Constant Base Current Research Articles

An Improved Proportional Base Driver for Minimizing Driver Power Consumption of SiC BJT Over Wide Current and Temperature Range

SiC bipolar junction transistor (BJT) still offers an attractive alternative to the more popular SiC MOSFET because of its several advantages, such as low fabrication cost, fast switching speed, and high temperature durability. However, it has not been widely accepted in the market partially because the large constant base current induces large power consumption on its base driver during on-state. In this paper, a new proportional base driver is proposed to provide adaptive base current for SiC BJT and thus minimize its driver power consumption when operating within a wide range of load current and operation temperature in SiC BJT-based power converters. It simply uses a silicon BJT serially connected with a current sensor as a current-/temperature-controlled current source in the base driver circuit, promptly supplying adaptive base current for SiC BJT whenever the SiC BJT’s junction temperature and/or operation current varies. The operation principle of this proportional base driver is theoretically analyzed and then experimentally verified in a 600-V/20-A SiC BJT-based dc/dc boost converter. Experimental results show that the proposed proportional base driver is capable of driving the SiC BJT much more efficiently than the GeneSiC’s constant base driver, greatly reducing the power consumption of base driver by more than 50% over wide ranges of operation currents and temperatures.

A New Proportional Base Drive Technique for SiC Bipolar Junction Transistor

As one of the most attractive postsilicon power semiconductor devices, SiC bipolar junction transistor (BJT) has been studied extensively and commercialized in the past few years. However, SiC BJT has not been widely accepted in the market partially because of high driver consumption in the on-state, which is induced by a relatively large constant base current in order to ensure it is fully turned on . In this paper, a new proportional base driver with a simple circuitry is proposed in order to reduce the steady-state base driver consumption of SiC BJT. It utilizes a silicon small-signal mosfet as a variable resistor placed between the driver and the base of the SiC BJT. The collector current of the SiC BJT is measured via a Hall current sensor whose output is connected to the gate of the mosfet . When the collector current decreases, the gate voltage of the mosfet decreases and its on-state resistance increases, resulting in the decrease of the actual base driver current of the SiC BJT. The operation principle of the proposed proportional base drive technique has been verified by theoretical analysis and experimental demonstration of a 1200 V 10 A-SiC BJT-based dc–dc boost converter. Theoretical calculation, PSpice simulation, and experimental results have shown that the steady-state power consumption of the SiC BJT's proportional base driver is reduced by approximately 70% under the light load condition compared to conventional base driver with a constant base current.

Characteristic Effects of Stochastic Oscillatory Forcing on Neural Firing: Analytical Theory and Comparison to Paddlefish Electroreceptor Data

Stochastic signals with pronounced oscillatory components are frequently encountered in neural systems. Input currents to a neuron in the form of stochastic oscillations could be of exogenous origin, e.g. sensory input or synaptic input from a network rhythm. They shape spike firing statistics in a characteristic way, which we explore theoretically in this report. We consider a perfect integrate-and-fire neuron that is stimulated by a constant base current (to drive regular spontaneous firing), along with Gaussian narrow-band noise (a simple example of stochastic oscillations), and a broadband noise. We derive expressions for the nth-order interval distribution, its variance, and the serial correlation coefficients of the interspike intervals (ISIs) and confirm these analytical results by computer simulations. The theory is then applied to experimental data from electroreceptors of paddlefish, which have two distinct types of internal noisy oscillators, one forcing the other. The theory provides an analytical description of their afferent spiking statistics during spontaneous firing, and replicates a pronounced dependence of ISI serial correlation coefficients on the relative frequency of the driving oscillations, and furthermore allows extraction of certain parameters of the intrinsic oscillators embedded in these electroreceptors.

Theory of Electrothermal Behavior of Bipolar Transistors: Part I—Single-Finger Devices

A detailed theoretical and numerical analysis of the electrothermal behavior of single-finger bipolar transistors is proposed. Two models of different complexities are introduced to investigate self-heating effects in bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBTs) biased with a constant base-emitter voltage source or with a constant base current source. In the constant base-emitter voltage case, simple relations are derived for determining the onset of the flyback behavior in the output characteristics which defines the boundary of the safe operating region. The model indicates that the flyback behavior disappears at high V/sub BE/ values, and predicts a thermal hysteresis phenomenon at high currents. It is also shown that at high current levels the electrothermal behavior is dominated by ohmic base pushout. If a constant base current is applied, the model shows that both BJTs and HBTs are unconditionally thermally stable. The transient behavior is also considered, and the temperature evolution is investigated for different bias conditions. The model shows that, if the device is biased in the thermally unstable region, thermal breakdown occurs within a finite time instant in the limit case of a zero ballast resistance. Finally, the reduction in the safe operating area due to avalanche effects and to the temperature dependence of thermal conductivity is discussed, and a simplified model is proposed.

Theory of Electrothermal Behavior of Bipolar Transistors: Part II—Two-Finger Devices

A thorough theoretical and numerical analysis of the electrothermal behavior of two-finger bipolar transistors is presented. It is shown that thermal feedback and coupling effects introduce an additional singularity in the output I-V characteristics, namely a current bifurcation, which manifests itself as multiple solution branches emanating from a branching point. As a result, when the bifurcation condition is reached, the device is triggered in an asymmetrical operation mode in which one device carries most of the current. A unified formulation for the electrothermal behavior of a two-finger device is derived for different bias conditions at the input port: constant voltage, constant base current and constant emitter current. The analysis proofs that the critical condition defining the onset of current bifurcation is the same for all kinds of bias conditions. While operation under a constant base-emitter voltage is limited by the flyback condition, the current bifurcation condition defines the boundary of the normal operation region for the constant base current and constant emitter current cases. Finally, a rigorous method for identifying the conditions of thermal instability for an arbitrary number of emitter fingers is outlined. As an example, the method is used to derive the thermal instability conditions for the general case of temperature dependent thermal conductivity in a two-finger device.

The influence of BF/sub 2/ and F implants on the 1/f noise in SiGe HBTs with a self-aligned link base

A study is made of 1/f noise in SiGe heterojunction bipolar transistors (HBTs) fabricated using selective growth (SEG) of the Si collector and nonselective growth (NSEG) of the SiGe base and Si emitter cap. The transistors incorporate a self-aligned link base formed by BF/sub 2/ implantation into the field oxide below the p/sup +/ polysilicon extrinsic base. The influence of this BF/sub 2/ implant on the 1/f noise is compared with that of a F implant into the polysilicon emitter. Increased base current noise S/sub IB/ and base current are seen in transistors annealed at 975/spl deg/C, compared with transistors annealed at 950 or 900/spl deg/C. At a constant collector current, both the BF/sub 2/ and F implants reduce S/sub IB/, whereas at a constant base current, only the BF/sub 2/ implant reduces S/sub IB/. This result indicates that the BF/sub 2/ implant decreases the intensity of the base current noise source whereas the F implant decreases the base current. The proposed explanation for the increased 1/f noise is degradation of the surface oxide by viscous flow at 975/spl deg/C under the influence of stress introduced during selective Si epitaxy. The influence of the BF/sub 2/ implant on the noise is explained by the relief of the stress and hence the prevention of viscous oxide flow.

DC extraction of the base and emitter resistances in polysilicon-emitter npn BJTs

Five DC techniques of extracting the base and emitter resistances of polysilicon-emitter npn bipolar junction transistors (BJTs) are presented and compared. The five techniques include three previously published techniques and two new techniques, constant base current and IB–IE plane fitting. Application of the five methods to a 0.8 × 16 μm2 npn BJT shows that all but the method of impact ionization yield comparable Rc and Rbb values at high currents. The impact ionization method, which extracts Rc and Rbb in the impact ionization region and at low base currents, yields markedly different Rc and Rbb values, indicating that the values of the parasitic resistances depend on the current range over which the extraction is performed. Thus the choice of which method is best to use depends on the current range over which Rc and Rbb are to be measured, and the validity of the assumptions used in the method when applied to the device.

Study on d.c. bias mode for common emitter HBT

The output voltage/current wave forms and load lines of a common emitter HBT under constant base current and constant base voltage bias conditions at several input power levels were simulated with a non-linear simulator. The simulated output power characteristics of each set of bias conditions were compared with measured data. It was learnt that good gain linearity and good efficiency were obtained across a wide input power range, by using a constant base current bias mode. This is because operation of the HBT changes from class A to class C with a constant base current bias.

Thermal effects on the characteristics of AlGaAs/GaAs heterojunction bipolar transistors using two-dimensional numerical simulation

Two-dimensional numerical simulations incorporating the spatial distributions of the energy band and temperature were used to study AlGaAs/GaAs heterojunction bipolar transistor characteristics. It was found that the negative differential resistance and the reduction of the base-emitter voltage for a constant base current in the active region are due to thermal effects. The differential current gain and cutoff frequency decrease when the transistor is operated at high power levels. The temperature distribution of the transistor operated in the active region shows a maximum at the collector region right beneath the emitter mesa. When the transistor is operated in the saturation region, the emitter contact region may be at a slightly lower temperature than the heat sink temperature. This thermoelectric cooling effect results from the utilization of the thermodynamically compatible current and energy flow formulations in which the energy band discontinuities are part of the thermoelectric power.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Using constant base current as a boundary condition for one-dimensional AlGaAs/GaAs heterojunction bipolar transistor simulation

Using constant base current as a boundary condition, a one-dimensional numerical simulation technique based on the conventional transport equations was developed. Transistor current-voltage curves with base current density as the variable parameter, similar to the experimental data taken from a curve tracer, are obtained. The simulation also produces transistor characteristics, especially base modulation effects, that cannot be obtained easily by using constant base voltage as a boundary condition.

Second breakdown prediction by two-dimensonal numerical analysis of BJT turnoff

An isothermal two-dimensional numerical calculation of the potential and current distribution in an n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -p-n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> bipolar power transistor driving an inductive load during its turnoff transient has been carried out. The transistor is initially considered to be in a heavily saturated ON-state and is then turned off by extracting a nearly constant base current. The simulation shows that during the turnoff transient, current constriction to the center of the emitter together with the increasing collector-emitter voltage produce a high electric field near the collector n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> junction which can initiate avalanche injection. It has been found that the collector-current density is not uniform vertically (from collector to emitter) due to the current spreadout in the collector n-region. Previous one-dimensional analytical analyses of second breakdown did not consider this important effect. Thus, for an accurate prediction of reverse second breakdown voltage, the two-dimensional current flow should be considered.

Strength-duration properties of a rapidly adapting insect sensory neuron

1. A technique was developed to measure the strength-duration relationship for action potential initiation in the rapidly adapting sensory neuron of the femoral tactile spine in the cockroach,Periplaneta americana. Extracellular current was applied via a microelectrode adjacent to the axon where it leaves the soma. A computer-controlled successive approximation algorithm reliably determined threshold levels for rectangular current pulses of varying duration. 2. Experiments were conducted under resting conditions and also with constant depolarizing and hyperpolarizing base currents. Strength-duration data were well fitted by theoretical relationships derived for these conditions and assuming a constant charge threshold for the axon. The fitted equations gave values of the rheobasic current (threshold current for an infinitely long pulse) and the membrane time constant. The time constant was in good agreement with previous estimates obtained from this preparation using different techniques. 3. Both the rheobasic current and the time constant were functions of the constant base current. The rheobasic current increased strongly with depolarization so that it was always more positive than the base current. This gives a quantitative explanation of the observation that no depolarizing stimulus can sustain continuous action potential firing in this neuron. 4. With mild hyperpolarizing currents the rheobasic current decreased but with strong hyperpolarizations it increased and the time constant was reduced. This suggests that additional ionic conductance limits threshold behavior with strong hyperolarizations. 5. Experiments with blocking agents and ionic substitution suggested that a potassium A-current is involved in the threshold behavior with strong hyperpolarizations. Some evidence for increased chloride conductance under these conditions was also obtained. 6. No evidence was found that potassium or chloride currents are involved in the threshold behavior or rapid adaptation under normal or depolarized conditions.

On the fall time of silicon planar transistors with constant base current drive

In the active region transient response of a transistor in common emitter configuration with constant base current drive, there is some discrepancy in theoretically predicted and experimentally observed fall time. It has been explained by taking into account an additional turn-off base current drive which is usually ignored due to the emitter-base voltage of the transistor in the fall time expression using charge-control model of the transistor. The calculated values of the fall time of various silicon planar transistors are verified experimentally.

Second breakdown in the forward and reverse base current region

At a constant forward or reverse base current, the pulse length of a collector current pulse, which forces a transistor under test into second breakdown, is measured. For all transistors measured, the pulse length <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\log \Delta t</tex> is plotted vs. the collector pulse height, with the forward or reverse base current as a parameter. According to our observations, one can distinguish three types of transistors, as far as their secondary breakdown characteristics are concerned. 1) Good epitaxial transistors show an approximately linear dependence of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\log \Delta t</tex> to I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> with a negative slope. The curves for reverse base current are displaced parallel to the ones for forward base current, the energy of the pulses not being much different in both cases. 2) Bad epitaxial transistors show an S-shaped curve of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\log \Delta t</tex> vs. I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> for reverse base currents. A deep minimum in second breakdown energy is observed at low collector currents. These transistors will readily fail in practical circuits, where the base current is allowed to reverse. 3) With homogeneous material, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\log \Delta t</tex> vs. I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> curves for forward and reverse base currents practically coincide. This method has revealed that a large number of transistor types, including overlay structures, show a decrease of energy handling capability of two to three orders of magnitude at collector currents of about 100 mA when the base current is reversed. It has also proved to be an indispensable tool in the development of high-frequency high power devices, which combine good electrical and second breakdown properties.