Alan Seabaugh

Electronics based on two-dimensional materials

The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industrys quest for devices and/or architectures based on new materials. Here, we provide a review of electronic devices based on two-dimensional materials, outlining their potential as a technological option beyond scaled complementary metal-oxide-semiconductor switches. We focus on the performance limits and advantages of these materials and associated technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. We also discuss the use of two-dimensional materials as an enabling factor for flexible electronics and provide our perspectives on future developments.

Low-Voltage Tunnel Transistors for Beyond CMOS Logic

Steep subthreshold swing transistors based on interband tunneling are examined toward extending the performance of electronics systems. In particular, this review introduces and summarizes progress in the development of the tunnel field-effect transistors (TFETs) including its origin, current experimental and theoretical performance relative to the metal-oxide-semiconductor field-effect transistor (MOSFET), basic current-transport theory, design tradeoffs, and fundamental challenges. The promise of the TFET is in its ability to provide higher drive current than the MOSFET as supply voltages approach 0.1 V.

Low-subthreshold-swing tunnel transistors

A formula is derived, which shows that the subthreshold swing of field-effect interband tunnel transistors is not limited to 60 mV/dec as in the MOSFET. This formula is consistent with two recent reports of interband tunnel transistors, which show lower than 60-mV/dec subthreshold swings and provides two simple design principles for configuring these transistors. One of these principles suggests placing the gate adjacent to the tunnel junction. Modeling of this configuration verifies that sub-60-mV/dec swing is possible.

Direct extraction of the electron tunneling effective mass in ultrathin SiO2

Electron transport in ultrathin (tox<40 A) Al/SiO2/n−Si structures is dominated by direct tunneling of electrons across the SiO2 barrier. By analyzing the tunneling currents as a function of the SiO2 layer thickness for a comprehensive set of otherwise identical samples, we are able to extract an effective mass for the tunneling electron in the SiO2 layer. Oxide films 16–35 A thick were thermally grown in situ in a dry oxygen ambient. The oxide thicknesses were determined by capacitance–voltage measurements and by spectroscopic ellipsometry. The tunneling effective mass was extracted from the thickness dependence of the direct tunneling current between an applied voltage of 0 and 2 V, a bias range that has not been previously explored. Employing both a parabolic and a nonparabolic assumption of the E−κ relationship in the oxide forbidden gap, we found the SiO2 electron mass to be mP*=0.30±0.02me, and mNP*=0.41±0.01me, respectively, independent of bias. Because this method is based on a large sample set, t...

Tunnel Field-Effect Transistors: State-of-the-Art

Progress in the development of tunnel field-effect transistors (TFETs) is reviewed by comparing experimental results and theoretical predictions against 16-nm FinFET CMOS technology. Experiments lag the projections, but sub-threshold swings less than 60 mV/decade are now reported in 14 TFETs. The lowest measured sub-threshold swings approaches 20 mV/decade, however, the measurements at these lowest values are not based on many points. The highest current at which sub-threshold swing below 60 mV/decade is observed is in the range 1-10 nA/μm. A common approach to TFET characterization is proposed to facilitate future comparisons.

A monolithic 4 bit 2 GSps resonant tunneling analog-to-digital converter

The combination of resonant-tunneling diodes and heterostructure field-effect transistors provides a versatile technology for implementing microwave digital and mixed-signal applications. Here we demonstrate and characterize the first monolithic flash analog-to-digital converter (ADC) in this technology. The first-pass ADC achieved 2.7 effective bits at 2 gigasamples per second (Gsps) for a 220-MHz input signal. The one-bit quantizer achieved a single-tone spurious free dynamic range greater than 40 dB at 2 Gsps for a 220-MHz single-tone input with dithering.

Device and Architecture Outlook for Beyond CMOS Switches

Sooner or later, fundamental limitations destine complementary metal-oxide-semiconductor (CMOS) scaling to a conclusion. A number of unique switches have been proposed as replacements, many of which do not even use electron charge as the state variable. Instead, these nanoscale structures pass tokens in the spin, excitonic, photonic, magnetic, quantum, or even heat domains. Emergent physical behaviors and idiosyncrasies of these novel switches can complement the execution of specific algorithms or workloads by enabling quite unique architectures. Ultimately, exploiting these unusual responses will extend throughput in high-performance computing. Alternative tokens also require new transport mechanisms to replace the conventional chip wire interconnect schemes of charge-based computing. New intrinsic limits to scaling in post-CMOS technologies are likely to be bounded ultimately by thermodynamic entropy and Shannon noise.

Graphene Nanoribbon Tunnel Transistors

A graphene nanoribbon (GNR) tunnel field-effect transistor (TFET) is proposed and modeled analytically. Ribbon widths between 3 and 10 nm are considered to effect energy bandgaps in the range of 0.46 to 0.14 eV. It is shown that a 5-nm ribbon width TFET can switch from on to off with only 0.1-V gate swing. The transistor achieves 800 muA/mum on -state current and 26 pA/mum off-state current, with an effective subthreshold swing of 0.19 mV/dec. Compared to a projected 2009 n MOSFET, the GNR TFET can provide 5times higher speed, 20times lower dynamic power, and 280 000times lower off-state power dissipation. The high performance of GNR TFETs results from their narrow bandgaps and their 1-D nature.

Transistors with chemically synthesized layered semiconductor WS2 exhibiting 105 room temperature modulation and ambipolar behavior

We report the realization of field-effect transistors (FETs) made with chemically synthesized multilayer crystal semiconductor WS2. The Schottky-barrier FETs demonstrate ambipolar behavior and a high (∼105×) on/off current ratio at room temperature with current saturation. The behavior is attributed to the presence of an energy bandgap in the ultrathin layered semiconductor crystal material. The FETs also show clear photo response to visible light. The promising electronic and optical characteristics of the devices combined with the chemical synthesis, and flexibility of layered semiconductor crystals such as WS2 make them attractive for future electronic and optical devices.

Room temperature operation of epitaxially grown Si/Si0.5Ge0.5/Si resonant interband tunneling diodes

Resonant interband tunneling diodes on silicon substrates are demonstrated using a Si/Si0.5Ge0.5/Si heterostructure grown by low temperature molecular beam epitaxy which utilized both a central intrinsic spacer and δ-doped injectors. A low substrate temperature of 370 °C was used during growth to ensure a high level of dopant incorporation. A B δ-doping spike lowered the barrier for holes to populate the quantum well at the valence band discontinuity, and an Sb δ-doping reduces the doping requirement of the n-type bulk Si by producing a deep n+ well. Samples studied from the as-grown wafers showed no evidence of negative differential resistance (NDR). The effect of postgrowth rapid thermal annealing temperature was studied on tunnel diode properties. Samples which underwent heat treatment at 700 and 800 °C for 1 min, in contrast, exhibited NDR behavior. The peak-to-valley current ratio (PVCR) and peak current density of the tunnel diodes were found to depend strongly on δ-doping placement and on the annea...