Nonuniform Doping Profile Research Articles

Depletion Layer Modeling for Short Gate Length Non-Uniformly Doped GaAs MESFET Under Dark and Illuminated Condition

This paper presents an analytical expression for the depletion region height of short gate length GaAs MESFET with non-uniform doping profile in the channel region. Both, dark as well as illuminated conditions have been considered for model formulation. Depletion region height sensitivities on the doping parameters have also been demonstrated.

High-Speed Silicon Mach–Zehnder Optical Modulator With Large Optical Bandwidth

We report a carrier-depletion silicon Mach-Zehnder optical modulator with large optical bandwidth by adopting two symmetric arms. The fiber-to-fiber loss of the device is about 7.2-8 dB in the wavelength range from 1525 to 1565 nm. We have used the truncation method to accurately measure the loss of the optical splitter and combiner, and the on-chip loss is about 3.8 dB. The dynamic extinction ratios at the speed of 40 Gb/s are 4.9-6.4 dB in the wavelength range from 1529 to 1565 nm. By analyzing the dependence of the optical bandwidth on the optical path difference between the two arms, we find that there is an unexpected optical path difference of around 3.3 μm between the two arms, which is considered to originate from the nonuniform morphologies of the waveguide and the nonuniform doping profiles along the two arms and is responsible for the slight wavelength dependence of the static and dynamic response of the silicon Mach-Zehnder optical modulator.

A quasi two-dimensional compact model for trapezoidal FinFETs with non-uniform doping profiles using the perturbation method

A quasi two-dimensional analytic model for trapezoidal fin field-effect transistors (FinFETs) with non-uniform doping profiles is developed. The perturbation method, combined with a variable transformation technique, is applied to solve the nonlinear Poisson’s equation analytically. Our approach lumps the depletion and inversion terms together to capture the effects of nonlinear coupling and spatial variation of an arbitrary doping profile. Trapezoidal boundary conditions are reformulated accordingly and a continuous analytic current–voltage (I–V) model for doped trapezoidal FinFETs is derived. More accurate prediction of the surface potential and the I–V characteristics is achieved by setting the operating point of a Taylor expansion at the channel surface and introducing the higher-order correction. The short-channel effect is also studied and verified with technology computer-aided design (TCAD) simulations, which indicates a high accuracy of this new model.

Surface depletion effects in semiconducting nanowires having a non-uniform radial doping profile

Doping profile has a significant effect on nanowire (NW) electrostatics, an effect that is expected to influence NW contact and transport properties. Herein, the electrostatic potential of nanowires (NWs) of non-uniform radial doping is calculated by two means: depletion approximation and a numerical calculation. Two profiles are considered: linear and exponential, corresponding to shallow and abrupt distributions; the results are compared to planar systems with similar doping profiles, and to uniformly doped NW systems. For a given average doping distribution, a non-uniform doping profile results in significantly lower carrier concentrations, an effect which intensifies with doping non-uniformity. Furthermore, in some cases, band diagrams obtained for the exponential doping profile vary greatly from any uniform doping, indicating that unique properties are expected for such NWs.

Effects of Ge profile on thermal characteristics of SiGe heterojunction bipolar transistor with non-uniform doping profile in base region

As is well known, base region is the design focus of bipolar junction transistor (BJT). Generally, the doping distribution in base is non-uniform. In this paper, the effects of different Gaussian dopings in the base on bulk temperature distribution, temperature dependences of current gain and cut-off frequency are first studied. It is found that current gain and cut-off frequency of BJT have positive temperature coefficients, and the temperature in bulk is high. Then, the effect of Ge composition distribution on these device parameters is investigated. It is found that the SiGe heterojunction bipolar transistors (HBTs) with box Ge composition distribution and trapezoidal Ge composition distribution have negative temperature coefficients of current gain and cut-off frequency, and have good bulk temperature distributions. Furthermore, the SiGe HBT with trapezoidal Ge profile has higher current gain and cut-off frequency, and its temperature insensitivity is kept. The good trade-off is made among the magnitudes of current gain and cut-off frequency, temperature sensitivity and bulk temperature distribution.

A Subthreshold Swing Model for Fully-Depleted Silicon-on-Insulator Metal–Oxide–Semiconductor Field Effect Transistors with Vertical Gaussian Profile

A compact subthreshold swing model is proposed for the short-channel fully-depleted (FD) silicon-on-insulator (SOI) metal–oxide–semiconductor field effect transistors (MOSFETs) with vertical Gaussian doping profile in this paper. Considering the channel potential which is influenced by the non-uniform doping profile, the model is derived by using the concept of effective conduction path effect (ECPE). The validity of the proposed model is shown by comparing the calculated results with the Sentaurus technology computer-aided design (TCAD) numerical simulation data. The model is believed to provide a better physical insight and understanding of FD SOI MOSFETs with a non-uniform doping profile operating in the subthreshold regime.

A Subthreshold Current Model of Fully-Depleted Silicon-on-Insulator Metal–Oxide–Semiconductor Field Effect Transistors with Vertical Gaussian Profile

A subthreshold current of fully-depleted (FD) silicon-on-insulator (SOI) metal–oxide–semiconductor field effect transistors (MOSFETs) with vertical Gaussian profile is presented in the paper. The model is based on analytical approximated solution of two-dimensional Poissons' equation and Boltzmann transport equation. The front and the back channel currents are effectively derived using a novel inversion layer model and the boundary conditions which take the nonuniform doping into account. The results are matched well with the numerical simulation results obtained by Sentaurus Technology Computer-Aided Design (TCAD). The model is believed to provide a deep physical insight and understanding of FD-SOI MOSFETs with a non-uniform doping profile operating in the subthreshold regime.

Design Issues for Piezoresistive Nanocantilever Sensors with Non-uniform Nanoscale Doping Profiles

A modeling method for piezoresitive nanocantilevers using their geometrical parameters is described. The spring constant and the effective mass of piezoresistive nanocantilevers are formulated using theoretical and approximation equations, which shows a good agreement with the results of finite element analysis. The displacement sensitivity and minmum detectable force of piezoresistive nanocantilevers with non-uniform doping profiles are calculated by introducing a piezoresistive efficiency factor.

Obtaining Uniform Dopant Distributions in VLS-Grown Si Nanowires

Semiconducting nanowires grown by the vapor-liquid-solid method commonly develop nonuniform doping profiles both along the growth axis and radially due to unintentional surface doping and diffusion of the dopants from the nanowire surface to core during synthesis. We demonstrate two approaches to mitigate nonuniform doping in phosphorus-doped Si nanowires grown by the vapor-liquid-solid process. First, the growth conditions can be modified to suppress active surface doping. Second, thermal annealing following growth can be used to produce more uniform doping profiles. Kelvin probe force microscopy and scanning photocurrent microscopy were used to measure the radial and the longitudinal active dopant distribution, respectively. Doping concentration variations were reduced by 2 orders of magnitude in both annealed nanowires and those for which surface doping was suppressed.

N-Type Doping of Vapor-Liquid-Solid Grown GaAs Nanowires.

In this letter, n-type doping of GaAs nanowires grown by metal–organic vapor phase epitaxy in the vapor–liquid–solid growth mode on (111)B GaAs substrates is reported. A low growth temperature of 400°C is adjusted in order to exclude shell growth. The impact of doping precursors on the morphology of GaAs nanowires was investigated. Tetraethyl tin as doping precursor enables heavily n-type doped GaAs nanowires in a relatively small process window while no doping effect could be found for ditertiarybutylsilane. Electrical measurements carried out on single nanowires reveal an axially non-uniform doping profile. Within a number of wires from the same run, the donor concentrations ND of GaAs nanowires are found to vary from 7 × 1017 cm-3 to 2 × 1018 cm-3. The n-type conductivity is proven by the transfer characteristics of fabricated nanowire metal–insulator-semiconductor field-effect transistor devices.

Lateral drift-field photodiode for low noise, high-speed, large photoactive-area CMOS imaging applications

In this work a theoretical concept and simulations are presented for a novel lateral drift-field photodetector pixel to be fabricated in a 0.35 μm CMOS process. The proposed pixel consists of a specially designed n-well with a non-uniform lateral doping profile that follows a square-root spatial dependence. “Buried" MOS capacitor-based collection-gate, a transfer-gate, and an n-type MOSFET source/drain n + floating-diffusion serve to realize a non-destructive readout. The pixel readout is performed using an in-pixel source-follower pixel buffer configuration followed by an output amplifier featuring correlated double-sampling. The concentration gradient formed in the n-well employs a single extra implantation step in the 0.35 μm CMOS process mentioned and requires only a single extra mask. It generates an electrostatic potential gradient, i.e. a lateral drift-field, in the photoactive area of the pixel which enables high charge transfer speed and low image-lag. According to the simulation results presented, charge transfer times of less than 3 ns are to be expected.

Correlating dopant distributions and electrical properties of boron-doped silicon nanowires

Quantitative nonuniform radial doping profiles in vapor liquid solid grown boron-doped silicon nanowires are correlated with axial variations in electrical properties. Boron concentrations measured by atom probe tomography are lower for the core material grown from a gold catalyst than for material deposited on the nanowire surface. Transistors fabricated along a single nanowire exhibit a transition from nonlinear contact-dominated behavior to linear behavior with increasing thickness of the dopant-enriched surface layer. Simple models confirm that the surface is doped to a level that enables the contact resistance to become comparable to the channel resistance, suggesting that unintentional surface doping may play a role in lowering contact resistances in some nanowire devices.

High-Performance Uni-Traveling-Carrier Photodiodes With a New Collector Design

We report two uni-traveling-carrier photodiode (PD) structures, for high-power and high-linearity applications. Using a thick collection layer (500 nm), the fabricated 25-m-diameter PDs achieve a 3-dB bandwidth up to 29 GHz, and a maximum dissipated heat power of about 480 mW, simultaneously. A new collector design with a nonuniform doping profile is proposed to better relax the space charge effect. Its performances are compared to a uniformly doped collector layer. Saturation currents and third-order intermodulation distortion measurements at 20 GHz confirm the advantage of the new collector design for high-power and high-linearity performances: 1-dB saturation current as high as 120 mA and a third-order intercept point in excess of 35 dBm at 70 mA are recorded.

Effect of interface states on conversion efficiency of non-uniformly doped MIS solar cell

A comprehensive numerical model is prepared to simulate the performance of a MIS solar cell with nonuniform substrate doping profile. The effect of interface states properties on the device conversion efficiency are simulated and investigated by computing the J–V characteristics under dark and illumination conditions. Also the effect of some important device parameters such as insulator thickness, doping profile, and fixed insulator charge are quantitatively investigated.

Design of Ion-Implanted MOSFET's with Very Small Physical Dimensions

This paper considers the design, fabrication, and characterization of very small MOSFET switching devices suitable for digital integrated circuits using dimensions of the order of 1μ. Scaling relationships are presented which show how a conventional MOSFET can be reduced in size. An improved small device structure is presented that uses ion implantation to proVide shallow source and drain regions and a nonuniform substrate doping profile. Onedimensional models are used to predict the substrate doping profile and the corresponding threshold voltage versus source voltage characteristic. A two-dimensional current transport model is used to predict the relative degree of short-channel effects for different device parameter combinations. Polysilicon-gate MOSFETs with channel lengths as short as 0.5μ were fabricated, and the device characteristics measured and compared with predicted values. Ibe performance improvement expected from using these very small devices in highly miniaturized integrated circuits is projected. Reprintedfrom the IEEE Journal of Solid-State Circuits, Vol. SC-9, October 1974, pp. 256-268.]

A study of the effects of the base doping profile on SiGe heterojunction bipolar transistor performance for all levels of injection

The effects of two different base doping profiles on the current gain and cut-off frequency for all levels of current injection have been studied for NPN Si/SiGe/Si double heterojunction bipolar transistors (SiGe DHBTs). The two-dimensional simulation results for a SiGe DHBT with uniform base doping and a fixed base Gummel number are compared with a non-uniform base doping profile SiGe drift-DHBT device. The study explains the performance of SiGe HBTs at different injection levels by analysing the electron and hole mobility, drift velocity, electric field, junction capacitances and intrinsic and extrinsic base region conductivities. The base doping profile in the SiGe drift-DHBT is controlled in such a way that it creates a net accelerating drift field in the quasi-neutral base for minority electrons. This accelerating field subsequently improves the current gain and cut-off frequency for the SiGe drift-DHBT in comparison with the SiGe DHBT for all levels of injection.

New fundamental insights into capacitance modeling of laterally nonuniform MOS devices

In compact transistor modeling for circuit simulation, the capacitances of conventional MOS devices are commonly determined as the derivatives of terminal charges, which in their turn are obtained from the so-called Ward-Dutton charge partitioning scheme. For devices with a laterally nonuniform channel doping profile, however, it is shown in this paper that no terminal charges exist from which the capacitances can be derived. Instead, for such devices, a new model is presented for the capacitances themselves. Furthermore, a method is given to incorporate such a capacitance model into circuit simulators, which are traditionally based on terminal charge models. Comparison with two-dimensional device simulations and a segmentation model shows that for a constant mobility, the new capacitance model provides an accurate description for a MOSFET with a laterally diffused channel doping profile. Through a comparison with high-frequency measurements, the agreement between model and experimental results is discussed.

Scaling the modulation bandwidth and phase efficiency of a silicon optical modulator

We present an optimized design and detailed simulation of an all-silicon optical modulator based on a silicon waveguide phase shifter containing a metal-oxide-semiconductor (MOS) capacitor. Based on a fully vectorial Maxwell mode solver, we analyze the modal characteristics of the silicon waveguide. We show that shrinking the waveguide size and reducing gate oxide thickness significantly enhances the phase modulation efficiency because of the optical field enhancement in the voltage induced charge layers of the MOS capacitor, which, in turn, induce refractive index modulation in silicon due to free carrier dispersion effects. We also analyze the device speed by transient semiconductor device modeling. As both optical absorption and modulation bandwidth increase with increasing doping concentration, we show that, with a nonuniform doping profile in the waveguide, balance between the device operation speed and optical loss can be realized. Our simulation suggests that a TE-polarized optical phase modulator with a bandwidth of 10 GHz and an on-chip optical loss less than 2 dB is achievable in silicon.

Optimization of RF Linearity in DG-MOSFETs

RF linearity of double-gate (DG) MOSFETs is investigated using accurate two-dimensional simulations. It is shown that the asymmetric DG-MOSFET is more linear than the symmetric counterpart and that DG-MOSFET linearity can be improved by a careful optimization of channel thickness, nonuniform doping profile and gate work function. For optimum linearity, a nonuniform doping profile and a thick (/spl sim/20 nm) silicon-on-insulator (SOI) layer is required. An intuitive description of this optimization is presented.

A New 2-D Model for the Potential Distribution and Threshold Voltage of Fully Depleted Short-Channel Si-SOI MESFETs

A new two-dimensional (2-D) analytical model for the threshold voltage of a fully depleted short-channel Si-MESFETs fabricated on the silicon-on-insulator (SOI) has been presented in this paper. The 2-D potential distribution functions in the active layer of the device is approximated as a parabolic function and the 2-D Poisson's equation has been solved with suitable boundary conditions to obtain the bottom potential at the Si/oxide layer interface. The calculations have been carried out for both uniform and nonuniform doping profiles in two dimensions. The minimum bottom potential is used to monitor the drain-induced barrier lowering effect and consequently an analytical expression for the threshold voltage of the device has been derived. The numerical results for the bottom potential and threshold voltage considering a wide range of device parameters have also been presented. The model has been compared with the simulated results obtained by using the ATLAS Device Simulation Software to show the validity of the proposed model. For uniform doping profile, the numerical results have also been compared with the reported data in the literature and a good agreement is observed among the three. The proposed model is simple and easy to understand the behavior of the fully depleted short-channel SOI-MESFETs as compared to the other models reported in the literature.