Journal ArticleDOI

# 500 °C Bipolar SiC Linear Voltage Regulator

, Ana Rusu1
10 Apr 2015-IEEE Transactions on Electron Devices (IEEE Press)-Vol. 62, Iss: 6, pp 1953-1957

AbstractIn this paper, we demonstrate a fully integrated linear voltage regulator in silicon carbide NPN bipolar transistor technology, operational from 25 °C up to 500 °C. For 15-mA load current, this regulator provides a stable output voltage with <2% variation in the temperature range 25 °C–500 °C. For both line and load regulations, degradation of 50% from 25 °C to 300 °C and improvement of 50% from 300 °C to 500 °C are observed. The transient response measurements of the regulator show robust behavior in the temperature range 25 °C–500 °C.

Topics: Voltage regulator (52%)

### Introduction

• Therefore, much research has been conducted in SiC electronics for elevated temperatures.
• This work was supported by the Swedish Foundation for Strategic Research through the HOTSiC Project.
• The feedback resistors (as a voltage–voltage network) sense the output, and a division of the sensed voltage is compared with the reference voltage.

### A. Loop Gain Analysis

• Personal use is permitted, but republication/redistribution requires IEEE permission.

### B. Temperature Compensated Biasing

• The resistors values fall as temperature rises up to ∼200 °C.
• Assuming a constant biasing current for the OTA, decrease of the resistors RC1 and RC2 can significantly reduce the gain of the OTA.
• The biasing network can be designed in order to provide higher collector current for the gain stages as temperature increases.
• The current tail value for the OTA is derived from the equation of biasing network.
• (4) Considering the effect of Vbe (−2 mV/°C) and RE in (4), it is concluded that more tail current is available for the OTA at higher temperatures, which results in higher gain of the OTA.

### C. Stability Issue

• The system has two poles; one of them is associated with the base of the pass device and the other one with the output of the regulator.
• Owing to the low output impedance of the pass device in emitter-follower configuration, the pole associated with the output of the regulator is pushed to high frequencies.
• The OTA has only one gain stage; there is no any other low-frequency node in the loop, and the stability of the voltage regulator can be inferred.

### III. EXPERIMENTAL RESULTS AND DISCUSSIONS

• On-wafer characterization was performed on a temperaturecontrolled hot-stage.
• The measurements were conducted using either parameter analyzer or oscilloscope at each temperature.

### A. Device Characterization

• A single BJT and the pass device were characterized individually.
• The pass device consists of four devices in parallel and compared with the single BJT provides higher collector current ratings, as can be observed in Fig. 3(b).

### B. Circuit Characterization

• The feedback resistors ratio defines the relation between the output and the reference voltage: Vout = Vref · (1 + R f 1/R f 2).
• All of the measurements of this paper were performed with VCCOTA and Vin connected to the same power supply.
• The regulated voltage is fairly robust in the range 25 °C–500 °C. Fig. 4(b) presents the output variation in different load conditions.
• The load currents are measured at each temperature considering the temperature variation of the integrated resistors.

### C. Performance Evaluation

• Line and load regulations [( Vout/ Vin), ( Vout/ Iload)] are two performance metrics of voltage regulators.
• The line regulation is calculated based on ±5% variation from the nominal input voltage.
• The load regulation follows a similar trend and remains in the range 2%–5% for the whole temperature range up to 500 °C.
• To measure the transient response of the regulator to instantaneous current loads, the output voltage was initially measured at a nominal input voltage (23 V).
• A 1-k on-chip resistor was used as the load, whose one end is connected to the output of the regulator and the other end is floated.

### IV. CONCLUSION

• For 15 V output voltage and up to 15-mA load current, a stable output voltage with <2% variation with temperature is observed in the whole temperature range.
• In addition, the transient response of the regulator to a 15-mA load current shows no significant performance degradation with temperature increase.

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Citation for the original published paper (version of record):
Kargarrazi, S., Lanni, L., Saggini, S., Rusu, A., Zetterling, C-M. (2015)
500 degrees C Bipolar SiC Linear Voltage Regulator.
IEEE Transactions on Electron Devices, 62(6): 1953-1957
http://dx.doi.org/10.1109/TED.2015.2417097
N.B. When citing this work, cite the original published paper.
© 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for
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IEEE TRANSACTIONS ON ELECTRON DEVICES 1
500 °C Bipolar SiC Linear Voltage Regulator
Saleh Kargarrazi, Member, IEEE, Luigia Lanni, Stefano Saggini, Member, IEEE,
Ana Rusu, Member, IEEE, and Carl-Mikael Zetterling, Senior Member, IEEE
AbstractIn this paper, we demonstrate a fully integrated
linear voltage regulator in silicon carbide NPN bipolar transistor
technology, operational from 25 °C up to 500 °C. For 15-mA
load current, this regulator provides a stable output voltage with
<2% variation in the temperature range 25 °C–500 °C. For
to 300 °C and improvement of 50% from 300 °C to 500 °C are
observed. The transient response measurements of the regulator
show robust behavior in the temperature range 25 °C–500 °C.
Index TermsBipolar junction transistor (BJT),
high-temperature IC, regulators, silicon carbide (SiC).
I. INTRODUCTION
S
ILICON carbide (SiC) is suggested as an advantageous
candidate for high-temperature electronics, because of its
wide bandgap. High-temperature electronics are promoted by
industries, such as downhole drilling, automotive, aviation,
and aerospace. Therefore, much research has been
conducted in SiC electronics for elevated temperatures.
High-temperature integrated digital logic circuits [1],
operational ampliﬁers [2], [3], and Schmitt triggers [4] are a
few examples.
In this paper, we demonstrate a SiC bipolar linear voltage
regulator capable of solid performance from room temperature
up to 500 °C. Although this circuit topology is not
expected to have a high efﬁciency [5], it was selected,
since requiring only transistors and resistors, it could
be fabricated in the available process technology [6].
Other high-temperature linear voltage regulators have been
demonstrated up to date. A silicon-on-insulator (SOI) lin-
ear voltage regulator has reported a maximum operating
temperature of 200 °C [7], and an nMOS SiC regulator has
been successfully operated up to 300 °C [8].
II. SiC D
ESIGN
The linear voltage regulator circuit is shown in Fig. 1.
It consists of an error ampliﬁer stage, a pass device, feedback
resistive network, and output resistive loads, all integrated
Manuscript received December 20, 2014; revised March 17, 2015; accepted
March 24, 2015. This work was supported by the Swedish Foundation for
Strategic Research through the HOTSiC Project. The review of this paper
was arranged by Editor S. N. E. Madathil.
S. Kargarrazi, L. Lanni, A. Rusu, and C.-M. Zetterling are with the School of
Information and Communication Technology, KTH Royal Institute of Tech-
nology, Stockholm 164 40, Sweden (e-mail: salehk@kth.se; luigia@kth.se;
arusu@kth.se; bellman@kth.se).
S. Saggini is with the Dipartimento di Ingegneria Elettrica, Gestionale
e Meccanica, University of Udine, Udine 33100, Italy (e-mail: stefano.
saggini@uniud.it).
Color versions of one or more of the ﬁgures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identiﬁer 10.1109/TED.2015.2417097
Fig. 1. Linear voltage regulator schematic (the resistor values are at
room temperature).
on chip. The reference voltage is, however, provided externally
at this stage. This facilitates the measurement, because the
reference voltage can be tuned in order to achieve different
output levels. Nevertheless, since the feedback resistors are
integrated, the voltage regulator is categorized as a ﬁxed
voltage regulator. The feedback resistors (as a voltage–voltage
network) sense the output, and a division of the sensed voltage
is compared with the reference voltage. The error (difference
between reference and sensed voltage) is ampliﬁed by the error
ampliﬁer, which in turn sets the dc bias at the base of the pass
device. Consequently, any variation in the output will change
the dc bias of the pass device and hence the output will be
regulated.
The error ampliﬁer employs a single-stage operational
transconductance ampliﬁer (OTA). Unlike MOSFET ampli-
ﬁers, bipolar junction transistor (BJT) counterparts suffer from
high base current and lower input impedance. To alleviate this
drawback, the differential pair of the OTA is designed with a
Darlington conﬁguration. The pass device is the output stage
of the regulator, which has an emitter-follower conﬁguration.
It is a current ampliﬁer that does not provide voltage gain and
so has little role in the loop gain of the regulator.
A. Loop Gain Analysis
Regulator gain is deﬁned by (1), where A is the open-loop
gain of the OTA and f is the feedback gain given by
A
f
=
A
1 + A · f
(1)
f =
R
f 2
R
f 1
+ R
f 2
. (2)
0018-9383 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
2 IEEE TRANSACTIONS ON ELECTRON DEVICES
Assuming equal V
be
and forward current gain (β)forall
the BJTs, the dc loop gain in no-load condition is
G
loop
= A · f
1
8V
T
β
β + 1
V
in
V
be
1 +
R
bias1
R
bias2
+
R
bias1
+1)R
E
R
C1
R
E
· f.
(3)
The performance of the linear voltage regulator depends
on the dc loop gain. Equation (3) indicates that the dc loop
gain is temperature dependent, in which V
T
(= kT/q), β,
and V
be
(2 mV/°C) vary by temperature and inﬂuence the
equation.
B. Temperature Compensated Biasing
The resistors values fall as temperature rises up to 200 °C.
Assuming a constant biasing current for the OTA, decrease
of the resistors R
C1
and R
C2
can signiﬁcantly reduce the
gain of the OTA. The biasing network can be designed in
order to provide higher collector current for the gain stages as
temperature increases.
The current tail value for the OTA is derived from the
equation of biasing network. It can be approximated as
I
tail
V
in
V
be
1 +
R
bias1
R
bias2
· R
E
. (4)
Considering the effect of V
be
(2mV/°C)andR
E
in (4),
it is concluded that more tail current is available for the OTA
at higher temperatures, which results in higher gain of the
OTA. This effect opposes the OTA gain reduction due to
the temperature dependence of R
C1
and R
C2
. Therefore, the
biasing network helps to compensate the gain reduction caused
by temperature increase.
C. Stability Issue
The system has two poles; one of them is associated with
the base of the pass device and the other one with the output of
the regulator. Owing to the low output impedance of the pass
device in emitter-follower conﬁguration, the pole associated
with the output of the regulator is pushed to high frequencies.
The OTA has only one gain stage; there is no any other
low-frequency node in the loop, and the stability of the voltage
regulator can be inferred.
D. Chip Layout
The 4H–SiC process technology used for fabricating this
circuit has already been reported in [6] and [9]. Starting
from a six-layer epitaxial structure, vertical NPN transistors
and integrated resistors in the highly doped collector layer
were realized. The linear voltage regulator circuit consists
of ten BJT devices with the size of 125.5 μm × 85 μm.
Fig. 2 shows the fabricated regulator, and an extra pass device
that can be characterized separately.
III. E
XPERIMENTAL RESULTS AND DISCUSSIONS
On-wafer characterization was performed on a temperature-
controlled hot-stage. The measurements were conducted using
either parameter analyzer or oscilloscope at each temperature.
Fig. 2. Optical image of the fabricated chip (1.9mm× 1.4 mm).
Dashed box: extra pass device.
Fig. 3. Measured current gain plot with V
BC
= 0 of (a) single BJT (with
an indication to the operation point of Q
1
and Q
2
in OTA stage) and
(b) pass device (with an indication of operating range based on different
A. Device Characterization
A single BJT and the pass device were characterized
individually. Fig. 3 presents the current gain plot of a single
BJT and the pass device with V
BC
= 0V.Fig.3(b)also

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KARGARRAZI et al.: 500
C BIPOLAR SiC LINEAR VOLTAGE REGULATOR 3
Fig. 4. (a) Measured output variation at different temperatures
(25 °C–500 °C) in medium load condition (7 mA/2 k). (b) Output variation
at 300 °C in different load conditions.
highlights the operating range of the pass device in different
in parallel and compared with the single BJT provides higher
collector current ratings, as can be observed in Fig. 3(b).
B. Circuit Characterization
The feedback resistors ratio deﬁnes the relation between the
output and the reference voltage: V
out
= V
ref
· (1 + R
f 1
/R
f 2
).
Using a 10 V reference voltage and an input of 23 V, result
in 14.3 V output voltage in no-load condition. All of the
measurements of this paper were performed with VCC
OTA
and
V
in
connected to the same power supply. The input voltage is
swept and the output voltage is measured.
The measured output versus input of the regulator for a
medium load condition (7 mA/2 k) is shown in Fig. 4(a). The
regulated voltage is fairly robust in the range 25 °C–500 °C.
Furthermore, in order to investigate the load regulation, the
regulator was loaded with different on-chip resistors. Fig. 4(b)
presents the output variation in different load conditions. Each
current value in Fig. 4(b) corresponds to an integrated resistor
of Fig. 2. The load currents are measured at each temperature
considering the temperature variation of the integrated
resistors. Fig. 5(a) shows the variation of the 1-k resistor
in the temperature range 25 °C–500 °C.
C. Performance Evaluation
out
/V
in
), (V
out
/I
)]
are two performance metrics of voltage regulators. To further
explore the performance of the SiC NPN linear
Fig. 5. Measured temperature behavior. (a) Load resistor (corresponds
to the 1-k designed resistor). (b) Line regulation in full-load condition.
voltage regulator, the line regulation in full-load condition
(15 mA) and the load regulation are derived and presented
in Fig. 5(b) and (c), respectively. The line regulation is
calculated based on ±5% variation from the nominal input
voltage. It varies in the range 30–65 (mV/V) in no-load
condition and in the range 50–230 (mV/V) in full-load
condition. The load regulation follows a similar trend and
remains in the range 2%–5% for the whole temperature range
up to 500 °C. Both the line and the load regulations have the
maximum at 300 °C. After this temperature, both parameters
slightly improve. Moreover, the regulator output voltage
temperature variation is <2% in the range 25 °C–500 °C
[Fig. 5(d)], suggesting stable operation of the circuit in this
wide range of temperature.
To measure the transient response of the regulator to
instantaneous current loads, the output voltage was initially
measured at a nominal input voltage (23 V). The reference
voltage was adjusted in order to get 15 V at the output in
no-load condition. A 1-k on-chip resistor was used as the
load, whose one end is connected to the output of the regulator
and the other end is ﬂoated. The ﬂoated end was controlled
with a voltage pulse source providing a 10-kHz square wave
pulse with rise and fall times of 350 ns, switching between
GND and 15 V [Fig. 6(a)]. The measured output voltage
and current are shown in Fig. 6(b) and (c), respectively.
Because of the on-chip loads, this measurement method
accounts for the loads at high temperatures. However, the
current passing through the on-chip resistor cannot accurately

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
4 IEEE TRANSACTIONS ON ELECTRON DEVICES
Fig. 6. Transient response to 15-mA current. (a) Control voltage. (b) Load
be controlled due to the temperature dependence of the
resistor. Therefore, the output currents are calculated based
on the measurement of the on-chip resistor at each
temperature.
In turn
ON and turn OFF, the load voltage settles after
5 and 10 μs, respectively. Lower output resistance of regulator
in turn
ON leads to lower RC constant and faster transition in
comparison with turn
OFF. It has to be noted that the transient
measurements are also limited by the slow response of the
pulse source (350 ns) used as control voltage. In turn
OFF,
an overshoot 16 V is observed before the load voltage settles
at 15 V. The undershoot value in turn
ON is negligible. The
regulator shows a robust transient response versus temperature
with almost no transient performance degradation from room
temperature up to 500 °C.
The linear voltage regulator in no-load condition consumes
61–81 mW at 23 V power supply in the whole range of
temperature. In full-load condition, the loss in the pass device
is added (120 mW) that leads to an efﬁciency of 50% over
the entire temperature range. This is expected for a bipolar
linear voltage regulator with a pass device operating in active
region [5].
Although no other voltage regulator for temperatures
>300 °C has been reported so far, Table I compares this
design with a recent nMOS linear voltage regulator aimed
for high-temperature, high-power applications [8]. The nMOS
linear voltage regulator has been designed and tested for load
currents as high as 2 A, and therefore targets a wide range
of power applications, whereas the reported bipolar regulator
is designed for loads 15 mA, using a simpler OTA and a
smaller pass device together with on-chip feedback and load
resistors.
TABLE I
C
OMPARISON OF THIS WORK WITH A RECENT HIGH-TEMPERATURE
LINEAR VOLTAGE REGULATOR
To improve the performance of the voltage regulator
(i.e., line and load regulations), higher loop gain is desired, and
therefore an OTA with more gain stages should be employed.
In addition, to achieve higher output power, a pass device
with higher current capability (e.g., by paralleling more BJTs)
should be used. Higher load current also translates into higher
base current for the pass device. Considering the low current
gain of the BJT devices (50), using a pass device with
Darlington topology is suggested. However, high dropout
voltage of the Darlington pair (2 · V
be
) should also be
considered. Furthermore, the external reference voltage, as the
only off-chip component, can also be integrated on-chip in the
future attempts.
IV. C
ONCLUSION
A fully integrated linear voltage regulator in 4H–SiC
bipolar technology is demonstrated, and its operation in the
temperature range from 25 °C up to 500 °C is discussed.
For 15 V output voltage and up to 15-mA load current,
a stable output voltage with <2% variation with temperature
is observed in the whole temperature range. The line and
condition) and 2%–5%, respectively. The voltage regulator
circuit consumes 61–81 mW from a 23 V power supply
the regulator to a 15-mA load current shows no signiﬁcant
R
EFERENCES
[1]L.Lanni,B.G.Malm,M.Ostling, and C.-M. Zetterling, “500 °C
bipolar integrated OR/NOR gate in 4H-SiC, IEEE Electron Device Lett.,
vol. 34, no. 9, pp. 1091–1093, Sep. 2013.
[2] A. C. Patil, X.-A. Fu, M. Mehregany, and S. L. Garverick, “Fully-
monolithic, 600 °C differential ampliﬁers in 6H-SiC JFET IC technol-
ogy, in Proc. IEEE Custom Integr. Circuits Conf. (CICC), Sep. 2009,
pp. 73–76.
[3] R. Hedayati, L. Lanni, S. Rodriguez, B. G. Malm, A. Rusu, and
C.-M. Zetterling, “A monolithic, 500 °C operational ampliﬁer in 4H-SiC
bipolar technology, IEEE Electron Device Lett., vol. 35, no. 7,
pp. 693–695, Jul. 2014.
[4] S. Kargarrazi, L. Lanni, and C.-M. Zetterling, “Design and characteri-
zation of 500 °C Schmitt trigger in 4H-SiC, in Proc. Eur. Conf. Silicon
Carbide Rel. Mater., Grenoble, France, 2014.
[5] N. Mohan and T. M. Undeland, Power Electronics: Converters,
Applications, and Design. New York, NY, USA: Wiley, 2007.
[6] L. Lanni, B. G. Malm, M. Östling, and C. M. Zetterling, “SiC etching
and sacriﬁcial oxidation effects on the performance of 4H-SiC BJTs,
Mater. Sci. Forum, vols. 778–780, pp. 1005–1008, Feb. 2014.

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