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.     

A back-wafer contacted silicon-on-glass integrated bipolar process. Part II. A novel analysis of thermal breakdown

Analytical expressions for the electrothermal parameters governing thermal instability in bipolar transistors, i.e., thermal resistance R/sub TH/, critical temperature T/sub crit/ and critical current J/sub C,crit/, are established and verified by measurements on silicon-on-glass bipolar NPNs. A minimum junction temperature increase above ambient due to selfheating that can cause thermal breakdown is identified and verified to be as low as 10-20/spl deg/C. The influence of internal and external series resistances and the thermal resistance explicitly included in the expressions for T/sub crit/ and J/sub C,crit/ becomes clear. The use of the derived expressions for determining the safe operating area of a device and for extracting the thermal resistance is demonstrated.     

Electrothermal simulation of an IGBT PWM inverter

An electrothermal network simulation methodology is used to analyze the behavior of a full-bridge, pulse-width-modulated (PWM), voltage-source inverter, which uses insulated gate bipolar transistors (IGBTs) as the switching devices. The electrothermal simulations are performed using the Saber circuit simulator and include control logic circuitry, IGBT gate drivers, the physics-based IGBT electrothermal model, and thermal network component models for the power-device silicon chips, packages, and heat sinks. It is shown that the thermal response of the silicon chip determines the IGBT temperature rise during the device switching cycle. The thermal response of the device TO247 package and silicon chip determines the device temperature rise during a single phase of the 60-Hz sinusoidal output. Also, the thermal response of the heat sink determines the device temperature rise during the system startup and after load-impedance changes. It is also shown that the full electrothermal analysis is required to accurately describe the power losses and circuit efficiency     

A physics-based dynamic thermal impedance model for verticalbipolar transistors on SOI substrates

A physics-based compact model for the thermal impedance of vertical bipolar transistors, fabricated with full dielectric isolation, is presented. The model compares favorably to both three dimensional (3-D) ANSYS(R) transient simulations and measurements. Using the software package Thermal Impedance Pre-Processor (TIPP), a multiple-pole circuit can be fitted to the thermal impedance model. The thermal equivalent circuit is used in conjunction with a modified version of SPICE to give efficient electrothermal simulations in the dc and transient regimes     

Time-dependent carrier flow in a transistor structure under nonisothermal conditions

A time-dependent temperature-dependent two-dimensional model has been developed to illustrate the internal behavior of bipolar transistors. Electrothermal interactions within the device are calculated in an attempt to better understand thermal instability modes. Numerically computed results are presented showing the electrical and thermal effects in the transistor operating in the switching mode during turn-on along a resistive load line. Plots of current density, electrostatic potential, and temperature versus time illustrate the combined effects of electrical rise time, thermal delay time, electrothermal interaction, current crowding, current spreading, conductivity modulation, and base pushout in bipolar transistors.     

A back-wafer contacted silicon-on-glass integrated bipolar process. Part I. The conflict electrical versus thermal isolation

A novel silicon-on-glass integrated bipolar technology is presented. The transfer to glass is performed by gluing and subsequent removal of the bulk silicon to a buried oxide layer. Low-ohmic collector contacts are processed on the back-wafer by implantation and dopant activation by excimer laser annealing. The improved electrical isolation with reduced collector-base capacitance, collector resistance and substrate capacitance, also provide an extremely good thermal isolation. The devices are electrothermally characterized in relationship to different heat-spreader designs by electrical measurement and nematic liquid crystal imaging. Accurate values of the temperature at thermal breakdown and thermal resistance are extracted from current-controlled Gummel plot measurements.     

Analysis of fast electrothermal dynamics in power BJT

It is well known that the self heating phenomenon in power bipolar devices strongly influences the electrical behavior. Heating phenomenon is a dynamic process ruled by different time constants. While the thermal dynamics associated to external heat exchange is an order of magnitude slower than the electrical one, the thermal dynamics occurring at the silicon level near the active junction is relatively faster and can interact with the electrical one. In this paper, it is proved that such a fast thermal dynamics is responsible for unstable oscillating electrical transients that can be detected in power bipolar transistors. An electrothermal resonance phenomenon Is theoretically and experimentally verified on a commercial power BJT. It is explained how such a phenomenon ran be operatively employed to extract the parameters of the fast thermal dynamics, difficult to measure with conventional procedures     

Correction methods of the electrothermal coupling in analog blocks based on heterostructure bipolar transistors

The problem of taking into account the thermal processes in high-performance microcircuits based on heterostructural bipolar transistors (HBT) is considered. A new thermal model of the HBT is suggested, which differs from the known BJT504t model by the addition of thermal resistance and thermal capacity. The means to decrease the thermal resistance of the HBT and to correct the electrothermal coupling in the comparator are suggested. Simulation of the comparator using the simplified model of the transistor in a wide temperature range showed that the use of compensation makes it possible to attain a decrease in the magnitude of hysteresis by a factor of 10.     

Aluminum nitride for heatspreading in RF IC’s

To reduce the electrothermal instabilities in silicon-on-glass high-frequency bipolar devices, the integration of thin-film aluminum nitride as a heatspreader is studied. The AlN is deposited by reactive sputtering and this material is shown to fulfill all the requirements for actively draining heat from RF IC’s, i.e., it has good process compatibility, sufficiently high thermal conductivity and good electrical isolation also at high frequencies. The residual stress and the piezoelectric character of the material, both of which can be detrimental for the present application, are minimized by a suitable choice of deposition conditions including variable biasing of the substrate in a multistep deposition cycle. Films of AlN as thick as 4 μm are successfully integrated in RF silicon-on-glass bipolar junction transistors that display a reduction of more than 70% in the value of the thermal resistance.     

A Modified Ebers-Moll Transistor Model for RF-Interference Analysis

This paper develops analytical techniques for the study of nonlinear RF and microwave effects in semiconductor devices. Rectification in p-n junctions is discussed, and a novel large-signal transistor model is developed, based upon modifications to standard Ebers-Moll formulations for bipolar transistors. Use of the models in worst-case analysis is discussed, with ranges of parameters given based on a simplified analysis of rectification in ideal diodes.     

On the accuracy of direct extraction of the heterojunction-bipolar-transistor equivalent-circuit model parameters C/sub /spl pi//, C/sub BC/, and R/sub E/

Several basic small-signal equivalent-circuit models for bipolar transistors lead to simple analytical expressions for the model parameters in terms of measured values. This paper investigates the accuracy of these expressions for real transistors by applying the direct extraction equations to more complicated small-signal models. The extraction of the base/collector capacitance, base/emitter capacitance, and emitter resistance are considered. Analytically derived trends are illustrated using measurements on small-area high-speed InP/GaAsSb/InP double heterojunction bipolar transistors.     

Electrothermal model for an SOI-based LIGBT

Several vertical insulated gate bipolar transistor (IGBT) electrothermal models are currently available on circuit simulators. However, no reliable electrothermal models have been proposed for the lateral IGBT (LIGBT). In this paper, for the first time, an electrothermal model for an LIGBT structure based on a novel concept recently reported by Udrea (IEDM, p. 451, 2004), and here termed silicon-on-membrane, is presented. The model relies on a systematic study of both the isothermal and self-heating behaviors of the device. The model is further implemented in the SPICE circuit simulator language and validated against extensive Medici numerical simulations and experimental data.     

Analysis of the electrical breakdown in hydrogenated amorphoussilicon thin-film transistors

Electrical breakdown induced by systematic electrostatic discharge (ESD) stress of thin-film transistors used as switches in active matrix addressed liquid crystal displays has been studied using electrical measurements, electrical simulations, electrothermal simulations, and postbreakdown observations. Breakdown due to very short pulses (up to 1 μs) shows a clear dependence on the channel length. A hypothesis that electrical breakdown in the case of short channel TFTs is due to the punch-through is built on this dependence and is proved by means of electrical simulations. Further, the presence of avalanche breakdown in amorphous silicon thin-film transistors is simulated and confirmed. It is finally assumed that the breakdown is a thermal process. Three-dimensional (3-D) electrothermal simulations are performed in the static and transient regime, confirming the location of the breakdown spot within the TFT from the electrical simulations and postbreakdown observations     

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.     

Measurements of 1/f noise amplitude modulated by a large-signal carrier in bipolar junction transistors

A novel experimental system for the investigation of frequency-translated flicker noise in bipolar junction transistors is demonstrated. First, transistors are characterized for their base–emitter junction behavior and flicker noise parameters in steady state. Then, measurements of the frequency conversion of 1/f noise are performed by the application of a large sinusoidal signal to the base–emitter junctions of bipolar junction transistors. The applied signal is varied in frequency and amplitude, and the resulting conversion gains are reported. A novel measurement technique, which enables the detection of noise fluctuations near a much larger signal is demonstrated. These results are compared with harmonic balance simulation.     

Impact of self-heating and thermal coupling on analog circuits inSOI CMOS

This paper examines the influence of the static and dynamic electrothermal behavior of silicon-on-insulator (SOI) CMOS transistors on a range of primitive analog circuit cells. In addition to the more well-known self-heating close-range thermal coupling effects are also examined. Particular emphasis is given to the impact of these effects on drain current mismatch due to localized temperature differences. Dynamic electrothermal behavior in the time and frequency domains is also considered, measurements and analyses are presented for a simple amplifier stage, current mirrors, a current output D/A converter, and ring oscillators fabricated in a 0.7-μm SOI CMOS process. It is shown that circuits which rely strongly on matching, such as the current mirrors or D/A converter, are significantly affected by self-heating and thermal coupling. Anomalies due to self-heating are also clearly visible in the small-signal characteristics of the amplifier stage. Self-heating effects are less significant for fast switching circuits. The paper demonstrates how circuit-level simulations can be used to predict undesirable nonisothermal operating conditions during the design stage     

New device structure using array transistors and flexible U-grooves suitable for 18-V, high-performance SOI complementary bipolar LSIs

We have developed a new device structure suitable for high-performance and high-power mixed signal large scale integrations (LSIs) using 0.35-/spl mu/m SOI complementary bipolar transistors. The new structure is composed of array transistors for various operating currents and flexible U-groove (trench) layout for high-power transistors. Thermal simulation results showed that the thermal resistance could be reduced by 40% by using the flexible U-groove layout. Test structure measurements showed that the maximum operating currents of a double polysilicon self-aligned NPN transistor were improved by 2 and 3.5 times by using ballasting resistors and ballasting resistors with flexible U-groove layout, respectively. The effects of the transistor structure on the thermal resistance and the maximum operating current were discussed.     

Thermal impedance extraction for bipolar transistors

This paper describes a method for extracting the thermal impedance of bipolar transistors. The measurement is a two-step process: first the fractional temperature coefficients are calibrated at dc and then a transient step response is measured to extract the thermal spreading impedance. Measurement configurations and an example measurement cycle are shown. The measurement results can be fitted to multiple-pole models for use in compact circuit modeling in SPICE