Development of the Bandgap Voltage Reference Circuit, Featuring Dynamic-Threshold MOS Transistors (DTMOSTs) in 0.13um CMOS Technology .

1
Development of the Bandgap Voltage Reference
Circuit, Featuring Dynamic-Threshold MOS
Transistors (DTMOSTs)in 0.13um CMOS Technology .

• Vladimir Gromov
• Group of microelectronics
• NIKHEF, Amsterdam,
• the Netherlands

2
Highlights

1. The 0.13um CMOS Technology.
2. Implementation of Voltage Bandgap Reference in
   the 0.13um CMOS Technology.
3. Dynamic-Threshold MOS Transistors (DTMOST)
   structure. Features, characterization, modeling.
4. Circuit submitted in the experimental run.
5. Conclusions.

3
Features of the 0.13um CMOS Technology in respect
to the 0.25um CMOS .

• Reduced physical thickness of the gate oxide.
• Fine pitch of lithography.
• Supports many metal layers.
• Support a wide range of new devices of which
  Triple-well NFET, and high-quality resistors are
  the most important.
• High production cost.
• Reduced power supply voltage Vdd1.2V.

• Devices in this process are extremely robust to
  radiation damages (there is no need for the
  gate-around geometries). Standard commercial
  libraries cells become suitable for our
  applications.
• Allows to reduce areas of the devices as to
  design high-speed high-integrated circuits.
• Easies wiring on the design .
• Let us reduce substrate noise and make the design
  much more robust in the mixed-signal
  environment.
• .
• Some conventional circuits do not fit into the
  reduced power rails.

4
Voltage Reference Circuit.

• Requirements
• The reference voltage must be insensitive to
  temperature variations.
• The reference voltage must be insensitive to the
  power supply voltage variations.
• The reference voltage must be insensitive to the
  fabrication process variations.

5
Classical voltage summing Bandgap Reference .
Vdd
Vdd
Vref
K R
PTAT
I
I
R
CTAT
These diodes are p-diffusion in N-well
structures.
D
n D
U,volts
Vdd1.2V
1.12V
Vref
Vg(T0)Eg(T0)/e1.12V, Where Eg(T) is energy
gap between Conduction Band and Valence Band in
silicon.
CTAT
PTAT
Temp
6
Principle of operation of the Bandgap Reference
circuit.
Diode current-to-voltage characteristics are
Shockley equations. I2(V,T)Io(T)exp(V e/kT)
1 I1(V,T)n Io(T)exp(V e/kT) 1
Vdd
Vdd
Vref
K R
V
V
I1, I2
D
n D
I
I
I1
I2
V2-V1
R
V
D
n D
V2
V1
Diode voltage-to-current characteristics are
Shockley equations.
V2(I,T)kT/elnI/Io(T)
V1(I,T)kT/elnI/(nIo(T))
In this architecture I1I2I
therefore ln(A)-ln(B)ln(A/B) V2(I,T)-V1(I,T)kT/
e ln(I/Io(T))- ln(I/(nIo(T))
kT/e ln(n) I V2(I,T)-V1(I,T)/R
kT/e ln(n)/R
V1, V2
Current is proportional to absolute temperature
PTAT
I
7
Voltage summing Bandgap Reference, featuring
DTMOST structures.
Vdd
Vdd
Vref
K R
PTAT
I
I
R
CTAT
In place of diodes new structures have been used.
U,volts
Vdd1.2V
Vg(T0)0.43V, is effective Bandgap voltage for
the DTMOST structure.
Vref
0.43V
CTAT
PTAT
Temp, C
8
Dynamic-Threshold MOS Transistors (DTMOST)
structure.Anne-Johan Annema (1999)
Gate metal oxide
Drain
Substrate
Source
P-diffusion
P-diffusion
N-well contact
Floating N-well (substrate)
Conventional diode structure
I, A
DTMOST structure
Source
Gate
N well
U
Uthr DIODE 650mV
Drain
Uthr DTMOST 200mV
I
U, Volts
9
Characterization of the DTMOST structures.
Heraeus Temperature chamber
Keithley 487 Picoammeter/ voltage source
Heraeus Thermometer
Experimental set-up.
The DTMOSTs Current-to-voltage characteristics
at various temperatures.
Voltage across the DTMOST at various currents as
a function of temperature.
Temp80ºC
Temp70ºC
I, A
Estimated Bandgap voltage 410 mV Reference
Voltage
U, mV
Temp0ºC
I2µA
Linear fits
I1µA
I0.5µA
Temp, ºC
U, Volts
10
Characterization of the DTMOST structures(exponen
tial behaviour).
The DTMOSTs Current-to-voltage characteristics.
Exponential fit function I(U)37 10-10 EXP(30
U)-1
Conventional diode configuration
I, A
DTMOST configuration
Region of exponential (ideal diode)
behaviour 100mV .220mV
Exponential fit function I(U)30 10-10 EXP(30
U)-1
I, A
DTMOST
Exponential behaviour range 0.1µA2µA
U, Volts
U, Volts
11
Modeling of the DTMOST structures in SPECTRE.
The DTMOSTs Current-to-voltage characteristics.
// DEVICE 1 //simulator langspectre
insensitiveyes .model DTMOST1 diode level1
isw0.0000e00
n1.34e00 rs0.000e-00
ik2.7400e09 ikp1.0000e-08
ibv1.0000e-09 trs1.0000e-03
eg0.225 tnom0.000e00 xti6.6
tlev1.0000e00 area1.000e00
perim1.00e00 cjo7.5000e-15
mj1.0000e00 vj3.3000e-01
cjsw0 mjsw1.000e00
vjsw4.3000e-01 is1.62e-09
bv1.3100e01 cta8.0000e-04
ctp1.0000e-03 pta0.00
ptp2.0000e-03 fc0.99
tlevc1.00000 imax1e14 gap10
gap20
I, A
Measured points
Results of simulation in Affirma Spectre
U, Volts
Reference voltage (Vref) as a function of
temperature.
Vref393mV
Diode model (CADENCE simulations).
Temp, ºC
12
The circuit.
Main Specifications (simulations) Reference
voltage 393mV Temperature sensitivity 1.5mV
(within a temperature range from 0ºC to 80ºC
). Shift of the Reference voltage caused by
supply voltage variations 0.25mV (if the supply
voltage varies in range from 0.9V to 1.4V
). Power consumption 60uW (50uA _at_1.2V) Spread of
the Reference voltage due to fabrication process
variations s1.2mV. Occupied area on the
chip250um vs 60um
13
Layout of the DTMOST structures.
Source
T lt19gt w11.7394ul600n nf1 m1
Gate
N well
Drain
Source
T lt136gt w11.7394u l600n nf1 m1
N well
Drain
14
The experimental chip (NIKHEF part).
(the chip was submitted in CuTe2 MPW run on May
10, 2004.)
4-channel Preamp/Shaper/Buffer circuit for
silicon microstrip sensors. The circuit features
conventional NFET devices.
Bandgap voltage Reference, Featuring DTMOST
devices.
4-channel Preamp/Shaper/Buffer circuit for
silicon microstrip sensors. The circuit features
Triple-well NFET devices.
2mm
15
Conclusions

• The 0.13um CMOS Technology provides designers
  with a wide set of attractive options, in
  particular, those who develop electronics to
  operate in high radiation environment.
• The Voltage Bandgap Reference circuit needs
  revision in order to fit into the reduced power
  supply voltage range of the technology.
• Dynamic-Threshold MOS Transistors (DTMOST) are to
  replace diodes in the classical Voltage Bandgap
  Reference circuit as to be suited for the reduced
  power supply range .
• After characterization and modeling of the DTMOST
  structures, the circuit was designed and
  submitted in CuTe2 MPW run on May 10, 2004 with
  turnaround time of 5 months.

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The Preamp/Shaper circuit.Main
specifications1) Rise time of the output
signal 14ns.2) ENC
(Cd15pf) 900e.3) Charge sensitivity
100mv/2fC(1MIP).4) Power consumption 1.7mW per
channel. 5) Dynamic range 010MIPs.
Preamplifier
Shaper
In
Gain1000
Gain2000
Out
Dummy Preamplifier
Gain1000
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The charge-sensitive preamplifier.
18
The Shaper.
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Principle of operation of the Bandgap Reference
circuit.
Diode current-to-voltage characteristics are
Shockley equations. I2(V,T)Io(T)exp(V e/kT)
1 I1(V,T)n Io(T)exp(V e/kT) 1
Vdd
Vdd
Vref
K R
V
V
I1, I2
D
n D
I
I
I1
I2
V2-V1
R
V
D
n D
V2
V1
Diode voltage-to-current characteristics are
Shockley equations.
V2(I,T)kT/elnI/Io(T)
V1(I,T)kT/elnI/(nIo(T))
In this architecture I1I2I
therefore ln(A)-ln(B)ln(A/B) V2(I,T)-V1(I,T)kT/
e ln(I/Io(T))- ln(I/(nIo(T))
kT/e ln(n) I V2(I,T)-V1(I,T)/R
kT/e ln(n)/R
V1, V2
Current is proportional to absolute temperature
PTAT
I
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• Content.
• Slide1. Good morning. I am Vladimir Gromov
  from microelectronics group of NIKHEF, Amsterdam,
  the Netherlands. I am delighted to be here today
  to tell you about a novel circuit proposed by
  our group. Namely my talk is entitled as
  Development of the Bandgap Voltage Reference
  Circuit, Featuring Dynamic-Threshold MOS
  Transistors (DTMOSTs)in 0.13um CMOS Technology .
  
• Slide2. These are highlights of the talk.
• I will start with a brief description of the
  features of the 0.13um technology.
• Then I will tell you why it is not easy to
  implement classical Voltage Bandgap Reference in
  this Technology.
• Further I will tell you about a new device called
  Dynamic-Threshold MOS Transistors (DTMOST).
  Features of this device, characterization
  approach and modeling will be discussed.
• I will demonstrate you a complete Bandgap Voltage
  Reference Circuit, Featuring Dynamic-Threshold
  MOS Transistors (DTMOSTs) circuit that we
  submitted in the experimental MPW run on May 10
  2004.
• Finally I will draw some conclusions.
• Slide3. Since one year ago we have joined
  efforts of CERN microelecronics group to start
  prototyping in a new 0.13um CMOS technology. This
  technology offers a set of features attractive
  for analog designers.
• .
• Because of the reduced thickness gate oxide
  isolation does not stand voltage higher than
  1.2V. It means that the power supply voltage is
  limited to 1.2V and any design in this technology
  is confined in this dynamic range . Some
  conventional circuits although do not fit into
  the reduced power rails. Bandgap voltage
  reference is one of them.
• Slide4. Voltage Reference Circuit, in general,
  is an important building block for many
  architectures. It is a key component for high
  quality A/D and D/A converters, it is often used
  when a stable bias voltage or power supply
  source is needed.
• A High Quality Reference circuit is to meet the
  following requirements
• ..
• Slide5.Voltage summing Bandgap Reference is a
  commonly used architecture in chip design. It
  consists of two diodes of different sizes
  (formed by p-diffusion in N-well structures) ,
  two resistors and an OPAMP to control a pair of
  identical current sources in the feedback.
  Operation of the circuit rely on two basic
  features.
• Number one for a given current voltage drop on
  a diode is conversely proportional to absolute
  temperature (CTAT). In the vicinity of the
  absolute zero temperature it approaches value
  called BandGap voltage (1.12V). This value is
  determined by energy gap between Conduction Band
  and Valence Band and constitutes a built-in
  fundamental in silicon .
• Number two because of the exponential character
  of the diodes current-to-voltage characteristics
  current through the diodes in the architecture
  is proportional to absolute temperature.
• By summing of the voltage drop on the diode with
  the current determined voltage drop on the
  resistor a temperature insensitive reference
  voltage is delivered when the slopes of the curve
  are properly adjusted. The reference voltage gets
  the value very close to the BandGap voltage.
• In 0.13um CMOS technology gap between the
  reference voltage and the power supply voltage
  becomes so narrow that voltage left for the
  current sources is not enough to keep the
  transistors in saturation mode. I makes the
  whole concept to fail.

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Abstract.   A CMOS bandgap reference circuit,
featuring dynamic-threshold MOS transistors
(DTMOST's) has been developed in the 0.13um CMOS
technology.  Insensitive to temperature and power
supply variations this cell is going to be a key
component for high quality A/D and D/A
converters.   The proposed circuit fits well into
the low supply-voltage range of the current and
future deep sub-micron technologies.   We have
carried out pre-design characterizations of the
DTMOST structures taken from an experimental
submit.   Design and specifications of the
bandgap voltage reference circuit is presented.  
The circuit was submitted in CuTe2 MPW submit in
May 2004.