Investigation of the dominant 1/f noise source in silicon nanowire sensors

We analyzed 1/f noise in silicon nanowire ion-sensitive field-effect transistors (SiNW-ISFETs) having different wire widths ranging from 100 nm to 1 pin and operated under different gating conditions in order to determine the noise source and the sensor accuracy. We find that the gate-referred voltage noise S-VG (power spectral density) is constant over a large range of SiNWs resistances tuned by a DC gate voltage. The measurements of S-VG for SiNWs with two different gate-oxide thicknesses, but otherwise similar device parameters, are only compatible with the so-called trap state noise model in which the source of 1/f noise is due to trap states residing in the gate oxide (most likely in the interface between the semiconductor and the oxide). S-VG is found to be inversely proportional to the wire width for constant wire length. From the noise data we determine a sensor accuracy of 0.017% of a full Nernstian shift of 60 mV/pH for a SiNW wire with a width of 1 pm. No influence of the ions in the buffer solution was found. (C) 2013 Elsevier B.V. All rights reserved.

Junctionless silicon nanowire transistors for the tunable operation of a highly sensitive, low power sensor

Matthieu Berthomé, Elizabeth Buitrago Godinez, Montserrat Fernandez-Bolanos Badia, Mihai Adrian Ionescu

Silicon nanowire (SiNW) field effect transistors (FETs) have been widely investigated as biological sensors for their remarkable sensitivity due to their large surface to volume ratio (S/V) and high selectivity towards a myriad of analytes through functionalization. In this work, we propose a long channel (L > 500 nm) junctionless nanowire transistor (JNT) SiNW sensor based on a highly doped, ultrathin body field-effect transistor with an organic gate dielectric epsilon(r) = 1.7. The operation regime (threshold voltage V-th) and electrical characteristics of JNTs can be directly tuned by the careful design of the NW/Fin FET. JNTs are investigated through 3D Technology Computer Aided Design (TCAD) simulations performed as a function of geometrical dimensions and channel doping concentration N-d for a p-type tri-gated structure. Two different materials, namely, an oxide and an organic monolayer, with varying dielectric constants er provide surface passivation. Mildly doped N-d = 1 x 10(19) cm(-3), thin bodied structures (fin width F-w < 20 nm) with an organic dielectric (epsilon(r) = 1.7) were found to have promising electrical characteristics for FET sensor structures such as V-th similar to 0 V, high relative sensitivities in the subthreshold regime S > 95%, high transconductance values at threshold g(m),(Vfg=0V) > 10 nS, low subthreshold slopes SS similar to 60 mV/dec, high saturation currents I-d,I-max similar to 1-10 mu A and high I-on/I-off > 10(4)-10(10) ratios. Our results provide useful guidelines for the design of junctionless FET nanowire sensors that can be integrated into miniaturized, low power biosensing systems. (c) 2013 Elsevier B.V. All rights reserved.

Elsevier2013

Tri-gate silicon nanowire transistors for ultra-low pH resolution and improved scalability

Enrico Accastelli

The scalability of on-chip analytical systems based on field-effect transistors as pH sensors is limited by the degradation of the signal to noise ratio when decreasing the device size. Nano-sized tri-gate transistors such as silicon nanowires and nanoribbons exhibit improved coupling with the electrolyte environment thanks to their tri dimensional structure. Even though in the framework of solid-state tri-dimensional transistors an enhancement of the channel con-ductivity is achieved by reducing the device width, so far the sensitivity performance of liquid-gate devices has been mostly considered in terms of threshold voltage readout. In pH sensing applications, however, the threshold voltage change is independent on the device size, thus overlooking the potential advantage derived from the improved electrical properties of nano sized devices. In this thesis, sensitivity characterization coupled with a comprehensive noise analysis has been performed on devices ranging from 50 nm to 70 µm in width and from 350 nm to 4250 nm in length. Such comprehensive characterization is made possible thanks to the employement indus-trial top down CMOS compatible technology, which ensures high control over process variation and enables the fabrication of nanowires with a wide range of widths and lengths. The results show that reducing the width below few hundreds of nanometers results in an increase of the conductivity properties of silicon nanoribbons, which ultimately improves the sensitivity with respect to surface charge, hence to pH. This enhanced sensitivity of nanoscaled devices can be further exploited within a multi wire configuration, in which the exposed surface is increased and the noise reduced. We provide experimental evidence that multi wire devices maximize the signal to noise ratio achieving, in the presented technology, a resolution of 0.0028 pH·µm2. This result could have a great impact on the improvement of the scalability of on-chip analytical systems requiring high pH resolution, such as DNA sequencing and quantitative PCR.

EPFL2014

Sensing with Advanced Computing Technology: Fin Field-Effect Transistors with High-k Gate Stack on Bulk Silicon

Antonios Bazigos, Didier Bouvet, Elizabeth Buitrago Godinez, Mihai Adrian Ionescu, Sara Rigante, Paolo Scarbolo

Field-effect transistors (FETs) form an established technology for sensing applications. However, recent advancements and use of high-performance multigate metal-oxide semiconductor FETs (double-gate, FinFET, trigate, gate-all-around) in computing technology, instead of bulk MOSFETs, raise new opportunities and questions about the most suitable device architectures for sensing integrated circuits. In this work, we propose pH and ion sensors exploiting FinFETs fabricated on bulk silicon by a fully CMOS compatible approach, as an alternative to the widely investigated silicon nanowires on silicon-on-insulator substrates. We also provide an analytical insight of the concept of sensitivity for the electronic integration of sensors. N-channel fully depleted FinFETs with critical dimensions on the order of 20 nm and HfO2 as a high-k gate insulator have been developed and characterized, showing excellent electrical properties, subthreshold swing, SS similar to 70 mV/dec, and on-to-off current ratio, l(on)/l(off) similar to 10(6), at room temperature. The same FinFET architecture is validated as a highly sensitive, stable, and reproducible pH sensor. An intrinsic sensitivity close to the Nernst limit, S = 57 mV/pH, is achieved. The pH response in terms of output current reaches S-out = 60%. Long-term measurements have been performed over 4.5 days with a resulting drift in time delta V-th/delta t = 0.10 mV/h. Finally, we show the capability to reproduce experimental data with an extended three-dimensional commercial finite element analysis simulator, in both dry and wet environments, which is useful for future advanced sensor design and optimization.

Amer Chemical Soc2015

Tri-gate silicon nanowire transistors for ultra-low pH resolution and improved scalability

Enrico Accastelli

EPFL2014