Noise in SiGe HBT

outline

• Basic concepts and terminology about noise• Representation of noise in bipolar by linear two-

ports• RF noise in bipolar

– A unified approach to RF and microwave noise parameter modeling in bipolar transistors [3]

• Noise model at RF for CAD– Investigation of compact models for RF noise in SiGe

HBTs by hydrodynamic devices simulation [4]

Basic concepts

• The most sources of noise in circuits exhibit a constant power.

• There are tradeoffs between noise and power dissipation, speed, and linearity.

• Noise in transistors: thermal, shot, and flicker noise.

Basic concepts

Basic concepts

Terminology

• Power spectrum density: the spectrum shows how much power the signal carries at each frequency.

• Most of the noise sources of interest to us exhibit a predictable spectrum.

• The spectrum shows the power carried in a small bandwidth at each frequency, revealing how fast the waveform is expected to vary in the t-domain.

• White spectrum:– Does not exist in reality.– In practice, any nose spectrum that is flat in the band of interest

is usually called white.

Terminology (cont’d)

• Correlated and uncorrelated:

• Shot noise:

• Thermal noise:

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Terminology (cont’d)

• At room temperature (300K), the thermal noise spectral density in a 1-k resistor is 16x10-18 V2/Hz.

• The thermal noise-current generator of a 1k resistor at 300K is the same as that of 50A of direct current exhibiting shot noise.

• Flicker noise:

– K1=const. for a particular device– a=0.5~2– b~1

• Burst noise, avalanche noise

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IKi

b

a

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RF Noise in bipolar (cont’d)

• The traditional model is inaccurate at RF.

• At RF, the dominant noise sources are the base resistance-induced thermal noise, and the terminal current shot noises.

RF Noise in bipolar

• Strictly, the distributive nature of base current flow requires a distributive description of the shot noise.

• When f<fT, rb<<Zin, ZS,opt, ignoring the distributive effect makes little difference.

RF Noise in bipolar (cont’d)

• The base and collector shot noises ib and ic are in general correlated to each other.

• Thermal noise vb is independent of shot noises.

RF Noise in bipolar (cont’d)

• A common-base noise equivalent circuit: shot noises at the emitter and collector currents transport in the same way as the signal.

RF Noise in bipolar (cont’d)

• The collector current shows shot noise only because the electron current being injected into the collector-base junction from the emitter already has shot noise.

• ine: electron injection into the base.

• ipe: hole injection into the emitter.

• ine and ipe are independent.

RF Noise in bipolar (cont’d)

• n is the transit time associated with the transport of emitter-injected electron shot noise current, and includes both the transit time in the base and the transit time in the collector-base junction.

• Shot noise due to the random nature of crossing the E-B junction does not involve the capacitance charging processes.

• 2qIC :a dc drift current passing through a reverse-biased junction does not show shot noise.

RF Noise in bipolar (cont’d)

• n is extracted from fitting the frequency dependence of noise figure.

Noise model at RF for CAD

• Physics-based simulation: hydrodynamic (HD) model.

• Base resistance determination.

• Transit time determination.

Noise model at RF for CAD (cont’d)

• Extraction of base resistance: Impedance circle method (measurement).

•

•

Noise model at RF for CAD (cont’d)

Noise model at RF for CAD (cont’d)

Noise model at RF for CAD (cont’d)

Noise model at RF for CAD (cont’d)

Noise model at RF for CAD (cont’d)

References

1. B. Razavi, Design of analog CMOS integrated circuits.2. Gray, Hurst, Lewis, Meyer, Analysis and Design of Ana

log Integrated Circuits.3. Guofu Niu, John D. Cressler, Shiming Zhang, William

E. Ansley, Charles S. Webster, and David L. Harame, A unified approach to RF and microwave noise parameter modeling in bipolar transistors, 2001

4. Christoph Jungemann, Bukhard Neinhus, Bernd Meinerzhagen, Robert W. Dutton, Investigation of compact models for RF noise in SiGe HBTs by hydrodynamic devices simulation, 2004