C.G. Jakobson

1/f Noise in CMOS transistors for analog applications from subthreshold to saturation

Abstract Detailed noise measurements of the 1/ f noise in p - and n -mos transistors for analog applications are reported under various bias conditions ranging from subthreshold to saturation. The CMOS transistors under study have a relatively large area, exhibit long channel behavior and are fabricated in a commercial “low noise process”, as prescribed for analog applications. A clear methodology and useful models for the power spectral densities of the gate voltage and drain current are presented and are based on recent studies in sub-micron transistors that have established the physical origin of 1/ f noise in MOS transistors. In saturation, it is found that it is advisable to limit the bias voltages to values that are experimentally determined from the transconductance characteristics and correspond to a nearly constant channel mobility. The experimentally observed reduction in channel mobility indicates the existence of strong fields that induce additional oxide charging and hence an increase in the effective density of oxide traps and the noise. In the bias voltages where channel mobility is nearly constant, the measured input-referred noise power is practically constant. Below threshold voltage, a reduction is observed in the input-referred noise as gate voltage is decreased, corresponding to the prediction of the model and due to the exponential reduction of the inversion capacitance with gate voltage. This behavior is observed for both n -mos and p -mos transistors.

Low frequency noise and drift in Ion Sensitive Field Effect Transistors

Abstract Ion Sensitive Field Effect Transistors (ISFETs) are currently produced commercially and promise to become the platform sensors for important biomedical applications. The drift in ISFETs is still an important inherent problem that prevents its application to accurate in vivo measurements. The present paper presents measurements of the drift and the drain current power spectral density (PSD) of pH ISFETs in the very low frequency range from 5 mHz to 10 kHz. The measurements have been performed in buffer solutions with pH 4, 7 and 10, at room temperature. Above a corner frequency, the measured spectra correspond to 1/ f noise introduced by fluctuations at the channel current. Below this corner frequency that depends on the magnitude of the drift, the measured spectra correspond to 1/ f 2 . The observed corner frequency is ∼1 Hz for a drift of 2 mV/h and shifts to frequencies below 0.01 Hz for a drift of 0.1 mV/h. The measured drift is correlated to leakage currents as well as temperature fluctuations and the inherent behaviour of the ISFET. A method for quality evaluation based on frequency behaviour is introduced.

Ion-sensitive field-effect transistors in standard CMOS fabricated by post processing

Highly integrated ion-sensitive field-effect transistor (ISFET) microsystems require the monolithic implementation of ISFETs, CMOS electronics, and additional sensors on the same chip. This paper presents new ISFETs in standard CMOS, fabricated by post-processing of a standard CMOS VLSI chip. Unlike CMOS compatible ISFETs fabricated in a dedicated process, the new sensors are directly combined with state-of-the-art CMOS electronics and are subject to continuous technology upgrading. The ISFETs presented include an intermediate gate formed by one or more conducting layers placed between the gate oxide and the sensing layer. The combination of the highly isolating gate oxide of the MOS with a leaky or conducting sensing layer allows the use of low temperature materials that do not damage the CMOS chip. The operation of ISFETs with an intermediate gate and sensing layers fabricated at low temperature is modeled. ISFETs with a linear pH response and drift as low as 0.3 mV/h are reported.

CMOS low-noise switched charge sensitive preamplifier for CdTe and CdZnTe X-ray detectors

CdTe and CdZnTe X-ray detector arrays for imaging and spectroscopy provide low capacitance current sources with low leakage currents. The optimal shaping time for low-noise operation is relatively high in CMOS analog channels that provide the readout for these detectors. The shaper is centered at lower frequencies, and thus the 1/f noise from the electronics is the main noise source that limits the resolution of the channel. The optimal dimensions of the input stage MOSFET are determined by this noise. In this paper a design criterion for the optimization of the resolution and the power consumption in a 1/f noise dominated readout is introduced. A readout based on CMOS switched charge sensitive preamplifier without feedback resistor has been designed and fabricated in the CMOS 2-/spl mu/ low-noise analog process provided by MOSIS. This design provides high sensitivity and the possibility to integrate a large number of channels with low power consumption. Measurements of the performance of a first prototype chip are presented.

CMOS low noise switched charge sensitive preamplifier for CdTe X-ray detection

A CdTe detector readout based on MOS switched charge sensitive preamplifier without feedback resistor is presented. CMOS technology allows high integration level, relatively low power consumption and provides a nearly ideal switch that does not introduce significant drift current to the integration capacitor. No external components are required. CdTe detectors combine low current with high resistivity and low capacitance that reduces noise levels. Noise performance is compared for semi-Gaussian pulse shaping and a low pass filter limiting of the preamplifier output. The more direct second approach exhibits very similar performance in comparison to the first one, due to 1/f noise; and can be better for specific shaping and reading times.

1/f noise in ion selective field effect transistors compared to MOSFETs

pH ISFETs are very interesting sensors for biomedical microsystems including in vivo measurements of pH. The noise phenomena and related resolution of these sensors are still not understood. In this paper measurements of low frequency noise in pH ISFETs are presented and compared to similar measurements performed in MOSFETs. Various bias conditions are used corresponding to the gate voltage changing from subthreshold to saturation, in the frequency range between 1 Hz to 100 kHz ISFET noise measurements were performed in solutions with pH in the range of pH4 to pH10, at room temperature. The measured ISFETs exhibit clearly 1/f noise. The dependence of the drain current noise power spectral density upon average drain current follows the same behavior of 1/f noise that the observed in MOSFETs. The measured noise characteristics shown here indicate that the Si/SiO/sub 2/ interface dominates the noise behavior of the ISFET. No pH dependent effects are observed and the interface between the solution and the gate insulator does not contribute measurable noise.

Capacitance-voltage characteristics of floating gate electrolyte-insulator-semiconductor capacitors

This paper focuses on the floating gate structure that is the basis of the floating gate ISFET. The study is based on the Floating Gate Electrolyte Insulator Semiconductor (FGEIS) capacitor, which is the parallel structure to the Metal Oxide Semiconductor (MOS) capacitor found in MOSFETs. The floating gate consists on an aluminum layer on silicon dioxide covered by a sensing layer of aluminum oxide or silicon nitride. Both layers are widely used to obtain pH sensitivity. The devices are studied through Capacitance-Voltage characteristics, using a buffer electrolyte with pH=7. In this study a Teflon jig provides good mechanical contact and interface to the solution. The measurements are performed at 1 KHz. High frequency characteristics are observed. Depending on the fabrication process different hysteresis is observed in the counter-clockwise direction.

Gun muzzle flash detection using a single photon avalanche diode array in 0.18µm CMOS technology

In this study, a CMOS Single Photon Avalanche Diode (SPAD) 2D array is used to record and sample muzzle flash events in the visible spectrum, from representative weapons. SPADs detect the emission peaks of alkali salts, potassium or sodium, with spectral emission lines around 769nm and 589nm, respectively. The alkali salts are included in the gunpowder to suppress secondary flashes ignited during the muzzle flash event. The SPADs possess two crucial properties for muzzle flash imaging: (i) very high photon detection sensitivity, (ii) a unique ability to convert the optical signal to a digital signal at the source pixel, thus practically eliminating readout noise. The sole noise sources are the ones prior to the readout circuitry (optical signal distribution, avalanche initiation distribution and nonphotonic generation). This enables high sampling frequencies in the kilohertz range without significant SNR degradation, in contrast to regular CMOS image sensors. This research will demonstrate the SPAD’s ability to accurately sample and reconstruct the temporal behavior of the muzzle flash in the visible wavelength, in the presence of sunlight. The reconstructed signal is clearly distinguishable from background clutter, through exploitation of flash temporal characteristics and signal processing, which will be reported. The frame rate of ~16 KHz was chosen as an optimum between SNR degradation and temporal profile recognition accuracy. In contrast to a single SPAD, the 2D array allows for multiple events to be processed simultaneously. Moreover, a significant field of view is covered, enabling comprehensive surveillance and imaging.

Microsatellite gamma-ray spectroscopy experiment

Preliminary results from Gamma ray experiment installed on a micro-satellite, Techsat 1, are reported. The experiment is based on CdZnTe detectors coupled to custom designed CMOS electronics, which includes low noise charge sensitive preamplifiers, pulse shaping amplifiers and sampling circuits. It was realized as a mile stone towards a micro- satellite mounted Gamma ray space telescope. The experiment is a stand-alone spectroscopy system that measures the radiation inside the micro-satellite and transmits the spectra to ground station via the main satellite computer. The radiation level inside micro-satellites is expected to be significantly lower compared to that inside large satellites. Additional goal of the experiment is to test the CdZnTe detectors and the front-end electronics, implemented in a standard CMOS process, under space radiation environment. In particular, the degradation in performance will be monitored. The Techsat 1 micro-satellite has been designed and constructed at Technion-Israel Institute of Technology. The satellite is approximately 50 X 50 X 50 cm-3 cube with a total weight of about 50 kilograms. It was successfully launched in July 1998 to a 820 km orbit.

Electrostatically driven micro resonator with a CMOS capacitive read out

An electrostatically driven micro resonator is fabricated from single-crystal silicon, using micromachining techniques. The displacement amplitude is measured through differential capacitance measurement, implemented on a CMOS interface circuit, which also provides the excitation clocks for the resonator. The unique feature of this micromechanical system is the integration method applied for attaching the resonator to the CMOS chip. The integration is based upon indium bumps technology, which is now established in modules of infra-red focal plane arrays. A technology which is extended here for the first time, to the best of our knowledge, to microelectromechanical systems.