Base Transit Time Research Articles

Base transit time of an epitaxial n +pn −n + bipolar transistor considering Kirk effect

First an analytical expression for the base transit time in an epitaxial n +pn −n + bipolar transistor considering the field-dependent mobility and the electron velocity saturation before the onset of the Kirk effect is obtained. The base transit time is found to increase at the high collector current density. Whereas the existing analytical models show decrease of the transit time with the increase of the collector current density. The base transit time with the Kirk effect is also studied with a simple equation for transit time obtained after modifying De Graaff–Kloosterman formula for collector current. The Kirk effect is found to influence the base transit time of a bipolar transistor with highly doped base significantly at large collector current density.

Large-signal modeling and characterization of high-current effects in InGaP/GaAs HBTs

High-current effects in InGaP/GaAs heterojunction bipolar transistors (HBTs) were modeled and characterized. In addition to the self-heating effect, high currents were found to degrade large-signal performance mainly through Kirk and quasi-saturation effects. New formalisms in terms of base transit time and base-collector diffusion capacitance were used to modify the conventional Gummel-Poon model. This new model was verified against large-signal characteristics measured at 2 GHz. The validity of the new model for HBTs of different emitter geometry was also explored.

Generalization of Moll–Ross relations for heterojunction bipolar transistors

Moll–Ross relations for the current flow through the base region of a bipolar transistor, and for the base transit time, have been generalized for heterojunction bipolar transistors with a nonuniform energy bandgap in the base region. The effect of both heavy doping and carrier degeneracy has been taken into account.

Modeling of SiGe-base heterojunction bipolar transistor with gaussian doping distribution

The results of numerical modeling of the base transit time and collector current of SiGe-base heterojunction bipolar transistors with a Gaussian base doping profile and two Ge profiles (linearly graded and box) are presented for the first time. The importance of including the dependence of minority carrier mobility on the drift field and the dependence of the effective density of states on the Ge concentration along the base is demonstrated through the analysis of base transit time and collector current. A function describing the decrease of the density of states product in strained SiGe layers with increasing Ge concentration is proposed.

Calculation of figures of merit of Si/Si 1− x− yGe xC y/Si HBTs and their optimization

Incorporation of C in the SiGe base of a heterojunction bipolar transistor (HBT) prevents outdiffusion of B across the junctions, improves film stability by reducing strain, and leads to better process integration. This paper investigates theoretically how the values of figure of merit such as current gain ( β), cut-off frequency ( f T) related inversely to the base transit time ( τ b), and the Early voltage ( V A) change with a small amount (<1%) of C incorporation in the base. For this purpose, we have derived generalized expressions for these quantities using a box-type, trapezoidal and triangular material (Ge and C) profile and an exponential doping profile. The variation of saturation-drift velocity with Ge mole fraction has been used to calculate τ b. A comparison between the calculated values, keeping the strain the same in both SiGe and SiGeC bases, indicates that all values of figure of merit are enhanced with C incorporation. The value of V A increases in SiGeC HBTs with small amounts of C because of prevention of B outdiffusion across the collector–base junction and sharp fall off of the C profile at the same junction.

An analytical model of the inverse base width modulation effect in SiGe graded heterojunction bipolar transistors

The analytical model of the collector current ideality factor degradation at high Vbe in modern SiGe graded base heterojunction bipolar transistors (HBTs) has been developed for the first time. It is subsequently used for the analysis of the inverse base width modulation (IBWM) effects in SiGe HBTs with respect to collector current degradation, current gain premature roll-off and SiGe base transit time. Comparing the IBWM effects calculated for HBTs with various base impurity concentration doping profiles and the Ge grading, we have found that the careful optimization of SiGe base parameters may substantially minimize negative influence of the IBMW effects on the HBT electrical parameters.

On the iterative schemes to obtain base doping profiles for reducing base transit time in a bipolar transistor

This paper shows that base doping profiles obtained using any iterative scheme for reducing the base transit time /spl tau//sub b/ in bipolar transistors for a given neutral base width must take into account the heavy doping effects implicitly. Comparing our results with those reported earlier, we demonstrate that if the heavy doping effects are not implicitly included in the iterative scheme it will result in a completely different base doping profile leading to an overestimation of base transit time and underestimation of base resistance.

Profile design considerations for minimizing base transit time in SiGe HBTs for all levels of injection before onset of Kirk effect

An iteration scheme to calculate the base transit time (/spl tau//sub b/) for a given collector current density is developed in order to determine the optimal doping profile and Ge profile in the neutral base for minimizing the /spl tau//sub b/ of SiGe HBTs under all levels of injection before the onset of the Kirk effect. We adopt a consistent set of SiGe transport parameters, tuned to measurement data, and include important effects such as the electric-field dependency of the diffusion coefficient and plasma-induced bandgap narrowing in our study. The scheme has been verified with simulation results reported in the literature. Our study shows that under both low and high injection, for a given Ge dose, intrinsic base resistance, and base concentration near the emitter, a retrograde doping profile with a trapezoidal Ge profile gives the minimum /spl tau//sub b/.

Comparative study of electron transit times evaluated by DD, HD, and MC device simulation for a SiGe HBT

Transit times of a silicon/germanium heterojunction bipolar transistor (HBT) with a base width of 24 nm are investigated in the quasi-stationary limit for the first time by consistent drift-diffusion (DD), hydrodynamic (HD), and fullband Monte Carlo (MC) simulations. The quasi-ballistic transport in the base and collector leading to a strong velocity overshoot is well described by the HD model and corresponding transit times are in good agreement with the MC results. On the other hand, the DD model fails in this region and substantially overestimates the base transit time bearing the possibility of wrong guidelines for transistor design optimization. However, since the base transit time is no longer dominating the cutoff frequency of high-speed HBTs, the failure of the DD model leads to an underestimation of the peak cutoff frequency by only 10%. Close to high injection differences in the emitter transit times of the HD and MC model are observed which are mainly related to small differences in the Gummel plot.

Analysis of bipolar junction transistors with a Gaussian base-dopant impurity-concentration profile

A method for a quantitative charge-control analysis of bipolar base-junction transistors with a Gaussian dopant impurity-concentration profile is demonstrated. Analytical expressions for the base transit time are given for two different Gaussian impurity-concentration profiles with the peak concentration at the edge, and within the quasi-neutral base layer. It is also shown that approximating the Gaussian profile by a simple exponential profile results in only an insignificant error in the charge-control analysis.

Optimum base-doping profile for minimum base transit time considering velocity saturation at base-collector junction and dependence of mobility and bandgap narrowing on doping concentration

We used variational calculus, the trial function, and the iterative procedure method to evaluate the optimum base doping concentration profile that will yield the minimum base transit time /spl tau//sub B/. All methods were extended to include the velocity saturation at the base collector junction, and the dependence of mobility and bandgap narrowing on the base doping concentration. We showed that all methods produce almost the same /spl tau//sub B/, although the profiles differ. Among them, the iterative method always produces the minimum /spl tau//sub B/ and variational calculus clearly shows the dependence of /spl tau//sub B/ on physical parameters.

New expression for base transit time in a bipolar transistor for all levels of injection

A new and compact formula for the base transit time in an npn bipolar junction transistor is derived. The collector current density and minority carrier charge within the base are separately expressed as a function of injected electron concentration in the neutral base in order to find an expression for base transit time. The modeling of collector current density, base stored charge and base transit time is essential for the design of high speed bipolar transistors. The derived expressions are applicable for arbitrary injection regions before the onset of the Kirk effect and they are simple and straightforward to give a physical insight into device operation.

Analyzing the switching behavior of ESD-protection transistors by very fast transmission line pulsing

This work describes, how the very fast transmission line pulsing (VFTLP)-technique can be used to characterize the switching behavior of ESD protection elements. In a first application we investigate the behavior of a protection element consisting of a lateral and vertical transistor part. This element shows a good ESD performance under 100 ns-TLP and HBM conditions. Under CDM relevant conditions, however, we could identify by means of VFTLP a delayed triggering of the vertical transistor part, which leads to an increased maximum voltage and thus to a low-failure threshold. In the second application we propose a methodology for the extraction of the base transit time parameter which improves the accuracy of a compact transistor model during turn on.

AlGaN/GaN heterojunction bipolar transistor structures-design considerations

The potential of III-nitride materials for the fabrication of bipolar transistors is investigated theoretically. Several different AlGaN/GaN n–p–n heterojunction bipolar transistor structures are examined through calculations of their band profiles and majority carrier distributions in equilibrium and in forward active mode. Spontaneous and piezoelectric polarization charges are utilized to create large hole sheet carrier densities in the base layer, thus minimizing the base spreading resistance. At the same time, a large accelerating field in the base can help reduce the base transit time of the electrons and, hence, increase the current gains of these devices. The temperature dependence of the hole concentration in the base is also investigated.

Base transit time in abrupt GaN/InGaN/GaN HBT's

Base transit time in an abrupt GaN/InGaN/GaN HBT is reported. Temperature and doping concentration dependence of low field mobility is obtained from an ensemble Monte Carlo simulation. Base transit time, /spl tau//sub b/, decreases with increasing temperature. The low temperature /spl tau//sub b/ is dominated by the diffusion constant or, in other words, transport within the neutral base region. However, at elevated temperatures base transit time is dependent more upon the base-collector junction velocity or, in other words, by the transport across the heterointerface. /spl tau//sub b/ increases with In-mole fraction showing a stronger dependence at lower temperatures. Unity gain current cut-off frequency, f/sub T/, is a strong function of temperature and base doping concentration. An f/sub T/ of 20 GHz is obtained for a 0.05 /spl mu/m HBT.

Low frequency conductance voltage analysis of Si/Ge/sub x/Si/sub 1-x//Si heterojunction bipolar transistors

Low-frequency-conductance-voltage (LFGV) method for analysis of heterojunction bipolar transistors (HBTs) is presented. The method gives accurate quantitative values for the important minority-carrier transport parameters that underlie the transistor performance, such as the base diffusion length, lifetime, diffusion coefficient and transit time. The method also allows a detailed analysis of the current gain and emitter injection efficiency. The analytical model and experimental methodology are demonstrated for a Si/Ge/sub x/Si/sub 1-x//Si HBT with a trapeziodal and linearly graded Ge profiles in the base. The LFGV method is general and can be applied to other bipolar transistors, including those based on III-V materials.

SiGe heterojunction bipolar transistor with 213 GHz fT at 77 K

An Si/Si0.65Ge0.35 abrupt heterojunction bipolar transistor with transit frequencies fT of 133 and 213 GHz at 300 and 77 K, respectively, is reported. The corresponding maximum oscillation frequencies fmax are 81 and 115 GHz. The fT of 213 GHz is the highest value yet reported for any silicon-based bipolar transistor. A detailed analysis of the intrinsic delay times reveals that the base transit time plays the dominant role.

Analytical base transit time of integrated bipolar transistors in quasi-saturation and hard saturation

Based on the assumption of negligible recombination within the thin epitaxial collector layer of an integrated bipolar transistor switch in quasi-saturation, solutions to the collector minority carrier profile and transit time in the induced base are derived. In contrast to Dai and Yuan's analysis (1997), the present analysis takes both the drift and diffusion currents into account and is valid for all levels of injection. Dependence of transit time on characteristics of the epitaxial-substrate interface and recombination at the interface is studied for the transistor driven into hard saturation. At high effective surface recombination velocity, recombination at the interface cannot be neglected. The study shows that transit time increases more rapidly with collector current when the transistor operates in hard saturation and the interface is highly reflecting.

Design of AlGaN/GaN Heterojunction Bipolar Transistor Structures

ABSTRACTThe potential of III-nitride materials for the fabrication of bipolar transistors is investigated theoretically. Several different pseudomorphic AlGaN/GaN n-p-n heterojunction bipolar transistor structures are examined through calculations of their band profiles and majority carrier distributions in equilibrium. Spontaneous and piezoelectric polarization charges are utilized to create large hole sheet carrier densities in the base layer, thus minimizing the base spreading resistance. At the same time, a large accelerating field in the base can help reduce the base transit times of the electrons and, hence, increase the current gains of these devices. The effect of strain due to substrate mismatch is also investigated.

High-performance, graded-base AlGaAs/InGaAs collector-up heterojunction bipolar transistors using a novel selective area regrowth process

Graded-base AlGaAs/InGaAs collector-up heterojunction bipolar transistors (C-up HBTs) were successfully fabricated using a novel selective area regrowth process to reduce the base resistance and their dc and microwave performances were evaluated. The base is compositionally graded to provide a quasi-built-in field which decreases the base transit time for high-frequency response and increases the base transport factor at low-temperature operation. A unity-gain cutoff frequency f/sub T/=55 GHz and a maximum frequency of oscillation f/sub MAX/=74 GHz for the C-up n-p-n HBT, and an f/sub T/=48 GHz and an f/sub MAX/= 39 GHz for the C-up p-n-p HBT were obtained for devices with a 5-/spl mu/m/spl times/10-/spl mu/m collector area. The nonself-aligned C-up HBT's reported here show great promise for future high-speed C-up complementary bipolar IC's.