Base Transit Time Research Articles

High-Breakdown-Voltage Submicron InGaAs/InP Double Heterojunction Bipolar Transistor with ft = 170 GHz and fmax = 253 GHz

The layer structure of InGaAs/InP double heterojunction bipolar transistor (DHBT) is designed to enhance the frequency performance and breakdown voltage. The composition-graded base structure is used to decrease the base transit time. The InGaAs setback layer and two highly doped InGaAsP layers are used to eliminate the conduction band spike of the collector. The submicron-emitter InGaAs/InP DHBT is fabricated successfully. The base contact resistance is greatly decreased by optimization of contact metals. The breakdown voltage is more than 6 V. The current gain cutoff frequency is as high as 170 GHz and the maximum oscillation frequency reached 253 GHz. The DHBT with such high performances can be used to make W-band power amplifier.

A STUDY ON TRANSIT TIME FLUCTUATION, AND CORRELATION BETWEEN LOW FREQUENCY PHASE AND CURRENT OR AMPLITUDE NOISES IN A TRANSISTOR

The low frequency phase and amplitude or average collector current noises are studied in a bipolar transistor. The transit time fluctuation is calculated from the diffusion coefficient fluctuation in the base. The phase noise is proportional to the base transit time fluctuation of the carriers. We discuss the relation between the phase and amplitude fluctuations and find both fluctuations correlate each other in low frequency. An experimental study is carried out in a bipolar transistor.

An iteration approach for base doping optimization to minimize the base transit time in triangular-Ge-profile SiGe HBTs

This work presents a fixed-point iterative approach to optimize the base doping profile inside the base of triangular-Ge-profile SiGe HBTs to achieve the minimum base transit time. The results show a consistent reduction in base transit time with increasing slope of triangular-Ge-profile in base region in conjunction with an optimized base doping profile. Moreover, the optimum base doping profile shows a continuous reduction in base Gummal number for an increasing Ge-slope inside base. The iterative methodology is extended to incorporate the shifted-Ge-profile approach in base to achieve further reduction in base transit time.

Continuity-Equation Analysis of Hot Electron Base Transport in InP/InGaAs Heterojunction Bipolar Transistors

The static and small-signal properties of the hot electron base transport in InP/InGaAs heterojunction bipolar transistors are analyzed theoretically within the framework of the continuity equation. By introducing a probability distribution function for a hot electron in the base, which is a continuous function of position, the nonequilibrium electron transport is integrated into the conventional drift-diffusion theory. The dc analysis of the continuity equation shows that the proposed theory is feasible for the seamless description of ballistic electron transport in thin bases and diffusion-like electron transport in thick bases. In the small-signal ac analysis, an analytic expression for the base transport factor is derived taking into account the hot electron transport. Moreover, a simple expression for the base transit time in the presence of hot electrons is obtained through the analysis of the derived base transport factor. The expression is given as a linear combination of the transit time for ballistic transport of hot electrons and the transit time for diffusive transport of energy-relaxed electrons.

Base transit time of a bipolar transistor considering field dependent mobility

An empirical expression for the base transit time, τb , of a bipolar transistor with exponential base doping profile is derived considering doping and field dependence of mobility. In the analysis, bandgap-narrowing effect, high injection effect and carrier velocity saturation at the base edge of the collector-base junction are also incorporated. The collector current density, Jc , and minority carrier stored charge, Qb , in the base are separately expressed as a function of the injected electron density no in the base in order to find an empirical expression for τb . The modelling of Jc , Qb and τb is essential for the design of a modern bipolar transistor. The base transit time calculated analytically is compared with simulation and numerical results in order to demonstrate the validity of the assumptions made in deriving the expression. The base transit time is found to be different if the field dependent mobility is considered.

Base-transit-time model considering field dependent mobility for BJTs operating at high-level injection

An analytical model for the base transit time tau <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> for an exponentially doped base is developed assuming a small change in electron concentration in the base of a bipolar junction transistor at high injection from its low injection value. The model is valid in all levels of injection before the onset of the Kirk effect. In this analysis, bandgap-narrowing effect, high-injection effect, and carrier velocity saturation at the base edge of the base-collector junction, and also doping and field dependence of mobility, are incorporated. The base transit time calculated analytically is compared with simulation and numerical results, and also with experimental data in order to demonstrate the validity of the assumptions made in deriving the expression. The base transit time is found to be different if the field dependent mobility is considered

Improved VBIC model for SiGe HBTs with an unified model of heterojunction barrier effects

An improved bipolar transistor model considering heterojunction barrier effect (HBE) in SiGe double heterojunction bipolar transistors is developed. The effect of barrier formation due to high level injection, which is related to the rapid degradations of the dc current gain (/spl beta/) and cutoff frequency (f/sub T/), is carefully investigated and analyzed. As the collector current becomes high, the conduction band barrier is induced and increased. It causes the saturation of collector current (J/sub C/) due to the blocking of carrier transport, the sharp increase of base transit time (/spl tau//sub B/) due to the additional charge storage, the increase of base current (J/sub B/) due to the increased recombination, and the decrease of intrinsic base resistance (R/sub bi/) due to the increased charge and base pushout. Those phenomena are included into a vertical bipolar intercompany model (VBIC) compact model by employing a unified model of the HBE on J/sub C/, J/sub B/, /spl tau//sub B/, and R/sub bi/. Furthermore, portions of /spl tau//sub B/ and R/sub bi/ from the Kirk effect itself are modeled according to the high current model description and the new formulation of widened base, respectively. A full extraction of parameters has been performed and the modified VBIC model is applied. The modeling accuracy is significantly improved at the high current region for the dc and RF characteristics.

Promoting emitter diffusion process and optimization of vertical profiles for high-speed SiGe HBT/BiCMOS

A high-temperature anneal-resistant process, which enables high-speed SiGe HBTs to embed scaled CMOS, is optimized in SiGe BiCMOS technology. This process, called promoting emitter diffusion (PED), is based on enhanced phosphorous diffusion from poly-Si emitter electrodes at high temperature to fabricate thin base layers and shorten the base transit time. By investigating the dependence of high-frequency performance on diffusion temperature, as-grown base layer thickness, and Si cap thickness, the methodology for PED optimization was yielded. In addition, this PED process is effective in reducing an extrinsic base resistance due to deep boron diffusion from poly-Si base electrodes. This indicates that the PED process is very effective at improving the tradeoff relationship between cutoff frequency f/sub T/ and maximum oscillation frequency f/sub max/ in self-aligned SiGe HBTs using selective epitaxial growth. As a consequence, both f/sub T/ and f/sub max/ of more than 200GHz were successfully obtained.

Analytical model for base transit time of a bipolar transistor with Gaussian-doped base

The present analysis for base transit time τ b of a modern high-speed npn bipolar transistor is done for Gaussian-doped base considering doping dependence of mobility, bandgap narrowing effect and carrier velocity saturation effect at base–collector junction. First the minority carrier current density and electron density equations incorporating all the effects are analytically solved. The collector current density J c and base stored charge Q b are then separately expressed as a function of the injected electron density n(0) in the base in order to find base transit time. The modeling of J c, Q b and τ b are essential for the design of high-speed bipolar transistor. The equations are applicable for low level of injection. The base transit time calculated analytically is compared with numerical results in order to demonstrate the validity of the assumptions made in deriving the equations. The closed form equations for collector current density and base transit time offer a physical insight into device operation and are a useful tool in device design and optimization.

Optimal Doping Profiles via Geometric Programming

We first consider the problem of determining the doping profile that minimizes base transit time in a (homojunction) bipolar junction transistor. We show that this problem can be formulated as a geometric program, a special type of optimization problem that can be transformed to a convex optimization problem, and therefore solved (globally) very efficiently. We then consider several extensions to the basic problem, such as accounting for velocity saturation, and adding constraints on doping gradient, current gain, base resistance, and breakdown voltage. We show that a similar approach can be used to maximize the cutoff frequency, taking into account junction capacitances and forward transit time. Finally, we show that the method extends to the case of heterojunction bipolar junction transistors, in which the doping profile, as well as the profile of the secondary semiconductor, are to be jointly optimized.

High-Performance InGaP/GaAs HBTs With Compositionally Graded Bases Grown by Solid-Source MBE

N-p-n InGaP/GaAs heterojunction bipolar transistors (HBTs) with compositionally graded In/sub x/Ga/sub 1-x/As (Be doped) bases have been successfully grown by solid-source molecular beam Epitaxy (SSMBE) using a gallium phosphide (GaP) decomposition source. In this paper, the dc and RF characteristics of HBTs with different indium mole fractions in the graded In/sub x/Ga/sub 1-x/As base (x:0 /spl rarr/ ;0.1 and x:0 /spl rarr/ 0.05) are measured to investigate optimum-grading profiles. The measured average current gains, /spl beta/s of a control sample, a 10% graded-base sample and a 5% graded-base sample, are 162, 397 and 362, respectively. To our knowledge, these current gains are the highest values ever reported in compositionally graded-base InGaP/GaAs HBTs with a base sheet resistance R/sub sh/ of /spl sim/200 /spl Omega//sq establishing a new benchmark for InGaP/GaAs HBTs. Furthermore, these compositionally graded-base HBTs show higher unity current/gain cutoff frequency, f/sub T/ and maximum oscillation frequency, f/sub max/. Compared to the control sample with the same base thickness, the base transit time /spl tau//sub B/ of the graded sample is reduced by /spl sim/15% to /spl sim/20% by the induced built-in potential, resulting in an increase of f/sub max/ from 16 to 18.5 GHz in a device with an emitter size of 10/spl times/10 /spl mu/m/sup 2/. Additionally, for the 5% graded-base sample, with a 5/spl times/5 /spl mu/m/sup 2/ emitter region, f/sub T/ and f/sub max/ are 16.3 and 33.8 GHz, respectively, under low-level collector current. These results demonstrate that InGaP/GaAs HBTs with In/sub x/Ga/sub 1-x/As graded-base layers (x:0 /spl rarr/ 0.05) have the potential for high-speed analogue to digital converters.

Effects of self-heating on the microwave performance of SiGe HBTs

In this study, the effects of self-heating on the f T and f max frequencies have been investigated in NpN SiGe heterojunction bipolar transistors (HBTs) by two-dimensional numerical device modeling using a commercial simulator. The device studied was a single emitter stripe, double mesa configuration with self-aligned base contacts where a Gaussian distribution was assumed for the base’s boron profile. Increases in the base transit time and the collector delay time due to degradation in the electron mobility and saturation velocity and an increase in the base-collector depletion layer width were found to degrade the f T while increased base resistance also contributed to the reduction in f max. The onset of significant device self-heating and degradation in f T and f max were observed for collector current densities of 5 × 10 4 A/cm 2 for V CE = 2 V.

Determining GaInP/GaAs HBT Device Structure by DC Measurements on a Two-Emitter HBT Device and High Frequency Transit Time Measurements

A method to monitor the GaInP/GaAs HBT device structure including emitter ledge thickness is demonstrated in this paper. The base thickness and base doping density are obtained through base transit time and base sheet resistance measurements while the base transit time is measured through the cut-off frequency measurements at various bias points. A large size two-emitter HBT device is used to measure the ledge thickness. Emitter doping profile and collector doping profile are obtained by the large size HBT device through C-V measurements. An FATFET device formed by two emitters as drain and source terminals and the interconnect metal as the on-ledge Schottky gate between two emitters is used to measure the ledge thickness.

Empirical expression for base transit time in bipolar transistors

A new and compact formula for the base transit time, τ b , of a modern high speed npn bipolar transistor with exponential base doping profile is derived considering doping dependence of mobility, bandgap narrowing effect, high injection effect and carrier velocity saturation at the base edge of the collector–base junction. The collector current density, J c , and minority carrier stored charge, Q b , in the base are separately expressed as a function of the injected electron density n o in the base in order to find an empirical expression for τ b . The modelling of J c , Q b and τ b is essential for the design of high-speed bipolar transistor. The expressions are applicable for arbitrary injection before the onset of the Kirk effect and they are simple and straight forward to give insight into device operation. The base transit time calculated analytically is compared with simulation results in order to demonstrate the validity of the assumptions made in deriving the expression. The closed form expressions for collector current density and base transit time offer a physical insight into device operation and are a useful tool in device design and optimization.

Optimal Ge profile design for base transit time of Si/SiGe HBTs

The steep Ge profile in the SiGe base yields the tradeoff between a large accelerating field in the base region, and a low saturation velocity in the base region near the collector. The Ge mole fraction, electric field and doping concentration dependence of minority electron mobility is obtained from the Monte Carlo simulation. An analytical analysis of the base transit time is given and compared with energy balance simulation with consistency. The high Ge content at the base–collector junction produces large strain in the base and requires the low thermal budget process to avoid the dislocation generation, but does not always reduce the base transit time due to the low saturation velocity in the high Ge content region. The minimum Ge content needed at the base–collector junction of graded SiGe HBTs can be predicted by our analysis.

Reduction of base-transit time of InP-GaInAs HBTs due to electron injection from an energy ramp and base-composition grading

The dc current gain of InP-GaInAs heterojunction bipolar transistors with varying base thickness and composition was measured. Much larger composition grade values than previously reported were achieved using strain compensation. A simple two-parameter Monte Carlo simulation was developed to interpret the results. The simulation yields an accurate plot of the base transit time versus base thickness. Clear evidence for the reduction of base transit time due to hot electron injection was observed in devices with thin uniform bases. The current gain of 45-nm-thick graded base devices saturated as the grading was increased beyond standard values. Grading did not increase the gain of devices with a 20-nm-thick base.

Base transit time of graded-base Si/SiGe HBTs considering recombination lifetime and velocity saturation

The base transit time of Si/SiGe heterojunction bipolar transistor is analyzed including the effects of minority carrier recombination lifetime and velocity saturation. The reduction of recombination lifetime in the neutral base region increases the base transit time as compared to the infinite recombination lifetime, and the finite saturation velocity also degrades the base transit time, as compared to the infinite saturation velocity. This analytical analysis can obtain the optimum design of Ge profiles in the base to minimize the base transit time. The extremely heavy doping of the base can degrade the base transit time significantly if the base width is larger than the diffusion length.

Modelling of base transit time in Si/Si1−y−zGeyCz/Si HBTs and composition profile design issue for its minimization

From a practical point of view, incorporation of a small amount of C in the base of Si/SiGe/Si HBT eliminates the problems of B outdiffusion and of lattice mismatch. We have developed in this paper a generalized model for the calculation of base transit time τb for such HBTs. In our model, we have considered the effect of emitter–base and collector–base depletion widths and the variation of saturation-drift velocity in the base–collector depletion region with molar Ge content. It is observed that τb becomes smallest for triangular material (Ge and C) profiles as compared to box type and trapezoidal profiles. In addition the triangular Ge profile with C codoping does not affect film stability and device performance. Our calculation shows that for the same value of strain between Si and SiGe and between Si and SiGeC, the latter device yields the smaller value of τb.

Measurement of base and collector transit times in thin-base InGaAs/InP HBT

The influence of base thickness reduction on performances of heterojunction bipolar transistors (HBTs) is examined. HBT structures are grown, with a base thickness in the range 25-65 nm and doping concentration from 3/spl times/10/sup 19/ to 6/spl times/10/sup 19/ at/cm/sup 3/. Base transit time is accurately extracted from total base-collector transit time, and described using a simple drift-diffusion approach. This model, however basic, shows very good agreement with measurements when usual parameter values are used. A 0.13-ps transit time reduction is measured when thinning the base from 65 to 25 nm. The thinnest base structure presents a 0.08 ps transit time, allowing a 250 GHz f/sub t/ operation at 270 kA/cm/sup 2/ emitter current density.

Induced base transit time of an epitaxial n +pn −n + bipolar transistor in saturation

Minority carrier recombination, drift and diffusion currents are considered in determining induced base transit time of an epitaxial n +pn −n + bipolar transistor operated in quasi-saturation and hard-saturation. The dependence of transit time upon the characteristics of the epitaxial–substrate interface is also studied for the transistor driven into hard saturation. The study shows that transit time in the induced base is significant when the transistor is driven into hard saturation and the interface is highly reflecting. The induced base transit time calculated analytically is compared with numerically obtained results in order to demonstrate the validity of the assumptions made in deriving analytical expressions. The closed form expressions for collector current density and transit time offer a physical insight into device operations at various bias conditions and are a useful tool in device design and optimization.