Drive current enhancement in tunnel field-effect transistors by graded heterojunction approach

Drive current enhancement in tunnel field-effect transistors by gradedheterojunction approachNguyen Dang Chien and Luu The Vinh Citation: J. Appl. Phys. 114, 094507 (2013); doi: 10.1063/1.4820011 View online: http://dx.doi.org/10.1063/1.4820011 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v114/i9 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

Drive current enhancement in tunnel field-effect transistors by gradedheterojunction approach

Nguyen Dang Chien1,2,a) and Luu The Vinh3

1Department of Electrical Engineering, National Chi Nan University, Nantou 54561, Taiwan2Faculty of Physics, University of Da Lat, Lam Dong 671463, Vietnam3Faculty of Electronic Technology, Industrial University of Ho Chi Minh City,Ho Chi Minh City 727905, Vietnam

(Received 26 June 2013; accepted 16 August 2013; published online 4 September 2013)

The heterostructure technique has recently demonstrated an excellent solution to resolve the

trade-off between on- and off-state currents in tunnel field-effect transistors (TFETs). This

paper shows the weakness of abrupt heterojunctions and explores the physics of drive current

enhancement as well as generalizes the proposed graded heterojunction approach in both n-type

and p-type TFETs. It is shown that the presence of thermal emission barriers formed by abrupt

band offsets is the physical reason of the on-current lowering observed in abrupt heterojunction

TFETs. By employing graded heterojunctions in TFETs, the thermal emission barriers for

electrons and holes are completely eliminated to narrow the tunnel widths in n-type and p-type

TFETs, respectively. With the significant improvement in on-current, this novel approach of

graded heterojunctions provides an effective technique for enhancing the drive current in

heterostructure-based TFET devices. VC 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4820011]

I. INTRODUCTION

With an achievable sub-60 mV/decade subthreshold

swing at room-temperature, which is impossible in conven-

tional metal-oxide-semiconductor field-effect transistors

(MOSFETs) due to the kT/q limit, tunnel field-effect transis-

tors (TFETs) have been considered as an excellent alterna-

tive in ultra-scaled power integrated circuits.1,2 Always

operated in highly reversed conditions of p-n junctions,

TFETs also present a significantly low power dissipation

which is generally a critical issue in regular MOSFETs when

scaling into sub-100 nm regimes.3,4 Instead of applying the

electric field, in the other hand, the conduction current in

TFETs is essentially generated by the band-to-band tunnel-

ing (BTBT) of carriers through the forbidden-band gap. The

tunnel barrier and hence the tunnel probability are mainly

governed by the applied voltage to produce an ideal on-off

switching with a steep subthreshold swing.2,5 Furthermore,

TFETs also exhibit competitive advantages over standard

MOSFETs in term of scaling devices because of small

tunneling regions.6,7

Although numerous advanced techniques have been

proposed to more effectively control the tunnel width,5,8,9

Si-based TFETs exhibit a low on-state current because of

the high-bandgap of silicon. Enhancing the on-current

becomes the most challenge in TFET devices. To improve

the on-current and to suppress the ambipolar off-state

leakage, asymmetric structures of abrupt heterojunctions

using low-bandgap materials, such as SiGe and InGaAs,

have been introduced to TFETs.10,11 Since the bandgap

decreases with increasing Ge (In) concentration in SiGe

(InGaAs), it is expected that a higher Ge (In) concentration

can lead to a higher on-state tunneling current.10,12 In

abrupt Si/Si1�xGex p-type TFETs, however, increasing Ge

mole fraction is only helpful in increasing the on-current

up to a medium fraction approximately 0.3.13 The continu-

ing increase in the Ge fraction, by contrast, degrades the

on-current. This on-current lowering limits the exploitation

of the low-bandgap of Ge-rich source for enhancing

the tunneling current. Although it is phenomenologically

explained to be due to the extension of the tunnel width

at high Ge concentrations,6,13 the physical reason of the

tunnel width extension has not been adequately clarified.

Therefore, it is difficult to explain why the on-current low-

ering does not occur in SiGe-source n-type TFETs.10,14

Additionally, previous publications6,15 have shown that the

on-current lowering in abrupt Si/SiGe p-type TFETs can be

resolved by applying the graded heterojunction technique,

but the physical reason still remains uncertainty. The cur-

rent enhancement in graded p-type TFETs is simply attrib-

uted to the narrowed tunnel barriers. However, the question

why the tunnel barriers are narrowed has not been properly

understood. It is problematic to affirm whether the graded

heterojunctions are benefitable and when they are needed

in n-type TFET devices to enhance the on-current.

Consequently, the application of graded heterojunctions in

n-type TFETs has not been reported so far.

By two-dimensional simulations,16 this paper elucidates

the physics insight of the abrupt heterojunction limitation

and explores the physical principle of the drive current

enhancement by graded heterojunctions. With the physical

mechanism clarified in abrupt p-type TFETs, the on-current

lowering can similarly be predicted and relevant explana-

tions can be made for abrupt n-type TFETs. Importantly,

graded heterojunctions have been numerically demonstrated

a)Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0021-8979/2013/114(9)/094507/6/$30.00 VC 2013 AIP Publishing LLC114, 094507-1

JOURNAL OF APPLIED PHYSICS 114, 094507 (2013)

in n-type TFETs to generalize the graded heterojunction

approach for use in both n-type and p-type TFETs to resolve

the weakness of abrupt heterojunctions. The paper is divided

into five sections, including the Introduction (Sec. I) and the

Conclusion (Sec. V). Section II describes the device archi-

tectures and the physical models used in the study. The

weakness of abrupt heterojunctions is presented in Sec. III,

whereas the physical principle of current enhancement by

graded heterojunctions is clarified in Sec. IV.

II. DEVICE STRUCTURES AND PHYSICAL MODELS

Figure 1 shows the schematic views of p-i-n structure p-

type and n-type TFETs with using abrupt/graded Si/Si1�xGex

and In0.17Ga0.83As/InyGa1�yAs heterojunctions, respectively.

For all devices, a double-gate structure with 1 nm SiO2 gate-

dielectric and 20 nm body-thickness was utilized for mini-

mized subthreshold swing and maximized on-current.5,17 In

SiGe-based p-type TFETs [Fig. 1(a)], a metal gate work-

function of 5.2 eV was used. A heavily doped nþþ source of

1020 cm�3, a lightly doped pþ drain of 5� 1018 cm�3, and a

40 nm p-channel of 1017 cm�3 were employed to improve the

on-current, subthreshold swing, and ambipolar leakage.9

Compressive strained-Si1�xGex with minimized bandgaps to

ameliorate the tunneling current10 was assumed in both

abrupt and graded structures. Si1�xGex source was defined in

the abrupt TFETs with a 2 nm gate-source overlap for opti-

mizing the device performance.18 In the graded TFETs, the

source region was defined by strained-Si1�xGex, whereas the

distribution of Ge composition was gradually graded from x

value at the source-channel junction to zero at 20 nm away

from the source. For InGaAs-based n-type TFETs [Fig. 1(b)],

the gate work-function 4.5 eV was specified, and the polarity

and the magnitude of the doping were swapped compared to

those of p-type TFETs. Instead of using Si and Si1�xGex,

In0.17Ga0.83As and InyGa1�yAs were, respectively, defined at

the drain and the source with a 6 nm gate-source overlap in

abrupt TFETs. InGaAs compounds were investigated because

they are direct- and low-bandgap semiconductors to improve

the tunneling current.11,12 A fixed composition alloy of

In0.17Ga0.83As was adopted to give a same bandgap of silicon

for comparisons of tunneling currents.

Two-dimensional simulations16 were performed to ana-

lyze the electrical characteristics and to explore the physical

mechanisms in both abrupt/graded Si/Si1�xGex p-type

TFETs and abrupt/graded In0.17Ga0.83As/InyGa1�yAs n-type

TFETs. The BTBT generation rate (GBTBT) is considered

with sufficient band bending to determine the tunneling

current in TFETs as16,19

GBTBT ¼ Anc

E1=2g

exp �BE

3=2g

n

!; (1)

where c ¼ 2 for direct tunneling in InGaAs11 and c ¼ 5=2 for

indirect tunneling in indirect bandgap SiGe.20,21 Equation (1)

shows that the BTBT generation depends strongly on the

semiconductor bandgap (Eg) and the electric field (n) which

is nonlocally determined in the simulations. Calibrated

parameters A and B depend on the bandgap and the effective

masses of carriers.14 In all simulations, the bandgap narrow-

ing is included, but the quantum confinement effect is negli-

gible in TFETs with larger 10-nm body thicknesses.22

Additionally, to focus on the physical considerations, the

trap-assisted tunneling is not taken into account because the

traps and associated tunneling are process sensitive and they

are only considerable in the subthreshold region.10,23

III. WEAKNESS OF ABRUPT HETEROJUNCTIONS

In a typical TFET device, the on-state current is mainly

contributed by the tunneling at the source-channel junction

while the ambipolar tunneling leakage is originated from the

tunneling at the drain-channel junction. For single-material-

based TFETs, the use of low-bandgap semiconductors can

improve the on-current, it also causes a severe increase in

the ambipolar leakage current.9 The key idea of employing

abrupt heterojunctions in TFETs is to resolve the trade-off

between on- and off-state currents.24 The source-side region

is designed by a low-bandgap material to boost the on-

current, whereas the drain-side region is specified by a high-

bandgap semiconductor to suppress the off-state tunnelingFIG. 1. Schematic structures of (a) SiGe-based p-type TFETs and (b)

InGaAs-based n-type TFETs with using abrupt and graded heterostructures.

094507-2 N. D. Chien and L. T. Vinh J. Appl. Phys. 114, 094507 (2013)

current. The interface of high- and low-bandgap materials

has to be located within the channel. To optimize the on- and

off-currents, the heterojunction should be overlapped several

nanometers by the gate.18 Owing to the direct contact of two

different band structure materials, abrupt conduction and

valence band offsets are formed at the abrupt heterojunction

interface. These abrupt band offsets with specific situations

limit the use of low-bandgap materials to enhance the

on-current in abrupt heterojunction-based TFETs as shown

in this section.

Fig. 2 shows the current-voltage characteristics of abrupt

Si/Si1�xGex heterojunction p-type TFETs and abrupt

In0.17Ga0.83As/InyGa1�yAs heterojunction n-type TFETs. In

both n-type and p-type TFETs, the on-state currents first

increase with increasing the Ge and In fractions. The

increase in the on-currents in SiGe- and InGaAs-based

TFETs is definitively attributed to the decrease in the bandg-

aps of Si1�xGex and InyGa1�yAs when increasing x and y,

respectively. Unfortunately, these on-currents reach to max-

ima at medium x and y values, and then degrade at higher

Ge and In fractions. It seems that this on-current lowering

contradicts to the continuous decrease of Si1�xGex and

InyGa1�yAs bandgaps. This property limits the exploitation

of Ge- and In-rich compounds with low-bandgaps for boost-

ing the on-currents in abrupt heterostructure-based p-type

and n-type TFET devices.

In order to physically understand the enigmatic behavior

of the on-state tunneling currents under the variation of Ge

and In compositions, Fig. 3 plots the energy-band diagrams

at on-state of the abrupt TFETs with two values of each of

Ge and In fractions. As shown in Fig. 3(a) for the p-type

TFETs, the valence band offset at the junction interface

forms a thermal emission barrier for the movement of holes

from the channel to the drain side. The complete physical

transport includes a tunneling of electrons from the channel

valence band to the source conduction band and a thermal

emission of trapped holes to the drain valence band. The

thermal barrier height increases with increasing the Ge con-

centration. For small Ge fraction x¼ 0.3, the thermal barrier

is still relatively low so as not to considerably obstruct the

movement of holes generated by the band-to-band tunneling.

FIG. 2. Current-voltage characteristics of (a) abrupt Si/Si1�xGex p-type

TFETs and (b) abrupt In0.17Ga0.83As/InyGa1�yAs n-type TFETs with various

Ge and In mole fractions.

FIG. 3. On-state energy-band diagrams of (a) abrupt Si/Si1�xGex p-type

TFETs and (b) abrupt In0.17Ga0.83As/InyGa1�yAs n-type TFETs with two

values of each of x and y. The solid-line arrows show the movement direc-

tions of electrons/holes generated by the band-to-band tunneling.

094507-3 N. D. Chien and L. T. Vinh J. Appl. Phys. 114, 094507 (2013)

In high Ge fraction x¼ 0.5 TFET, the high thermal barrier

severely obstructs holes to move from the channel to the

drain. Numerous holes stuck in the well mitigate the band

bending in the channel, which consequently causes an

extended tunnel barrier and a narrowed tunneling region.

Therefore, the on-state tunneling current is decreased in

x¼ 0.5 TFET irrespective of the decreased bandgap. The

decrease in the bandgap is only helpful till a relatively small

value of Ge fraction. Based on understanding the physical

mechanism of the on-current lowering in the p-type TFETs,

a similar situation can be predicted and relevant explanations

can be made for the n-type TFETs in Fig. 3(b). Since the

band offset of In0.17Ga0.83As/InyGa1�yAs heterojunctions

considerably occurs at the conduction band, significant

thermal barriers for electrons are formed in abrupt

In0.17Ga0.83As/InyGa1�yAs heterojunctions. The complete

physical transport in these abrupt heterojunction n-type

TFETs includes a tunneling of electrons from the source

valence band to the channel conduction band followed by a

thermal emission of trapped electrons to the drain conduction

band. Holes generated in the tunneling process move back-

ward to the source. A higher In mole fraction causes a higher

thermal barrier which explains for the current lowering in

the In-rich TFET. Notably, in the n-type TFETs, only con-

duction band offset forms a thermal barrier for electrons,

whereas the valence band offset does not influence the flow

of electrons and holes at all. Similarly in the p-type TFETs,

only valence band offset causes an undesirable thermal

barrier for holes. Therefore, it can be generalized that the

on-current lowering only occurs in abrupt heterojunction

n- and p-type TFETs if the conduction and valence band

offsets are significant in abrupt heterojunctions, respectively.

Because most of the band offset of silicon-germanium heter-

ojunctions occurs at the valence band, the conduction

band offset is negligible. The on-current lowering can only

be observed in abrupt Si/Si1�xGex p-type TFETs. No such

on-current lowering is observed in n-type TFET

counterparts.10,14

In order to further confirm the presence of the thermal

barriers, Fig. 4 shows the current-voltage curves of the abrupt

Si/Si0.7Ge0.3 and In0.17Ga0.83As/In0.35Ga0.65As heterojunction

TFETs at various operating temperatures. In both the abrupt

p-type and n-type TFETs, the on-state currents significantly

increase with increasing the temperature. The strong tempera-

ture dependence of the on-current confirms the presence of

thermal emission barriers for holes and electrons, respec-

tively. The temperature dependence of the on-current is

stronger in the Si/Si0.7Ge0.3 TFET compared to that in the

In0.17Ga0.83As/In0.35Ga0.65As TFET because its thermal bar-

rier is higher.

IV. GRADED HETEROJUNCTION APPROACH

As explained in Sec. III, the weakness of using abrupt

heterojunctions is the formation of thermal emission barriers.

The thermal barriers are formed because of the abrupt band

offsets at the junction interfaces. With the presence of ther-

mal barriers, even if a low-bandgap is assigned to the source

to generate a high tunneling rate, generated electrons/holes

are still obstructed to limit the on-current. To retain the basic

idea of applying heterojunctions, such thermal barriers have

to be eliminated in TFET devices. In the other hand, the

physical mechanism elucidated in abrupt TFETs allows us to

generalize the graded heterojunction approach to n-type

TFETs to enhance the on-state tunneling current. Similarly

to p-type TFETs, the on-current lowering in n-type TFETs is

also attributed to the thermal emission barriers which are

formed in abrupt heterojunctions with abrupt conduction

band offsets. Therefore, the idea of graded heterojunction

approach to eliminate the thermal barriers is also applicable

in n-type TFETs. The graded band offsets resulted from

graded heterojunctions are demonstrated in this section to be

an excellent solution to resolve the weakness of abrupt heter-

ojunctions in both p- and n-type TFETs.

Fig. 5(a) shows the current-voltage characteristics of

graded Si/ Si1�xGex p-type TFETs, whereas those of graded

In0.17Ga0.83As/InyGa1�yAs n-type TFETs are displayed in

Fig. 5(b). For small Ge and In mole fractions x and y, the

on-currents are regularly increased with increasing x and y.

Similar to the abrupt TFET counterparts, this is due to the

decrease of bandgaps with increasing Ge and In fractions.

FIG. 4. Current-voltage characteristics of (a) p-type and (b) n-type TFETs

based on abrupt heterojunctions at different operating temperatures.

094507-4 N. D. Chien and L. T. Vinh J. Appl. Phys. 114, 094507 (2013)

Importantly, the on-currents of the graded TFETs continue

to be enhanced at high Ge and In fractions, which is obvi-

ously attributed to the low-bandgaps of Ge- and In-rich com-

pounds. By employing the graded heterojunctions in TFETs,

the weakness of the abrupt heterostructures has basically

been resolved to further enhance the on-current. Using the

graded heterojunction approach, the drive currents can be

maximized in a graded Si/Ge p-type TFET and a graded

In0.17Ga0.83As/InAs n-type TFET.15 As also seen in Fig. 5,

the on-state currents of InGaAs-based TFETs are signifi-

cantly higher than those of SiGe-based TFETs by approxi-

mately two orders of magnitude even though the bandgaps

are almost similar. This is due to the fact that the direct tun-

neling process always results in a considerably higher proba-

bility compared to the indirect tunneling for a same energy

bandgap. Therefore, InGaAs exhibits a suitable and promis-

ing material for use in TFET devices to achieve high drive

currents.

To interpret how the weakness of abrupt heterojunctions

is successfully resolved, Fig. 6 plots the energy-band

diagrams at on-state of the graded TFETs with two values of

each of Ge and In mole fractions. With the gradually graded

band offsets, the thermal barriers for holes and electrons are

not formed in the channels in both p-type and n-type TFETs.

Because all electrons/holes generated by the tunneling are

immediately moved to the source/drain, there are no elec-

trons/holes stuck in the channels to inhibit the channel band

bending as observed in the abrupt TFETs. Similar to single-

material TFETs without the accumulation of electrons/holes

in the channel, the channel band bendings in the graded

TFETs are strongly controlled by gate voltage to narrow the

tunnel widths. In addition, the high Ge and In fractions also

reduce the tunnel widths, thus the on-state tunneling currents

are significantly enhanced. With the disappearance of ther-

mal barriers, higher Ge and In fractions can flexibly be used

to further upgrade the drive currents in the graded TFETs. In

short, the graded heterojunction approach should be utilized

to enhance the on-current whenever the thermal emission

barrier is presented in heterojunction-based TFET devices

regardless of n-type or p-type FET.

FIG. 5. Current-voltage characteristics of (a) graded Si/Si1�xGex p-type

TFETs and (b) graded In0.17Ga0.83As/InyGa1�yAs n-type TFETs with some

values of x and y.

FIG. 6. On-state energy-band diagrams of (a) graded Si/Si1�xGex p-type

TFETs and (b) graded In0.17Ga0.83As/InyGa1�yAs n-type TFETs with two

values of each of Ge and In fractions. The solid-line arrows show the move-

ment directions of electrons/holes generated in tunneling processes.

094507-5 N. D. Chien and L. T. Vinh J. Appl. Phys. 114, 094507 (2013)

As a confirmation for the absence of thermal emission

barriers, Fig. 7 shows the current-voltage curves of graded

Si/Si0.7Ge0.3 and In0.17Ga0.83As/In0.35Ga0.65As heterojunction

TFETs at different operating temperatures. It is inferred from

Fig. 7 that there are no such thermal barriers in the graded

TFETs because their on-currents are almost independent of

the temperature. Slight differences in the on-currents at dif-

ferent temperatures originate in the temperature dependence

of bandgap.11

V. CONCLUSION

This study explores the physical mechanisms to show

the weakness of abrupt heterojunctions and to generalize the

graded heterojunction approach proposed for enhancing

the drive current in both n-type and p-type TFETs. The

on-current lowering in abrupt heterojunction TFETs is

mainly attributed to the thermal emission barriers which

detain electrons/holes in the channel and hence extend the

tunnel barriers. Graded heterojunctions are used instead of

abrupt counterparts to eliminate thermal emission barriers.

Without electrons/holes stuck in the channel, the tunnel

width is further narrowed to significantly enhance the on-

state tunneling current. This graded heterojunction approach

is useful in improving the drive current in heterojunction-

based TFET devices.

ACKNOWLEDGMENTS

N.D.C. gratefully acknowledges the support of a Ph.D.

stipend from the National Chi Nan University.

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FIG. 7. Current-voltage curves of (a) p-type and (b) n-type TFETs based on

graded heterojunctions at various operating temperatures.

094507-6 N. D. Chien and L. T. Vinh J. Appl. Phys. 114, 094507 (2013)