DESIGN OF THREE BIT ANALOG-TO-DIGITAL CONVERTER (ADC) USING SPATIAL WAVEFUNCTION SWITCHED (SWS) FETS

International Journal of VLSI design & Communication Systems (VLSICS) Vol.4, No.3, June 2013
DOI : 10.5121/vlsic.2013.4301 1
DESIGN OF THREE BIT ANALOG-TO-DIGITAL
CONVERTER (ADC) USING SPATIAL WAVE-
FUNCTION SWITCHED (SWS) FETS
Supriya Karmakar
Intel Corporation, Hillsboro, OR, 97124
mr.karmakar@yahoo.com
ABSTRACT
The spatial wave-function switched field effect transistor (SWSFET) has two or three low band-gap
quantum well channels inside the substrate of the semiconductor. Applied voltage at the gate region of the
SWSFET, switches the charge carrier concentration in different channels from source to drain region. The
switching of electron wave function in different channels can be explained by the device model of the
SWSFET. A circuit model of SWSFET is developed in BSIM 4.0.0. The design of three bit analog-to-digital
converter (ADC) using one three wells SWSFET is explained in this work. Analog-to-digital converter
(ADC) circuit design using less number of SWSFET will reduce the device count in future analog and
digital circuit design.
KEYWORDS
Spatial wave-function switched FET, Integrated circuit, VLSI, analog-to-digital converter (ADC), digital-
to-analog Converter (DAC)
1. INTRODUCTION
Basic building block of any integrated circuit is metal-oxide-semiconductor field effect transistor
(MOSFET) which acts as a switch based on the applied voltage in its gate terminal. The cross
sectional schematic of a conventional MOSFET is shown in Fig. 1. The charge flow between the
source and drain region of the MOSFET is controlled by the applied voltage in its gate terminal.
The applied voltage in the gate terminal either enhances the channel formation or depletes the
channel formation between the source and drain region of the MOSFET. The density and the
performance of the MOSFET can be increased by decreasing it different parameters such as
channel length, channel width, gate oxide thickness and other dimensions. Research is ongoing to
improve MOSFET performance by controlling the different parameters like device structures [1-
3], gate dielectric materials [4-6], substrate doping, and source-drain doping profile of the device.
But when the feature sizes approach towards nm range, one of the major challenge is the gate
dielectric thickness which needs to be decreased to increase the gate capacitance and thereby the
drive current and the device performance.
The gate dielectric thickness below 2 nm, increases the leakage currents due to the direct
tunnelling of charge carriers which increases the power consumption and reduces device
reliability [7-9]. Other major scaling issues besides the gate dielectric thickness are gate-channel
interface states and surface charge doping fluctuations in the source and drain region, different
kinds of short channel effects such channel length modulation, quantum confinement in the
inversion layer etc. which deviates the transistor characteristics in the sub-nm range.

2
Figure 1: Cross-sectional schematic of conventional FET
The performance of integrated circuit can also be increased by only single polar based transistor
circuit operation because of the high electron mobility of the charge carriers. In SWSFET, the
charge population in the different channels depends on the applied voltage in its gate terminal
[10]. In this work we have shown the design of three bit analog-to-digital converter (ADC) based
on SWSFET. Before discussing the analog-to-digital converter (ADC), the SWSFET will be
introduced in section II. The fabrication and theory of operation of SWSFET is discussed in brief
in section III. The circuit model of SWSFET is discussed in section IV. The architecture of
membership function using SWSFET is discussed in section V which is followed by conclusion
in section VI.
(a)
Gate
Source Drain
Lower Channel(Channel #1)
Drain aSource
Substrate
Gate Insulator
Upper Channel(Channel #3)
Insulator
Insulator
(b)
Figure 2: Cross-sectional schematic of SWSFET (a) Two well (b) Three well
2. SPATIAL WAVE-FUNCTION SWITCHED FIELD EFFECT TRANSISTOR
(SWSFET)
Spatial wave-function switched (SWS) FET is a field effect transistor where two or three separate
low band gap channels are separated by a high band gap material between them. The cross
sectional schematic of a two-channel SWSFET and a three-channel SWSFET are shown in Fig. 2.
The applied gate voltage of a SWSFET switches the charge carrier concentration between the
channels. Different channels are connected to two different drain terminals. Current flows
through different drain terminals based on the applied voltage in its gate terminal. Three bit

3
analog-to-digital converter (ADC) can be designed by using one three well SWSFET which is the
minimum number of circuit elements than the existing any other architecture. The high electron
mobility of SWSFET also makes this circuit faster than the CMOS based conventional circuit
architecture.
3. FABRICATION AND THEORY OF OPERATION
Metalorganic chemical vapor deposition (MOCVD) growth of the InGaAs-AlInAs three
quantum- well structure on a p-InGaAs/p-InP wafer was the first step for substrate formation of
the SWSFET structure, which was followed by the selective regrowth of an n+- InGaAs layer to
form the source and drain regions.
The formation of source and drain regions was followed by the opening of the gate region. This is
followed by epitaxial growth of II–VI gate insulators. on the InGaAs top well layer using photo-
assisted metalorganic chemical vapor deposition (MOCVD).
Multiple-layer stack of ZnSe-ZnSZnMgS using ultraviolet (UV) radiation was deposited to form
the gate insulator. The first layer (serving as the buffer) was a ZnSe layer. This layer was grown
for 30 s with dimethylzinc (DMZn) and dimethylselenide (DMSe) growing a buffer layer of ZnSe
(as thin as possible).
This was followed by the formation of another stack layer of ZnS/ZnMgS/ZnS/ZnSe using
MOCVD technique.
The source - drain contact was formed using Gold-Arsenic following annealing in N2
environment at 300o
C. Finally aluminum metal gate was formed on top of the gate region of the
transistor.
The energy band-diagram of a two well SWSFET is schematically shown in Fig. 3. The charge
flows in a SWSFET through different channels based on the applied gate voltage. When the gate
voltage is low but above the threshold voltage (VTH1) of the device, charge carriers are confined in
the lower quantum well channel and flows from source to drain region. As the gate voltage is
increased (VTH2), charge carriers transfer from the lower channel to the upper quantum well
channel and current flows through the upper channel of the device. Based on the applied gate
voltage, the electron concentration as well as the electron wave function switches from one
channel to the other and current flows through different drain terminals. Fig. 4 shows the electron
wave-function switching between different quantum well channels in the SWSFET when the gate
voltage increases gradually from (a) to (b). Fig. 5 shows the charge density variations in different
channels with respect to gate voltage.
Fig. 6 shows the capacitance-voltage (C-V) characteristics of a fabricated two quantum well
channel InGaAs-AlInAs SWSFET [10-13]. The C-V curves show distinct peak before the
accumulation regime (gate voltage less than -2V) where the capacitance becomes constant. The
capacitance of the SWS device reaches to first maximum value at a gate voltage ~ -1 V. This
occurs when electrons are in the lower quantum well channel (channel#1). When the gate voltage
is increased further, electrons are transferred from the first quantum well layer (channel#1) to the
second quantum well layer (channel #2). The capacitance decreases as the carriers are in the
proximity of gate until total inversion is reached. The detailed device operation is already
published elsewhere [10-13].

4
Figure 3: Energy band diagram of SWSFET
(a) (b)
Figure 4: Device simulation for two well SWSFET [13]

5
Figure 5: Transfer characteristics from device simulation [13]
Figure 6: C-V characteristics of SWSFET [13]
Figure 7 shows the transfer of electron wave function in a three well SWSFET based on the
applied gate voltage. Based on the applied gate voltage, the electron wave function switches
between different channels and the drain current flows through different drai terminals. The
transfer characteristics of a three well SWSFET is shown in Fig.8.

6
Figure 7: Device simulation for three well SWSFET [10]
Figure 8: Transfer characteristics of a three well SWSFET from device simulation
4. SWSFET CIRCUIT MODEL
In this model the amount of charge in the different channels of the SWSFET is represented with
the current level in through different channels. In three well SWSFET, we consider channel#3 as
the channel which is further from the gate, and channel#1, which is closest to the gate. Channel#2
is in between these two.
Figure 9 shows the IDS-VGS characteristics of a three well SWSFET. Channel #3 turn on first, then
Channel#2 and Channel#1 respectively. The charge transfer occurs from bulk semiconductor to
the channel#3 first, then from channel#3 to channel#2 and channel#2 to channel#1. So initially in
channel#3, current increases after threshold voltage corresponding to that channel, and then it
reaches maximum value. Channel#2 turn on based on two mechanism: one based on its threshold

7
voltage, accumulation of charge in this channel and tunnelling of carriers from channel #3 to
channel#2. The maximum value on current in channel#3 is the mutual effect of charge population
in channel#3 because of charge accumulation from bulk semiconductor and tunnelling to
channel#2.
Similar explanation is true for channel#2. Here charge population is corresponding to three
effects: tunnelling from channel#3, accumulation of charge from bulk semiconductor and
tunnelling to channel#1. Channel#1 behaves like a inversion channel in conventional FET, only
difference is charge accumulation effect. Here additional charge accumulation is due to charge
tunnelling form channel#2.
Different circuit model parameters are shown in Table 1.
The threshold voltage in channel ‘a’ can be expressed as
Vtha = Vtha when VGSeff < VqL (a)
Vtha = Vtha + α(VGSeff – VqL) when VGSeff > VqL (b)
where α is a matching parameter and
( )
( )qLq
qLGSeff
VV
VV
−
−
=
1
α (c)
Simmilarly the threshold voltage in channel ‘b’ can be expressed as
Vthb = Vthb when VGSeff < Vq2 (d) (1)
Vthb = Vthb + β(VGSeff – Vq2) when VGSeff > Vq2 (e)
where β is a matching parameter and
( )
( )2
2
qqH
qGSeff
VV
VV
−
−
=β (f)
The threshold voltage in channel ‘c’ can be expressed as
Vthc = Vthc when VGSeff < VqH (g)
Voltage across the polysilicon gate can be expressed as
( )
2
62
62
1
10
22
1
2
10








−
−−
+=
OXgates
fFBGSOXgates
Poly
Cnq
VVCnq
V
ε
ϕε
(h)
Since the voltage across the poly-silicon gate does not exceed the silicon bandgap
voltage, the effective voltage across the poly-silicon gate is
( ) ( )



 +−−+−−−= 12.1..412.112.15.012.1
2
δδδ PolyPolyPolyEff VVV (i)
The effective gate voltage can be expressed as
PolyEffGSGSeff VVV −= (j)
where
ngate is the poly silicon gate doping concentrations
VqL is the transition voltage
Vq1 is the voltage corresponding to peak current in channel 3.

8
Vq2 is the transition voltage
VqH is the voltage corresponding to peak current in channel 2
Vtha is the threshold voltage of the channel 3
Vthb is the threshold voltage of the upper channel 2
Vthc is the threshold voltage of the upper channel 1
α is a matching parameter
VGS is the gate-source voltage
VGSeff is the effective gate-source voltage
VPolyEff is the voltage drop in the Poly Si gate
COX is the gate capacitance
VFB is the flat band voltage
Φf is the surface potential
q is the electron charge
δ = 0.01 is the parameter for DC VDSeff
ϵs is the permittivity
The drain current
( ) 





−−





=
2
2
DS
DSthaGSnOXDS
V
VVVC
L
W
I µ (2)
Table 1
SWSFET parameters
Parameter Value
Minimum L
Minimum W
Vtha
Vthb
Vthc
VqL
Vq1
Vq2
VqH
VDD
5.0 µm
10 µm
0.2 V
0.7 V
2.25 V
0.6 V
1.5 V
2.0 V
2.5 V
3.0 V
5. ANALOG-TO-DIGITAL CONVERTER (ADC)
The threshold voltages of three channels of SWSFET are different. Based on the input voltages,
different channels of the FET conduct for different input voltages.
Based on this charge transfer concept different channel has different amount of accumulated
charge based of gate voltage. When gate voltage is below the threshold voltage of the
channel#3(further from gate), no charge accumulation, the three well SWSFET is off, no current
in output.
State assignment concept sequence is as (channel#1 channel#2 channel#3). In the OFF state of the
FET, all channels are OFF and assignment of that state is (000) [0]. When some charge
accumulated in channel#3 and channel#2 is off, that state is assigned as (001) [1]. When charge
tunnelling starts from channel#3 to channel#2, channel#2 starts to conduct, get some current in

9
channel#2 as well as channel#3. Initially channel#3 charge is more, more current corresponding
to this channel than channel#2, this state is assigned as (011+
) [2]. When gate voltage increases,
charge carriers tunnel more from channel#3 to channel#2, sometimes, channel#3 current will be
less than channel#2, that state is assigned as (01+
1) [3]. Gradually, channel#3 will be empty of
charge, all charge will be in channel#2 at some point, and in this gate voltage range, only
conducting channel is channel#2. This state is assigned as (010) [4]. For more gate voltage,
tunnelling of charge carriers from channel#2 to channel#1 will start and channel#1 will start to
conduct. Based on the same concept between channel#3 and channel#2, different states are
assigned as (11+
0) [5], (1+
10) [6] and (100) [7].
Figure 9 shows the transfer characteristics from the model of three state SWSFET and assignment
of different states with the current levels of different channels of the FET.
Figure 9: Assignment of different states IDS-VGS (Transfer) Characteristics of triple FET
To identify the current levels in different channels, MOS inverter is used in the output of each
channel. Each channel is connected to the input of two comparator circuits of different threshold
voltages. The high threshold voltage comparator represents 1+
state, because it turns on at higher
reference voltage (for higher current level). Low threshold voltage comparator represents 1 state.
This step is equivalent to quantization of analog signal for analog-to-digital conversion. The
block diagram of the ADC circuit based on three well SWSFET is shown in Fig. 10.
CurrentinmA
Voltage in Volts

10
Input
signal
Comparator
Comparator
Comparator
Comparator
Comparator
Comparator
Three channel
SWSFET
ENCODER
MSB
LSB
Figure 10: Circuit diagram of analog-to-digital converter (ADC)
The encoder circuit comprised of a code converter and encoder. The code converter converts
different quantized input to different codes in three bit logic system.
The code converter activates anyone of 0-7 outputs at a time based on the A-F input waveforms
combination. Table 2 and table 3 represent this design.
The outputs show that, at a time only one output active, which represent the combination of
different channel charge status which depends on the input analog voltage applied in the gate of
the device.
The code converter is followed by the encoder. The encoder is designed based on the table 4. The
encoder design is shown in Fig. 11.
Table 2
A B C D E F Activated
output
0 0 0 0 0 0 0
1 0 0 0 0 0 1
1 1 Don’t care 0 0 0 2
0 0 1 Don’t care 0 0 3
0 0 1 1 0 0 4
0 0 1 1 1 0 5
0 0 1 Don’t care 1 1 6
0 0 0 0 1 1 7
A
B
C
D
E
F

11
Table 3
Table 4
Input to encoder MSB Central LSB
0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1
So MSB = 4 + 5 + 6 +7
Central = 2 + 3 + 6 +7
LSB = 1 + 3 + 5 + 7
Figure 11 shows the corresponding circuit implementation.
Activated output Input Combination based on Table#1
0 . . . . .A B C D E F
1 . . . . .A B C D E F
2 . .( ). . .A B C C D E F+
3 . . .( ). .A B C D D E F+
4 . . . . .A B C D E F
5 . . . . .A B C D E F
6 . . .( ). .A B C D D E F+
7 . . . . .A B C D E F

12
4
5
6
7
MSB
2
3
6
7
Central
1
3
5
7
LSB
Figure 11: Encoder Circuit diagram
The final Analog-to-Digital (ADC) output and input combination is shown in Figure 12.
The tabular form of ADC output is shown in Table 5.
Figure 12: Input and output waveform of the designed Analog-to-Digital converter

13
Table 5
Analog input
voltage (mV)
MSB Central LSB
0 – 62.5 0 0 0
62.5 – 135.0 0 0 1
135.0 – 197.5 0 1 0
197.5 – 260.0 0 1 1
260.0 – 312.5 1 0 0
312.5 – 375.0 1 0 1
375.0 – 437.5 1 1 0
437.5 – 500.0 1 1 1
6. CONCLUSION
In this paper the design of analog-to-digital converter based on spatial wave function switched
field effect transistor (SWSFET) is shown. The SWSFET can be fabricated using conventional
CMOS process. The basic advantage of using SWSFET is the number of circuit elements. Single
SWSFET based three bit ADC will give minimum device count for this circuit design. The
number of circuit element to design ADC circuit is reduced a lot compared to conventional
CMOS architecture. The SWSFET generally fabricated in InGaAs material systems. Because of
their higher electron mobility SWSFET based circuits are faster than other. The implementation
of three bit ADC using less number of circuit elements will make SWSFET a promising circuit
element in future communication circuit design.
ACKNOWLEDGEMENTS
Discussions with Prof. Faquir C. Jain and Dr. John A. Chandy, Dept. of Electrical and Computer
Engineering, University of Connecticut, Storrs, CT, are gratefully acknowledged.
REFERENCES
[1] Alan C. Seabaugh, William R. Frensley, John N. Randall, Mark A. Reed, Dewey L. Farrington and
Richard J. Matyi, “Pseudomorphic Bipolar Quantum Resonant-Tunneling Transistor”, IEEE
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[2] Jurgen Stock, Jorg Malindretos, Klaus Michael Indlekofer, Michael Pottgens, Arno Forster and Hans
Luth,”A Vertical Resonant Tunneling Transistor for Application in Digital Logic Circuits”, IEEE
Transaction on Electron Devices, Vol. 48, No. 6, June 2001.
[3] H. C. Lin, "Resonant Tunneling Diodes for Multi-Valued Digital Applications, “Proc. 24th IEEE Int.
Symp. Multiple –Valued Logic, pp. 188-195, 1994.
[4] Federico Capasso and Richard A. Kiehl, “Resonant tunneling transistor with quantum well base and
high –energy injection: A new negative differential resistance device”, Journal of Applied Physics,
Vol. 58, Issue 3, pp. 1366-68, 1985.
[5] Arno Forster, “Resonant tunneling diodes: The effect of structural properties on their performance”,
Advances in Solid State Physics, Vol. 33/1993, pp. 37-62.

14
[6] Jung-Hui Tsai, “High-performance AlInAs/GaInAs δ-doped HEMT with negative differential
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[7] Ru Huang, HanMing Wu, JinFeng Kang, DeYuan Xiao, XueLong Shi, Xia an, Yu Tian, RunSheng
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[11] Supriya Karmakar, Faquir C. Jain, John A. Chandy , Evan Heller “Circuit model of SWSFET to
implement multi-valued logic”, 21st CMOC Symposium, April 09, 2012, University of Connecticut,
Storrs, CT.
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[13] Supriya Karmakar, John A. Chandy and Faquir C. Jain, “Implementation of unipolar inverter based on
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Author
Dr. Supriya Karmakar received the M.Sc. degree in electronics and the M.Tech. degree in radio physics
and electronics, both from the University of Calcutta, Kolkata, India, in 1999 and 2001, respectively, and
the Ph.D. degree in electrical engineering from the University of Connecticut, Storrs, in 2011. His doctoral
research was on fabrication and circuit modeling of quantum dot gate FETs (QDGFETs). He joined Intel
Corporation, Hillsboro, OR, as an Engineer. His current research interests include the fabrication and
modeling of different semiconductor devices, such as QDGFETs, spatial wavefunction-switched FETs,
quantum dot channel FETs, and their application in multivalued logic.

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