probe used under similar conditions, such as silicon cantilevers and LERs.

Thermal Noise Models for Trigate Junctionless Transistors Including Substrate Bias Effects

Thermal noise power spectral density (PSD) models for trigate junctionless field-effect transistors (TG-JLFETs) incorporating substrate bias effects are developed in this article. The PSDs of drain current thermal noise, induced gate noise, and cross correlation between the two noises are derived for TG-JLFET using the modified Klaassen and Prins (KP) equation. An all-region drain current model of TG-JLFET is used to obtain the aforementioned thermal noises. Thermal noise PSD in TG-JLFET depends on channel conductance, and substrate bias voltage significantly modulates the channel conductance, and thus, the substantial impact of substrate bias can be observed on thermal noise of TG-JLFET. Model results are validated with the simulation results obtained using a 3-D technology computer-aided design (TCAD)-based device simulator from Synopsys.

A Low-Noise Chopper Amplifier Designed for Multi-Channel Neural Signal Acquisition

This paper proposed the design of a low-noise, low total harmonic distortion (THD) chopper amplifier for neural signal acquisition. A dc servo loop (DSL) based on active Gm-C integrator is proposed to reject the electrode-dc-offset (EDO). Architecture of a complementary input very low-transconductance (VLT) operational transconductance amplifier (OTA) was proposed and integrated in the active Gm-C integrator to improve the linearity as well as to reduce the noise, featuring a transconductance ranging from 45 pS to a few nS. The proposed amplifier was fabricated in a TSMC 0.18- $\mu \text{m}$ CMOS process, occupying an area of 0.2 mm 2, featuring a power consumption of $3.24~\mu \text{W}$ /channel under a 1.8-V supply voltage. The THD for a 5-mVpp input is lower than −61 dB. An input-referred thermal noise power spectral density (PSD) of 39 nV/ $\sqrt {\text {Hz}}$ is measured. The measured input-referred noise is $0.65~\mu \text {V}_{\text {rms}}$ in the 0.3–200-Hz frequency band and $2.14~\mu \text{V}_{\text {rms}}$ in the 200-Hz–5-kHz frequency band, respectively, leading to a noise-efficiency factor of 2.37 (0.3–200 Hz) and 1.56 (0.2 k–5 kHz). In addition, the high-pass corner frequency can be precisely configured and linearly adjusted with the external bias current from 0.35 to 54.5 Hz.

Time-Varying Communication Channel High Altitude Platform Station Link Budget and Channel Modeling

Because of the high BER (Bit Error Rate), low time delay and low channel transmission efficiency of HAPS (High Altitude Platform Station) in the near space. The link budget of HAPS and channel model are proposed in this paper. According to the channel characteristic, the channel model is set up, combined with different CNR (Carrier Noise Ratio), elevation angle, coding situations of wireless communication link by using Hamming code, PSK (Pulse Shift Keying) and Golay code respectively, then the situations of link quality and BER are analyzed. The simulation results show that the established model of the link budget and channel are suitable for the theoretical analysis results. The elevation of the HAPS communication link is smaller while the BER is increasing. The case of channel in the coding is better than in the un-coded situation. When every bit power and thermal noise power spectral density is larger, the BER of the HAPS communication link is becoming smaller.

Compact Modeling of Drain Current Thermal Noise in FDSOI MOSFETs Including Back-Bias Effect

In this paper, we present an analytical charge-based model for thermal noise power spectral density in fully depleted silicon on insulator (FDSOI) MOSFETs. Two important aspects particular to FDSOI technology, namely, different inversion charges and different effective mobilities at front and back interfaces, are considered in the model. Proposed model is valid from weak to strong inversion regions of operation. Velocity saturation and channel length modulation are also incorporated to properly capture the excess noise in deep submicrometer MOSFETs. To test the quality of the model, standard benchmark tests are performed and asymptotic behavior of the model is validated in all regions of operation. The model is implemented in SPICE and validated with calibrated TCAD simulations as well as with experimental data of high frequency noise for wide range of back biases.