Modifications of equivalent circuits for narrow-base transistors

Conventional equivalent circuits for the large-signal and small-signal behavior of bipolar transistors are all based on the assumption that carriers flow in the base region by drift and diffusion. For a very narrow base, this assumption becomes questionable, and the equivalent circuits based on it need modification. We find that the interconnections of circuit elements conventionally used still apply, independent of base width; but that the functional dependence of certain parameters of the equivalent circuits change as the base becomes narrower. For the limiting case of very narrow bases, we show that the Early effect vanishes; and we give expressions for the functional dependencies of Beta, saturation current, forward charge-control time constant, transconductance, and gain-bandwidth product. All of these parameters derive less benefit from decreasing base width than conventional theory predicts.     

Theory and small signal analysis for a new bipolar injectiontransit time device (BIPOLITT)

We have proposed and analyzed a new millimeter and submillimeter wave transit time device, the bipolar injection and transit time diode (BIPOLITT). The theory and small signal analysis for a two-terminal type rf operation is presented here. The device structure is similar to a heterojunction bipolar transistor (HBT) with an rf floating base. The relatively longer base is utilized to achieve an injection-phase delay close to 180°. The collector depletion region is used as drift region. As an example, a BIPOLITT diode for 300-400 GHz operation is designed, and its small signal specific negative resistance is calculated to be about -3×10^-7 Ω·cm²      

Diffusion near an absorbing boundary

Using a microscopic model for collisions, an analysis is given of the diffusion of minority carriers in the field free base of a transistor, in the proximity of the base/collector junction where the carrier density gradient and the electric field vary strongly over a mean free path. This is done both on an idealised model which assumes that no carriers going into the junction return to the base, (perfect sink) and on a more realistic model which takes an approximate account of backscattering. In the case of the idealised model a diffusion equation is obtained which contains a new term due to the sink. The minority carrier density n₀ at the boundary is limited by the thermal energy of the carriers, and the density gradient is increased in the immediate vicinity of the sink. This increase has, however, a very small effect on the value of the carrier densities and on the base transit time. The value n₀ of the carrier density at the boundary becomes larger in the presence of backscattering from the junction. Calculations show that n₀ lies within 25–50 per cent of the value obtained from the standard transport equation in the base/collector junction, which includes carrier heating effects. A more detailed model of backscattering would however be desirable to confirm the latter results.     

Space charge limited and emitter current limited injections in space charge region of semiconductors

If, in a space charge region of a semiconductor device, a unipolar drift current is a main component of the total current, an analogy with motion of electrons in vacuum exists. In the space charge limited emission, injection of minority carriers is limited by space charges, as in a punched-through p-n-p diode. On the other hand, in a transistor, minority carriers are injected into the depletion layer between the base and the collector, after diffusion in the neutral base region. This injection is limited by emitter current, and this mode of injection corresponds to a temperature limited emission in a vacuum tube. In this paper, equations of minority carrier impedances will be calculated in both these cases, and negative resistances can be expected at some transit angles. Then a generalized equation, which includes these two modes of injections as special cases, will be obtained. This generalized equation corresponds to the Llewellyn-Peterson equation of vacuum tubes, which include the space charge limited emission and the temperature limited emission as special cases.     

一种考虑复合电流的SiC LBJT行为模型改进

介绍了一种考虑基区SiC/SiO₂界面处复合电流的SiC LBJT改进模型。分析了横向碳化硅双极结型晶体管与其垂直结构之间的区别,将横向BJT的外延层和半绝缘机构等效为衬底电容。再引入一个平行于SiC BJT基极结的附加二极管来描述复合电流,以垂直SiC BJT的SGP模型为基础建立SiC LBJT行为模型。校准了LBJT模型的基区渡越时间,模型与实际器件的开关特性接近吻合。相较于未考虑复合电流的LBJT模型,改进后的模型输出特性曲线与实测数据精度误差较小。该模型可以较精确地描述受复合电流影响的LBJT器件行为。     

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.     

Transit time in the base region of drift transistors considering diffusion, recombination, and variable built-in electric field

The present work gives detailed analysis of the transit time of injected minority carriers in the base region of drift transistors, taking into consideration the effect of recombination as well as the variations in the built-in electric field. Some useful impurity-density distributions in the base region are analyzed in detail. Graphs of transit time for these distributions under different conditions are given. The work also includes the distribution that yields minimum transit time for a specified field parameter.     

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.     

Bipolar transistor: Two-dimensional effects on current gain and base transit time

Two-dimensional effects in bipolar devices with small dimensions are calculated analytically if simplifying assumptions are made e.g. uniformly doped regions and zero recombination. In this paper we treat effects of the emitter sidewalls on the current gain and the base transit time.

The current flows through the base and emitter are calculated for rectangular as well as curved emitter-base junctions. Current gain and base transit time are analysed as local variables, i.e. functions that vary along the 2-D emitter-base junction. In this way it can be seen where and how several parameters influence the overall performance of the device.

The total current gain is modified by the sidewalls; it increases if the distance between the emitter-base junction and the emitter metallisation is increased. However, this distance cannot be increased indefinitely because of finite recombination lifetimes.

The diffusion capacitance is strongly affected by the sidewalls. Because the sidewalls carry a considerable part of the current the transit time is less affected. However, the influence of the sidewalls is still noticeable. It is expected that in a transistor with a 1 μm wide and 0.15 μm deep emitter the transition frequency is already negatively influenced by the emitter sidewalls.     

Impact of compositionally graded base regions on the DC and RF properties of reduced turn-on voltage InGaP-GaInAsN DHBTs

Built-in drift fields are employed to enhance the performance of GaAs-based heterojunction bipolar transistors (HBTs) with reduced turn-on voltage. Specifically, we explore in detail the dc and RF device property improvements enabled by using compositionally graded GaInAsN base layers. Experimental results are compared to predictions of the standard drift-diffusion base transport model employing a finite exit velocity. In large area devices, graded base samples with built-in fields of /spl sim/7 kV/cm (i.e. 40 meV over 500 /spl Aring/) typically have a dc current gain 1.8/spl times/ larger than constant base composition samples. In small area devices, the peak cut-off frequency is typically 10%-15% higher than constant composition samples. These results are shown to agree reasonably well with predictions, thereby demonstrating that analytical drift-diffusion based models can be extended to HBTs with GaInAsN base layers.     

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.     

Comparison of heterostructure-emitter bipolar transistors (HEBTs) with InGaAs/GaAs superlattice and quantum-well base structures

The characteristics of InGaP/GaAs heterostructure-emitter bipolar transistors (HEBTs) including conventional GaAs bulk base, InGaAs/GaAs superlattice-base, and InGaAs quantum-well base structures are presented and compared by two-dimensional simulation analysis. Among of the devices, the superlattice-base device exhibits a highest collector current, a highest current gain and a lowest base–emitter turn-on voltage attributed to the increased charge storage of minority carriers in the InGaAs/GaAs superlattice-base region by tunneling behavior. The relatively low turn-on voltage can reduce the operating voltage and collector–emitter offset voltage for low power consumption in circuit applications. However, as to the quantum-well base device, the electrons injecting into the InGaAs well are blocked by the p⁺-GaAs bulk base and it causes a great quantity of electron storage within the small energy-gap n-type GaAs emitter layer, which significantly increases the base recombination current as well as degrades the collector current and current gain.     

Two integral relations pertaining to the electron transport through a bipolar transistor with a nonuniform energy gap in the base region

The two integral relations by Moll and Ross for the current flow through the base region of a bipolar transistor, and for the base transit time, are generalized to the case of a heterostructure bipolar transistor with a nonuniform energy gap in the base region.     

The two-dimensional model for the base surface current in integrated bipolar transistors and its applications

The recombination of minority carriers at the Si-SiO₂ interface has a great effect on bipolar devices. In this paper, the surface electrical properties of capacitors (MOS or MNOS) and gate-controlled npn transistors which are passivated by SiO₂ and Si0₂---Si₃N₄ films, respectively, are studied, and it is found that the Si0₂---Si₃N₄ dual dielectrical films can reduce the surface current in the base region. An improved theoretical model of the base surface current versus surface potential or gate voltage is set up on the basis of the work of Hillen and Holsbrink [Solid St. Electron. 26, 453–463 (1983)] for integrated bipolar transistors. The model successfully explains the experimental results of the variation of the base surface current of the gate-controlled integrated bipolar npn transistors with the gate voltage and provides a more accurate model for the computer simulation and the reliability analysis of the devices.     

Simulation of npn and pnp AlGaN/GaN heterojunction bipolartransistors performances: limiting factors and optimum design

The performance capabilities of npn and pnp AlGaN/GaN heterojunction bipolar transistors have been investigated by using a drift-diffusion transport model. Numerical results have been employed to study the effect of the p-type Mg doping and its incomplete ionization on device performance. The high base resistance induced by the deep acceptor level is found to be one of the causes of limited current gain values for npn devices. Reasonable improvements of the dc current gain β are observed by realistically reducing the base thickness and consequently the transit time, in accordance with processing limitations. Base transport enhancement is predicted by the introduction of a quasi-electric field in the base. The impact of the base resistivity on high-frequency characteristics is investigated for npn AlGaN/GaN devices. Simulation results reveal the difficulty to achieve decent current gain values at high current density for pnp HBTs in common emitter configuration. Despite the high electron mobility in the n-type base that aids in reducing the base resistance, a preliminary analysis for pnp devices indicates limited rf performances caused by the reduced minority hole transport across the base     

Fast-switching and shallow saturation bipolar power transistorsusing corrugated base junctions

A fast-switching and shallow saturation bipolar power transistor fabrication technology using corrugated base junctions, which does not require additional process steps, is proposed in this paper. Computer simulation shows that less excess minority and majority carriers stored in the base and the collector drift region cause the shallow saturation phenomena of the corrugated base transistors at the conduction stage, and that the corrugated base transistors have lateral built-in electric fields under the base electrode, which accelerate the movement of the minority carriers from the bulk to the surface and promote the recombination of excess electrons and holes in the base region. The turn-off times and the saturation voltages between the collector and the emitter are studied systematically as a function of the base masking oxide widths of the corrugated base region, which agree well with the simulation results     

Origin of 1/f noise in bipolar transistors

In planar n-p-n transistors fabricated in IC technology, the dependence of 1/fnoise on the base current density jB, the base width WB, and the emitter area FEwas measured. The power spectrumS_{iB}(f)of the base current fluctuations iBcan be represented by the empirical relationS_{iB} = const. jmin{B}max{gamma} cdot Fmin{E}max{beta} cdot wmin{B}max{-1} cdot f^{-1}where1 leq gamma leq 2and β has been found to be 1.3 or 2. The results of measurements on gate-controlled devices indicate that 1/fnoise cannot be explained by McWhorter's surface model. Therefore, a new model is proposed which assumes resistance fluctuations in the base region as the cause of 1/fnoise in bipolar transistors. The model establishes the relation forS_{iB}(f)as well as the magnitude of the coefficients β and γ.     

Carrier transport in the drift region of read-type diodes

The transport properties of the drift region significantly impact the conversion efficiency of Read-type IMPATT diodes. A detailed numerical study has been undertaken to gain insight into the carrier transport under large-signal conditions and the roles of depletion-width modulation and the carrier-induced electric field. A phase delay in the peak of the avalanche current results from assuming a sinusoidal voltage rather than a sinusoidal field and is incorporated in the model. induced current waveforms, transport factors, diode admittances, and conversion efficieneies are presented as functions of RF level, bias current density, and the small-signal transit angle. A region of partial collection of the drifting carriers exists between regular IMPATT operation and the onset of the premature collection mode, which provides a smooth transition between these two modes. The effects of premature collection are most pronounced at long transit angles where the corresponding large increase in the transport factor at a high RF level will give high-efficiency operation, but also a large hysteresis in the tuning characteristic. The displacement current has an appreciable effect on the induced external current under these conditions. The presented results aid the understanding of tuning behavior and tradeoffs in design for Read diodes.     

MPS二极管的少子特性仿真

混合pin/肖特基(MPS)二极管是广泛应用于电子电路中的快恢复功率器件,具有高击穿电压、快速开关和正向电流大等特性。对MPS二极管漂移区的少数载流子的特性进行了仿真分析。仿真结果表明,MPS二极管的p^+区向漂移区注入的少数载流子浓度随外加正向电压和pn结面积占元胞总面积比例的增大而增大。虽然漂移区的少数载流子改变了MPS二极管的工作模式,增大了电流,但是存储在漂移区的少数载流子增大了反向峰值电流和恢复时间,进而增大了功耗并降低了关断速度。折中考虑正向电流和反向恢复特性,可获得具有正向电流大、反向峰值电流小和反向恢复时间短的MPS二极管。     

Optimum doping distribution for minimum base transit time

The problem of determining the necessary impurity distribution function which will minimize the minority carrier emitter-to-collector transit time is considered. The use of this optimum profile affords a possibility of improvement in the overall high-frequency performance of diodes and transistors. A simple one-dimensional geometry is assumed, and the drift and diffusion components of minority current are considered. The mobility is assumed independent of electric field and is approximated bymu =a_{0} - b_{0} ln N(x). When optimum doping is used, the resulting electric field is found to be a monotonically decreasing function over the base region. When the ratio of impurity densities at the boundaries of the base region is 10³, the minority current is primarily drift over most of the base region and the base cutoff frequency is approximately 6.5 times higher than the value resulting when the base is uniformly doped.