Jose M. Marulanda

Carrier Density and Effective Mass Calculations for Carbon Nanotubes

The electronic structure of carbon nanotubes has been examined and recalculated using previous theoretical and experimental results. The effective mass and intrinsic carrier concentration have been calculated for different carbon nanotubes. These results show how different diameters and wrapping angles of carbon nanotubes can change their electronic properties. Since the intrinsic carrier concentration of a carbon nanotube can be adjusted, it allows die designer to use the same material (carbon nanotube) for a wide range of applications only by varying the chiral vector (n,m). The calculations performed in this work set an upper limit for a wide range of applications, including carbon nanotube interconnects and carbon nanotube field effect transistors.

Emerging carbon nanotube electronic circuits, modeling and performance

In this paper, a CNT interconnect model is presented and combined with a CNT-FET model to study the performance of CNT-FET circuits at very high frequencies. An all complementary CNT-FET inverter pair using CNT interconnection is modeled and characterized. Cadence/Spectre simulations show that CNT-FET circuits can operate at GHz frequencies and CNT interconnects are able to provide enough bandwidth for GHz operation of CNT-FETs circuits.

THRESHOLD AND SATURATION VOLTAGES MODELING OF CARBON NANOTUBE FIELD EFFECT TRANSISTORS (CNT-FETs)

We present analytical model equations for threshold voltage (Vth) and saturation voltage (Vds,sat) characterizing CNT-FETs. These model equations have been obtained from the charge and potential distributions between the gate and substrate in a CNT-FET. It is shown that both Vth and Vds,sat are strongly dependent on chiral vectors of CNTs. The results show close agreement between theoretical and graphical modeling techniques. It is also shown that the calculated Vth of a CNT-FET with chiral vector (3, 1) is in close agreement with the corresponding published work.

Numerical Modeling of the I-V Characteristic of Carbon Nanotube Field Effect Transistors (CNT-FETs)

Using derived equations for the potential description for carbon nanotube field effect transistors (CNT-FETs), basic semiconductor equations for carbon nanotubes have been used to model the charge transport. The carbon nanotube has been modeled as a line of charge, and a numerical model has been implemented for the current transport. This numerical model uses MATLAB capabilities to solve the given current and voltage equations numerically and presents I-V characteristics for any given CNT-FET.

Built-in Current Sensor for High Speed Transient Current Testing in Analog CMOS Circuits

In this paper, we present a new built-in current sensor (BICS) for high speed, low voltage degradation transient current (IDDT) testing. This sensor has been designed using forward bias technique to limit the supply voltage degradation caused during transient current peaks to 2% of the supply voltage. A CMOS operational amplifier designed for operation at plusmn2.5 V in 0.5 mum n-well CMOS process is used as the circuit under test (CUT). The faults simulating possible short and bridging defects are introduced using the fault injection transistors (FIT). A total of twenty short faults have been introduced into the CUT and nineteen of them were detected, giving 95% fault coverage.

Built-in Current Sensor for Quiescent Current Testing in Analog CMOS Circuits

In this paper we present a new built in current sensor (BICS) for quiescent current testing- IDDQ. This sensor has been designed using forward bias technique to limit the supply voltage degradation caused by quiescent current passing through the BICS to 2% of the supply voltage. A CMOS operational amplifier designed for operation at plusmn 2.5 V in 0.5 mum n-well CMOS process is used as the circuit under test (CUT). The faults simulating possible short and bridging defects are introduced using the fault injection transistors (FIT). A total of twenty short faults have been introduced into the CUT and nineteen of them been detected giving 95% fault coverage.

Current transport modeling in carbon nanotube field effect transistors (CNT-FETs) and bio-sensing applications

Current transport in carbon nanotube field effect transistors (CNT-FETs) has been modeled from charge distributions and the potential inside the carbon nanotube. Analytical equations describing I-V characteristics of the CNT-FETs have been obtained from the combination of diffusion and drift mechanisms in the channel region for normal and sub-threshold operations. It is shown that the electronic transport in semiconducting single-walled carbon nanotubes and field effect transistors can provide better understanding of their bio- and chemical sensing for the detection of traces of agents at molecular levels.

Transfer characteristics and high frequency modeling of logic gates using carbon nanotube field effect transistors (CNT-FETs)

In the present work, current model equations from an analytical model for carbon nanotube field effect transistors are utilized to calculate the transfer characteristics of different logic gates such as inverters, NAND gates, and NOR gates. A small signal model has been implemented and the necessary parameters have been calculated to describe the high frequency response for these logic gates. Results show that how different diameters and wrapping angles of carbon nanotubes, determined by the chiral vector (n,m), can change the transfer characteristics, small signal parameters, and high frequency response for devices based on carbon nanotubes.

Numerical Modeling of the I-V Characteristics of Carbon Nanotube Field Effect Transistors

Carbon nanotubes (CNTs) were first discovered in 1991 by Sumio Iijima (Iijima, 1991), and they have been a rapid and successful target of development by many researchers (Baughman et al., 2002). Especially with the end of Moore’s law in sight (Wind et al., 2002), CNTs are being considered as possible candidates substituting silicon in the fabrication and design of analog and digital integrated circuits (ICs) (Guo et al., 2002; Wong, 2002). Carbon nanotubes are basically two dimensional graphene sheets rolled into a one dimensional tubular structure (Martel et al., 1998). Their properties are determined by the chiral vector represented by the indices (n,m) (Tanaka et al., 1999; Wallace, 1947; Wildoer et al., 1998). Depending on the number of layers rolled, carbon nanotubes can be either single walled (one layer), or multi walled (two or more layers) (Dresselhaus et al., 2001). Single walled carbon nanotubes (SWNTs) are used in the fabrication of carbon nanotube field effect transistors (CNT-FETs), the first CNT-FETs were implemented in 1998 (Martel et al., 1998; Tans et al., 1998). The structure of a CNT-FET is similar to the structure of a typical MOSFET where the CNT forms the channel between two electrodes that work as the source and the drain of the transistor. The structure is build on top of an insulating layer and a substrate wafer that works as the back gate, (Nihey et al., 2003) (Wind et al., 2002). The basic structure of a CNT-FET based on a SWNT is shown in Fig. 1. Among the CNT-FETs reported (Martel et al., 1998; Wind et al., 2002), a 40 nm gate length transistor (Lin et al., 2005), a multistage complementary logic (Javey et al., 2002), oscillators (Chen et al., 2006), and an 80 GHz operating field effect transistor (Nougaret et al., 2009) have been achieved. In addition, carbon nanotubes are one-dimensional conductors (1D), which confines the electrons to only back-scattering effects. This property provides a large electron mean free path in metallic carbon nanotubes of usually a few micrometers (White & Todorov, 1998). Carbon nanotubes also exhibit large current capabilities of ~10-9 A/cm2, (Wei et al., 2001; Yao et al., 2000) and it has also been reported that doping can be avoided in the CNT fabrication process, yet still achieving complementary CNT-FETs (Zhang et al., 2007). With carbon nanotubes interconnects being also a major target of research (Koo et al., 2007; Xu et al., 2008) and a completely carbon nanotube based integrated circuit (IC) already reported 11

I-V Characteristics Modeling and Parameter Extraction for CNT-FETs

In the quest for nanoscale technology, carbon nanotube field effect transistors (CNT-FETs) have emerged as promising devices with unique electronic properties. With the end of silicon-based transistors scaling perhaps in sight, CNT-FETs are being considered as alternative transistors for continuing improvements in the density and performance of electronic systems [1]. This calls for an urgent need to develop practical models characterizing CN-FETs. In [2, 3], model equations for the resistance in metallic carbon nanotubes have been developed. In the present work, we have used these model equations to characterize CNT-FETs. The resistance of a CNT can be expressed as follows [4]: