Ion-implanted threshold tailoring for insulated gate field-effect transistors

This paper develops a general threshold equation for long-channel insulated gate field-effect transistors which accounts for the effects of ion-implant profiles used in threshold tailoring. Although any integrable function used to describe the nonuniform doping will yield an analytical expression for threshold, a Gaussian function is chosen because it accurately describes the implanted profile following proper annealing, regardless of subsequent high-temperature steps during fabrication. Most profiles of interest are quasi-neutral, i.e., the spatial dependence of the majority carriers in the undepleted bulk is adequately described by the doping profile, but the threshold equation is shown to be an excellent approximation for non quasi-neutral profiles as well. Comparison with experimental results show the analytical expression to be in good agreement with data over a wide range of implant conditions and starting substrate resistivity.     

Analysis and Improvement of Intermodulation Distortion in GaAs Power FET's

Tailoring of the doping profile is a powerful tool in reducing the intermodulation distortion (IMD) in GaAs power FET's. Reproducible and uniform preparation of the required profiles is a difficult task for epitaxial techniques. This shortcoming has motivated the present investigation of fabricating highly linear power FET's by ion implantation. An analytical divice model was developed for exploring the relationship between the active layer profile and the IMD. These calculations revealed a complex behavior in the variation of the distortion levels due to partial correlation between the contributions arising from nonlinear transconductance and output conductance. The device model was used to identify implant doses and energies for approaching an optimum active layer profile. Based on the results, a deep Se implant followed by a shallow compensating Be implant to reduce the doping level close to the surface was used in the device fabrication. The IMD of the transistors was measured by the two-tone method. Conventional epitaxial FET's with a flat doping profile were evaluated for comparison purposes. This comparison demonstrated that a 4-dB increase in the intercept point for the third-order intermodulation product can be realized by using the tailored implanted profile. The experiments demonstrated that the tuning conditions for maximum output power and minimum IMD are virtually identical for the implanted transistors, in contrast to the behavior of conventional devices with flat doping profiles. These performance advantages, coupled with the high levels of uniformity and reproducibility of doping parameters, show ion implantation to be a powerful technique in the fabrication of highly linear power FET's.     

Determination of a microwave transistor model based on an experimental study of its internal structure

Based on an analysis of the physical structure of a bipolar interdigitated microwave transistor, a simple model is developed which accurately describes the high frequency small signal operation. The transistor model contains 15 elements. The values for ten of these parameters are determined by technological measurements using a special test pattern mask. The values of four of the parameters are determined through measurements made on the transistor, and one parameter is adjusted using the microwave S parameters measurements.A circuit model for the transistor package is also developed. The emitters are all arsenic doped, and the transistor structure does not exhibit a push effect. However, our measurements indicate that the emitter depth varies significantly with the width of the emitter window. This result shows that doping profiles measured on a uniformly doped test wafer may be quite different from the doping profile in the active base of a fine geometry microwave transistor. This implies that much recent work on microscopic models treating charge carrier transport may be invalid, since calculations are based on inaccurate doping profiles. The model in its present form neglects the physical mechanisms related to high current injection, but it describes accurately the transistor characteristics at normal operating currents and provides a valuable guide in optimizing the fabrication technology.     

Transformation of a Gaussian doping profile in a temperature field as influenced by the temperature dependence of the diffusion coefficient

An analytical solution is obtained for the dopant diffusion equation with a uniform temperature gradient and a Gaussian initial doping profile. It essentially takes into consideration the temperature dependence of the diffusion coefficient. With large temperature gradients, this factor is shown to lead to marked divergence of the tails of doping profiles corresponding to temperature gradients of opposite sign. The particular features of doping profiles as influenced by spatial temperature variation are explained.     

Improved device performance by multistep or carbon co-implants

High-energy ion implantation is used for forming the collector in vertical bipolar transistors in a BiCMOS process. Secondary defects, remaining after annealing the implant damage, give rise to an increased leakage current and to collector-emitter shorts. These shorts reduce the transistor yield. The use of multiple step implants or the introduction of a C gettering layer are demonstrated to avoid dislocation formation. Experimental results show that these schemes subsequently lower the leakage current and dramatically increase device yield. The presence of C can cause increased collector/substrate leakage, indicating that the C profile needs to be optimized with respect to the doping profiles     

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.     

Read versus flat doping profile structures for the realization ofreliable high-power, high-efficiency 94 GHz IMPATT sources

An investigation of the potential RF performance of various types of silicon IMPATT homojunction structures was carried out to identify the most efficient type for reliable high-power, high-efficiency CW generation in the 94 GHz window. The study used an IMPATT oscillator model accounting, in a self-consistent manner, for both thermal limitation and diode impedance matching. The main result is that in contrast to lower operating frequencies, the realization of a Read doping profile does not improve the RF performance level compared to structures with a flat doping profile. Operating conditions were optimized for high RF emitted performance conditions. In addition, the fundamental effects on RF performance of both the diode thermal resistance and RF losses have been quantified     

Profile design considerations for minimizing base transit time inSiGe HBT's

A unified closed form analytical model for base transit time of SiGe HBT's for uniform and exponential base dopant distributions with different Ge profiles in the base (e.g., box, trapezoidal, triangular) is reported. The model is subsequently used to study the design of Ge profile for different base doping profiles, including that of epitaxial base transistors. Consistent with the reported results, our unified model predicts that beyond a certain total Ge content, there is very little reduction in τ_b.SiGe. It is further demonstrated that the trapezoidal Ge profile with X_T~0.8W_B gives near optimal base transit time for all doping profiles considered. Our analysis shows that 1) for a given base width and intrinsic base resistance, the exponential base doping profile with Ge yields the least value of τ_{b, SiGe} and 2) for a given peak base doping concentration and the intrinsic base resistance, the uniform base doping with Ge gives minimum τ_{b, SiGe}. Also, the need for keeping the total base Ge content constant while optimizing the Ge profile in the base is emphasized by showing that a false minimum for τ_{b, SiGe} may appear if the total Ge content is not kept constant     

The influence of a doping profile on the characteristics of an ion-implanted GaAs field-effect transistor with a Schottky barrier

A GaAs field-effect ion-implanted transistor with a Schottky barrier is simulated. The doping profile obtained when doping through an insulator mask is determined and the dependences of the static transistor characteristics on the parameters of the doping profile are calculated and analyzed. The physical processes controlling the transistor characteristics in the case of a variation in the parameters of its doping profile and the coefficient of compensation of the substrate are studied. Based on calculations, the optimal doping-profile parameters ensuring the best characteristics for transistors are predicted.     

New analytical expressions for dark current calculations of highlydoped regions in semiconductor devices

We studied highly doped quasi-neutral regions of semiconductor devices with position dependent doping concentration in the absence of illumination. An important parameter of a highly doped region is its dark current. To clarify how the doping profile influences the dark current, simple analytical expressions are useful. To this end, we first transformed the transport equations to a simple dimensionless form. This enables us to write already existing analytical expressions in an elegant way. It is demonstrated how, from any analytical dark current expression, a direct counterpart can be derived. Next, we derived a dimensionless form for a nonlinear first-order differential equation for the effective recombination velocity. Starting from the analytical solution of this differential equation for uniformly doped regions and using linearization techniques, we obtained two new simple and accurate expressions for the dark current. The expressions are valid for general doping profiles with different minority carrier transparencies. The exact solution is included between both new approximate solutions. The new expressions are compared with previous approximate solutions     

Comparison of doping profiles in capless annealed and dielectrically capped — Annealed ion implanted GaAs

Recent results of a capless method of annealing ion implanted GaAs are reported. The physical mechanism and effectiveness of the process are described and comparison of doping profiles from wafers annealed with a reactively sputtered Si₃N₄ dielectric encapsulation and with the capless process is given. Capless annealing is shown to consistently result in narrower profiles for various dopants and implant energies. The observed differences are shown to be consistent with enhanced diffusion in the dielectric capped samples, and the effective diffusion coefficients, which are of the order of 10^?15 cm²/s for Se, differ by as much as a factor of two.     

Ion implantation in semiconductors—Part I: Range distribution theory and experiments

Ion implantation in semiconductors provides a doping technique with several potential advantages over more conventional doping methods. Among the most important of these are: 1) the ability to introduce into a variety of substrates precise amounts of nearly any impurity element desired; 2) the ability to control doping profiles in three dimensions by modulating the energy, current, and position of the ion beam; and 3) the possibility of avoiding certain undesirable effects that accompany the high-temperature diffusion process. Ion implantation can also be used in conjunction with other fabrication techniques to produce device structures that no one process can produce simply by itself. Current research in the field is directed toward several problems that must be solved before the full impact of ion implantation on semiconductor technology can be soundly predicted. In particular, it is necessary to be able to predict the distribution profiles of the implanted ions accurately, to know which crystalline sites the implanted ions occupy, to know the nature of the damage centers that are introduced by the implantation process, and to determine the extent to which these defects can he removed by appropriate annealing procedures. Theoretical and experimental work pertinent to the problem of predicting impurity distribution profiles in ion-implanted material are reviewed here. A review of current research on the other problems listed will be given in Part II, together with the characteristics of a number of interesting semiconductor devices that have already been fabricated by ion implantation.     

Improvement of Temperature Coefficient of Resistance by Co-Implantation of Argon or Xenon or Fluorine in Boron Implanted Polysilicon Resistors

This paper proposes a new method to decrease the absolute value of temperature coefficient of resistance (TCR) in P-type boron implanted polysilicon resistors, at a given intermediate sheet resistance values, by selecting an optimized combination of boron doping implant conditions with co-implantation conditions. The co-implantation ion species that were investigated are fluorine, argon and xenon. Each of the co-implantation species was studied at three different co-implantation conditions and two different boron doping implant conditions of dose and energy.      

Channeling, exponential tails, and analytical modeling of Siimplants into GaAs

The depth distribution N(x) of Si²⁹ ion beam implanted into GaAs for a wide range of implant doses, energies, and SiN randomizing-layer thicknesses is discussed. It is found that the variation of implant angle with position on the wafer (typically encountered in scanned-ion-beam implantation systems) results in significant channeling-induced nonuniformity of the doping profiles, even in some cases where a randomizing layer was used. Because of the existence of exponential tails in virtually all implants, an extension of the standard LSS theoretical profile is necessary for accurate characterization of the implants and prediction of the device parameters. The measured profiles were fitted to two different modified-Gaussian analytical forms: the Pearson four-parameter form, which offers excellent flexibility in fitting arbitrary distributions but is relatively devoid of physical content, and a new three-parameter form that is an extension of a Gaussian to profiles hyperbolic in log N. In almost all cases, the hyperbolic-Gaussian form fits the data as well as the Pearson form, and the hyperbolic-Gaussian form is simpler. Parameters characterizing the implants for both types of fit are presented     

Large-Signal Equivalent Circuits of Avalanche Transit-Time Devices

A large-signal analysis for IMPATT diodes is derived, which allows carrier multiplication by impact ionization to occur at every point in the diode. Therefore, the operating characteristics of IMPATT diodes with a wide range of realistic doping profiles can be investigated. For a given operating frequency, RF voltage, dc bias current, and doping profile, the admittance, power output, efficiency, bias voltage of a diode can be obtained. An equivalent circuit the diode package, microwave circuit mount and diode, is obtained experimentally. Using this circuit, the admittance of the diode is measured by a reflection-type circuit and an oscillator circuit as a function of the RF voltage, dc bias current, and frequency.     

A simplified fully implanted bipolar VLSI technology

An implanted n-p-n bipolar transistor structure named Isoplanar Z II (currently, being marketed as FAST-Z technology) with reduced process and masking steps is described. The simplification is achieved by employing self-aligned-transistor (SAT) masking, ion-implantation techniques to provide impurity doping, and using one common annealing cycle for collector, base, and emitter implantations. The device structure reduces design constraints through use of self-aligned field implantation and SAT mask for contact window definition. Submicrometer emitter widths are obtained by step and repeat optical photolithographic tool and two-dimensional effect on current gain due to sidewall injection is also studied. This technology is used to demonstrate 13-15 ns TAA, 4K static RAM and minimum delay of 250 ps per gate, gate array products.     

Ion implantation and rapid thermal processing of Ill-V nitrides

Ion implantation doping and isolation coupled with rapid thermal annealing has played a critical role in the realization of high performance photonic and electronic devices in all mature semiconductor material systems. This is also expected to be the case for the binary III-V nitrides (InN, GaN, and A1N) and their alloys as the epitaxial material quality improves and more advanced device structures are fabricated. In this article, we review the recent developments in implant doping and isolation along with rapid thermal annealing of GaN and the In-containing ternary alloys InGaN and InAlN. In particular, the successful n- and p-type doping of GaN by ion implantation of Si and Mg+P, respectively, and subsequent high temperature rapid thermal anneals in excess of 1000°C is reviewed. In the area of implant isolation, N-implantation has been shown to compensate both n- and p-type GaN, N-, and O-implantation effectively compensates InAlN, and InGaN shows limited compensation with either N- or F-implantation. The effects of rapid thermal annealing on unimplanted material are also presented.     

Novel low-temperature C-V technique for MOS doping profiledetermination near the Si/SiO2 interface

An inverse modeling technique for doping profile extraction from MOS C-V measurements is presented. The method exploits the “kink” effect observed near flat bands in low-temperature C-V curves to accurately estimate the dopant concentration at the oxide-silicon surface. The inverse modeling approach, based on a self-consistent Schrodinger-Poisson solver, overcomes the limitations of previous analytical methods. The accuracy of the doping extraction is demonstrated by successfully reconstructing doping profiles from simulated C-V curves, including abrupt variations of doping in the vicinity of the oxide interface. When applied to experimental data from boron- and phosphorus-doped samples, the technique is shown to provide a substantial improvement in resolution with respect to room-temperature C-V measurements     

Nonlinear properties of IMPATT devices

The basic principles of IMPATT diodes as microwave devices are reviewed and the current status of these devices concerning power output and efficiency is given. The main purpose of this paper, however, is to discuss the nonlinear properties of these diodes which are useful in the design of amplifiers, oscillators, and other microwave devices. The main results of this paper are obtained from a digital computer analysis where an approximate, but realistic, diode model is employed. A detailed comparison of complementary silicon diodes as well as GaAs diodes concerning power output and efficiency is given. The effects of doping profile, current density, temperature, and material parameters on the performance of these devices have been investigated and are summarized. Saturation effects which limit the efficiency and power output of these devices are described and optimum efficiencies which can be achieved for various doping profiles are given. A comparison between single-sided and double-drift diodes in both silicon and GaAs is also presented.     

A two-dimensional analytical threshold voltage model for MOSFET's with arbitrarily doped substrates

A threshold voltage model is presented which is valid for short- and long-channel MOSFET's with a nonuniform substrate doping profile. The model is based upon an approximate two-dimensional analytical solution of Poisson's equation for a MOSFET of arbitrary substrate doping profile which takes into account the effect of curved junctions of finite depth. The analytical model is compared to MINIMOS simulations showing that it can accurately predict short-channel threshold voltage falloff and threshold voltages in this vicinity without the use of fitting parameters.