An accurate analysis of noise in rectangular bipolar transistors including current crowding

An accurate analysis of noise in rectangular bipolar transistors is developed from a distributed model using a collective approach and the transport noise theory. In this model, emitter current crowding effect are taken into account and noise behaviour at intermediate and low values of source impedance is precisely described. The structure of teh equivalent lumped circuit is established, and the analytical relationships characterizing its elements in an extended range of current and frequency are given. It is shown that; (a) the active base region must be represented by a nonlinear impedance with a generalized thermal noise source; (b) for low source impedances the equivalent input voltage shot noise generator is higher than predicted by low injection theories. Furthermore it is found that emitter crowding induces a uniform and important decrease in (a) base impedance (b) thermal noise and (c) the correlation between shot noise generators of the equivalent lumped circuit. Finally it appears that classical low injection theories are valid when crowding occurs in transistors biased with high source impedances.     

Sub-ft gain resonance of InP/InGaAs-HBTs

Advanced npn-InP/InGaAs HBTs are often operated at high current levels for optimum high-speed performance. Because of velocity modulation effects, these transistors may operate in base-pushout although measurements of the cut-off frequency f_t indicate the opposite. We show that the low mobility of the holes has a strong effect on the transistor operation in this regime, which is only revealed from a dynamic analysis: The unilateral power gain peaks far below f_t followed by a -40 dB/dec roll-off. The effect was thoroughly analyzed and as a result, we present a simple equivalent circuit model that successfully describes transistors operating in pushout up to very high frequencies     

Broad-band determination of the FET small-signal equivalent circuit

A method to determine the broadband small-signal equivalent circuit of field-effect transistors (FETs) is proposed. This method is based on an analytic solution of the equations for the Y parameters of the intrinsic device and allows direct determination of the circuit elements at any specific frequency or averaged over a frequency range. The validity of the equivalent circuit can be verified by showing the frequency independence of each element. The method can be used for the whole range of measurement frequencies and can be applied to devices exhibiting severe low-frequency effects     

Thermal feedback in power semiconductor devices

Thermal feedback (TF) is an important aspect for the thermal management of semiconductor devices and high-power density integrated circuits. Different features of positive and negative TF in transistors are reviewed and summarized for the macroscopic domain. The thermal feedback mechanism is applied to the microscopic domain of noise fluctuations in semiconductor devices. It is argued that TF may be responsible for a major part of 1/fflicker or excess noise. Some experimental evidence is presented which supports this thermal feedback 1/f-noise theory for bipolar and MOS field effect transistors. Device and circuit design rules for the minimization of transmitter noise are given.     

1/f noise due to temperature fluctuations in heat conduction in bipolar transistors

Noise is an important consideration in the reliability of microelectronic circuits and often sets a lower bound on their sensitivity and limits operation. High device temperature which result from high power operation is shown to result in an additional noise source or mechanism which can become important, become a limiting factor on circuit operation, and limit reliability. Power dissipation at high currents and voltages in bipolar transistors results in significant heat generation and heat conduction towards the heat sink. As might be expected the device temperature is only an average value and there are, as a consequence of the diffusion equation for heat flow itself, temperature fluctuations about this average value. It will be shown that these temperature fluctuations can result in 1/f noise at moderately low frequencies where these frequencies are determined by the physical dimensions over which the heat flows and the diffusion transit time. This physical phenomena is another mechanism which can be explained by the equivalent circuit representations to obtain frequency dependent solutions to the diffusion equation. The results presented here are then related to the shot noise or white noise due to the collector current allowing a determination of the l/f noise corner frequency.     

A thin-film inductance using thermal filaments

A particular thin-film structure is analyzed to determine the conditions for thermal filament formation. Assuming that these conditions are satisfied and that the structure is electrically biased so that a thermal filament exists, the current-voltage characteristic and small-signal equivalent circuit for a general conductivity-temperature characteristic in the thin film are determined. It is shown that an abrupt or discontinuous change in conductivity with temperature of the type observed in materials exhibiting semiconductor-metal transitions is not necessary to obtain thermal filaments. It is also shown that if there is no thermal hysteresis in the conductivity -temperature characteristic of the thin film, the filament equivalent circuit for the particular structure analyzed is closely approximated by a resistance in parallel with an inductance. The thin-film prop. erties required for this inductance to be independent of both the ambient temperature and the bias current are defined. If thermal hysteresis exists, the analysis shows that small-signal distortions occur, the inductance will become frequency dependent at low frequencies, and a nonlinear resistance must be added to the equivalent circuit in series with the inductance. Measurements on this structure using VO2as the thin-film material are presented and discussed, and are shown to verify the conclusions based on the analysis.     

Properties of the super-alpha composite transistor at high frequencies

The equivalent circuit of the composite transistor, the input impedance and the current gain have been developed by analysing its equivalent circuit, in terms of the various parameters of the individual transistors in the common base and common emitter configuration. It is found that in the C.E. configuration, the short-circuit input impedance hi_e has a negative resistance component over a frequency range in which the current gain in the C.B. configuration h/b is greater than 1.     

Some new aspects of thermal instability of the current distribution in power transistors

An important characteristic of second breakdown in p-n junctions is the current constriction to a small region. This may be caused by a thermal feedback mechanism, as discussed by Scarlett and Shockley, and by Bergmann and Gerstner. A brief review of this theory is given, illustrated by experimental results of a simple model arrangement consisting of three thermally coupled transistors. The essential parameters influencing the thermal stability of the current distribution are device geometry, power density, and temperature dependence of current. It is widely known that second breakdown occurs at high voltages at a much lower power level than at low voltages. To allow a more detailed discussion of this effect in view of thermal stability, we determined experimentally the temperature coefficient of transistor current for various Si planar transistors as a function of current, voltage, and junction temperature. The experimental procedure is described and the results are discussed. The experimental values of the temperature coefficient range from 0.08 to 0.01 1/°C. The values for high currents are much lower than predicted by the theory of Ebers and Moll. It thus can easily be understood why, in the case of high current, and low voltage, the thermal stability of the current distribution is much better than in the case of low current and high voltage.     

Measurements and modeling of the harmonic distortion in InGaAsPdistributed feedback lasers

Measurements of the second harmonic distortion in 1.3 μm InGaAsP distributed feedback lasers are reported. At low modulation frequencies (~50 MHz), the second harmonic distortion exhibits a minimum at a certain bias current. The minimum, in general, is not observed at high modulation frequencies (~1 GHz). A model of the harmonic distortion, based on the equivalent electrical circuit of the laser, is presented to explain the observed behavior. The spatial hole burning effect in the active region introduces additional nonlinearity, its effects can be significant compared to that of the leakage path     

A functional GaAs FET noise model

It is the purpose of this paper to develop a theory upon which the design of low noise FET amplifiers can be based. This is not a fundamenta model of the noise mechanisms in GaAs FET's, but rather, an endeavor to relate physically measurable device capacitances and resistances to the device noise figure and optimum noise source impedance. I will be shown that the noise performance of an FET can be adequately described by two uncorrelated noise sources. One, at the input of the FET, is the thermal noise generated in the various resis, tances in the gate-source loop. This noise source is frequency dependent and it can be calculated from the equivalent circuit of the FET. The second noise source, in the Output of the FET, is frequency independent, and not recognizably related to any measured parameters. This output nise is a function of drain current and voltage. The decomposition of the FET noise into two uncorrelated sources simplifies the design of broad-band low noise amplifiers. Once the equivalent circuit of a device and its noise figure at one frequency are known, the optimum noise source impedance and noise figure over a broad range of frequencies may be calculated. For the device designer this model also may be helpful in balancing input-output noise tradeoffs.     

The noise performance of microwave transistors

Expressions for the noise parameters of microwave transistors are derived. The theory is based on a small-signal common-emitter equivalent circuit which includes a new basic noise equivalent circuit and the dominanting header parasitics. The theory is verified experimentally in the L-band (1 to 2 Gc/s) frequency range using Ge and Si microwave transistors. It is found that the header parasitics have little influence on the minimum noise figure, but do have large effects on the equivalent noise resistance and the optimum source admittance in the frequency region above about one-half of the series-resonant frequency resulting from the parasitics in conjunction with wafer parameters. For a quick evaluation of the noise performance, new approximate expressions are also given for the noise figure and for the optimum current which produces the lowest value.     

RF figures-of-merit for process optimization

Today, transistor y-parameters are routinely being measured for the determination of the current-gain cut-off frequency f/sub T/ and the maximum oscillation frequency f/sub max/. In this paper, it is shown that a much wider use of y-parameter measurements can be made for the RF characterization of transistors. A method is presented to determine the small-signal behavior of actual RF circuit-blocks from the measurements of the y-parameters of the individual circuit components. This is applied to define additional RF figures-of-merit for basic building blocks of analogue and digital RF circuits. No equivalent transistor circuit or compact-model parameters are needed, which is important for giving quick feedback to process developers. This approach is illustrated on three basic RF circuit blocks using bipolar transistors.     

Small-signal, high-frequency theory of field-effect transistors

In this theory a field-effect transistor of planar geometry is considered as an active, distributed, nonuniform transmission line. The wave equation of this line is determined and solved by an approximation method. From this solution theyparameters are determined. By comparing the results for y11with van der Ziel's expression for the high-frequency gate noise of field-effect transistors, it is shown that the noise temperature of y11is of the order of the device temperature. The conductance part g11of y11varies as ω²over a wide frequency range. The high-frequency cutoff of the field-effect transistor is determined.     

Three-dimensional base distributed effects of long stripe BJT's: ACeffects on input characteristics

The small-signal voltage and current distributed effects in the polysilicon and intrinsic base regions of long stripe bipolar junction transistors (BJT's) at high frequencies are investigated, and simple analytic equations describing the voltage and current distribution in these regions are derived. It is shown that the frequency-dependent debiasing effects in the polysilicon contacts and intrinsic base region change the current behavior and modulate the input admittance. The current and voltage distributions in the polysilicon region are nonuniform and vary with frequency. Conventional two-dimensional (2-D) device simulations cannot accurately predict this three-dimensional (3-D) effect. A quasi-3D simulation scheme combining a 2-D device simulator and the distributed model is presented to properly and efficiently describe the input characteristics of the device at high frequencies. It is also shown the use of various polysilicon sheet resistances, geometry sizes, and layout structures changes the distributed characteristics and modulates device performance at high frequencies. The impact of the layout structure and geometry size on RF circuit design due to the distributed effects is also studied     

Scaling in npn and pnp heterostructure bipolar transistors

Calculating the cutoff frequency f_T of bipolar transistors from the emitter-to-collector delay neglects the heavy influence of parasitic reactances on the frequency response of realistic transistors. A more complete equivalent circuit modelling reveals that the speed advantage of npn against pnp heterojunction bipolar transistors of common geometry, base width, and doping profile decreases as the transistor is scaled up in size. For power applications, the f_T of InP/GaInAs pnp devices may even surpass that of npn transistors     

Base-width modulation and the high-frequency equivalent circuit of junction transistors

The effect of base-width modulation on the exact small-signal, high-frequency equivalent circuit of p-n-p (and n-p-n) junction transistors is examined. It is found that (except for the effect on the base spreading resistance) base-width modulation only helps to determine the magnitude of one element. Failure to recognize this fact has resulted in the past in the "discovery" of "new" elements and the creation of a multiplicity of equivalent circuits. Through the use of an engineering approach, the most important of these circuits are related to the exact circuit, and hence their degree of approximation becomes apparent. The effect of a high injection level of minority carriers is also mentioned. Finally, the equivalent circuit of p-n-i-p (and n-p-i-n) transistors is discussed.     

A New Extraction Technique for the Complete Small-Signal Equivalent-Circuit Model of InGaP/GaAs HBT Including Base Contact Impedance and AC Current Crowding Effect

In this paper, both ac current crowding and base contact impedance are considered and included in the T-type small-signal equivalent circuit of InGaP/GaAs heterojunction bipolar transistors. The ac current crowding effect and base contact impedance are modeled as a parallel$RC$circuit, respectively. Devices parameters of the equivalent circuit are obtained by a new parameters extraction technique. The technique is to directly analyze the two-port parameters of multibias conditions (cutoff-bias, open-collector, and active-bias modes). The parallel capacitances ($C_B$and$C_ bi$), base resistances ($R_B$and$R_ bi$), and base inductance$(L_B)$are especially determined under the active-bias mode without numerical optimization. In addition, the small-signal equivalent circuits of cutoff-bias and open-collector modes are directly derived from the active-bias mode circuit for consistency. By considering base contact impedance and intrinsic base impedance effects in the presented small-signal equivalent circuit, the calculated$S$-parameters agree well with the measured$S$-parameters. The observed difference in the slope for the unilateral power gain$(U)$versus frequency at high frequency is mainly attributed to the ac emitter current crowding effect and well modeled in this study.     

Maximum operating frequency in Si bipolar master-slave toggleflip-flop circuit

This paper presents a propagation-delay equation which is expressed as the influence of the individual device and circuit parameters on the maximum operating frequency of Si bipolar master-slave toggle flip-flop (MS-TFF) circuit with double feedback emitter followers. This equation shows that optimizing the size of individual transistors can enhance the operating speed. Test results show a 10% increase in operating frequency by adopting this design technique. It is also shown that the time constants R_BC_jC, R_B C_D, T_F, R_LCj_C, and R _CCj_C of the upper-level current switch and T_F of the second feedback emitter follower greatly affect the operating speed of circuits using recently developed Si bipolar transistors. The results predicted by the equation are in good agreement with both the experimental ones and SPICE simulations     

有源电阻RTIA红外焦平面读出电路

提出了一种基于有源电阻的电阻反馈跨导放大器(RTIA)红外焦平面读出电路,该电路采用工作在亚阈区的MOS管实现1011 以上的有源大电阻,不仅能与热释电红外探测器的高阻抗良好匹配,而且配合两管共源放大器可针对热释电微弱信号进行高增益电流放大。同时,简单的三管单元结构能够方便地置于像元之下,相比于采用特殊高阻材料实现的RTIA,不附加材料和工艺。经上华0.5 ?滋m CMOS工艺流片验证,在5 V电源电压下,该电路增益40 dB,输出摆幅3 V,在高低温测试下表现出了良好的增益带宽稳定性,适用于PZT和BST等热释电大阵列探测器。