M. Radosavljević

Drain voltage scaling in carbon nanotube transistors

Decreasing the oxide thickness in carbon nanotube field-effect transistors (CNFETs) improves the turn-on behavior. However, we demonstrate that this also requires scaling the range of the drain voltage. This scaling is needed to avoid an exponential increase in off-current with drain voltage, due to modulation of the Schottky barriers at both the source and drain contact. We illustrate this with results for bottom-gated ambipolar CNFETs with oxides of 2 and 5 nm, and give an explicit scaling rule for the drain voltage. Above the drain voltage limit, the off-current becomes large and has equal electron and hole contributions. This allows the recently reported light emission from appropriately biased CNFETs.

Unexpected scaling of the performance of carbon nanotube Schottky-barrier transistors

We show that carbon nanotube transistors exhibit scaling that is qualitatively different than conventional transistors. The performance depends in an unexpected way on both the thickness and the dielectric constant of the gate oxide. Experimental measurements and theoretical calculations provide a consistent understanding of the scaling, which reflects the very different device physics of a Schottky barrier transistor with a quasi-one-dimensional channel contacting a sharp edge. A simple analytic model gives explicit scaling expressions for key device parameters such as subthreshold slope, turn-on voltage, and transconductance.

High performance of potassium n-doped carbon nanotube field-effect transistors

We describe a robust technique for the fabrication of high performance vertically scaled n-doped field-effect transistors from large band gap carbon nanotubes. These devices have a tunable threshold voltage in the technologically relevant range (−1.3 V⩽Vth⩽0.5 V) and can carry up to 5–6 μA of current in the on-state. We achieve such performance by exposure to potassium (K) vapor and device annealing in high vacuum. The treatment has a twofold effect to: (i) controllably shift Vth toward negative gate biases via bulk doping of the nanotube (up to about 0.6e−/nm), and (ii) increase the on-current by 1–2 orders of magnitude. This current enhancement is achieved by lowering external device resistance due to more intimate contact between K metal and doped nanotube channel in addition to potential reduction of the Schottky barrier height at the contact.

Carbon nanotubes as potential building blocks for future nanoelectronics

We present recent experiments on carbon nanotube field-effect transistors and their characteristics and compare the performance of these devices with state-of-the-art silicon MOSFETs. By reducing the gate dielectric film thickness and working with high-k dielectric materials such as HfO2, we are able to effectively reduce the operational voltages below 1 V. The electrical characteristics obtained clearly indicate excellent device performance in both the on- and off-state wit of the nanotube transistor. On/off-current ratios of almost 104 are achieved along with a maximum transconductance of around 425 µS/µm and drive currents of 270 µA/µm at Vgs -- Vth = -0.6 V. Since device parameters are not fully optimized, significant performance improvements can be expected making carbon nanotubes particularly promising as building blocks for future nanoelectronics.

Electrical properties and transport in boron nitride nanotubes

We have fabricated electronic devices based on single-walled boron nitride nanotubes (BNNTs). Our measurements indicate that all BNNTs are semiconducting, and p-doped. Temperature dependence of two terminal transport experiments suggests that at low drain fields, transport is dominated by thermionic emission over 250–300 meV Schottky contact barriers. Gate-induced barrier modulation was observed in vertically scaled devices, resulting in field-effect transistor operation.

Transistor structures for the study of scaling in carbon nanotubes

We report on the fabrication and electrical characterization of two uniquely structured carbon nanotube field-effect transistors (CNFETs). These devices have been designed to emphasize different aspects of their electrical switching behavior, so that we may learn more about their transport properties. In one structure, back-gate CNFETs were built to study the effects of vertical scaling by varying the thickness of an underlying high-quality gate dielectric. An unexpected dependence of the subthreshold slope and transconductance on the gate dielectric thickness was observed, and was attributed to Schottky barrier switching at the metal/nanotube contacts. A second structure incorporates a top gate electrode with multiple, individually addressable segments of different width. This design decouples the interior of the device from the contacts, thereby enabling the study of lateral scaling within the same physical device structure. Clear indications of bulk switching were observed in this device, with strong e...

The role of Schottky barriers on the behavior of carbon nanotube field‐effect transistors

We discuss recent advances in the fabrication of carbon nanotube field‐effect transistors (CNTFETs) and demonstrate that already they can outperform corresponding silicon devices. We then reexamine the switching mechanism of the nanotube devices and find it to be different than that of Si MOSFETs. Specifically, we present evidence that the CNTFETs are Schottky barrier (SB) transistors. A number of outstanding questions, such as the p‐type nature of most CNTFETs and the effects of gases on their function, can be answered on the basis of the SB model. Finally, we briefly discuss the unique characteristics of the SBs in 1D systems on the basis of studies on carbon and boron nitride nanotubes.

Thermal Carrier Injection into Ambipolar Carbon Nanotube Field Effect Transistors

We demonstrate thermal injection of charge carriers into ambipolar carbon nanotube field‐effect transistors with significantly smaller Schottky barrier to the conduction band of the nanotube than to the valence band. The asymmetry of the observed transfer characteristics with drain voltage can be used to find the crossover between the tunneling and the thermal regime. The experimental data can be explained based on a semiclassical model.