N. Nenadovic

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.

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.

Extraction and modeling of self-heating and mutual thermal coupling impedance of bipolar transistors

A measurement system comprised of an ultra-low-distortion function generator, lock-in amplifier, and semiconductor parameter analyzer is used for sensitive extraction of the small-signal thermal impedance network of bipolar devices and circuits. The extraction procedure is demonstrated through measurements on several silicon-on-glass NPN test structures. Behavioral modeling of the mutual thermal coupling obtained by fitting a multipole rational complex function to measured data is presented.

RF power silicon-on-glass VDMOSFETs

Applicability of vertical double-diffused MOSFETs for future base station power amplifiers has been demonstrated by characterizing the first devices fabricated in a substrate transfer silicon-on-glass technology. For a gate length of 0.8 /spl mu/m and gate width of 350 /spl mu/m, the measured f/sub T//f/sub max/ is 6/10 GHz, and the breakdown voltage approaches 100 V. The devices feature an output power of 12 dBm at the 1-dB compression point, excellent linearity (IM3/IM5 of -50/ -70 dBc at 10-dB backoff) and high power gain (14 dB) at 2 GHz, and are the first vertical DMOSFETs suitable for 2-GHz power applications. Excellent heat sinking and no significant degradation of the quiescent current due to hot-carrier injection ensure thermal stability and good long-term reliability of the fabricated devices.

Restabilizing mechanisms after the onset of thermal instability in bipolar transistors

The electrothermal behavior of single- and two-finger bipolar transistors at medium- and high-current operations is studied through theoretical modeling, experimental measurements, and computer simulations. Bias conditions that border thermally stable and unstable operation regimes are described by novel analytical formulations, which for the first time include simultaneously all relevant parameters that weaken the electrothermal feedback at high currents such as ballasting resistors, current dependence of the base-emitter-voltage temperature coefficient, and high-injection effects. Hence, besides giving a correct description of thermal instability mechanisms, the developed formulations also allow the prediction and physical understanding of restabilization phenomena. The models are supported by measurements on silicon-on-glass n-p-n bipolar junction transistors and by simulation results from a novel SPICE-based electrothermal macromodel for bipolar transistors. Furthermore, the models are employed to analyze the influence of the germanium percentage in the base of SiGe heterojunction bipolar transistors on the thermal ruggedness of the device.

Bulk-Micromachined Test Structure for Fast and Reliable Determination of the Lateral Thermal Conductivity of Thin Films

A novel bulk-micromachined test structure is presented for the fast and reliable determination of the lateral thermal conductivity of thin films. The device is composed of a heater resistor and thermocouples that are fabricated in polysilicon (poly-Si), and the associated processing and DC measurement procedures are straightforward. The validity of the method is supported by numerical simulations and verified by experimental determination of the lateral thermal conductivity of aluminum (Al), aluminum nitride (AlN), p-doped poly-Si, and silicon nitride (SiN) thin films. For Al, an average value of 217 W m-1 K-1 was found for 1-mum thick layers. For the other layers, a number of thicknesses were studied, and the increase of thermal conductivity with thickness was effectively detected: for AlN, values from 7 to 11.5 W m-1 K-1 were found, and for p-doped poly-Si, values went from 21 to 46 W m-1 K-1 for thicknesses from 0.15 to 1 mum. For SiN, a value of 1.8 was extracted for layers thicker than 0.5 mum.

Electrothermal limitations on the current density of high-frequency bipolar transistors

In this paper, electrothermal consequences of downscaling bipolar transistors, reducing the emitter resistance and implementing substrate modifications are examined by means of electrical measurements, numerical simulations and analytical calculations. A formulation is given for the optimum current density that can be run through the device and still maintain both sufficient transconductance and thermal stability. This expression sets a theoretical limit on the current density and therefore also on the speed of the given technology node. Particularly the lowering of the emitter resistivity is a trade-off between transconductance and thermal stability, and the optimum choice can be estimated from these results along with the maximum emitter area that will allow unconditional thermal stability.

Thermal instability in two-finger bipolar transistors

A novel analytical formulation for the biasing condition leading to thermal instability in a two-finger bipolar transistor is derived. The temperature dependence of the current gain as well as the mutual thermal coupling is accounted for. Different technologies are compared by varying the model parameters. It is demonstrated that SiGe devices are more thermally stable than comparable Si devices. Finally, the influence of the substrate modification on thermal instability is investigated. It is shown that thermally conducting substrates are needed to electro-thermally stabilize bipolar transistors.

PVD Aluminium Nitride as Heat Spreader in SilicononGlass Technology

Physical-vapor-deposited aluminium nitride has been integrated in a silicon-on-glass NPN BJT process. Deposition conditions have been developed for which suitable electrical, mechanical and thermal properties are achieved. Electrothermal device characterization is used to demonstrate the effective heat spreading of this thin-film material

MEMS test structure for measuring thermal conductivity of thin films

A novel MEMS test structure and measurement procedure is presented with which the lateral thermal conductivity of thin films can be easily and accurately extracted. The extraction procedure is discussed in detail and supported by numerical simulations. Experimental examples are given for the determination of the lateral thermal conductivity of aluminium (Al), aluminium nitride (AlN), and p-doped polysilicon (polySi) thin films.