Cem Alper

Quantum Mechanical Study of the Germanium Electron–Hole Bilayer Tunnel FET

The electron-hole bilayer tunnel field-effect transistor (EHBTFET) is an electronic switch that uses 2-D-2-D sub-band-to-sub-band tunneling (BTBT) between electron and hole inversion layers and shows significant subthermal swing over several decades of current due to the step-like 2-D density of states behavior. In this paper, EHBTFET has been simulated using a quantum mechanical model. The model results are compared against transactions on computer-aided design simulations and remarkable differences show the importance of quantum effects and dimensionality in this device. Ge EHBTFET with channel thickness of 10 nm results as a promising device for low supply voltage, subthreshold logic applications, with a super steep switching behavior featuring SSavg ~ 40 mV/dec up to VDD. Furthermore, it has been demonstrated that high on current levels ( ~ 40 μA/μm) can be achieved due to the transition from phonon-assisted BTBT to direct BTBT at higher biases.

Assessment of field-induced quantum confinement in heterogate germanium electron–hole bilayer tunnel field-effect transistor

The analysis of quantum mechanical confinement in recent germanium electron–hole bilayer tunnel field-effect transistors has been shown to substantially affect the band-to-band tunneling (BTBT) mechanism between electron and hole inversion layers that constitutes the operating principle of these devices. The vertical electric field that appears across the intrinsic semiconductor to give rise to the bilayer configuration makes the formerly continuous conduction and valence bands become a discrete set of energy subbands, therefore increasing the effective bandgap close to the gates and reducing the BTBT probabilities. In this letter, we present a simulation approach that shows how the inclusion of quantum confinement and the subsequent modification of the band profile results in the appearance of lateral tunneling to the underlap regions that greatly degrades the subthreshold swing of these devices. To overcome this drawback imposed by confinement, we propose an heterogate configuration that proves to suppress this parasitic tunneling and enhances the device performance.

Impact of Asymmetric Configurations on the Heterogate Germanium Electron–Hole Bilayer Tunnel FET Including Quantum Confinement

We investigate the effect of asymmetric configurations on the heterogate germanium electron-hole bilayer tunnel FET (TFET) and assess the improvement that they provide in terms of boosting the typically very low ON-current levels of TFET devices in the presence of field-induced quantum confinement. We show that when a very strong inversion for holes is induced at the bottom of the channel, the formation of the inversion layer for electrons is shifted to higher gate voltages, which in turn enhances the electrostatic control of the band bending at the top of the channel. As a result, the pinning of the quantized energy subbands is prevented for a wider range of gate voltages, and this allows vertical band-to-band tunneling distances to be further reduced compared with the conventional symmetric electron-hole bilayer configurations.

Assessment of pseudo-bilayer structures in the heterogate germanium electron-hole bilayer tunnel field-effect transistor

We investigate the effect of pseudo-bilayer configurations at low operating voltages (≤0.5 V) in the heterogate germanium electron-hole bilayer tunnel field-effect transistor (HG-EHBTFET) compared to the traditional bilayer structures of EHBTFETs arising from semiclassical simulations where the inversion layers for electrons and holes featured very symmetric profiles with similar concentration levels at the ON-state. Pseudo-bilayer layouts are attained by inducing a certain asymmetry between the top and the bottom gates so that even though the hole inversion layer is formed at the bottom of the channel, the top gate voltage remains below the required value to trigger the formation of the inversion layer for electrons. Resulting benefits from this setup are improved electrostatic control on the channel, enhanced gate-to-gate efficiency, and higher ION levels. Furthermore, pseudo-bilayer configurations alleviate the difficulties derived from confining very high opposite carrier concentrations in very thin s...

New tunnel-FET architecture with enhanced I ON and improved Miller Effect for energy efficient switching

Tunneling Field Effect Transistors (TFET) are promising devices to respond to the demanding requirements of future technology nodes. The benefits of the TFETs are linked to their sub-60mV/decade sub-threshold swing, a prerequisite for scaling the supply voltage well below 1V. Main research efforts are currently dedicated to improving the on current (ION) level in a TFET. However, from the circuit point of view the device capacitances are equally important. It is known that the drain-to-gate capacitance in a TFET is almost equal to the gate capacitance in moderate and strong inversion regimes. Due to enhanced Miller Effect, they are known to exhibit large over/undershoot in transient operation as compared to CMOS. Therefore, the effort on improving ION should be simultaneous to an effort of reducing the Miller capacitance (CMILLER). This work proposes a new architecture which addresses both these issues.

A Steep-Slope Transistor Combining Phase-Change and Band-to-Band-Tunneling to Achieve a sub-Unity Body Factor

Steep-slope transistors allow to scale down the supply voltage and the energy per computed bit of information as compared to conventional field-effect transistors (FETs), due to their sub-60 mV/decade subthreshold swing at room temperature. Currently pursued approaches to achieve such a subthermionic subthreshold swing consist in alternative carrier injection mechanisms, like quantum mechanical band-to-band tunneling (BTBT) in Tunnel FETs or abrupt phase-change in metal-insulator transition (MIT) devices. The strengths of the BTBT and MIT have been combined in a hybrid device architecture called phase-change tunnel FET (PC-TFET), in which the abrupt MIT in vanadium dioxide (VO2) lowers the subthreshold swing of strained-silicon nanowire TFETs. In this work, we demonstrate that the principle underlying the low swing in the PC-TFET relates to a sub-unity body factor achieved by an internal differential gate voltage amplification. We study the effect of temperature on the switching ratio and the swing of the PC-TFET, reporting values as low as 4.0 mV/decade at 25 °C, 7.8 mV/decade at 45 °C. We discuss how the unique characteristics of the PC-TFET open new perspectives, beyond FETs and other steep-slope transistors, for low power electronics, analog circuits and neuromorphic computing.

Conformal mapping based DC current model for double gate tunnel FETs

In this work, the conformal mapping technique is applied to obtain an analytical closed form solution of the 2D Poissons equation for a double-gate Tunnel FET. The generated band profiles are accurate in all regions of device operation. Furthermore, the current levels are estimated by implementing the non-local band-to-band tunneling model from Synopsys Sentaurus TCAD. A good agreement with simulations for varying device parameters is demonstrated and the advantages and limitations of the new modeling approach are investigated and discussed.

Underlap counterdoping as an efficient means to suppress lateral leakage in the electron–hole bilayer tunnel FET

The electron-hole bilayer tunnel (EHBTFET). has been proposed as a density of states (DOS) switch capable of achieving a subthreshold slope lower than 60mV/decade at room temperature; however, one of the main challenges is the control of the lateral band-to-band tunneling (BTBT) leakage in the OFF state. In this work, we show that by using oppositely doped underlap regions; the unwanted penetration of the wavefunction into the underlap region at low gate biases is prevented; thereby drastically reducing the lateral BTBT leakage without any penalty on the ON current. The method is verified using a full-quantum 2D Schrodinger-Poisson solver under the effective mass approximation. For a channel thickness of 10 nm, an In0.53Ga0.47As EHBTFET with counterdoping can exhibit an ON-current up to 20 mu A/mu m and an average subthreshold swing (SS) of about 30 mV/dec. Compared to previous lateral leakage suppression solutions, the proposed method can be fabricated using template-assisted selective epitaxy.

Impact of device geometry of the fin Electron-Hole Bilayer Tunnel FET

We study the impact of quantum mechanical effects on the fin Electron-Hole Bilayer Tunnel FET (EHBTFET) considering different geometries. Through quantum simulations based on the effective mass approximation (EMA), it is found that the fin EHBTFET is affected by the corner effects at the substrate-fin interface, due to reduced electrostatic control that causes a dramatic reduction of the ON current. Three different solutions; corner smoothing, corner doping and trapezoidal fins; are proposed and their efficiency are assessed. The corner smoothing turns out to be ineffective whereas trapezoidal fins entail a device performance trade-off. Utilizing corner doping is the most viable choice to achieve a large ON current.

The Electron-Hole Bilayer TFET: Dimensionality Effects and Optimization

An extensive parameter analysis is performed on the electron-hole bilayer tunnel field-effect transistor (EHBTFET) using a 1-D effective mass Schrödinger-Poisson solver with corrections for band non-parabolicity considering thin InAs, In_0.53Ga_0.47As, Ge, Si_0.5Ge_0.5, and Si films. It is found that depending on the channel material and channel thickness, the EHBTFET can operate either as a 2-D-2-D or 3-D-3-D tunneling device. InAs offers the highest I_ON, whereas for the Si and Si_0.5Ge_0.5 EHBTFETs, significant current levels cannot be achieved within a reasonable voltage range. The general trends are explained through an analytical model that shows close agreement with the numerical results.