Aerospace and Electronic Systems - August 2018 - 27

Varizat et al.
Around room temperature, the electron density is stable but below 200
K it starts to decrease, it's the freezeout effect. It decreases about ten times
from 300 K down to 77 K, implying
that the current flowing into transistors decreases rapidly according to the
temperature. Fermi potential variations
is one of the main parameter affecting
the threshold voltage, discussed in the
Thermal Study section of this article.
At room temperature, the charge
carrier mobility variation takes over
charge carrier density variations involving an increase in transistors current
drain. Conversely, at cryogenic temperature, the mobility slows down while
the density starts to decrease involving
a diminution in transistors current.

BSIM3.3 PARAMETERS
EXTRACTION

Figure 6.

IDVG and IDVD measurements (dots) and extracted based model simulations (lines) of a standard 50 μm ×
BSIM refers to a MOSFET transistor
10 μm NMOS at 100 K. Curves correspond to sweep voltage for VGS ∈ [0 V; 3.3 V], VDS ∈ [0 V; 3.3 V],
model for integrated circuit design. It's
and VBS = 0 V, −0.3 V and −0.6 V, respectively.
a precise model with 142 parameters in
order to consider transistors threshold
δ V1
δ V2
voltage, drain current, intrinsic capacities, mobility, etc. However, the
=α
= −α
(2)
δT
δT
BSIM3.3 model [11] doesn't take in account charge carrier density
variations, so low temperature behaviours are misrepresented like the
The sum of these two Complementary to Absolut Temperature
freeze-out effect. Recent works present new models allowing to take
voltage produce a temperature-independent voltage, like:
in account the freeze-out effect [12], [13], requiring modifications in
EDA tools. In this work, we took in account the freeze-out effect by
δ Vout δ V1 δ V2
(3)
=
+
= α −α = 0
adjusting BSIM3.3 parameters. Figure 6 shows IDVG and IDVD curves
δT
δT δT
from extracted based model simulations and measurements at 100
The large majority of bandgap design are based on bipolar tranK. The measurements are performed when sweeping VGS and VDS
sistors [14] in order to generate a voltage around 1.205 V, thanks
from 0V to 3.3V and for three VBS bias points (0V, −0.3V and −0.6V,
to the bandgap energy of silicon. These bandgaps voltage circuits
respectively). We observe a good agreement between the extracted
present a typically drift around 25-50 ppm/K [15] and some recent
model and measurements. With a maximum error rate of 2%, we test
study can reach 4 ppm/K [16].
this model on a complex circuit, detailed in the next section.
Because their low radiation tolerance, bipolar transistors-based
circuits require a dedicated qualification process when implemented in space electronic. The use of full CMOS design is preferred.
VOLTAGE REFERENCE CIRCUIT FOR SPACE APPLICATION
Recent studies [17], [18] present full CMOS voltage reference circuits accurate enough for a space instrumentation application but
For the need of space front-end electronics, it is essential to design
are not meant to work at low temperature and over a wide thermal
stable reference voltage circuits over a wide temperature range.
range. We designed a full CMOS hard rad bandgap circuits workIn a later section, we present a bandgap circuit design based on
ing from room temperature down to LNT.
the extracted model. A bandgap reference voltage circuit produces
a constant voltage regardless of temperature changes and power
supply variations.

CIRCUIT DESIGN

STATE OF THE ART
Bandgap circuit principle is to cancel out two opposing variations
caused by temperature. If we consider two components with a voltage Proportional To Absolut Temperature (PTAT), like:
AUGUST 2018

Extracted models were used to design a full CMOS bandgap circuit as a core function of a voltage reference circuit. The bandgap
circuits are optimized to work in the 100 K-150 K range and has to
be accurate enough to work at 300 K in order to test the front-end
electronics and its instrument at room temperature.

IEEE A&E SYSTEMS MAGAZINE

Aerospace and Electronic Systems - August 2018

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