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Noise in SiGe HBT
outline
• Basic concepts and terminology about noise
• Representation of noise in bipolar by linear twoports
• 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:
1
Pav  lim
T  T
T
2
 x t   x t 
2
1

2
T
2
• Shot noise:
1
dt  Pav1  Pav 2  lim
T  T
T
2
 2 x t x t dt
1

T
2
i  I  I D   2qI D f
2
2
1
• Thermal noise: v  4kTRf , i  4kT f
R
2
2
2
Terminology (cont’d)
• At room temperature (300K), the thermal noise spectral
density in a 1-kW resistor is 16x10-18 V2/Hz.
• The thermal noise-current generator of a 1kW resistor at
300K is the same as that of 50mA of direct current
exhibiting shot noise.
a
I
• Flicker noise: i 2  K
f
1
b
f
– K1=const. for a particular device
– a=0.5~2
– b~1
• Burst noise, avalanche noise
Noise in bipolar
vb2  4kTrb f
ic2  2qI C f
I Ba
i  2qI B f  K1 f
f
2
c
2qI C f
v  4kTrb f 
2
gm
2
i

IC 
I Ba
i  2q  I B  K1 ' 

2
f
  j  

2
i

0

1 j

RF Noise in bipolar (cont’d)
• The traditional model
is inaccurate at RF.
• At RF, the dominant
noise sources are the
base resistanceinduced 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)
• tn 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)
• tn 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.
2.
3.
4.
B. Razavi, Design of analog CMOS integrated circuits.
Gray, Hurst, Lewis, Meyer, Analysis and Design of
Analog Integrated Circuits.
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
Christoph Jungemann, Bukhard Neinhus, Bernd
Meinerzhagen, Robert W. Dutton, Investigation of
compact models for RF noise in SiGe HBTs by
hydrodynamic devices simulation, 2004