Nandita DasGupta

Subthreshold current model of FinFETs based on analytical solution of 3-D Poisson's equation

The potential variation in the channel obtained from analytical solution of three-dimensional (3-D) Poissons equation is used to calculate the subthreshold current and threshold voltage of fin field-effect transistors with doped and undoped channels. The accuracy of the model has been verified by the data from 3-D numerical device simulator. The variation of subthreshold slope and threshold voltage with device geometry and doping concentration in the channel has been studied.

Analytical Model of Subthreshold Current and Slope for Asymmetric 4-T and 3-T Double-Gate MOSFETs

In this paper, analytical models of subthreshold current and slope for asymmetric four-terminal double-gate (DG) MOSFETs are presented. The models are used to study the subthreshold characteristics with asymmetry in gate oxide thickness, gate material work function, and gate voltage. A model for the subthreshold behavior of three-terminal DG MOSFETs is also presented. The results of the models show excellent match with simulations using MEDICI. The analytical models provide physical insight which is helpful for device design.

Gate Leakage Mechanisms in AlGaN/GaN and AlInN/GaN HEMTs: Comparison and Modeling

The gate leakage mechanisms in AlInN/GaN and AlGaN/GaN high electron mobility transistors (HEMTs) are compared using temperature-dependent gate current-voltage (IG-VG) characteristics. The reverse bias gate current of AlInN/GaN HEMTs is decomposed into three distinct components, which are thermionic emission (TE), Poole-Frenkel (PF) emission, and Fowler-Nordheim (FN) tunneling. The electric field across the barrier in AlGaN/GaN HEMTs is not sufficient to support FN tunneling. Hence, only TE and PF emission is observed in AlGaN/GaN HEMTs. In both sets of devices, however, an additional trap-assisted tunneling component of current is observed at low reverse bias. A model to describe the experimental IG-VG characteristics is proposed and the procedure to extract the associated parameters is described. The model follows the experimental gate leakage current closely over a wide range of bias and temperature for both AlGaN/GaN and AlInN/GaN HEMTs.

An analytical expression for sheet carrier concentration vs gate voltage for HEMT modelling

Abstract A simple expression of the Fermi potential ( E F ) variation with the sheet carrier concentration ( n s ) in the two-dimensional electron gas at the heterojunction of a High Electron Mobility Transistor (HEMT) is presented. This particular approximation is shown to lead to an analytical expression for n s in termks of the applied gate voltage ( V G ). Comparisons with the exact solutions of n s vs E F and n s vs V G as well as with several previous approximations show that our results are more accurate for a wider range of values of n s at different temperatures. This single analytical expression for n s as a function of V G , valid from subthreshold to high conduction, can be used for better analytical modelling of HEMTs.

Threshold Voltage model for mesa-isolated small geometry fully depleted SOI MOSFETs based on analytical solution of 3-D Poisson's equation

A threshold voltage model for mesa-isolated fully depleted silicon-on-insulator (FDSOI) MOSFETs, based on the analytical solution of three-dimensional (3-D) Poissons equation is presented for the first time in this paper. The separation of variables technique is used to solve the 3-D Poissons equation analytically with appropriate boundary conditions. Simple and accurate analytical expressions for the threshold voltage of the front and the back gate are derived. The model is able to predict short channel as well as narrow width effects in mesa-isolated FDSOI MOSFETs. The model is validated by comparing with the experimental results as well as with the numerical results available in the literature.

Positive Shift in Threshold Voltage for Reactive-Ion- Sputtered Al 2 O 3 /AlInN/GaN MIS-HEMT

AlInN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) have been fabricated with reactive-ion-sputtered (RIS) Al2O3 as a gate dielectric. Significant reduction in the gate leakage current is achieved upon insertion of RIS-Al2O3. MIS-HEMTs also show better transconductance, drain characteristics, and ION/IOFF ratio. Most interestingly, a positive shift in threshold voltage is observed for MIS-HEMTs indicating the presence of net negative charge at oxide-semiconductor interface. The origin and stability of the negative charge at the interface is discussed in this letter.

Effect of sulfur passivation and polyimide capping on InGaAs-InP PIN photodetectors

It is shown in this work that sulfur passivation followed by a deionized water rinse reduces the dark current of InGaAs-InP PIN photodetectors significantly. This reduction in dark current is shown to be due to reduced recombination at the exposed mesa surface. Detectors with polyimide capping after sulfur passivation showed no degradation in characteristics with time.

Improvement in electrical characteristics of ultrathin thermally grown SiO 2 by selective anodic oxidation

The presence of weak spots and pinholes in ultrathin gate oxides significantly increases the leakage current, thereby degrading the device performance. This paper proposes a method, which identifies the weak spots in thermally grown gate oxide and repairs them by selective anodization. By controlling the applied voltage, it is ensured that current flows only through the weak spots in the oxide during anodization. Anodic oxide therefore grows over these weak spots, improving the reliability of the oxide without increasing the gate oxide thickness. Significant improvement in electrical characteristics was observed in the gate oxides treated by anodic oxidation.

A codoping route to realize low resistive and stable p-type conduction in (Li, Ni):ZnO thin films grown by pulsed laser deposition.

We report on the growth of Li-Ni codoped p-type ZnO thin films using pulsed laser deposition. Two mole percent Li monodoped ZnO film shows highly insulating behavior. However, a spectacular decrease in electrical resistivity, from 3.6 × 10(3) to 0.15 Ω cm, is observed by incorporating 2 mol % of Ni in the Li-doped ZnO film. Moreover, the activation energy drops to 6 meV from 78 meV with Ni incorporation in Li:ZnO lattice. The codoped [ZnO:(Li, Ni)] thin film shows p-type conduction with room temperature hole concentration of 3.2 × 10(17) cm(-3). Photo-Hall measurements show that the Li-Ni codoped p-ZnO film is highly stable even with UV illumination. XPS measurements reveal that most favorable chemical state of Ni is Ni(3+) in (Li, Ni): ZnO. We argue that these Ni(3+) ions act as reactive donors and increase the Li solubility limit. Codoping of Li, with other transitional metal ions (Mn, Co, etc.) in place of Ni could be the key to realize hole-dominated conductivity in ZnO to envisage ZnO-based homoepitaxial devices.

A new SPICE MOSFET Level 3-like model of HEMT's for circuit simulation

A fully analytical model for the current-voltage (I-V) characteristics of HEMTs is presented. It uses a polynomial expression to model the dependence of sheet carrier concentration (n/sub s/) in the two-dimensional electron gas (2-DEG) on gate voltage (V/sub G/). The resultant I-V relationship incorporates a correction factor /spl alpha/ analogous to SPICE MOSFET Level 3 model and is therefore more accurate than models assuming a linear n/sub s/-V/sub G/ dependence leading to square law type I-V characteristics. The model shows excellent agreement with experimental data over a wide range of bias. Further, unlike other models using nonlinear n/sub s/-V/sub G/ dependence, it neither uses fitting parameters nor does it resort to iterative methods at any stage. It also includes the effects of the extrinsic source and drain resistances. Due to its simplicity and similarity in formulation to the SPICE MOSFET Level 3 model, it is ideally suited for circuit simulation purposes.