IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 3, MARCH 2009
517
Dual-Material-Gate Technique for Enhanced
Transconductance and Breakdown Voltage
of Trench Power MOSFETs
Raghvendra S. Saxena and M. Jagadesh Kumar, Senior Member, IEEE
Abstract—In this brief, we propose a new dual-material-gatetrench power MOSFET that exhibits a significant improvement in
its transconductance and breakdown voltage without any degradation in on-resistance. In the proposed structure, we have split the
gate of a conventional trench MOSFET structure into two parts
for work-function engineering. The two gates share the control
of the inversion charge in the channel. By using 2-D numerical
simulation, we have shown that by adjusting the lengths of the two
gates to allow equal share of the inversion charge by them, we get
the optimum device performance. By using N+ poly-Si as a lower
gate material and P+ poly-Si as an upper gate material, approximately 44% improvement in peak transconductance and 20%
improvement in breakdown voltage may be achieved in the new
device compared to the conventional trench MOSFET.
Index Terms—Breakdown voltage, dual
on-resistance, power MOSFET, trench gate.
material
gate,
I. INTRODUCTION
T
RENCH-GATE technology [1]–[9] makes the MOSFET
a more attractive device for low-to-medium voltage power
applications as it provides reduced conduction power losses and
lower forward voltage drop due to the absence of parasitic JFET
region as compared to the planar power MOSFET. A trenchgate power MOSFET can be designed to have ultralow onresistance [1]–[7] with high switching speed [2], [3] and can be
fabricated easily using the standard Si process technology [7]–
[9]. In addition, since high packing densities can be achieved
using trench power MOSFETs [8], the on-resistance can be reduced significantly by the parallel conduction of a large number
of unit cells in a given chip area. These features make the trench
MOSFETs suitable for control switching, dc–dc converters,
automotive electronics, microprocessor power supplies, etc.
In various power electronic applications, low on-resistance,
higher drive current, low gate-to-drain capacitance, high
transconductance, and high breakdown voltage are the desired
Manuscript received August 12, 2008; revised November 6, 2008. Current
version published February 25, 2009. The review of this brief was arranged by
Editor M. A. Shibib.
R. S. Saxena is with the Department of Electrical Engineering, Indian
Institute of Technology, New Delhi 110 016, India, and also with the Solid
State Physics Laboratory, Delhi 110 054, India.
M. J. Kumar is with the Department of Electrical Engineering, Indian Institute of Technology, New Delhi 110 016, India (e-mail: mamidala@ieee.org).
Color versions of one or more of the figures in this brief are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2008.2011723
Fig. 1. Schematic DMGT MOSFET device. (a) Top view. (b) Cross-sectional
view along cut line AA .
features [10]–[13]. Designing a power MOSFET for a specific
application is a compromise among these parameters because
they are linked together by the technology and any attempt
to improve one may adversely affect the other, making the
device unsuitable in many other applications. The most desired
performance specifications in all the applications are low onresistance and high breakdown voltage. However, when we attempt to improve the breakdown voltage of a power MOSFET,
the on-resistance increases drastically [13], [14]. Therefore, to
overcome this difficulty, we propose a dual-material-gate trench
(DMGT) MOSFET structure that shows improvement not only
in breakdown characteristics but also in transconductance without any degradation in its on-resistance. A high transconductance makes the device suited for RF power amplification also
[11], [15]. By using 2-D numerical simulation in ATLAS device
simulator [16], we have analyzed the improved performance
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518
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 3, MARCH 2009
Fig. 2. Process steps to fabricate DMGT device.
of the proposed DMGT MOSFET by comparing it with a
conventional trench-gate MOSFET.
II. DEVICE STRUCTURE AND PROPOSED
FABRICATION PROCEDURE
Fig. 1(a) shows the schematic top view of the proposed
DMGT device, and Fig. 1(b) shows its cross-sectional view
along the cut line AA . As apparent from the figure, DMGT
device contains two sections of its trench gate. The upper
section (G1) has a higher work function gate material, whereas
the lower section (G2) has the smaller work function material.
The lengths of the two sections of the gate are denoted as L1
and L2 in Fig. 1. These sections are electrically connected
by a metal. The lower section of the gate material is chosen
to be N+ poly-Si (work function, φG2 = 4.17 eV), and the
upper gate material is chosen to be P+ poly-Si (work function,
φG1 = 5.25 eV). However, to analyze how the work function
difference between the two gate materials (ΔφG = φG1 −
φG2 ) affects the device performance, we have also varied the
value of φG1 from (φG2 + 0.25 eV) to (φG2 + 1.25 eV) in our
simulations.
The fabrication process of the DMGT structure is shown in
Fig. 2. First, by using some initial processing steps of a conventional trench-gate MOSFET fabrication [7]–[9], we create
a structure, as shown in Fig. 2(a), consisting of the N+ substrate (ND = 1 × 1019 cm−3 ) as the drain, a 0.1-μm-thick N+
source (ND = 1 × 1019 cm−3 ) on the top side, a 2.7-μm-thick
N-type drift region (ND = 1 × 1016 cm−3 ), and a 0.7-μmthick P-type body region (NA = 5 × 1017 cm−3 ). It also has a
1.2-μm-wide and 1.2-μm-deep trench with a 50-nm-thick gate
oxide layer grown inside the trench. In the next step, N+ polySi is done and CMP is carried out to get the structure shown in
Fig. 2(b). A selective etching of the upper part of the poly-Si,
followed by the oxide etching, forms the N+ poly lower gate
electrode as shown in Fig. 2(c). Again, by growing a 50-nmthick gate oxide followed by P+ poly deposition and CMP, we
realize the structure shown in Fig. 2(d). Finally, to take the gate
contact, we make a contact hole in the middle of the main trench
as shown in Fig. 2(e), having a depth just enough to reach the
lower gate material. It is then filled with a metal using standard
metallization process. The final structure is shown in Fig. 2(f).
For device simulation, we have created the DMGT structure in ATLAS and compared it with the conventional device
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SAXENA AND KUMAR: DUAL-MATERIAL-GATE TECHNIQUE FOR TRANSCONDUCTANCE AND BREAKDOWN VOLTAGE
519
Fig. 3. (a) Surface potential profile along the channel for different work function differences between G1 and G2. (b) Electric field profile along the channel
for different work function differences between G1 and G2. (c) Electric field profile for the conventional and DMGT devices with gate length L1 = 0.3 μm and
0.5 μm while keeping the total channel length L1 + L2 constant at 1.1 μm. (d) The corresponding electron velocity in the channel for the conventional and the
DMGT devices for different values of L1.
having the same parameters as mentioned earlier except that
the conventional device has only one gate material of N+
poly-Si.
III. SIMULATION RESULTS AND DISCUSSION
The performance of DMGT device depends on the work
function difference ΔφG of the two gates and their lengths L1
and L2. We have first simulated the DMGT device by varying
φG1 , keeping φG2 constant at 4.17 eV. For this analysis, we
kept L1 fixed at 0.3 μm. We have also analyzed the effect
of gate length variation of the DMGT device by varying L1
but keeping the total gate length constant (L1 + L2 = L).
With this analysis, we found the optimum gate length L1
to be 0.3 μm for achieving the best performance. Then, we
compared the optimized DMGT device with the conventional
device for current–voltage characteristics, transconductance,
on-resistance, and breakdown voltage.
The difference in the work function of the two gates causes
a change in the surface potential as shown in Fig. 3(a). It is
clear from the figure that as ΔφG increases, the step in the
surface potential profile also increases, modifying the channel
electric field as shown in Fig. 3(b). The channel electric field
for different values of L1 is shown in Fig. 3(c) for a fixed
ΔφG (1.08 eV, poly-Si gate case) in comparison with the
conventional device. In addition to the peaks at the body-source
junction and at the body-drift junction, the channel electric field
has one more peak in the DMGT device at the location where
gate G1 ends, whereas in a conventional device, the electric
field increases gradually in the channel. This modified electric
field profile results in a higher acceleration and higher velocity
of the charge carriers coming from the source, as shown in
Fig. 3(d), in the DMGT device (ΔφG = 1.08 eV). Hence,
a higher transconductance is expected in the DMGT device.
The additional peak in the channel electric field also provides
screening to the increased drain voltage, making the DMGT
device better from the hot-carrier suppression point of view.
The value of this peak has almost linear dependence on L1,
as shown in Fig. 4.
If the gate length is fixed at L = L1 + L2, for L1 = 0,
the device behaves like a conventional trench-gate MOSFET
with N+ poly gate length L = L2, and for L2 = 0, it again
becomes a conventional device with P+ poly gate length L =
L1. Therefore, the best advantage of the dual material gate is
expected to be in between these two extremes.
Because of the increase in peak channel electric field with
the increasing L1 (as shown in Fig. 4), the carrier transport
efficiency from the source to the channel is improved. However,
beyond a certain value of L1, this improvement reduces due
to the lowering of the second peak of the electric field at the
body-drift junction. As a result, compared to the conventional
single-gate case (having only N+ poly gate), the drive current
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520
Fig. 4. Peak value of the channel electric field in DMGT device as function
of gate1 length (L1).
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 3, MARCH 2009
At higher drain voltages, the electric field peak near the
end of the trench is responsible for the breakdown in both
conventional and DMGT devices and may be considered as the
main peak of the electric field. The additional electric field peak
in the channel of the DMGT device results in the reduction
of the main peak as compared to the conventional device,
and therefore, we observe an improvement in the breakdown
voltage. Furthermore, as L1 increases, the peak channel electric
field also increases (as shown in Fig. 4), giving a higher
breakdown voltage until L1 approaches the value of L (when
it reduces to a single-gate device), because at that situation, the
additional peak merges with the main peak of the electric field
to make it further high. The breakdown behavior of the DMGT
device, for various values of L1, along with the conventional
device (having only one N+ poly gate), is shown in Fig. 5(b),
indicating not only a significant increase in the breakdown
voltage in the DMGT device but also a monotonic increase
in the breakdown voltage with increasing L1 (of course, until
L1 < L), as expected.
The influence of the variation in L1 is summarized in Fig. 6,
where various performance parameters such as threshold voltage, transconductance, on-resistance, and breakdown voltage
are plotted as functions of L1. Fig. 6(a) shows the increase in
the threshold voltage in the DMGT device that can be taken care
of in the design by selecting appropriate body doping or gate
oxide thickness. The on-resistance and the transconductance of
the device, evaluated at VDS = 1.0 V, are shown as functions
of L1 in Fig. 6(b) and (c), respectively, indicating that the
best values are achieved at L1 = 0.3 μm. The breakdown
voltage variation with L1 is shown in Fig. 6(d) that indicates
a monotonic improvement in the breakdown voltage with increasing L1, as anticipated earlier.
From the earlier discussion, it may be inferred that the best
performance is achieved when L1 is kept at about half of the
total length of the p-region (∼0.3 μm), giving equal control of
the inversion charge in the p-region to both the gates G1 and G2
as shown in Fig. 1. When we compare the performance of the
optimized DMGT device (L1 = 0.3 μm) with the conventional
device, we find 44% improvement in peak transconductance,
20% improvement in breakdown voltage, 6% improvement in
drive current, and 0.5% improvement in on-resistance. Usually,
an improvement in breakdown voltage adversely affects the onresistance [11]–[13]. However, in the proposed device, although
there is a 20% improvement in the breakdown voltage, it does
not affect the on-resistance.
IV. CONCLUSION
Fig. 5. (a) Transfer characteristics of conventional device and DMGT device
and (b) the breakdown performance of conventional and DMGT devices with
different gate1 lengths (L1) and constant total channel length L1 + L2 =
1.1 μm.
first increases with increasing L1 and starts reducing when L1
is increased beyond the limit of 0.3 μm as shown in Fig. 5(a),
which shows the drive current of DMGT device (for different
values of L1) along with the conventional device as functions
of overdrive gate voltage (VGS − VTh ).
By using 2-D numerical simulations, we have demonstrated
that in a trench-gate power MOSFET, the sectioning of the
gate into two parts gives the flexibility of gate work-function
engineering. By using P+ poly in upper section and N+ poly
in deeper section of the gate, we have shown a significant
improvement in the device performance. The optimized performance is achieved when both the gates have equal control of the
inversion charge of the channel. We obtained 20% improvement
in the breakdown voltage and 44% improvement in the peak
transconductance in the optimized DMGT device over the
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SAXENA AND KUMAR: DUAL-MATERIAL-GATE TECHNIQUE FOR TRANSCONDUCTANCE AND BREAKDOWN VOLTAGE
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Fig. 6. Effect of gate1 length (L1) variation in DMGT device on (a) threshold voltage, (b) on-resistance, (c) transconductance, and (d) breakdown voltage with
constant total channel length (L1 + L2 = 1.1 μm).
conventional device. An additional electric field peak obtained
in the channel of the DMGT device also makes it better from
hot-carrier suppression point of view.
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Raghvendra S. Saxena received the B.E. degree
in electronics and communication engineering from
G. B. Pant Engineering College, Pauri Garhwal,
India, in 1997, and the M.Tech. degree in microelectronics from the Indian Institute of Technology,
Bombay, India, in 2003. He is currently working
toward the Ph.D. degree in the Department of Electrical Engineering, Indian Institute of Technology,
New Delhi, India.
Since 1998, he has been a Scientist with the Solid
State Physics Laboratory, Delhi, India, working on
the design, modeling, and characterization of infrared detectors and their
readout circuits. His current fields of interest are power electronic devices,
nanoscale VLSI devices, and infrared detectors. He has published about ten
papers in various journals and conference proceedings.
Mr. Saxena is a member of the Institution of Electronics and Telecommunication Engineers, India.
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 3, MARCH 2009
M. Jagadesh Kumar (M’95–SM’99) was born in
Mamidala, Andhra Pradesh, India. He received the
M.S. and Ph.D. degrees in electrical engineering
from the Indian Institute of Technology, Madras,
India.
From 1991 to 1994, he was with the Department
of Electrical and Computer Engineering, University
of Waterloo, Waterloo, ON, Canada, where he performed postdoctoral research on the modeling and
processing of high-speed bipolar transistors. While
with the University of Waterloo, he also did a research on amorphous-silicon TFTs. From July 1994 to December 1995, he was
initially with the Department of Electronics and Electrical Communication Engineering, Indian Institute of Technology, Kharagpur, India, and then, he joined
the Department of Electrical Engineering, Indian Institute of Technology,
New Delhi, India, where he became an Associate Professor in July 1997 and
a Full Professor in January 2005. His research interests include nanoelectronic
devices, modeling and simulation for nanoscale applications, integrated-circuit
technology, and power semiconductor devices. He has authored three book
chapters and more than 125 publications in refereed journals and conference
proceedings. His teaching has often been rated as outstanding by the Faculty
Appraisal Committee of IIT Delhi.
Dr. Kumar is a fellow of the Indian National Academy of Engineering and the
Institution of Electronics and Telecommunication Engineers (IETE), India. He
is an IEEE Distinguished Lecturer of the IEEE Electron Devices Society (EDS).
He is also a member of the EDS Publications Committee and EDS Educational
Activities Committee. He is an Editor for the IEEE TRANSACTIONS ON
ELECTRON DEVICES and Editor-in-Chief of IETE Technical Review. He is also
on the Editorial Board of the Journal of Computational Electronics, Recent
Patents on Nanotechnology, Recent Patents on Electrical Engineering, Journal
of Low Power Electronics, and Journal of Nanoscience and Nanotechnology.
He is the Lead Guest Editor of the joint special issue of IEEE TRANSACTIONS
ON E LECTRON D EVICES and IEEE T RANSACTIONS ON N ANOTECHNOLOGY
on “Nanowire Transistors: Modeling, Device Design, and Technology”
(November 2008 issue). He has extensively reviewed for different international
journals. He was the recipient of the 29th IETE Ram LalWadhwa GoldMedal
for his distinguished contribution in the field of semiconductor device design
and modeling and the 2008 IBM Faculty Award. He was the first recipient of
ISA-VSI TechnoMentor Award given by the India Semiconductor Association
in recognition of a distinguished Indian academician or researcher for playing
a significant role as a Mentor and Researcher.
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