IEEE Solid-States Circuits Magazine - Spring 2021 - 30

voltage one could employ if self-biasing
were not used.
Following initial results indicating
that the gain of a differential BiCMOS
amplifier operating at low collectorcurrent
density was insensitive to
the supply voltage for collector-emitter
voltages as low as 100 mV [68], a
detailed study was carried out to
understand the potential of reducing
the power consumption of SiGe cryogenic
LNAs by reducing the collector
supply voltage [67]. At low enough
V ,CE
the base-collector junction be -
comes sufficiently forward biased
that the device enters the saturation
regime. In the limit of deep saturation,
the noise of a SiGe HBT deteriorates
due to both the extra shot
noise associated with conduction
through the base-collector diode and
the degraded high-frequency performance
associated with the increased
depletion and diffusion capacitances
of the base-collector junction.
However, prior to carrying out this
study, it was unclear how gradual the
onset of these effects was; in addition,
while it was known that when
operated cryogenically, SiGe HBTs
require V 1V,
value of VBC
BE .
the approximate
corresponding to the
forward biasing of the base-collector
junction was not known. As such, a
noise model describing the performance
of an HBT into the saturated
regime was developed and used to
study the performance of a sample
transistor from the Global Foundries
BiCMOS8HP technology platform.
Example results demonstrating
the dc and ac terminal characteristics
of the sample transistor as a function
of collector-emitter voltage
appear in Figure 8(a)-(d). Remarkably,
when biased in the region typically
employed for high-performance SiGe
cryogenic LNAs (J 1mA/ m),
C #
n
2
the device entered deep saturation
only at collector voltages below
about 100 mV. However, before this
occurred, the ac performance began
to deteriorate due to the increase in
C .CB
Nonetheless, it was found that,
2
C #
dropped
for (.J 092mA/nm), the minimum
noise temperature [Figure 8(e) and (f)]
began to rise only when VCE
below 150 mV, implying that a power
savings of approximately seven times
was feasible simply by reducing
V .CC
To validate the experimentally
modeled dependence of the cryogenic
noise of a SiGe HBT as a function
of
V ,CE a simple 1.8-3.6-GHz proof-ofconcept
amplifier was designed. As
depicted in Figure 9, the amplifier is
a two-stage discrete transistor design
with transmission line matching networks
employed to facilitate accurate
modeling. At its nominal bias point
corresponding to a dc power consumption
of just
290W ,n the amplifier
achieved a noise temperature and
gain of better than 5 K and 27 dB over
the entire frequency band, respectively.
The simulated and measured
noise temperature at 3 GHz is plotted
as a function of supply voltage in Figure
9(b). The measured performance
agrees well with the simulation, validating
the approach to model noise
performance including the effects of
saturation. Moreover, the noise performance
was found to depend only
weakly on VCC
as 125 mV (PDC 180W),n
=
down to values as low
further
supporting this approach as a viable
1.2
1.4
1.6
1.8
Ta = 7 K
0.2
0.4
0.6
0.8
1
(a)
10-4
10-3
10-2
10-1
100
10-5
0.2
0.4
0.6
VCE (V)
(b)
0.8
1
(b)
100
150
200
250
300
350
50
0.2
0.4
0.6
VCE (V)
(d)
0.8
1
100
150
200
250
50
1
(c)
(e)
100
10
f = 1 GHz
10
1
0.1
0.2
f = 10 GHz
0.3
VCE (V)
(f)
FIGURE 8: Cryogenic HBT performance as a function of VCE at a physical temperature of 7 K. The (a) collector current density, (b) base current
density, (c) ft, (d) fmax, and the modeled minimum noise temperature at (e) 1 GHz and (f) 10 GHz. These plots were taken using a Global Foundries
8HP transistor and were originally reported in [67]. The applied base voltages correspond to collector current densities of 0.22 (solid blue
line), 0.46 (dashed green line), 0.92 (dashed red line), and 1.67 mA/nm2 (dashed purple line) for VCB = 0.
30
SPRING 2021
IEEE SOLID-STATE CIRCUITS MAGAZINE
0.4
JB (mA/µm2)
JC (mA/µm2)
fmax (GHz)
ft (GHz)
TMIN (K)
TMIN (K)
IEEE Solid-States Circuits Magazine - Spring 2021

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