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Journal ArticleDOI

A p-Channel GaN Heterostructure Tunnel FET With High ON/OFF Current Ratio

23 May 2019-IEEE Transactions on Electron Devices (Institute of Electrical and Electronics Engineers (IEEE))-Vol. 66, Iss: 7, pp 2916-2922

AbstractA novel mechanism to achieve a nonambipolar tunnel FET (TFET) is proposed in this paper. The method relies on polarization charge induced in semiconductors, such as group III nitrides, to enhance the electric field across the junction and facilitate unidirectional tunneling based on the polarity of the applied gate bias. This also enables enhanced control over the tunneling distance, reducing it significantly in comparison to a conventional TFET. The proposed p-channel device implemented in a novel vertical GaN nanowire geometry facilitates a reduction of footprint while still maintaining comparable performance to that of conventional E-mode p-channel devices in GaN. This opens up possibilities for E-mode p-channel GaN devices.

Topics: Gallium nitride (51%)

Summary (2 min read)

I. INTRODUCTION

  • Forming a tunnel junction in GaN with a low tunnel resistance is challenging, in part due to the large band gap that increases the tunneling barrier height and electric field required to produce sharp band bending.
  • Therefore, a thin layer of either AlN [9] , [10] , InGaN [11] - [13] or InN [14] is sandwiched between pand n-type GaN regions to introduce additional polarization charge at each of its interfaces to raise the electric field between the p-and n-type regions, thereby facilitating tunneling.
  • Moreover, conventional TFETs suffer from ambipolarity which results in high offcurrent [19] and limits their applicability in complementary circuits [20] .
  • The work was partially funded by ENIAC-JU project E2SG under grant contract number 296131.

II. MODEL AND CALIBRATION

  • All results are obtained using Silvaco TCAD [25] , where the inbuilt non-local band-to-band tunneling (BBT) model along with III-nitride specific field-dependent mobility model [26] , [27] , Shockley-Read-Hall, and Augur recombination are selected for the tunneling current and device electrical characteristics.
  • Unlike the local tunneling models, where the tunneling rate at each point is calculated from the localised value of electric field, a non-local BBT model that includes a local variation of energy bands is employed in the interests of accuracy [27] , while quantum confinement effects are neglected.
  • In conventional tunneling devices, such as Zener diodes in Si, degenerately doped regions located adjacently, achieve high band bending that facilitates carrier tunneling through the energy band gap.
  • To overcome this, a thin layer of AlN is sandwiched between p-and n-type regions (inset Fig. 1 (b) ), which introduces polarization charge at each of its interfaces with GaN.
  • This is because a thin AlN layer fails to provide sufficient band bending required for the BBT, while a thicker AlN increases the length of the tunnel barrier, resulting in exponential degradation of the tunnelling current.

III. NON-AMBIPOLAR OPERATION OF P-CHANNEL GAN HTFET

  • Fig. 3 (a) shows a schematic diagram of the p-channel GaN HTFET with its vertical direction along [0001] .
  • As shown in the corresponding band diagram in Fig. 3 (b), the u-GaN acts as channel and maintains the valence band sufficiently lower than both the hole quasi fermi level (ℎ + 𝑄𝐹𝐿) as well as the conduction band in the n-GaN, thus preventing the tunneling of carriers when the gate bias is zero.
  • The bias requirement of this device also increases to raise the energy of the valence band for alignment with the conduction band across the AlN layer.
  • A lightly doped channel could also be employed to reduce the operating bias of this device.
  • As can be observed, the drain current remains orders of magnitude lower at positive gate bias than at negative gate bias, thus confirming non-ambipolar behaviour, as indicated by the band diagrams (Fig. 3 (d) ).

IV. OPTIMISED CYLINDRICAL P-CHANNEL GAN HTFET

  • The most common technique to improve the on-current in TFETs is to introduce a highly doped pocket of opposite polarity in the vicinity of the source edge of the channel to enhance the electric field across the tunneling junction [39] , [40] .
  • The current is normalised to the diameter of the cylindrical geometry.
  • In the absence of the thin AlN layer at the top edge of the gate, a positive gate bias alone is insufficient to produce a large band bending at this interface, hence the device continues to remain non-ambipolar.
  • Owing to the higher recombination, the maximum drain current of the device degrades.
  • This increase comes at the cost of increase in SS and leakage current, which arises from an inability of the gate to maintain the same potential across a wider channel.

V. ANALYSIS OF TUNNEL DISTANCE

  • To contrast the operation of the PITJ HTFET, from a conventional TFET employing group IV semiconductor such as Si, in Figs. 7 (a ) and (b), the band diagrams during the OFF and ON states are compared with a conventional double gated p-i-n TFET in Si.
  • The figures indicate the tunnel distance, defined as the minimum horizontal distance between the valence and conductance bands.
  • The tunneling region also moves away from the gated channel region therefore resulting in a weaker gate control.
  • To further highlight the distinction in operation, the, transfer characteristics and tunneling distances of the two devices are compared in Fig. 8 .
  • Due to a large bandgap even though the maximum on-current is smaller, a wider band gap in GaN as well as a better control of the tunneling distance, limited only by the thickness of the tunnel barrier, lead to a higher ON/OFF current ratio and a steeper SS.

VI. CONCLUSION

  • In summary, an analysis of a p-channel heterostructure TFET in GaN reveals that owing to a polarization induced tunnel junction, transfer characteristics do not suffer from ambipolarity.
  • Unlike contemporary p-channel MOSHFETs in GaN, the transfer characteristics show normally-off operation with a threshold voltage greater than |−4| 𝑉, along with a subthreshold swing of 36 𝑚𝑉/𝑑𝑒𝑐.
  • In addition, since the region of tunneling is pinched to the location of the PITJ, a better electrostatic control over the tunneling region via the gate and reduction in the tunnel distance by a factor of 2 are shown in the present device compared to the conventional TFETs.
  • Further improvements in the on-current and reduction in the supply voltage are expected for the PITJ based on smaller band gap materials such as InGaN or InN instead of AlN.

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ON/OFF current ratio.
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Version: Accepted Version
Article:
Kumar, A. orcid.org/0000-0002-8288-6401 and De Souza, M.M. orcid.org/0000-0002-
7804-7154 (2019) A p-channel GaN heterostructure tunnel FET with high ON/OFF current
ratio. IEEE Transactions on Electron Devices, 66 (7). pp. 2916-2922. ISSN 0018-9383
https://doi.org/10.1109/ted.2019.2915768
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A p-channel GaN Heterostructure Tunnel FET with
High ON/OFF Current Ratio
Ashwani Kumar and Maria Merlyne De Souza
AbstractA novel mechanism to achieve a non-ambipolar
Tunnel FET (TFET) is proposed in this work. The method relies
on polarization charge induced in semiconductors, such as group
III nitrides, to enhance the electric field across the junction and
facilitate unidirectional tunneling based on the polarity of applied
gate bias. This also enables enhanced control over the tunneling
distance, reducing it significantly in comparison to a conventional
tunnel FET. The proposed p-channel device implemented in a
novel vertical GaN nanowire geometry facilitates a reduction of
footprint while still maintaining a comparable performance to
that of conventional E-mode p-channel devices in GaN. This opens
up possibilities for E-mode p-channel GaN devices.
Index TermsBand-to-band tunneling, tunnel field effect
transistor, subthreshold swing (SS), wide band gap materials, III-
nitrides, tunneling resistance, Heterojunction TFETs.
I. INTRODUCTION
Increasing attention is being divested currently in low
resistance tunnel junctions in III-nitrides in order to improve
the efficiency of visible and ultraviolet light-emitting diodes
(LEDs) [1][3] by elimination of p-type contacts in GaN [4]
[6]. Forming a tunnel junction in GaN with a low tunnel
resistance is challenging, in part due to the large band gap that
increases the tunneling barrier height and electric field required
to produce sharp band bending. An increased diffusion of Mg
ions at high temperature [7] and a large activation energy of
 [8] also make it difficult to achieve degenerately
doped p-type GaN, necessary to form a tunnel junction with
abrupt band bending. Therefore, a thin layer of either AlN [9],
[10], InGaN [11][13] or InN [14] is sandwiched between p-
and n- type GaN regions to introduce additional polarization
charge at each of its interfaces to raise the electric field between
the p- and n- type regions, thereby facilitating tunneling. This
type of tunnel junction which is aided by polarization charge
is referred to as polarization-induced tunneling junction (PITJ)
[10].
A PITJ with a thin InN in a novel n-channel tunnel FET
(TFET) in GaN was predicted with an on-current of
 in a simulated fin geometry and a SS of
 with an ON/OFF current ratio  orders of
magnitude in a sidewall-gated cylindrical geometry,
respectively [14]. In another TCAD based simulation study, an
inline-gated rectangular TFET with InN based PITJ
demonstrated an on-current of  with a SS of
, and an ON/OFF ratio of 5 orders of magnitude
[15], while the maximum bias was kept at .
Recent progress in p-type doping in excess of 



facilitated by low temperature MBE growth has led to
demonstration of a direct tunnel junction between degenerately
doped p- and n-type GaN [6], [16]. This device achieved a
differential resistivity of 


[16], an order of
magnitude lower than the lowest reported resistivity in a PITJ
using In
0.25
Ga
0.75
N [13]. Based on this study the realisation of
GaN based TFETs without PITJs, can also be envisioned. In
this work however, we have analysed TFETs with embedded
PITJ for facilitating transport, rather than as low resistance
contacts.
Conventional TFETs suffer from poor on-current, because
the tunneling mechanism introduces an additional resistance in
the source-drain path relative to a MOSFET. To date, no one
has yet demonstrated a TFET of comparable current level to a
MOSFET with subthreshold slope (SS) below 
[17]. The best reported electrical characteristics were achieved
in a vertical nanowire InAs/GaAsSb/GaSb TFET, which
showed an on-current of  at  of SS
[18]. At a minimum SS of  the maximum current
degraded to  in this device. Moreover, conventional
TFETs suffer from ambipolarity which results in high off-
current [19] and limits their applicability in complementary
circuits [20]. To address ambipolarity, short-gated TFET [21],
asymmetric doping, and band gap engineering [22], have been
proposed.
In this work, a p-channel heterostructure tunnel FET
(HTFET), utilising a thin layer of AlN as a PITJ, is introduced.
Despite the larger band gap of AlN, which results in higher
tunneling resistance in comparison to InGaN or InN, a thin
layer of AlN is adopted [5]. Since the polarity of polarisation
charge arising from InN or InGaN is opposite to that of AlN,
employing either InN or InGaN in forming a PITJ can be
accommodated by simply inverting the device geometry.
However, a significant lattice mismatch between GaN and InN
 [23] results in strain that can introduce challenges to
the growth of InN or InGaN on GaN. In comparison, a lattice
mismatch  between GaN and AlN [23] implies that up
to  of fully strained AlN can be grown on GaN without
the introduction of microcracks [24].
This article is organised as follows: In section II, the model
of the tunneling current and its calibration with reported
experimental results from the literature are presented [10].
Sections III and IV are dedicated to explaining the non-
ambipolar operation of the p-channel HTFET. Section V
presents a unique behaviour of the tunneling region and tunnel
distance in this device in contrast to that in the conventional
The authors are with the Department of Electronic & Electrical
Engineering, The University of Sheffield, George Porter
Building, Broad Lane, Sheffield S3 7HQ, UK (email:
m.desouza@sheffield.ac.uk). The work was partially funded by
ENIAC-JU project E2SG under grant contract number 296131.

TFETs. Finally, in section VI, key results of our analysis are
summarised.
II. MODEL AND CALIBRATION
All results are obtained using Silvaco TCAD [25], where the
inbuilt non-local band-to-band tunneling (BBT) model along
with III-nitride specific field-dependent mobility model [26],
[27], Shockley-Read-Hall, and Augur recombination are
selected for the tunneling current and device electrical
characteristics. Unlike the local tunneling models, where the
tunneling rate at each point is calculated from the localised
value of electric field, a non-local BBT model that includes a
local variation of energy bands is employed in the interests of
accuracy [27], while quantum confinement effects are
neglected. W. Li et al. [15] have previously reported that a
negligence of such quantum effects in TCAD models leads to
an underestimation of the drain current compared to
nonequilibrium Green’s function (NEGF) simulations of their
HTFET utilising an InN based PITJ.
TABLE I
Summary of Parameters Used in the Simulations
Parameter
Description
Value

Electron tunnel mass in
GaN

a

Hole tunnel mass in GaN

a

Electron tunnel mass in AlN

a

Hole tunnel mass in AlN

a

Permittivity of Al
2
O
3
gate dielectric
 [27]
Polarisation charge
density at AlN/GaN
interface





[28]
Maximum hole mobility in
GaN

 [29]

Band gap of GaN
 [30]

Band gap of AlN
 [30]

󰇛󰇜
Activation energy for
donor dopant
 [31]

(Mg)
Activation energy for
acceptor Mg dopant
 [29]
Density of donor (Si)
doping




Density of acceptor (Mg)
doping





Energy level of traps
with respect to valence
band in TAT simulation

b
a
Calibrated from the values reported in [32]
b
Consistent with Mg
+
and other cation traps in GaN [33]
In all simulations, the maximum hole mobility is limited
to
 [29], while an activation energy of  is
used for acceptor dopants, consistent with the reported
activation energy of Mg in GaN. Unless stated otherwise, the
doping density in all n- and p- type regions is kept at




. A list of all the important material parameters
employed in the simulations are provided in Table I.
In conventional tunneling devices, such as Zener diodes in
Si, degenerately doped regions located adjacently, achieve high
band bending that facilitates carrier tunneling through the
energy band gap. Fig. 1 (a) and the inset therein show the
simulated energy band diagrams for a degenerately doped
(




) p-n junction in GaN. The
depletion width at the junction in this case is more than ,
which greatly suppresses band to band tunneling of carriers on
either side. To overcome this, a thin layer of AlN is sandwiched
between p- and n- type regions (inset Fig. 1 (b)), which
introduces polarization charge at each of its interfaces with
GaN. This results in a high electric field across AlN
( [10]), thereby providing a sharp band bending
to enable band-to-band tunneling, as observed in Fig. 1 (b).
This mechanism has been utilized to implement tunnel diodes
and light emitting diodes [9][11].

Fig. 2. (a) Schematic of a  
GaN Zener diode with a 2.8
nm AlN barrier layer sandwiched between p-GaN and n-GaN, (b)
Comparison of our simulation model with the reported experiment data
reported from [10] (Adapted from Fig. 3 (a) with permission from [10]
Copyright (2009) by the American Physical Society). The inset shows
the simulation results of on-current for different AlN thicknesses.
The tunneling rate via the non-local BBT model is
benchmarked by adjusting the effective electron tunnel mass
and hole tunnel mass
, using reported I-V data for a
 
GaN tunnel diode, a two-terminal device, with a
2.8 nm AlN barrier and  and  ohmic contacts for
p- and n- GaN, shown in Fig. 2 (a) from [10]. This device
showed a total specific resistivity (including the tunnel
resistance) of 
. Effective masses
 and
 for GaN and
 and
 for AlN
[23], [32], produce simulated I-V results that closely follow
experimental characteristics, as shown in Fig. 2 (b), plotted for
different contact resistivity values of the p-GaN contact. A
good match between the model and experimental results is
achieved with a contact resistivity anywhere between 




, which agrees with contact resistivities in the range
of 




for  [34], [35]. Since better
contacts to p-GaN, with
as low as 


have been
realised with a proper choice of metal stack [36], we
optimistically employ 


as the contact resistivity in
all the simulations of a three-terminal p-channel HTFET. With
this value of contact resistivity, the behaviour of the on-current
(current at  of applied bias) at different thicknesses of AlN
is plotted in the inset in Fig. 2 (b). As seen, the device current
remains close to zero for AlN thickness less than  or
greater than . This is because a thin AlN layer fails to
provide sufficient band bending required for the BBT, while a
thicker AlN increases the length of the tunnel barrier, resulting
in exponential degradation of the tunnelling current. The on-
current of the device peaks around  of AlN thickness,
resulting in a minimum total specific resistivity of 



.
III. NON-AMBIPOLAR OPERATION OF P-CHANNEL GAN
HTFET
Fig. 3 (a) shows a schematic diagram of the p-channel GaN
HTFET with its vertical direction along [0001]. From the
bottom to the top, the structure consists of a  n-GaN
source,  AlN tunneling barrier,  undoped GaN (u-
GaN) channel and  p-GaN. The energy of carriers in the
u-GaN channel is modulated by a  Al
2
O
3
separated gate,
either in rectangular geometry with double gate or cylindrical
geometry with a gate-all-around architecture. Unless stated
otherwise, the width of the device in either rectangular or
cylindrical geometry is kept at , in-line with the
minimum reported GaN nanowire widths of 14 nm or 10 nm
reported via experiment in [37], [38]. As shown in the
corresponding band diagram in Fig. 3 (b), the u-GaN acts as
channel and maintains the valence band sufficiently lower than
both the hole quasi fermi level (
) as well as the
conduction band in the n-GaN, thus preventing the tunneling
of carriers when the gate bias is zero.
A negative gate bias, raises the energy of the bands in the
channel, moving the valence band closer to the hole quasi
fermi level (
), as shown in Fig. 3 (c), which leads to an
increase in hole concentration. At sufficiently large negative
gate bias, the valence band in the channel aligns with the
conduction band of n-GaN, hence enabling tunneling across the
AlN barrier, as indicated by the arrow in Fig. 3 (c) and turning
the device on. On the other hand, a positive gate bias reduces
the energy levels of the conduction and valence bands in the u-

GaN channel, as shown in Fig. 3 (d). However, since no AlN
layer is present at the interface between p-GaN and u-GaN, the
band bending between p-GaN and u-GaN occurs over a large
distance, which greatly suppresses the tunneling current to
maintain the device in the off-state.
The drain to source current

with respect to

for a
rectangular or fin geometry is plotted in Fig. 3 (e) for a device
or channel width of . In contrast to the n-channel GaN
HTFET reported in [14] where the channel is also doped,
utilisation of u-GaN as a channel layer reduces the leakage by
10 orders of magnitude. However, the bias requirement of this
device also increases to raise the energy of the valence band for
alignment with the conduction band across the AlN layer. A
lightly doped channel could also be employed to reduce the
operating bias of this device. As can be observed, the drain
current remains orders of magnitude lower at positive gate bias
than at negative gate bias, thus confirming non-ambipolar
behaviour, as indicated by the band diagrams (Fig. 3 (d)).
Owing to the wide band gap nature of GaN, the off-current of
the device remains much lower at


 
even for a
small channel length of . A large band gap of AlN
however, also introduces a large tunneling barrier height,
which makes it difficult for the charge carriers to tunnel across
the AlN. Hence the maximum drain current or on-current
remains limited to  in a fin geometry.
IV. OPTIMISED CYLINDRICAL P-CHANNEL GAN HTFET
The most common technique to improve the on-current in
TFETs is to introduce a highly doped pocket of opposite
polarity in the vicinity of the source edge of the channel to
enhance the electric field across the tunneling junction [39],
[40]. Moreover, a better electrostatic gate control is expected
in cylindrical or nanowire geometry. Hence, in this section, we
analyse the electrical characteristics of an optimised cylindrical
GaN HTFET, which utilises a thin () and highly doped
(




) pocket at the interface between channel
and an AlN barrier of 1.7 nm thickness of GaN HTFET, as
shown in Fig. 4, while the rest of the dimensions are kept the
same as that for the device in Fig. 3 (a).
The electrical characteristics of the p-channel GaN HTFET
are presented in Figs. 4 (a) and 4 (b), respectively. The current
is normalised to the diameter of the cylindrical geometry.
Owing to the thin p-GaN pocket, the maximum drain on-
current 

in Fig. 4 (a) is more than double in comparison to
a rectangular device without the pocket (Fig. 3 (e)). An
improved gate control in the cylindrical geometry also leads to
a much steeper subthreshold slope (SS) of , in the
absence of defect or trap states. In the absence of the thin AlN
layer at the top edge of the gate, a positive gate bias alone is
insufficient to produce a large band bending at this interface,
hence the device continues to remain non-ambipolar. In Fig. 4
(b), it can be noted that a higher 

is required to turn-on the
device as 

is increased. This is because a high 

not
only raises the energy of the valence band in the channel but
also increases the energy of the conduction band in n-GaN
across the AlN barrier layer (Fig. 3 (c)), thus requiring a higher


to align the bands to turn the device on.
The impact of traps with densities 



and




in AlN and resulting trap-assisted tunneling
(TAT) is analysed in Fig. 4 (c). Owing to the higher
recombination, the maximum drain current of the device
degrades. Due to the increased leakage caused by TAT, the SS
of the device no longer remains below . Even
though the device turns on at a relatively smaller 

, it

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Abstract: In this study, tunnel field-effect transistor (TFET) which has surface Ge-rich SiGe nanowire as a channel has been demonstrated. There are improvements in terms of on-current and subthreshold swing (SS) comparing with control groups (constant Ge concentration SiGe TFET and Si TFET) fabricated by the same process flow except for the channel formation step. In order to obtain the concentration-graded SiGe channel, Ge condensation method which is a kind of oxidation is adopted. The rectangular shape of the channel becomes a rounded nanowire through the Ge condensation process. The TFET with the concentration-graded SiGe channel can improve drive current due to a smaller band gap at the Ge-condensed surface of the channel compared to Si or non-condensed SiGe channel TFET.

8 citations


Proceedings ArticleDOI
01 Dec 2019
Abstract: In this work a vertical p-channel heterojunction GaN tunnel field effect transistor (TFET) has been reported. A thin layer of InGaN is filled in between source and channel to enhance the performance of the device. The mechanism of current conduction in the proposed device is based on the physics related to polarization charge induced in III-nitrides compound semiconductor such as GaN, InN, AlN etc. The effect of polarization increases the electric field at the source-channel interface, which opens the door for unidirectional tunneling depending on the voltage applied across gate to source terminal. The other significance of this polarization concept is to reduce the tunneling width in comparison to conventional TFET as well as to minimize ambipolar current. The performance in terms of I ON /I OFF and SS are also investigated for the proposed device.

2 citations


Cites background from "A p-Channel GaN Heterostructure Tun..."

  • ...In this paper, we have reported a vertically grown pchannel heterojunction GaN TFET where a thin layer of InGaN is introduced as a PITJ to enhance the ON current [27]....

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Journal ArticleDOI
TL;DR: This article highlights the scalability of III-V TFETs, influence of thickness and permittivity of gate dielectric, interface trap density, other geometrical dimensions, material properties and various TFET architectures on the ON and OFF state performance ofIII-VTFETs.
Abstract: Tunnel Field Effect Transistors (TFETs) have emerged as serious contenders for the replacement of traditional MOSFET technology for the future ultra low power Analog/Digital circuit applications because of their unique properties such as Sub-60 mV/decade subthreshold swing (SS), negligible short channel effects (SCEs) and very low off current (IOFF). This review article intensively studies the RF/Analog and DC performance of III-V materials based TFETs. This article highlights the scalability of III-V TFETs, influence of thickness and permittivity of gate dielectric, interface trap density, other geometrical dimensions, material properties and various TFET architectures on the ON and OFF state performance of III-V TFETs. This paper also point outs the impact of temperature, strain, gate metal work function, source-gate overlap and underlap, doping concentration and supply voltage scaling on the DC, RF/Analog characteristics of III-V TFETs.

1 citations



References
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Journal ArticleDOI
Abstract: We present a comprehensive and up-to-date compilation of band parameters for all of the nitrogen-containing III–V semiconductors that have been investigated to date. The two main classes are: (1) “conventional” nitrides (wurtzite and zinc-blende GaN, InN, and AlN, along with their alloys) and (2) “dilute” nitrides (zinc-blende ternaries and quaternaries in which a relatively small fraction of N is added to a host III–V material, e.g., GaAsN and GaInAsN). As in our more general review of III–V semiconductor band parameters [I. Vurgaftman et al., J. Appl. Phys. 89, 5815 (2001)], complete and consistent parameter sets are recommended on the basis of a thorough and critical review of the existing literature. We tabulate the direct and indirect energy gaps, spin-orbit and crystal-field splittings, alloy bowing parameters, electron and hole effective masses, deformation potentials, elastic constants, piezoelectric and spontaneous polarization coefficients, as well as heterostructure band offsets. Temperature an...

2,350 citations


"A p-Channel GaN Heterostructure Tun..." refers background or methods in this paper

  • ...In comparison, a lattice mismatch 2.5% between GaN and AlN [23] implies that up to 5 nm of fully strained AlN can be grown on GaN without the introduction of microcracks [24]....

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  • ...However, a significant lattice mismatch between GaN and InN > 10% [23]...

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  • ...69 for AlN [23], [32] produce simulated I–V results that closely follow experimental characteristics, as shown in Fig....

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  • ...5% between GaN and AlN [23] implies that up...

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Journal ArticleDOI
Abstract: The macroscopic nonlinear pyroelectric polarization of wurtzite AlxGa1-xN, InxGa1-xN and AlxIn1-xN ternary compounds (large spontaneous polarization and piezoelectric coupling) dramatically affects the optical and electrical properties of multilayered Al(In)GaN/GaN hetero-, nanostructures and devices, due to the huge built-in electrostatic fields and bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. Models of polarization-induced effects in GaN-based devices so far have assumed that polarization in ternary nitride alloys can be calculated by a linear interpolation between the limiting values of the binary compounds. We present theoretical and experimental evidence that the macroscopic polarization in nitride alloys is a nonlinear function of strain and composition. We have applied these results to interpret experimental data obtained in a number of InGaN/GaN quantum wells?(QWs) as well as AlInN/GaN and AlGaN/GaN transistor structures. We find that the discrepancies between experiment and ab initio theory present so far are almost completely eliminated for the AlGaN/GaN-based heterostructures when the nonlinearity of polarization is accounted for. The realization of undoped lattice-matched AlInN/GaN heterostructures further allows us to prove the existence of a gradient in spontaneous polarization by the experimental observation of two-dimensional electron gases?(2DEGs). The confinement of 2DEGs in InGaN/GaN QWs in combination with the measured Stark shift of excitonic recombination is used to determine the polarization-induced electric fields in nanostructures. To facilitate inclusion of the predicted nonlinear polarization in future simulations, we give an explicit prescription to calculate polarization-induced electric fields and bound interface charges for arbitrary composition in each of the ternary III-N alloys. In addition, the theoretical and experimental results presented here allow a detailed comparison of the predicted electric fields and bound interface charges with the measured Stark shift and the sheet carrier concentration of polarization-induced 2DEGs. This comparison provides an insight into the reliability of the calculated nonlinear piezoelectric and spontaneous polarization of group III nitride ternary alloys.

905 citations


Proceedings ArticleDOI
01 Dec 2008
Abstract: The main challenges for Tunnel FETs are experimentally demonstrating SS<60 mV/dec, high ON currents and solving their ambipolar behavior. We have experimentally demonstrated a Double-Gate, Strained-Ge, Heterostructure Tunneling FET (TFET) exhibiting very high drive currents and SS<60 mV/dec. Due to small bandgap of s-Ge and the electrostatics of the DG structure, record high drive current of 300 uA/um (the highest ever reported experimentally for a TFET) and a subthreshold slope of ~50 mV/dec was observed. In addition, to address the ambipolar problem and examine the scalability of TFETs, we have developed a sophisticated TFET simulator that uses a Quantum transport model, Non-local BTBT, complete Bandstructure (real and complex) information, and includes all transitions (direct and phonon assisted). Using this simulator, we have studied the scalability of three asymmetric DG TFET configurations (underlapped drain, lower drain doping and lateral heterostructure) in terms of their ability to solve the ambipolar behavior and achieve high ON and low OFF currents.

468 citations


"A p-Channel GaN Heterostructure Tun..." refers background in this paper

  • ...To address ambipolarity, short-gated TFET [21], asymmetric doping, and bandgap engineering [22] have been proposed....

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Journal ArticleDOI
Abstract: Progress in the development of tunnel field-effect transistors (TFETs) is reviewed by comparing experimental results and theoretical predictions against 16-nm FinFET CMOS technology. Experiments lag the projections, but sub-threshold swings less than 60 mV/decade are now reported in 14 TFETs. The lowest measured sub-threshold swings approaches 20 mV/decade, however, the measurements at these lowest values are not based on many points. The highest current at which sub-threshold swing below 60 mV/decade is observed is in the range 1–10 nA/ \({{\mu }}\) m. A common approach to TFET characterization is proposed to facilitate future comparisons.

440 citations


"A p-Channel GaN Heterostructure Tun..." refers background in this paper

  • ...To date, no one has yet demonstrated a TFET of a comparable current level to a MOSFET with subthreshold slope (SS) below 60 mV/dec [17]....

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Book
09 Jan 2003
Abstract: I. Fundamentals 1. Introduction to Semiconductors 2. Electron Energy Bands 3. Carrier Transport 4. Optical Waves 5. Photon Generation 6. Heat Generation and Dissipation II. Devices 7. Edge-Emitting Laser 8. Vertical- Cavity Lasers 9. Nitride Light Emitters 10. Electroabsorption Modulator 11. Amplification Photodetector

422 citations