Asymmetric Doping Research Articles

Influence of contact doping on graphene nanoribbon heterojunction tunneling field effect transistors

We investigated the device performance of graphene nanoribbon tunneling field-effect transistors with heterogeneous channel as a function of the contact doping concentrations. The simulations were carried out based on the non-equilibrium Green’s function, coupled with a Dirac Hamiltonian model, and the roles of symmetric and asymmetric contact doping concentrations on the device performance were identified. It was observed that the device performances such as OFF-state currents (IOFF), ON-state currents (ION) and subthreshold slopes (SSs) were greatly influenced by the source doping concentrations, while variations in drain doping concentrations changed mainly the IOFF. By applying proper asymmetric source and drain doping concentrations, low SS and large ION/IOFF ratio can be achieved, indicating that it is an alternative route to effectively enhance the device performance.

Multiple silicon nanowire complementary tunnel transistors for ultralow-power flexible logic applications

Based on experimental and simulation studies to gain insight into the suppression of ambipolar conduction in two distinct tunnel field-effect transistor (TFET) devices (that is, an asymmetric source-drain doping or a properly designed gate underlap), here we report on the fabrication and electrical/mechanical characterization of a flexible complementary TFET (c-TFET) inverter on a plastic substrate using multiple silicon nanowires (SiNWs) as the channel material. The static voltage transfer characteristic of the SiNW c-TFET inverter exhibits a full output voltage swing between 0 V and Vdd with a high voltage gain of ∼29 and a sharp transition of 0.28 V at Vdd = 3 V. A leakage power consumption of the SiNW c-TFET inverter in the standby state is as low as 17.1 pW for Vdd = 3 V. Moreover, its mechanical bendability indicates that it has good fatigue properties, providing an important step towards the realization of ultralow-power flexible logic circuits.

Transport properties of InGaAs/GaAs Heterostructures with δ-doped quantum wells

The lateral transport of electrons in single- and double-well pseudomorphic GaAs/n-InGaAs/GaAs heterostructures with quantum wells 50–100 meV deep and impurity δ-layers in the wells, with concentrations in the range 1011 < N s < 1012 cm−2, has been investigated. Single-well structures with a doped well at the center exhibit a nonmonotonic temperature dependence of the Hall coefficient and an increase in low-temperature electron mobility with an increase in the impurity concentration. The results obtained indicate that the impurity-band electron states play an important role in the conductivity of these structures. Involvement of the impurity band also allows to explain adequately the characteristics of the conductivity of double-well structures; in contrast to single-well structures, band bending caused by asymmetric doping is of great importance. The numerical calculations of conductivity within the model under consideration confirm these suggestions.

Strong asymmetrical doping properties of spinel CoAl2O4

Using first-principles density-functional theory, we have investigated the intrinsic and extrinsic doping properties of CoAl2O4 by calculating the transition energies and formation energies of intrinsic and extrinsic defects. We find that CoAl2O4 exhibits strong asymmetrical doping properties: Although excellent p-type conductivity can be achieved by Li or Na doping at O-rich growth condition, n-type conductivity cannot be achieved by any intrinsic or extrinsic dopants at any growth conditions. These asymmetrical doping properties are attributed to the formation of intrinsic defects, particularly AlCo, which has very low formation energy at all growth conditions. Our results suggest that for better use of CoAl2O4, the electronic devices should require CoAl2O4 to exhibit p-type conductivity.

A study on the performance of metal-oxide-semiconductor-field-effect-transistors with asymmetric junction doping structure

Diode currents of MOSFET were studied and characterized in detail for the ion implanted pn junction of short channel MOSFETs with shallow drain junction doping structure. The diode current in MOSFET junctions was analyzed on the point of view of the gate-induced-drain leakage (GIDL) current. We could found the GIDL current is generated by the band-to-band tunneling (BTBT) of electrons through the reverse biased channel-to-drain junction and had good agreement with BTBT equation. The effect of the lateral electric field on the GIDL current according to the body bias voltage is characterized and discussed. We measured the electrical doping profiling of MOSFETs with a short gate length, ultra thin oxide thickness and asymmetric doped drain structure and checked the profile had good agreement with simulation result. An accurate effective mobility of an asymmetric source–drain junction transistor was successfully extracted by using the split C–V technique.

Electronic properties and STM images of doped bilayer graphene

Electronic structures and scanning tunneling microscopy (STM) patterns of boron- and nitrogen-doped bilayer graphene are predicted using state-of-the-art first-principles calculations. Asymmetric doping is considered, leading to different charge-carrier densities on each graphene layer and to a band-gap opening. When lying on the top layer, the local STM patterns of the dopant are predicted to be similar to images observed in monolayer graphene. In contrast, the local electronic states of the buried dopant are not directly detectable by STM. However, by analyzing the charge transfer between graphene layers and its effect on the contrast of the STM image, the chemical doping is found to affect the symmetry (hexagonal and triangular) of the observed lattice for both the top doped surface and the buried doped layer. Consequently, our ab initio simulations predict a possible indirect detection of N or B dopants buried in bilayer graphene when such a contrast is revealed by a series of STM images or using scanning tunneling spectroscopy.

Highly reproducible tunnel currents in MBE-grown semiconductor multilayers

Tunnel currents through semiconductor tunnel barriers have proved very difficult to control to the extent that device-to-device variability and wafer-to-wafer irreproducibility have prevented electronic devices based on tunnelling from ever going into production. With reference to a single tunnel barrier of AlAs in a GaAs multilayer structure with an asymmetric doping profile, it is shown that, with careful attention to detail, diodes from equivalent sites on three separate wafers can be produced whose average current in forward bias is within 1 % while the total in-wafer standard deviation of current at the same fixed bias (0.5 V) is 6 %, dominated by a systematic cross-wafer variation that is described. This level of reproducibility now enables these devices to be used in pick-and-place systems for the manufacture of low-cost hybrid integrated microwave circuits.

Asymmetrically Doped FinFETs for Low-Power Robust SRAMs

We propose FinFETs with unequal source and drain doping concentrations [asymmetrically doped (AD) FinFETs] for low-power robust SRAMs. The effect of asymmetric source/drain doping on the device characteristics is extensively analyzed, and the key differences between conventional and AD FinFETs are clearly shown. We show that asymmetry in the device structure leads to unequal currents for positive and negative drain biases, which is exploited to achieve mitigation of read-write conflict in 6T SRAMs. The proposed device exhibits superior short-channel characteristics compared to a conventional FinFET due to reduced electric fields from the terminal that has a lower doping. This results in significantly lower cell leakage in AD-FinFET-based 6T SRAM. Compared to the conventional FinFET-based 6T SRAM, AD-FinFET SRAM shows 5.2%-8.3% improvement in read static noise margin (SNM), 4.1%-10.2% higher write margin, 4.1%-8.8% lower write time, 1.3%-3.5% higher hold SNM, and 2.1-2.5 lower cell leakage at the cost of 20%-23% higher access time. There is no area penalty associated with the proposed technique.

Trade-Offs Between RF Performance and Total-Dose Tolerance in 45-nm RF-CMOS

The hot carrier and ionizing radiation responses of 45-nm SOI RF nMOSFETs are investigated. Devices with “tight” source/drain (S/D) contact spacing have improved RF performance but degraded hot carrier reliability and radiation tolerance. Devices with “loose” gate finger-to-gate finger spacing have improved RF performance and also improved hot carrier and radiation tolerance. The effects of finger width on the hot carrier stress and ionizing radiation degradation of strained silicon-on-insulator RF MOSFETs are also investigated. Enhanced degradation is observed for devices with wide finger widths and is attributed to the greater channel-region mechanical stress induced impact ionization. This result is contrary to the previous studies which showed that narrow channel width devices should exhibit greater damage. Taken together, these results have serious consequences for RF circuits that require large widths for sufficient RF gain. Finally, devices with symmetric halo doping are observed to exhibit greater total-dose degradation than devices with asymmetric halo doping.

Vertically Integrated Unidirectional Biristor

The unidirectional read-out characteristics of a two-terminal biristor are investigated through numerical simulations and experiments. The base doping profile in the biristor, which is analogous to an open-base bipolar junction transistor (BJT), is a key parameter to control both the multiplication factor and the common-emitter gain of the open-base BJT. The simulated results indicate that the asymmetric base doping produces a difference in the latch-up voltage according to the reading direction. A unidirectional conduction path is thereby implemented in a crossbar-array configuration that consists of only the two-terminal biristor. The experimental results based on a vertical structure with local charge injection support that the leakage path through the reverse read direction can be blocked by the asymmetric base doping structure with the selection of proper bias conditions.

Bidirectional Two-Terminal Switching Device for Crossbar Array Architecture

We propose a bilateral switching poly-Si junction device to realize a crossbar array with a perpendicular spin-transfer torque (STT) magnetic random access memory (MRAM). An N+ /P/N+ bilateral junction device with two bias terminals provides bidirectional current flow enough to write STT MRAM by a drain induced barrier lowering under a reverse bias of N+/P. In addition, asymmetrical doping for two N+ terminals provides a high on-off ratio of 107 under read condition, which is acceptable for a crossbar array. From this work, it is expected that a bilateral poly-Si junction will be a promising switch device to achieve crossbar architecture with a perpendicular STT MRAM.

Boron and Nitrogen Doping Induced Half-Metallicity in Zigzag Triwing Graphene Nanoribbons

The electronic and magnetic properties of quasi-1D zigzag triwing graphene (ZZ-TWG) ribbons with boron and nitrogen (BN) doping were investigated by means of a spin-polarized density functional theory approach. Our results showed that asymmetric BN doping can give rise to half-metallicity and suppress spin polarization of the doped wings, making the BN-doped ZZ-TWGs potential application in spintronics devices. Heavily doping can turn metallic ZZ-TWG ribbons into semiconductors. A critical ratio of BN to C and appropriate doping site (e.g., ribbon’s edges) are required for achieving specific electronic properties. An effective path of engineering electronic properties of ZZ-TWGs by BN doping was suggested.

Gap opening by asymmetric doping in graphene bilayers

We study the energy gap opening in the electronic spectrum of graphene bilayers caused by asym- metric doping. Both substitutional impurities (boron acceptors and nitrogen donors) and adsorbed potassium donors are considered. The gap evolution with dopant concentration is compared to the situation in which the asymmetry between the layers is induced by an external electric field. The effects of adsorbed potassium are similar to that of an electric field, but substitutional impurities behave quite differently, showing smaller band gaps and a large sensitivity to disorder and sublattice occupation.

Positive quasiclassical magnetoresistance and quantum effects in germanium quantum wells

Changes in the conductivity of p-type quantum-well heterostructures of Si0.05Ge0.95 alloy are studied at temperatures ranging from 0.352–7.1K and magnetic fields of up to 11T. The distinctive feature of the sample was asymmetric doping, with layers of Si0.4Ge0.6 with boron impurity concentrations of 2⋅1018 and 8⋅1018cm−3 positioned on opposite sides of the quantum well. Shubnikov–de Haas oscillations were observed clearly against the background of a high quasiclassical positive magnetoresistance. The field dependence of the magnetoresistance is well described by a function of the form ρxx(B)∕ρxx(0)∝B12∕7, as predicted by a theory including the combined effect of both short- and long-range disorder. The contribution to the temperature and magnetic field dependences of the resistance owing to quantum corrections associated with weak localization and charge carrier interactions is determined. Strong spin-orbital scattering of holes on the quantum well is revealed by analyzing these corrections. A study of the variations in the amplitude of the Shubnikov–de Haas oscillations with temperature and magnetic field (including the monotonic behavior of the resistance with changing magnetic field) makes it possible to determine the effective mass of the charge carriers, m*=0.17m0 The temperature dependence of the hole-phonon relaxation time was found by studying the overheating of charge carriers by an electric field.

Electronic Transport in Laterally Asymmetric Channel MOSFET for RF Analog Applications

In this paper, an ensemble Monte Carlo investigation of the static and dynamic performances in the high-frequency domain of laterally asymmetric channel (LAC) bulk metal-oxide-semiconductor field-effect transistors (MOSFETs) is presented. A detailed comparison with a homogeneously doped bulk device is also included. The results presented show that the use of an asymmetric doping within the channel enhances nonequilibrium features as velocity overshoot, thus significantly improving the transconductance of the device. The gradual variation of doping is also responsible for a modification of the electrostatic conditions and the inversion charge profiles, provoking the reduction of the gate-to-source capacitance, a minor influence of surface scattering, reduced transit times, and higher mean free paths. A noticeable enhancement (as compared to a conventional device) in the RF and microwave frequency range of the dynamic performance of the transistors is also evidenced. This is mainly due to a better transconductance-to-current ratio, Early voltage, and open-loop gain, which are the results of the improvement of the charge transport conditions in the device at a microscopic level. Therefore, LAC MOSFETs can be a viable option to enhance the figures of merit of bulk silicon technology for high-frequency analog applications.

Simulation of asymmetric doped high performance silicon on insulator metal oxide semiconductor field effect transistors for very large scale integrated complementary metal oxide semiconductor technologies

Asymmetric halo and extension implantations are examined by simulation for their usability in 45 and 32 nm technology high performance silicon on insulator metal oxide semiconductor field effect transistors (SOI-MOSFETs). Tilted halo and extension implantations from the source side show higher saturation currents and lower drain overlap and junction capacitances, which improve the intrinsic MOSFET power delay product. Furthermore the asymmetric doping profile leads to an inverter chain speed benefit. The stronger short channel effect, present in these devices, can be reduced by a low dose drain side halo implantation simultaneously maintaining a transistor performance improvement from asymmetric doping. This optimized transistor design is successfully transferred from the 45 into the 32 nm technology.

Impact of Lateral Asymmetric Channel Doping on 45-nm-Technology N-Type SOI MOSFETs

Lateral asymmetric channel doping is applied to 45-nm technology NFET devices. The measured effective drain-current enhancement over coprocessed symmetric control devices is 10%. Analysis reveals that the dominant physical mechanism, which accounts for two-third of the total enhanced drain current, is an 8% increase in the source-side injection velocity. The remaining one-third is attributed to the decreased drain-induced barrier lowering. This paper concludes with an analysis of the switching characteristics of CMOS inverters composed of an asymmetric NFET and a companion symmetric PFET and shows a 5% improvement in the delay. The improvement is explained in terms of the increased velocity and 30% reduction in drain junction capacitance.

An extended drain current conductance extraction method and its application to DRAM support and array devices

In this paper an extended drain-current conductance method is introduced, which can be used for extraction of separated drain and source resistors, forward and reversed modes carrier mobilities and effective channel length of MOSFET. The new single device extraction method is successfully applied to logic devices with symmetric doping profiles and DRAM array devices with highly asymmetric doping profiles. Furthermore a new physical definition is introduced for effective channel length which provides a better understanding of the device.

Computational Study of Tunneling Transistor Based on Graphene Nanoribbon

Tunneling field-effect transistors (FETs) have been intensely explored recently due to its potential to address power concerns in nanoelectronics. The recently discovered graphene nanoribbon (GNR) is ideal for tunneling FETs due to its symmetric bandstructure, light effective mass, and monolayer-thin body. In this work, we examine the device physics of p-i-n GNR tunneling FETs using atomistic quantum transport simulations. The important role of the edge bond relaxation in the device characteristics is identified. However, the device has ambipolar I-V characteristics, which are not preferred for digital electronics applications. We suggest that using either an asymmetric source-drain doping or a properly designed gate underlap can effectively suppress the ambipolar I-V. A subthreshold slope of 14mV/dec and a significantly improved on-off ratio can be obtained by the p-i-n GNR tunneling FETs.

Low frequency noise in Co/Al2O3〈Si〉/Py magnetic tunnel junctions

AbstractLow frequency noise and dynamic tunneling resistance have been studied in Co(80 Å)/Al2O3(12 Å)/Py(100 Å) magnetic tunnel junctions (MTJs) with and without asymmetric Si doping of the insulating barrier (Si ≤ 1.8 Å). Variation of the dynamic resistance and tunneling resistance with Si doping and applied bias in these MTJs indicate a transition from the Si‐doped regime to Si cluster formation above a δ ‐layer thickness of about 1.2 Å, close to 1 monolayer coverage. The measurements show anomalously strong enhancements of the low frequency noise for Si thickness above 1.2 Å, mainly due to the appearance of random telegraph noise. A simple model, which considers suppression of Coulomb blockade in the array of Si dots, opening two‐step tunnel channels, qualitatively explains the variation of both conductivity and noise with Si content. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)