Base Transit Time Research Articles

Graded base profiles on the performance of AlGaAs HBTs

Effects of graded base profiles on the performance of AlGaAs HBTs have been examined. The exponential graded base gives highest electron current density for given base-emitter voltage. The parabolic base profile results in lowest base transit time followed by exponential, Gaussian, square root and linear base profiles. For the base resistivity, however, the linearly graded base gives lowest base sheet resistance for given base concentration.

Quantitative analysis of Si/Ge x Si 1– x /Siheterojunction bipolar transistors with linearly graded Ge profile

A new experimental methodology for the analysis of Si/GexSi1–x/Si HBT yields quantitative values for the important transistor parameters, such as the energy gap reduction and the built-in electric field due to Ge, and the minority-carrier base diffusion length, lifetime and transit time. The method also enables a charge-control analysis of the base current to be made. A demonstration is presented for a GeSi heterojunction bipolar transistor with a linearly graded Ge profile in the base. The methodology can be extended for arbitrary material composition and doping profile in the base.

Analytical expressions of base transit time for SiGe HBTs with retrograde base profiles

We present a regional base transit time ( τ b) model which results in a set of closed-form analytical expressions for an exponentially-doped retrograde base of a SiGe-base HBT with a linearly graded Ge profile. This model considers the retarding built-in electric field due to the retrograde region, heavy doping effects, the effect of velocity saturation at the collector-base junction, the effects of the doping dependency and the electric-field dependency of the electron diffusivity, and the electric field induced by the Ge concentration gradient. The impact of these different effects on τ b is assessed. Our model results show that the retarding field due to the retrograde region in the base does not increase τ b as much as Gao et al. reported, especially when a high Ge grading exists and the retrograde region is small compared to the total basewidth. For a high Ge grading across the base, the effect of the electric-field dependency of the electron diffusivity on τ b becomes very pronounced and must be considered in the model. We also show that this model results in Kroemer’s analytical expression for the special case of a uniformly-doped base. Our results agree well with the published simulation data of Patton et al. for their 75-GHz SiGe-base HBT.

On the transit time of polysilicon emitter transistors

Polysilicon emitter transistors are now being widely used in high speed bipolar circuits because of their extremely low emitter transit time τ E. A model has been developed here to predict the relative contributions of the neutral polysilicon and monosilicon regions of the emitter to τ E, for various device parameters. Studies have been made for both uniform and exponential doping profiles in the mono-emitter region. Relative contributions of τ E and base transit time τ B towards the total forward transit time τ F for various device parameters have also been studied.

Novel Ge-profile design for high-speed SiGe HBTs: modelling and analysis

The authors investigate the optimisation of the Ge profile in SiGe HBTs, with the aim of enhancing current gain without degrading the base transit time at different ambient temperatures and points of film stability. Using a new box-triangular Ge profile, they show that a current gain enhancement of ~3 is achievable at T = 300 K for ytot = 14% Ge without degrading the base transit time corresponding to that of a triangular Ge profile. The effect of an electric field on the base transit time is also studied. It is shown that for WB = 50 nm, the limiting base electric field is ~40 kV/cm, beyond which any reduction in the base transit time is offset by the electron mobility degradation.

Base Transit Time in Abrupt GaN/InGaN/GaN and AlGaN/GaN/AlGaN HBTs

Base transit time, τb , in abrupt npn GaN/InGaN/GaN and AlGaN/GaN/AlGaN double heterojunction bipolar transistors (DHBTs) is reported. Base transit time strongly depends not only on the quasi-neutral base width, but also on the low field electron mobility, μn, in the neutral base region and the effective electron velocity, Sc, at the edge of base-collector heterojunction. μn and Sc are temperature-dependent parameters. A unity gain cut-off frequency of 10.6 GHz is obtained in AlGaN/GaN/AlGaN DHBTs and 19.1 GHz in GaN/InGaN/GaN DHBTs for a neutral base width of 0.05um. It is also shown that non-stationary transport is not required to study τb for neutral base width in the range of 0.05um for GaN-based HBTs.

A fast CAD model of plasma spread in thyristors using a seminumerical time-domain approach

This paper presents a fast CAD model to calculate plasma-spreading velocity in large area thyristors. The model is based on the two-dimensional two-transistor circuit model of a thyristor. At first, a simplified numerical solution of the semiconductor equations is developed to obtain the current amplification factor and the base transit time of each transistor of the thyristor model as a function of the emitter current density. The turn-on recurrence relations governing the evolution of the anode current at different lateral points are then derived by the transit function method. The time constant of the anode current rise and the plasma spreading velocity are calculated as functions of the anode current density. The results of simulation are in close agreement with the corresponding measured values of plasma spreading velocity by the electrical probes method. This agreement confirms the validity of our model over a large scale of anode current densities.

Implications of hole vs electron transport properties for high speed Pnp heterojunction bipolar transistors

This paper examines the implications of differences in hole and electron transport properties on the microwave performance potential of InP-based Pnp HBTs. In particular, the carrier mobility, diffusion length, saturation velocity and ballistic transport properties for holes vs electrons are compared and their implications for Pnp device performance, such as the current gain and high frequency performance. The effects of the doping level on the mobility and minority carrier lifetime are included using empirical models fit to the available experimental data. The results show that, while the hole's transport properties are inferior to those for the electron, the minority carrier lifetime is larger for holes than electrons so that the hole diffusion length in the base is smaller but comparable to the electron's. As a result, Pnp HBTs with adequate current gain can be achieved with only a modest reduction in the base width compared with the Npn. For the device's microwave performance, the hole's characteristics determine the base transit time and the collector delay time, which limit the Pnp HBT's high frequency performance.

A new set of initial conditions for fast and accurate calculation of base transit time and collector current density in bipolar transistors

A new set of initial conditions for fast and accurate calculation of base transit time and collector current density in bipolar transistors

Quasiballistic transport in GaAs-based heterojunction and homojunction bipolar transistors

An iterative approach to solving an implicit integral relation for the electron distribution function in the base of a bipolar transistor is exploited to achieve a solution to the field-free Boltzmann transport equation. The method, which is based on one previously applied to Si homojunction transistors, is extended here to hetero- and homojunction transistors in the GaAs material system. This involves incorporating tunneling and reflection into the boundary condition for the injected flux at the emitter end of the quasineutral base, and considering anisotropic and inelastic scattering mechanisms. The ballistic, scattered, and reflected portions of the distribution are examined as the base width is reduced to values where quasiballistic transport is evident. Numerical results are presented for the carrier concentration and velocity profiles, and for the base transit time.

A thermionic-emission-diffusion model for graded base Pnp heterojunction bipolar transistors

An analytical model which matches thermionic-emission-diffusion of holes across the emitter-base heterojunction with drift-diffusion transport across a graded base has been developed and used to examine the performance capabilities of InP-based Pnp heterojunction bipolar transistors (HBTs). Hole drift-diffusion across the emitter-base space charge region is shown to be of comparable importance to thermionic emission in controlling hole injection into the base. The effects of compositional base grading on the recombination currents is also taken into account. Compositional grading of the base is shown to enhance the device’s current gain by as much as a factor of 10 by reducing recombination in the quasi-neutral base. More importantly, compositional base grading significantly reduces the base transit time which improves the device’s peak cutoff frequency by as much as a factor of 1.5. A cutoff frequency as high as 35 GHz is found to be possible. The analysis indicates that composition grading of the base can be useful in developing high performance Pnp InP-based HBTs.

Modelling the influence of high currents on the cutoff frequency in Si/SiGe/Si heterojunction transistors

A one-dimensional self-consistent bipolar Monte Carlo simulation code has been used to model carrier mobilities in strained doped SiGe and the base-collector region of Si/SiGe/Si and SiC/Si heterojunction bipolar transistors (HBTs) with wide collectors, to study the variation of the cutoff frequency with collector current density . Our results show that while the presence of strain enhances the electron mobility, the scattering from alloy disorder and from ionized impurities reduces the electron mobility so much that it is less than that of Si at the same doping level, leading to larger base transit times and hence poorer performance for large for an Si/SiGe/Si HBT than for an SiC/Si HBT. At high values of , we demonstrate the formation of a parasitic electron barrier at the base-collector interface which causes a sharp increase in and hence a dramatic reduction in . Based on a comparison of the height of this parasitic barrier with estimates from an analytical model, we suggest a physical mechanism for base pushout after barrier formation that differs somewhat from that given for the analytical model.

BASE TRANSIT TIME MINIMIZATION WITH FIXED BASE DOPANT DOSE

The problem of determining the doping profile minimizing base transit time τ B in a BJT is investigated for the case in which the net active dopant dose Q in the quasi-neutral base is held fixed, but the width W of the base is allowed to vary. It is found that the minimum τ B is obtained, for reasonable values of Q, by shrinking the base width until the profile becomes box-shaped with doping equal to the solid solubility. This occurs at so narrow a base width, however, that in any practical device it will always be favourable to grade the base doping profile.

Effect of bandgap discontinuity on the cut-off frequency, base transit time and junction capacitance of npn AlGaAs/GaAs heterojunction bipolar transistors

The effect of the quasi-Fermi level discontinuity and the electrostatic potential nonlinearity on the transconductance, junction capacitance, base transit time and the emitter charge time of heterojunction bipolar transistors (HBT) are investigated. The analysis takes into account the effect of the thermionic emission boundary condition at the heterojunction interface on the base transit time, the transconductance and the junction capacitance. It is evident from the analysis that the base transit time and the transconductance are functions of the electrons injected from the emitter. Calculation shows that the effect of a high level of electron injection to the base on the base transit time is less effective for an abrupt bandgap emitter than that for a graded-gap emitter HBT.

Profile design considerations for minimizing base transit time in SiGe 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 /spl tau//sub b.SiGe/. It is further demonstrated that the trapezoidal Ge profile with X/sub T//spl sim/0.8W/sub 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 /spl tau//sub b, SiGe/ and 2) for a given peak base doping concentration and the intrinsic base resistance, the uniform base doping with Ge gives minimum /spl tau//sub 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 /spl tau//sub b, SiGe/ may appear if the total Ge content is not kept constant.

Simulation and design of InAlAs/InGaAs pnp heterojunction bipolar transistors

The performance capabilities of InP-based pnp heterojunction bipolar transistors (HBT's) have been investigated using a drift-diffusion transport model based on a commercial numerical simulator. The low hole mobility in the base is found to limit the current gain and the base transit time, which limits the device's cutoff frequency. The high electron majority carrier mobility in the n/sup +/ InGaAs base allows a reduction in the base doping and width while maintaining an adequately low base resistance. As a result, high current gain (>300) and power gain (>40 dB) are found to be possible at microwave frequencies. A cutoff frequency as high as 23 GHz and a maximum frequency of oscillation as high as 34 GHz are found to be possible without base grading. Comparison is made with the available, reported experimental results and good agreement is found. The analysis indicates that high-performance pnp InP-based HBT's are feasible, but that optimization of the transistor's multilayer structure is different than for the npn device.

GaInP/AlGaAs/GaInP double heterojunction bipolar transistors with zero conduction band spike at the collector

Al/sub 0.11/Ga/sub 0.89/As was used in the base, next to the GaInP collector of a DHBT, to eliminate the conduction band spike. The DHBT's demonstrated high breakdown voltages, BV/sub CEO/ and BV/sub CBO/ of 44.5 V and 54.5 V (gain/spl ap/20), respectively, for a 1-/spl mu/m-thick collector doped to 2/spl times/10/sup 16/ cm/sup -3/ with no voltage dependence of the current gain. Magneto-transport measurements were made on the AlGaAs bases and indicated limitations on the maximum practical base doping due to the inferior minority electron mobility and lifetime when compared with equivalently doped GaAs. Grading in the base from Al/sub 0.11/Ga/sub 0.89/As at the collector to Al/sub 0.21/Ga/sub 0.79/As at the emitter introduced a quasielectric field in the base, reduced the base transit time by a factor of /spl sim/2.5, and improved the gain over ungraded devices with the same average Al concentration.

Base Transit Time in Abrupt GaN/InGaN/GaN and AlGaN/GaN/AlGaN HBTs

AbstractBase transit time, τb, in abrupt npn GaN/InGaN/GaN and AlGaN/GaN/AlGaN double heterojunction bipolar transistors (DHBTs) is reported. Base transit time strongly depends not only on the quasi-neutral base width, but also on the low field electron mobility, μn, in the neutral base region and the effective electron velocity, Sc, at the edge of base-collector heterojunction. μn and Sc are temperature-dependent parameters. A unity gain cut-off frequency of 10.6 GHz is obtained in AlGaN/GaN/AlGaN DHBTs and 19.1 GHz in GaN/InGaN/GaN DHBTs for a neutral base width of 0.05um. It is also shown that non-stationary transport is not required to study τb for neutral base width in the range of 0.05um for GaN-based HBTs.

Optimum Ge profile for base transit time minimization of SiGe HBT

Optimization of the trapezoidal Ge profile to minimize base transit time of the SiGe bipolar transistor was examined. A closed-form equation to find the optimum point of the trapezoidal Ge profile for base transit time minimization as a function of temperature is derived. Experimental data published in the literature are used to compare the model predictions with measurement.

Effects of base and emitter doping gradients on the electrical performance of heterojunction bipolar transistors

Abstract The analytical equations of heterojunction bipolar transistors with nonuniform doping in the base and emitter was developed. Device characteristics between linearly and uniformly doped HBTs was evaluated. The linearly doped HBT has higher current gain, lower base transit time, higher cutoff frequency, and lower emitter-base junction capacitance than those of the uniformly doped HBT.