Superlinear Onset Research Articles

An Analytical Model of Gate-All-Around Heterojunction Tunneling FET

A compact analytical drain current model considering the inversion layer and source depletion is developed for the gate-all-around (GAA) heterojunction tunneling FET (H-TFET) with staggered-gap alignment. Poisson’s equations are solved to obtain the continuous surface potential profile for the GAA H-TFET first, then the drain current is derived based on Kane’s model by using the tangent line approximation method, and finally, the model is verified by TCAD simulation using GaAs0.5Sb0.5/In0.53Ga0.47As GAA H-TFET and published data. The impacts of bias, gate oxide dielectric constant, and interface fixed charge on the surface potential, electric field, and ${I} _{{\textsf {DS}}}$ – ${V} _{{\textsf {GS}}}$ can be well predicted by the proposed model. The super-linear onset and saturation characteristics of ${I} _{{\textsf {DS}}}$ – ${V} _{{\textsf {DS}}}$ curves are also obtained.

SiGe based line tunneling field-effect transistors

In this paper we report on our progress with SiGe gate-normal / line tunneling FETs, highlighting recent advancements by the example of three transistor concepts. We demonstrate the unique characteristics shared by these transistors, such as the on-current proportionality to the source-gate-channel overlap area and explain the obstacles imposed by fringing fields leading to parasitic tunneling at the edges of the tunneling area. Our experimental results show that adding counter doping to the channel provides an efficient means to mitigate penalties to the subthreshold swing caused by parasitic tunneling paths and additionally helps to improve the on-current and Ion/Ioff-ratio. Moreover, we point out the dependence of the superlinear onset on the tunneling transmission probability with a focus on the doping profile at the tunneling junction. We consider the role of traps on the subthreshold swing within the scope of temperature dependent electrical measurements. Furthermore, we show that by avoiding ion implantation and hence crystal defects as much as possible, smaller minimum subthreshold swings can be reached. At last, taking the experience acquired on the three transistors concepts into consideration, we propose an advanced TFET concept.

Bandgap-dependent onset behavior of output characteristics in line-tunneling field-effect transistors

The onset behavior of output characteristics in tunnel field-effect transistors (TFETs) importantly determines the performance of digital TFET-based circuits. In this paper, we analytically and numerically examine the dependence of the onset behavior of output characteristics on the bandgap of semiconductors in line-tunneling TFETs. The qualitative and quantitative analyses show that the output onset behavior in line-tunneling TFETs strongly depends on the bandgap because the roles of two factors, including the incident electron number and tunneling probability, in determining the variation of tunneling current under increasing drain voltage change oppositely when varying the bandgap. Particularly, the superlinear onset in line-tunneling TFETs can be effectively suppressed to reduce the saturation drain voltage by using low-bandgap semiconductors. Together with the advantages in on-current and subthreshold swing as shown previously, the significant superiority in output characteristic makes the use of low-bandgap materials to be an efficient approach for simultaneously enhancing the device and circuit performances of advanced line-tunneling TFETs.

Benchmarking of Homojunction Strained-Si NW Tunnel FETs for Basic Analog Functions

This paper reports a compact ambipolar model for homojunction strained-silicon (sSi) nanowire (NW) tunnel FETs (TFETs) capable of accurately describing both ${I}$ – ${V}$ and ${G}$ – ${V}$ characteristics in all regimes of operation, n- and p-ambipolarity, the superlinear onset of the output characteristics, and the temperature dependence. Experimental calibration on long channel (350 nm) complementary n- and p-type sSi NW TFETs has been performed to create the model, which is used to systematically benchmark the main analog figures of merit at device level: ${g}_{m}/{I}_{\text {d}}$ , ${g}_{m}/{g}_{ds}$ , ${f}_{T}$ and ${f}_{T}/{I}_{d}{V}_{d}$ , and their temperature dependence from 25 °C to 125 °C. This allows for a direct comparison between 28-nm low-power Fully Depleted Silicon on Insulator (FD-SOI) CMOS node and 28-nm double-gate (DG) TFET. We demonstrate unique advantages of sSi DG TFET over CMOS, in terms of: 1) reduced temperature dependence of subthreshold swing; 2) higher transconductance per unit of current with peaks close to $40~\text {V}^{-1}$ , for currents lower than 10 nA/ $\mu \text{m}$ ; and 3) higher unity gain frequency per unit power for currents below 10 nA/ $\mu \text{m}$ .

Impact of Super-Linear Onset, Off-Region Due to Uni-Directional Conductance and Dominant <inline-formula> <tex-math notation="LaTeX">$\mathrm{C}_{\text {GD}}$ </tex-math></inline-formula> on Performance of TFET-Based Circuits

This paper investigates the consequences of several distinctive device characteristics of tunnel FETs (TFET), namely super-linear onset, uni-directional conduction, and the dominant gate-drain capacitance, regarding the energy consumption, propagation delay, and noise resilience. Simulations have shown that these TFET specific characteristics have a detrimental effect on the dynamic response. We also report that their impact remains significant when operating voltage is scaled. Thus device level optimizations are required to eliminate these attributes to take full advantage of TFETs small subthreshold swing and low voltage operation.

Universal analytic model for tunnel FET circuit simulation

A simple analytic model based on the Kane–Sze formula is used to describe the current–voltage characteristics of tunnel field-effect transistors (TFETs). This model captures the unique features of the TFET including the decrease in subthreshold swing with drain current and the superlinear onset of the output characteristic. The model also captures the ambipolar current characteristic at negative gate–source bias and the negative differential resistance for negative drain–source biases. A simple empirical capacitance model is also included to enable circuit simulation. The model has fairly general validity and is not specific to a particular TFET geometry. Good agreement is shown with published atomistic simulations of an InAs double-gate TFET with gate perpendicular to the tunnel junction and with numerical simulations of a broken-gap AlGaSb/InAs TFET with gate in parallel with the tunnel junction.

Understanding the Superlinear Onset of Tunnel-FET Output Characteristic

In this letter, we report that the source and channel Fermi-Dirac distributions in interband-tunneling-controlled transistors play a fundamental role on the modulation of the injected current. We explain the superlinear onset of the output characteristics based on the occupancy function modulation. Thus, we point out that, along with the tunneling barrier transparency, the availability of carriers and empty states, at the beginning and at the end of the tunneling path, respectively, should be always taken into account for a proper modeling of tunnel FETs.

Drain voltage dependent analytical model of tunnel field-effect transistors

Tunnel field-effect transistors (TFETs) are potential successors of metal-oxide-semiconductor FETs because they promise superior input characteristics. However, the output characteristics of TFETs are poorly understood, and sometimes a superlinear onset, undesirable for circuit design, is observed. We present the first analytical model to include the impact of the drain voltage on the TFET performance. The model is developed for both a pure line-tunneling TFET and a pure point-tunneling TFET. Good agreement is observed with device simulations, especially for line-tunneling TFETs. Our model highlights and explains the superlinear onset of the output characteristics, thereby enabling an improved analysis of experimental data. Increasing the source doping level and switching to a smaller bandgap material can remove the undesired onset. We confirm this finding with our experimental data.