Buffer Trenched Gate MOSFET

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GaN-BTG MOSFET: A TCAD
Numerical and comparative Study
By Harshit Soni
Under the supervision of Dr. (Prof) MM Tripathi
Department of Electrical Engineering
Delhi Technological University
(Delhi College of Engineering)
Bawana Road, Delhi-110042, India.
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In M.Tech course I have published following research papers
under the guidance of Dr. (Prof) MM Tripathi
• “Thermal Reliability of GaN-BTG-MOSFET for High-Performance Applications in
Integrated Circuits," in 2020 IEEE 40th International Conference on Electronics
and Nanotechnology (ELNANO) Ukraine, 2020, pp. 220-223: IEEE
• “Numerical Simulation and Parametric Assessment of GaN Buffered Trench Gate
MOSFET for Low Power Applications”.. IET Circuits, Devices & Systems. 2020 Apr
20, Wiley Publication
• “Novel GaN Buffered Trench Gate (GaN-BTG) MOSFET: A TCAD Numerical Study” in
8th International Conference on Computing, Communication and Sensor
Networks, 2019.

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Outline
Overview
Scaling of MOSFETs
Trenched Gate MOSFET
GaN-BTG MOSFET
Results and Conclusion
References
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3
6
5
7
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1
Miscellaneous
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Overview of MOSFET
MOSFETs are generally used in integrated circuits
because of the following reasons:
 Simpler fabrication process
 High package density
 High input impedance and lower power dissipation
 Unipolar device
 Very high input impedance
 4 terminal device: G, S, D, B

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Scaling of MOSFETs
 To achieve increased circuit density
 To increase device speed.
 Power dissipation
 Maximum operational frequency
 Production cost
Moore’s Law
Number of Transistors on an integrated circuit chip doubles every 18 months.

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Problem of scaling
 The internal electric fields in the device would increase
if the power supply voltages are kept the same.
 The longitudinal electric fields and the transverse
electric fields across the gate oxide, increase with
MOSFET scaling.
 Problems known as hot carrier effects and short
channel effects.
THE PROBLEMS ASSOCIATED WITH SCES:
 Threshold Voltage (Vth ) roll-off
 Hot-carrier effect
 Punch-through
 Gate tunneling/leakage current

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Conventional Trenched Gate(CTG) MOSFET
 Trenched Gate MOSFET is a promising candidate
for suppressing the various SCEs and HCEs.
 Two potential barriers are formed at the two
corners due to high density of electric field lines.
Carriers in the channel now requires more energy
to surmount these barriers, which limits its carrier
transport efficiency and hence, lowering the current
driving capabilities of the device.

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Contd….
Parameter Unit
Oxide Thickness (tox) 5nm
Buffer Thickness 2nm
Gate Length (Lg) 26 nm
Length of channel (Lch) 40 nm
Device Length (l) 100 nm
Device Breadth(b) 70nm
Vgs/Vds 2V/0.2V
Permittivity of SiO2 ( ɛox ) 3.9
Device doping (ND
+) 5×1019 cm-3
Electrode Thickness
S: 10nm
D: 10nm
G: 26nm
MOSFET PARAMETERS

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Simulation
Results
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Comparison between CTG, BTG and
GaN-BTG MOSFET’s Characteristics
Use of high-K buffer increases the gate oxide
capacitance (Cox), which leads to the increased Gm and
results in a higher electric field, Electric field further
increases for GaN BTG MOSFET because of higher
electron mobility of GaN as compared to Silicon
Change in electron mobility of material a higher
electron velocity is observed in case of GaN-BTG-
MOSFET.

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Rate of increase of Drain current is
highest for GaN BTG MOSFET
because of improved control of
gate bias voltage.
Transconductance (gm1) is
increased in the channel with the
introduction of High-K buffer.

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An improvement of 8.33% in SS
of BTG-MOSFET and an overall
improvement of 43.85% in SS of
GaN-BTG-MOSFET
The threshold voltage curve reveals that
Vth of BTG-MOSFET observed a decrement
of 1.8% and that of GaN-BTG-MOSFET
reduced by 9.83% when compared to that
of a conventional MOSFET.

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Comparison between CTG, BTG and GaN-BTG
MOSFET’s Characteristics with variation in
channel length from 15 nm to 40 nm
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Drain current reduces slightly
with reduction in channel length
of MOSFET because of increased
scattering in the channel.
Transconductance (gm1) of GaN-BTG
MOSFET reduces with reduced channel
length as mobility of electrons in the
channel is reduced with increase in
scattering.

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High switching ratio of a device
is preferred for high frequency applications.
Here we observe the maximum switching ratio for
40nm channel length for GaN-BTG-MOSFET, which
decreases with reducing channel length of MOS.
With a reduction in channel length, GaN BTG
MOSFET works at a lower threshold voltage and
depicting its power efficient behavior. At 15nm
channel length, it can be observed threshold voltage
value is 0.34V which is 57% of the threshold voltage
at 40 nm channel length.

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Comparison between CTG, BTG and GaN-
BTG MOSFET’s Characteristics with variation
in doping concentration as depicted in table
below:
Doping Level P-Type N-Type
Doping 1 1*1017 1*1018
Doping 2 5*1017 7*1018
Doping 3 1*1018 1*1019

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Transconductance for different
doping Concentration. The
transconductance of GaN-BTG-
MOSFET stays zero if doping
concentration is expanded past level 2
doping i.e. GaN-BTG-MOSFET behaves
as open circuit if doping concentration
is increased past level 2, because of
increase in on resistance of MOSFET
when doping concentration is
improved. High On resistance is
because of increase in scattering of
charge carriers in MOSFET

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Transfer characteristics for different
doping Concentraion. The transfer
characteristics of GaN-BTG-MOSFET
stays zero (of order nA) if doping
concentration is increased beyond
level 2 doping i.e. GaN-BTG-MOSFET
behaves as open circuit when doping
concentration is increased past level
2, indicating high on resistance of
MOS

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Here we observe that switching ratio increases
with increased doping concentration in MOS,
i.e. in order to design a MOSFET for high
frequency applications, device doping
concentration should be high.
The threshold voltage of the device has been
increased by 392.85% and is noted to be 2.415V,
i.e. MOSFET consumes more power and is not
power efficient in case of increased Doping
concentration because of increase in On
Resistance of MOS

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Comparison between CTG, BTG and GaN-BTG
MOSFET’s Characteristics with variation in
oxide thickness (HfO2) 1nm,2nm and 3nm
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Fig. infers that the decrement in the thickness of the oxide
increases the transconductance of the device. Here 33.3%
improvement is observed in triode region while diminishing
effective thickness of the oxide (tox) from 3nm to 1nm.
With the decrease in effective oxide thickness (tox), gate
capacitance Cox improves which results in better gate control.
With increased Cox, the transconductance of the device is
improving which results in better channel current Ids and is a
result that drain current for 1nm is high compared to oxide
thickness of 2,3nm.

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On diminishing the thickness of oxide to 1nm,
threshold voltage reduces to 0.465V (0.8% reduction of
2nm), and on expanding thickness of oxide to 3nm, the
threshold voltage of 0.481V is recorded (2% increase of
2nm). This information shows that with variation in oxide
layer thickness, there is minute effect on the operational
behavior of GaN-BTG-MOSFET and GaN-BTG MOSFET can
be scaled with less thick oxide layer.

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Contour Plots for Electron Velocity
CTG MOSFET BTG MOSFET
GaN-BTG MOSFET

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Contour Plots for Electron Mobility
CTG MOSFET BTG MOSFET
GaN-BTG MOSFET

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With increase in temperature drain current of both the devices
increases because of increase in electron mobility. Higher drain current is
observed in case of GaNBTG MOSFET as compared to CTG MOSFET as
electron mobility of GaN is more than electron mobility of Si.
Transfer characteristics

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Transconductance
Transconductance of GaN BTG is more than CTG MOSFET because of high electron mobilityof GaN. With
increase in temperature electron scattering increases resulting in reduced transconductance for both CTG
and GaN BTG MOSFET

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Resistance
At 300K GaN BTG MOSFET has high cutoff resistance and a very low saturation region resistance,
depicting that use of GaN benefits in switching. With increase in temperature, resistance of GaN BTG
MOSFET reduces but still curve of GaN BTG MOSFET is better than CTG MOSFET.

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CONCLUSION
I. GaN-BTG MOSFET exhibit superior electric field, electron velocity, drain
current and higher transconductance.
II. GaN-BTG MOSFET has lower threshold voltage.
III. Lower off-current is observed for GaN-BTG-MOSFET, resulting in less energy
consumption.
IV. GaN BTG MOSFET serves as a promising device for reducing short channel
effects (SCEs).
V. Thus, signify that GaN-BTG MOSFET is more reliable and serves as
a promising candidate for analog and low power high linearity applications.

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REFERENCES
[1] N. Arora, "MOSFET Models for VLSI Circuit Simulation Theory and Practice Springer," New York, 1993.
[2] S. Veeraraghavan and J. G. Fossum, "Short-channel effects in SOI MOSFETs," IEEE Transactions on Electron Devices, vol. 36, pp. 522- 528,
1989.
[3] A. Kumar, "Effect of trench depth and gate length shrinking assessment on the analog and linearity performance of TGRC-
MOSFET," Superlattices and Microstructures, vol. 109, pp. 626-640, 2017/09/01/ 2017.
[4] A. Kumar, N. Gupta, and R. Chaujar, "Analysis of novel transparent gate recessed channel (TGRC) MOSFET for improved analog
behaviour," Microsystem Technologies, vol. 22, pp. 2665-2671, 2016.
[5] A. Kumar, M. Tripathi, and R. Chaujar, "Investigation of parasitic capacitances of In2O5Sn gate electrode recessed channel MOSFET for ULSI
switching applications," Microsystem Technologies, vol. 23, pp. 5867-5874, 2017.
[6] A. Kumar, M. Tripathi, and R. Chaujar, "Reliability Issues of In2O5Sn Gate Electrode Recessed Channel MOSFET: Impact of Interface Trap
Charges and Temperature," IEEE Transactions on Electron Devices, vol. 65, pp. 860-866, 2018.
[7] A. Kumar, N. Gupta, and R. Chaujar, "TCAD RF performance investigation of Transparent Gate Recessed Channel MOSFET," Microelectronics
Journal, vol. 49, pp. 36-42, 2016.
[8] J. Appenzeller, R. Martel, P. Avouris, J. Knoch, J. Scholvin, J. A. del Alamo, et al., "Sub-40 nm SOI V-groove n-
MOSFETs," IEEE Electron Device Letters, vol. 23, pp. 100-102, 2002.
[9] R. S. Pengelly, S. M. Wood, J. W. Milligan, S. T. Sheppard, and W. L. Pribble, "A review of GaN on SiC high electron-
mobility power transistors and MMICs," IEEE Transactions on Microwave Theory and Techniques, vol. 60, pp. 1764-1783, 2012.

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[10] I. SILVACO, "ATLAS User’s Manual," Santa Clara, CA, Ver, vol. 5, 2011.​
[11] K.-S. Im, J.-B. Ha, K.-W. Kim, J.-S. Lee, D.-S. Kim, S.-H. Hahm, et al.,
"Normally off GaN MOSFET based on AlGaN/GaN heterostructure with extremely high 2DEG density grown
on silicon substrate," IEEE Electron Device Letters, vol. 31, pp. 192-194, 2010.​
[12] C. Tsai, T. Wu, and A. Chin, "High-Performance GaN MOSFET With High-$kappa $$hbox {LaAlO} _ {3}/hbox {SiO} _
{2} $ Gate Dielectric," IEEE Electron Device Letters, vol. 33, pp. 35-37, 2011.​
[13] B. J. Baliga, "Trench gate lateral MOSFET," ed: Google Patents, 1995.
[14] C.-Y. Tsai, T. Wu, and A. J. I. e. d. l. Chin, "High-Performance GaN MOSFET With High-$kappa $$hbox {LaAlO} _
{3}/hbox {SiO} _ {2} $ Gate Dielectric," vol. 33, no. 1, pp. 35-37, 2011.
[15] S. Veeraraghavan and J. G. J. I. T. o. E. D. Fossum, "Short-channel effects in SOI MOSFETs," vol. 36, no. 3, pp. 522-528,
1989.
[16] N. D. Arora, MOSFET models for VLSI circuit simulation: theory and practice. Springer Science & Business Media,
2012.
[17] B. Doris et al., "Extreme scaling with ultra-thin Si channel MOSFETs," in Digest. International Electron Devices
Meeting, 2002, pp. 267-270: IEEE.

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33. Simulation Models
Following models involved during simulation:
S.No. Physical Models Description
1. Mobility Models Lombardi CVT and Constant Low Field Mobility Model.
2.
Recombination
Model
Shockley Read Hall (SRH) Recombination is included to
incorporate minority recombination effects with carrier
lifetime=1×107s.
3. Statistics
Boltzmann Transport model, The use of Boltzmann
statistics is normally justified in semiconductor device
theory, but Fermi-Dirac statistics are necessary to account
for certain properties of very highly doped (degenerate)
materials.
Major 2020 A1

34. Contd…
S.No. Physical Models Description
4.
Impact ionization and
Tunneling Model
Such model is used for evaluating hot-carrier
performance.
5. Energy Transport Model
Hydrodynamic Model is used as it includes all
nonlocal effects and is more accurate than the drift-
diffusion method. Drift diffusion Model show short
comings as channel length scales down to 50nm.
Major 2020 A2