Debarshi Basu

High-mobility bottom-contact n -channel organic transistors and their use in complementary ring oscillators

The electrical characteristics of bottom-contact organic field-effect transistors fabricated with the air-stable n-type semiconductor N,N′-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) are described. The mobility, threshold voltage, subthreshold swing, and Ion∕Ioff ratio(VDS=40V, VG=0∼40V) are 0.14cm2∕Vs, 1.6V, 2.0V/decade, and 1.2×103, respectively. The effect of electrode/dielectric surface treatment on these devices is also examined, with a combination of 1-octadecanethiol and hexamethyldisilazane. Organic complementary five-stage ring oscillators were fabricated using pentacene and PDI-8CN2, and operated at an oscillation frequency of 34kHz and a propagation delay per stage of 3μs.

Effect of edge roughness on electronic transport in graphene nanoribbon channel metal-oxide-semiconductor field-effect transistors

Results of quantum mechanical simulations of the influence of edge disorder on transport in graphene nanoribbon metal-oxide-semiconductor field-effect transistors (MOSFETs) are reported. The addition of edge disorder significantly reduces ON-state currents and increases OFF-state currents, and introduces wide variability across devices. These effects decrease as ribbon widths increase and as edges become smoother. However, the band gap decreases with increasing width, thereby increasing the band-to-band tunneling mediated subthreshold leakage current even with perfect nanoribbons. These results suggest that without atomically precise edge control during fabrication, MOSFET performance gains through use of graphene will be difficult to achieve in complementary MOS applications.

Electric-field-dependent charge transport in organic thin-film transistors

This article reports the experimental study of the electric-field-dependent charge transport mechanisms in polycrystalline organic thin-film field-effect transistors. This work represents the quantitative measurement of the temperature and electric-field dependences of the mobility in organic thin-film transistors with scaled device geometry when carrier densities are at levels of practical importance. The true behavior of field-dependent mobility was extracted by minimizing contact effects consistently over a range of channel lengths. In these partially ordered systems, experimental data suggest that thermally activated and field-assisted hopping transport between disorder-induced localized states dominates over intrinsic polaronic transport seen in organic single crystals. The experimental results were found to exhibit a Frenkel-Poole-type dependence consistently over a wide range of channel lengths, fields, and temperatures.

Pentacene field-effect transistors with sub-10-nm channel lengths

The field effect in pentacene thin-film transistors was studied using bottom-contact devices with channel lengths below 10nm. To suppress spreading current in these devices, which have a small channel width-to-length (W-L) ratio, we employed a pair of guarding electrodes as close as 20nm to the two sides of the channel. The responses of these nanometer scale transistors exhibit good gate modulation. Mobilities of 0.046cm2∕Vs and on/off ratios of 97 were achieved in sub-10-nm transistors. We find that the device response is strongly influenced by the nature of the metal-semiconductor contact.

Organic field effect transistor mobility from transient response analysis

An electronic method for analyzing the transient response of a pentacene organic field effect transistor at time scales below 100ns is presented with analysis that allows extraction of estimated field-dependent device mobility from the measured carrier velocity. A second technique we propose is the use of T-SPICE simulations of transient response data of the device behavior between ∼100ns and ∼3μs. These results are compared with lower field-effect mobilities extracted from the transient data at 250μs and the dc drain current (Id) versus source-drain voltage (Vds) characteristics in the saturation regime. This trend of decreasing mobility with increasing time is perhaps due to the absence of the bias stress effect at small time scales.

Surface roughness exacerbated performance degradation in silicon nanowire transistors

Scaling of silicon devices is fast approaching the limit where a single gate will fail to retain an effective control over the channel region. Of the alternative devices being researched, silicon nanowire transistors (SNWTs) show great promise in terms of scalability, performance, and ease of fabrication. In this work, the authors present the results of self-consistent, three-dimensional fully quantum mechanical simulations of SNWTs to show the role of surface roughness (SR) on the device parameter variation of SNWTs. The authors find additional quantum interference to take place when SR is taken into account in addition to a discrete impurity distribution. Due to this, the variations of the SNWT operational parameters increase about their mean value, indicating a performance concern. However, it is also seen that the quantum interferences are dependent on the dopant locations to a large extent, and for devices with preferential dopant configurations, these effects can be overcome to obtain nearly ballist...

Direct measurement of carrier drift velocity and mobility in a polymer field-effect transistor

An electronic method to measure the drift velocity and mobility of charge carriers in polymer thin film transistor has been developed. The measurement is based on the movement of a packet of carriers injected into the channel. This technique can be used to explore trap states and therefore obtain a comprehensive understanding of charge transport in these materials. Drift mobility of 0.52cm2∕Vs is obtained from the transit time which is a factor of 3 higher than the field-effect transistor mobility.

Drift Velocity and Drift Mobility Measurement in Organic Semiconductors Using Pulse Voltage

This chapter is focused on the electrical characterization of organic field effect transistors. Conventional methods for characterizing mobility in organic semiconductors include field-effect mobility measurement and optical time-of-flight measurement of drift mobility. In his presentation This chapter describes a new method that combines the advantages of both these methods. It involves the injection of carriers at the source of a transistor using a voltage pulse followed by their subsequent extraction at the drain. The delay between the two events is used to extract the velocity of carriers. The electronic time-of-flight method is a fast, simple and direct method to determine the charge transport properties of the semiconductor. In combination with the prevalent methods the electronic time-of-flight method presents itself as a source of information for understanding injection into the semiconductor and determining the trap distribution

Actuation of water droplets driven by an organic transistor based inverter

The authors report the implementation of a microfluidic device for actuation of water droplets driven by an organic transistor based inverter. The research reported in this letter combines the advantages of discrete droplet based microfluidic systems along with simple fabrication procedures for fabricating organic transistor based circuits. An organic transistor based inverter fabricated using pentacene and N,N′-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboxyimide) (Northwestern University) is used to drive water droplets on a simple microfluidic device not employing the use of a top plate. Rapid and repeatable motion of droplets is observed for output voltages of 50–95V.

Novel double layer graphene transistors-bilayer pseudospin FETs and 2D-2D tunnel FETs

In this paper, bilayer pseudospin FET (BiSFET) is fabricated and tested for the condensate using Coulomb drag measurements in the double layer graphene system. The basic BiSFET structure can also be used as 2D-2D single particle tunnel FET, and the single particle h-h and e-e 2D-2D tunnel FETs, which is graphenes single-atom thickness could lead to more ideal interlayer tunneling characteristics provided the layers can be aligned. Single particle tunneling current calculations have been performed which show NDR characteristics, reminiscent of the BiSFET, albeit with higher operating powers.