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

An iteration scheme to calculate the base transit time (τ_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 τ_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 τ_b     

Optimum base doping profile for minimum base transit time

The author investigates the base doping profile which gives the minimum base transit time. The peak base concentration is selected so that the base resistance is the same at any base width for all profiles: box, Gaussian, and exponential. The exponential profile always gives the minimum base transit time at any base width. Thus this profile is the best if there is no other base-concentration-limiting factor, which is the case for heterojunction-bipolar transistors. The shallowest base width was achieved with the box profile, and the base transit time at this base width is smaller than that of other profiles at each profile's allowable base width. Consequently, for homojunction transistors, the optimum base doping profile changes from exponential to Gaussian and from Gaussian to box as the base width decreases. The author also investigated the base transit time of epitaxial base transistors with a 50-nm base width and found that the base transit time increases by a factor of 1.5 when the peak base concentration is located at a depth of 20 nm     

Analytical Modeling of Base Transit Time of SiGe HBTS Including Concentration Dependent Bandgap Narrowing Effect

Heterojunction Bipolar Transistors with SiGe base and Si emitter and collector have increasingly become important in high speed applications in electronics due to better performance of these devices with a modest increase in complexity of fabrication process.Speed of these devices is mainly determined by transit time of minority carriers across the device.Base transit time is the most important component of the total transit time.An analytical model is developed here to predict the variation of base transit time with Ge content,base doping concentration,temperature,and other device parameters.Studies have been made for both uniform and exponential doping distributions with different Ge profiles in the base region.Band gap narrowing effect due to high doping concentration is also taken into account in the model.     

Effect of base profile on the base transit time of the bipolartransistor for all levels of injection

Comparing the uniform base profile with the exponential base profile in low injection, the uniform base profile gives a lower base transit time for a given base resistance and peak base concentration, while the exponential base profile gives a lower base transit time for a given base resistance and base width at large base widths. At high injection the uniform base profile always gives the minimum base transit time. The uniform base doping is the optimal base profile for BiCMOS circuits in which the bipolar transistors are operated under high injection     

Reduction of ft by nonuniform base bandgapnarrowing

Analytical and simulation results are presented to illustrate qualitatively the effect of doping on base transit time. Nonuniform base bandgap narrowing (BGN) in silicon bipolar transistors can give rise to an electric field that is comparable to and against the built-in field. The base transit time τ is subsequently increased, leading to a deterioration of the cutoff frequency f₁. It is shown that the BGN effectively reduces the impurity profile grading factor K and subsequently the transit-time coefficient η. Physically, the minority carriers can be thought of as moving in a new profile characterized by a reduced η but in the absence of BGN. Unlike earlier investigations which also consider effective BGN dopings but ignore the field effects, this treatment includes their impact on the minority-carrier base transit time. For a steep exponential profile with strong BGN, an increase of η by a factor 3.57 at 300 K is calculated. Device simulations predict a smaller f_t reduction factor of 1.5 for more general profiles     

Effect of hot-electron injection of high-frequency characteristicsof abrupt In0.52(Ga1-xAlx)0.48As/InGaAs HBT's

The effect of hot-electron injection energy (E_i) into the base on the high-frequency characteristics of In_0/52(Ga_1-xAl_x)_0.48 As/InGaAs abrupt heterojunction bipolar transistors (HBTs) is investigated by changing the composition of the emitter. There exists an optimum E_i at which a maximum current gain cutoff frequency (ft) is obtained. Analysis of hot-electron transport in the base and collector by Monte Carlo simulation is carried out to understand the above phenomenon. The collector transit time (τ_c ) increases with E_i, because electrons with higher energy transfer from the Γ valley into the upper L and X valleys. At first, the base transit time (τ_b ) decreases with E_i at the low E_i region. However, τ_b does not decrease monotically with E_i, because of the nonparabolicity in the energy-band structure of InGaAs. Consequently, there exists a minimum in the sum of τ_b and τ_c , in other words a maximum f_t, at an intermediate value of E_i     

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, τ_b, decreases with increasing temperature. The low temperature τ_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. τ_b increases with In-mole fraction showing a stronger dependence at lower temperatures. Unity gain current cut-off frequency, f_T, is a strong function of temperature and base doping concentration. An f_T of 20 GHz is obtained for a 0.05 μm HBT     

Significant time constants defined by high-current charge dynamicsin advanced silicon-based bipolar transistors

Analytical expressions for the time constants in advanced silicon-based bipolar transistors defined by the charge dynamics in the base-collector junction space-charge region (τ_C) and by the charge modulation in the quasi-neutral base (τ_BM) are derived based on an accounting for the high-current-induced perturbation of the space-charge region. The derivations show that voltage drops in the intrinsic and extrinsic collector regions and in the extrinsic emitter region are important in defining τ_C and τ_BM, and that τ_BM is approximately proportional to collector-current density. Application of the results to an aggressive SiGe-base HBT technology shows that τ_C and τ_BM are comparable to the base transit time, and hence that they are significant in defining high-current speed of the HBT     

SiGe HBT基区渡越时间研究

对基于SiGe HBT基区本征载流子浓度和电子迁移率依赖于掺杂、基区Ge组分分布和速度饱和效应的基区渡越时间进行了研究。结果表明,相同Ge组分条件下,基区渡越时间bτ随WB由100 nm减薄到50 nm,降低了74.9%;相同WB,Ge组分为0.15比0.1 Ge组分的bτ减小了33.7%。该研究与其他文献的结果相吻合,可为SiGe HBT基区设计提供一定的理论指导。     

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.     

Analysis of device parameters for pnp-type AlGaAs/GaAs HBTsincluding high-injection using new direct parameter extraction

Device parameters of the small-signal T equivalent circuit for pnp-type AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are obtained using a new direct parameter extraction technique. These parameters are analyzed not only under the low-current conditions but also under high-current conditions so as to understand the RF-performance fall-off after base pushout occurs. In this analysis, the intrinsic and extrinsic small-signal parameters which affect RF performance are directly determined using several steps without numerical optimization in order to properly analyze device parameters. The T equivalent circuit model determined by the method shows excellent agreement with the mean errors of 3.5-6.9% under both low-and high-current conditions. The analysis showed that the intrinsic transit time, which is the sum of the base transit time (τ_b) and the collector depletion layer transit time (τ_c), small-signal emitter resistance (r_e), small-signal base resistance (r_b) and collector-base capacitance (C_BC) all increase under high-current conditions. In addition, we found that the intrinsic transit time is the dominant parameter for the fall-off of the cut-off frequency (f_t) under high-current conditions, and there is little effect of r_b and C_BC in the fall-off of the maximum oscillation frequency (f_max) under high-current conditions. Judging from these results, device parameters are successfully obtained under a wide current range by a new parameter extraction technique and circuit modeling for HBTs under a wide current range can be achieved using the small-signal T-equivalent circuit     

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.     

Delay time analysis of submicron InP-based HEMT's

The intrinsic delay time of submicron InP-based HEMT's has been evaluated by coupling the delay time analysis with a 2D Ensemble Monte Carlo Simulation. The relationship between the delay time and the transit time is explained. It is shown that the delay time can be quite different from the transit time depending on the velocity modulation. The delay from each segment of the HEMT is calculated to study the distribution of the delay inside the device. The delay from the gate region was the major contributor while that from the drain region was also important. The bias dependence of the delay in each region of the device was calculated to explain the bias dependence of the total intrinsic delay time. The intrinsic delay time increase at low V_gs was due to the increase of τ_d and τ_s and the increase at high V_ds was due to the increase of τd. As a means of validation, the simulated data have been compared with experimental intrinsic delay time data at various bias points. Good agreement was found over a wide V_gs and V _ds range     

On the operation configuration of SiGe HBTs based on power gain analysis

The power gain difference, under different device stability conditions, between common-emitter (CE) and common-base (CB) bipolar junction transistors (BJT) is analyzed comprehensively. The analysis reveals that the CB configuration offers higher maximum available power gain than the CE configuration in the device's high operation frequency range, while the inverse relation holds in the very low frequency range. In the intermediate frequency range, the base resistance value, mainly affected by the base doping concentration, determines which configuration offers higher maximum stable power gain (MSG). These analyses have explicit implications on the operation configurations of SiGe heterojunction bipolar transistors (HBTs). Employing a typical doping profile of Si bipolar junction transistors with a trapezoidal Ge profile in SiGe HBTs usually results in a larger base resistance than the emitter resistance. For these devices, the CE configuration exhibits higher MSG than the CB configuration. Employing a higher base doping concentration than the emitter with a box-type Ge profile considerably reduces the base resistance and thus favors the CB configuration for power amplification in this frequency range. The analysis are quantitatively verified with simulation and measurement results from SiGe HBTs of representative Ge and base doping profiles.     

Emitter and base transit time of polycrystalline silicon emittercontact bipolar transistors

The author formulates transit time in the neutral emitter region, τ_E, and in the neutral base region τ_B, of polycrystalline silicon emitter contact bipolar transistors. An analytical theory derived for τ_E of polysilicon emitter contact bipolar transistors and its dependence on the emitter junction depth, the polysilicon thickness, and the base width are presented. The influence of bandgap narrowing on τ_E and τ_B is analyzed. Bandgap narrowing increases τ_E , but τ_B is insensitive to it. τ_E is proportional to base width W_B and τ_B to W²_B. τ_E is not negligible compared to τ_B when W_B is less than 100 nm. Reducing emitter junction depth and polysilicon thickness is indispensable to developing shallow base bipolar transistors     

Optimum germanium profile for SiGe/int

The thermal stability of an SiGe base HBT is closely related to the integrated Ge content in the base. It is therefore appropriate to consider what form the Ge profile should take to minimise the base transit time tau /sub B/ for a given total Ge content. It is shown that a linear increase in Ge content from the base-emitter to the base-collector junction is required to minimise.<>     

High-low polysilicon-emitter SiGe-base bipolar transistors

Self-aligned heterojunction bipolar transistors with a high-low emitter profile consisting of a heavily doped polysilicon contact on top of a thin epitaxial emitter cap have been fabricated. The low doping in the single-crystal emitter cap allows a very high dopant concentration in the base with low emitter-base reverse leakage and low emitter-base capacitance. The thin emitter cap is contacted by heavily doped polysilicon to reduce the emitter resistance, the base current, and the emitter charge storage. A trapezoidal germanium profile in the base ensures a small base transit time and adequate current gain despite high base doping. The performance potential of this structure was simulated and demonstrated experimentally in transistors with near-ideal characteristics, very small reverse emitter-base leakage current, and 52-GHz peak f_max, and in unloaded ECL and NTL ring oscillators with 24- and 19-ps gate delays, respectively     

Optimum impurity distribution for minimum base transit time in junction transistors

Considering the effect of field dependent carrier mobility, an optimum impurity doping distribution for minimum base transit time has been obtained using variational methods. It is shown that the exponential doping distribution in the base region offers minimum transit time.     

Reduction of the current gain of the n-p-n transistor component ofa thyristor due to the doping concentration of the p-base

The reduction of the current amplification factor of a wide-base transistor, with growing doping concentration in the base region, is investigated. A method for the determination of the minority-carrier lifetime τ_n in the base region and the emitter Gummel number G_e is developed. The method is based on transistor structures differing only in the base width. It was found that the lifetime τ_n decreases according to the power law τ_n~N^-0.45_A. This result is analyzed for different recombination processes. Good agreement is obtained if shallow impurities acting as recombination centers are assumed. The injection-limited current gain β_γ decreases significantly with an increase in the total number of the doping concentration of the base, reaches a broad maximum, and then falls slowly. The maximum value of G_e is found to be 1.1×10¹⁴ cm^-4-s in good agreement with theoretical results. Finally, the contribution of the injection efficiency γ and the transport factor α_T to the current gain α are determined. It is found that α is limited mainly by the injection efficiency γ     

Base transport factor calculations for transistors with complementary error function and Gaussian base doping profiles

By taking account of the carrier mobility degradation at high impurity concentrations, the high-frequency base transport factor of an n-p-n germanium base transistor was computed numerically for different base doping levels. The doping profiles under consideration were Gaussian and complementary error functions. The base doping level adjacent to the emitter was optimized for minimum base transit time. The optimum values are 4×10¹⁷atom/ cm³for complementary error function profile and 2×10¹⁷atom/cm³for Gaussian profile. The effects of emitter barrier capacitance, base resistance, collector barrier capacitance, and the collector depletion layer on the overall frequency response of a junction transistor are also discussed.