Nonuniform Doping Profile Research Articles

A new method for extracting the counter-implanted channel profile of enhancement-mode p-MOSFETs

A new methodology is proposed to extract the nonuniform channel doping profile of enhancement mode p-MOSFETs with counter implantation, based on the relationship between device threshold voltage and substrate bias. A selfconsistent mathematical analysis is developed to calculate the threshold voltage and the surface potential of counter-implanted long-channel p-MOSFET at the onset of heavy inversion. Comparisons between analytic calculation and two-dimensional (2-D) numerical analysis have been made and the accuracy of the developed analytic model has been verified. Based on the developed analytic model, an automated extraction technique has been successfully implemented to extract the channel doping profile. With the aid of a 2-D numerical simulator, the subthreshold current can be obtained by the extracted channel doping profile. Good agreements have been found with measured subthreshold characteristics for both long- and short-channel devices. This new extraction methodology can be used for precise process monitoring and device optimization purposes.

Accurate modelling of the accumulation region of a double barrier resonant tunnelling diode

We have developed a computer program which calculates the electron concentration, potential profile and sub-band energies in the accumulation region of a double barrier resonant tunnelling diode. The calculations employ a local density approximation to calculate the 3D electron density and an expansion in orthogonal functions for the 2D wave functions. The total potential is separated into two parts: one part originating from 2D electrons and the other part due to 3D electrons and doping effects. The numerical profile of the latter part is modelled by a simple analytical function in a satisfactory way even for non-uniform doping profiles. Also, the potential caused by 2D electrons may be written in an appropriate functional form. With the help of both potential models, we are able to build an electron Hamiltonian in which the effects of 2D and 3D electrons, as well as the doping, are taken into account. In this way, the wave functions of the 2D electrons may be obtained semi-analytically by diagonalizing the Hamiltonian and solving Poisson's equation to a restricted degree of self-consistency. Tunnelling of electrons through the barrier is taken into account. The routine is combined with an analysis of the tunnelling process and a model for the depletion region to give an accurate description of the current-voltage characteristics of double barrier resonant tunnelling diodes.

Numerical Study of Collector-Base Junction Design for Ultra-High-Speed InP/InGaAs Heterojunction Bipolar Transistors

High-frequency operation of nonplanar and planar InP/InGaAs heterojunction bipolar transistors (HBTs) with various collector doping profiles has been investigated using a Monte Carlo particle simulation. It is shown that vertical scaling of the collector does not provide a substantial improvement in HBT overall speed performance due to the dramatic increase in the collector capacitance charging time with decreasing collector layer thickness. A considerable improvement in HBT high-frequency performance has been obtained in HBTs with buried subcollector and nonuniform collector doping profile.

Optical characterization of diamond MIS capacitors

The transient photoresponse properties of diamond metal-insulator-semiconductor (MIS) capacitors have been characterized for the first time. Capacitors were fabricated on natural diamond using an electrochemical cleaning step with a CVD SiO/sub 2/ dielectric and an optional carbon implantation to create a nonuniform doping profile. Devices were found to function as integrating photodetectors and were evaluated by the spectral dependence of the transient photocapacitance (PC). We discuss a model that distinguishes between the responses due to inversion layer population and that due to bulk trap occupancy changes. Inversion charge generation was observed at all wavelengths investigated and it dominated the PC transient at photon energies above 3 eV. Possible reasons for this result are discussed and analyzed. We could not demonstrate a suitable way to use carbon implantation to form a surface n-type layer in a MIS device without degrading the device IV properties and eliminating the integrating photoresponse observed on non-implanted devices. These results suggest that diamond charge-storage devices can function only if the diamond surface is prepared properly before device fabrication.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Avalanche multiplication in forward- and reverse-active mode bipolar junction transistors

Impact ionization in the reverse-biased base-collector space-charge layer of the bipolar junction transistor (BJT) can cause an avalanche collector current as well as a reverse (or negative) base current. We develop analytical models to predict and compare avalanche phenomena in the BJT biased under forward-active and under reverse-active operations. The models consider a position-dependent electric field in the space-charge layer, the effect of the non-uniform doping profile, and the excess free carriers associated with the current passing through the space-charge layer. For the specified device make-up and parameters used, our results suggest that avalanche is more prominent in the reverse-active BJT than the forward-active BJT. The physics underlying this occurrence is given, and experimental data obtained from an advanced BJT is included in support of the models.

Formulation of mobility fluctuation [formula omitted] noise in bipolar junction transistors

In the past, the model for mobility fluctuation 1 f current noise in bipolar junction transistors (BJTs) has been developed based on uniform doping profiles. In advanced BJTs, either heavily doped or with polyemitter, the doping profiles and nonuniform. This paper presents analytical expressions for mobility fluctuation 1 f current noise in modern BJTs considering the effect of nonuniform doping profiles.

Unified quasi-static MOSFET capacitance model

An accurate calculation of MOSFET capacitance-voltage (C-V) characteristics has to account for the bulk charge which is affected by nonuniform doping profiles and short-channel effects. In an approach based on the unified charge control model (UCCM), the voltage dependencies of the bulk charge are related to the standard parameters of the body plots which are routinely measured during MOSFET characterization. The results of the C-V calculations based on this model are in good agreement with experimental data and calculations based on the standard BSIM model. Compared to the BSIM simulations, the present model more accurately describes capacitances related to the bulk charge and the device subthreshold behavior, and it is suitable for incorporation into circuit simulators. >

The influence of nonuniform doping profile on I-V characteristics of MOS transistors

The influence of nonuniform doping on I-V characteristics of a MOS transistor is estimated. A modification of the Pao-Sah model (1966) is presented, which extends its validity on a box doping profile. >

Current-voltage characteristics of submicrom GaAs MESFETs with nonuniform channel doping profiles

We present a physics-based model for GaAs metal semiconductor field-effect transistors (MESFETs). The present model improves an existing model by allowing the possibility that the electric field near the source region can exceed a critical field, which is likely in an advanced submicrom MESFET operated in the quasi-saturation or saturation region. Furthermore, a realistic and nonuniform channel doping profile was considered in our calculations. Experimental data measured from a low-noise, ion-implanted MESFET are included in support of the model. The effects of different doping profiles and velocity profiles in the channel on the MESFET current-voltage characteristics were also investigated.

Limits on the use of tunneling to describe the Pd–Ge ohmic contact to GaAs

Tunneling theory predicts a break in the slope of the contact resistivity (ρc) versus the inverse of the temperature at the transition from the thermionic to the field emission regime. Although temperature dependent data of the newly developed Pd–Ge ohmic contact to GaAs has often been described by this model, we found that the low barrier heights obtained from such data, are not consistent with the approximations of the method used. We also found that it was not possible to fit the experimental results using the complete thermionic‐field expression for ρc. Numerical calculation based on the Wentzel–Kramers–Brillouin (WKB) approximation for a nonuniform doping profile gave also a negative result in reproducing the experimental curves. Then, either the WKB method is not precise enough, or another transport mechanism is playing an important role. We suggest that an alternative transport mechanism to explain the experimental data is the interface states assisted tunneling. We also demonstrate that the low experimental activation energies found cannot be explained without taking the temperature dependence of the Fermi level into account.

Numerical study of a silicon permeable base transistor with a non-uniform doping profile

We present the results of the 2D numerical simulation of a silicon permeable base transistor. We correlate the slight degradations of the transconductance and transit frequency, as well as the large improvement of the power capability which were obtained with a non-uniform doping profile, to the physical mechanisms involved, and showed that a significant net improvement of the f T V B product can be gained by this means.

Numerical model for electrical charge injection in the acoustic charge-transport device

A two dimensional model is developed to study the electrical charge injection process at the input of a GaAs buried-channel acoustic charge-transport device. The model allows for nonuniform impurity doping profiles, variable epitaxial layer configurations, and arbitrary structural designs of the input electrode architecture. The acoustic wave potential is incorporated as a time- and space-varying doping density that adds directly to the impurity doping density. The wave-induced doping density is obtained from the piezoelectric displacement charge that accompanies the acoustic wave. The partial differential equations which form the mathematical basis of the charge injection process are derived from the semiconductor transport equations and solved numerically. The algorithm for simulating charge injection and the results of a simulation are presented. This model provides a means for characterizing the electrical performance of the acoustic charge-transport device input circuit in terms of device physics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Exponential thermal emission transients from D X centers in heavily Si-doped GaAs

The kinetics for the thermal emission of electrons from DX levels are shown to be exponential in heavily Si-doped GaAs. Isothermal voltage transients, obtained at constant capacitance, show a perfect exponential behavior. In contrast, a clear deviation from a single exponential function is observed when the transients are recorded at constant voltage, due to the nonuniform doping profile in these structures. The exponential emission kinetics seen in GaAs support the proposal that nonexponential emission kinetics observed at constant capacitance in AlxGa1−xAs are due to different emission rates for DX levels having different local atomic configurations in the alloy.

Optimization of the doping profile in Si permeable base transistors for high-frequency, high-frequency, high-voltage operation

The effects of variations in doping profiles along the direction of current flow in Si permeable base transistors have been investigated. Numerical simulations of Poisson's equation and the electron-current-continuity equation have been used to determine the fundamental tradeoff between the unity-current-gain frequency f/sub T/ and the breakdown voltage V/sub B/ for a variety of doping profiles. These range from a uniform profile (4*10/sup 16/ cm/sup -3/) to a highly nonuniform profile in which the doping in the emitter region is greater than 100 times that in the collector region. Although f/sub T/ decreases significantly with increasing collector-to-emitter voltage for the uniformly doped case, it is nearly independent of collector-to-emitter voltage for the nonuniform doping profile. In addition, nonuniform doping profiles produce devices with higher V/sub B/ than uniform doping profiles for a given f/sub T/. A class A power analysis performed using simulated current-voltage characteristics showed that output power, power-added efficiency, and large-signal gain can be increased with devices having nonuniform doping profiles. Experimental devices with nonuniform vertical doping profiles have been fabricated using high-energy (300-400 keV) P implantation into a high-resistivity (4- Omega -cm) epitaxial layer. Although present processing technology limits the f/sub T/ and V/sub B/ to 60% and 80% of the simulated values, respectively, an f/sub T/>20 GHz at a base-to-collector bias of 16 V and an f/sub T/ of 12 GHz at a base-to-collector bias of 26 V have been obtained. >

A new model for the dual-gate GaAs MESFET

The development of a novel GaAs dual-gate MESFET model suitable for the design and analysis of microwave circuits is described. This quasi-two-dimensional physical model is numerically efficient due to a unique formulation of the carrier transport equations. The model includes a comprehensive description of the geometric and material parameters accounting for recess structures, nonuniform doping profiles, current injection into the buffer layer, forward-biased gate conduction, and surface depletion. The accuracy of the model under DC, small-signal, and large-signal operating conditions is assessed by comparing simulated and measured performance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Neutron radiation effects in GaAs ion-implanted metal-semiconductor field-effect transistors

Neutron radiation investigations have been carried out on ion-implanted GaAs metal-semiconductor field-effect transistors (MESFETs). Device characteristics were measured before irradiation and following neutron irradiations with fluences from 5×1013 to 2×1015 n/cm2. At 5×1013 n/cm2, the device degradation is negligible, while at 2×1015 n/cm2 the threshold voltage shift was 0.7 V and the device transconductance was 30% of its original value. Degradation parameters needed to explain these results are larger than what has been previously reported; this discrepancy is, in part, due to the nonuniform doping profiles in the FET channel. The details of a new model applicable to nonuniform carrier distributions and to density dependent carrier removal rates are presented and results compared to those in the literature. A carrier removal rate of approximately 20 cm−1 is required to explain the degree of observed device degradation.

The effect of channel implants on MOS transistor characterization

MOS device characterization involves the extraction of parameters from electrical measurements. A nonuniform channel doping profile can make such characterization ambiguous since device parameters are usually based upon a uniform doping profile model. In this paper, we solve the one-dimensional Poisson's equation for several doping profiles and show the impact of a nonuniform doping profile on the threshold surface potential, threshold voltage, normal field mobility degradation, and transconductance.

Physical and CAD models for the implanted-channel VLSI MOSFET

A new approach is described for the modeling of practical MOS transistors that have nonuniform substrate doping profiles. The threshold characteristic is used to provide an accurate measure of body charge and thereby to give operating point dependences of the threshold voltage, body effect, mobility, and weak-inversion conduction. The results are incorporated into a simple and flexible CAD model suitable for existing and foreseeable devices. The model has continuity of current and all derivatives throughout all regions of operation. It provides an accurate representation of real transistors and minimizes numerical problems of convergence and stability. It has been implemented as level-4 in SPICE 2G.5 and is freely available for VLSI circuit design.

A Low-Loss Ku-Band Monolithic Analog Phase Shifter

A GaAs monolithic Ku-band analog phase shifter integrating 90° branch line coupler with planar varactor diodes has been fabricated for the first time. A phase shift of 109° +- 3° with an insertion loss of 1.8+-0.3 dB was measured from 16 to 18 GHz. A 360° phase shifter with 4.2+-0.9 dB insertion loss was realized in the same frequency range by connecting three phase-shifter chips in series. To our knowledge, this is the lowest insertion loss obtained by a 360° Ku-band phase shifter using monolithic circuits. In addition, hyperabrupt varactors using nonuniform doping profiles increased the phase shift by more than 30° and produced a more linear dependence of phase shift on control voltage.

Superior low-noise GaAs MESFET's with graded channel grown by MBE

GaAs MESFET's with superior microwave performance were fabricated with uniform and nonuniform doping profiles grown by molecular-beam epitaxy (MBE). At 8 GHz the devices with graded doping exhibited about 1 dB higher gain at the minimum noise figure point compared to flat doping profile devices. The noise figure of 1.38 dB with an associated gain of 8.6 dB at 8 GHz was obtained for the best device with a graded profile.