Short channel InGaSb-on-insulator FET: with and without Junctions Md. Nur Kutubul Alam, Muhammad Shaffatul Islam, Md. Raifqul Islam
Department of Electrical and Electronic Engineering Khulna University of Engineering & Technology Khulna-9203, Bangladesh
[email protected] Abstract-In
this study the ballistic performance of III-Von
insulator (XOI) and "junction less XOI" (JLXOI) nFET are investigated
and
compared
by
NEGF
formalism,
taking
Ino.3Gao.7Sb as channel material. At 15nm gate length and O.5nm EOT of gate dielectric the JLXOI shows significant improvement in threshold voltage (V)I and ION with a fine tuned IoFF• Also the subthreshold
slope
(SS)
reduced
from
82.35mVd / ec
to
68.088mV/dec along with imporoved DIBL performance and simplified fabrication process. Keywords-XOI;NEGF;Ballistic transport; InGaSb;InAsSb (key words)
I.
INTRODUCTION
In the field of nano electronics, there is an adage "Smaller is better", and it has been the key motivation to reduce transistor size in chips. In last 4 decades Silicon MOSFET's gate length has reduced from 10 f.!m to 22 nm, and it is now approaching to its physical limit [1-3]. Various adverse effects including short channel and hot electron effects, parasitic capacitance, leakage current, transistor isolation etc causes the overall performance deterioration at this ultra scaled regime. In search for a solution, the SOl structure came into business by drastically improving the device performance and suppressing these adverse effects [4,5]. In fact replacement of Si by III-V materials in the channel would further improve the device performance by lowering the operating voltage due to their superior transport properties [6,7]. Keeping this in mind and taking the advantage of available process technology [8], n-type InAs FETs were experimentally demonstrated on Si/Si02 substrate by Javey et. al [9] and the structure is termed as "XOI" to represent compound semiconductor-on-insulator platform. However, among different III-V materials, Ga rich ternaries like InGaSb gives the best performance when device operates at ballistic limit (less than 100nm gate length) [10]. But all of the device structures from bulk MOSFET to XOI have been using junctions to control the carrier flow through the channel. The p-n junction is most common, which is formed by n type contacts and a p-type channel region or vice versa. This junction forms a barrier along the channel, height of which is controlled by gate bias. Thereby modulation of the amount of carrier flow is being done [11]. But when device size shrinks to 22 nm regime or even further, formation of junctions becomes very challenging. Extremely sharp doping
concentration gradients are required in junctions. Typically the n type concentration must switch from 1 x 1019 cm-3 to p-type concentration of 1 x 1018 cm-3, within a couple of nanometers. This imposes severe limitations on the processing thermal budget and necessitates the development of costly millisecond annealing techniques [11,12]. The possible solution is making a device with no junction, in which the doping concentration in the channel is exactly same as that in source and drain region. When the channel with such doping profile is thin and narrow enough to allow the full depletion of carriers, performance of the device can be made comparable with traditional devices having junctions [11]. Availing the advantage of junction less architecture, performance of double gate and multi gate transistor has been studied recently [12]-[15]. But all of those devices has 3D structure and therefore fabrication process is complicated. In contrast, XOI has the planner architecture and its performance with junction less channel has not been well studied. In this paper, the performances of 15 nm XOI and junction less XOI (JLXOI) FETs are investigated using non equilibrium greens function (NEGF) method. It is found that JLXOI FET significantly outperforms the XOI. II.
QUANTUM BALLISTIC SIMULATION
A. Device structure
The device structure used in our study is shown in Fig. 1, and all the numerical parameters used are listed in Table-I.
y Fig. 1.
Substrate Structure of XOI and JLXOI FET. For XOI, channel region right
bellow the oxide is left undoped. For JLXOI, n type doping is done with uniform concentration throughout the channel.
To study the electronic transport of the device, we used Non Equilibrium Green's Function (NEGF) method in the mode space approach with effective mass approximation. TABLE I.
PARAMETERS USED IN THE SIMULA nON OF THE DEVICE STRUCTURE SHOWN IN FIG. I.
Parameter name
Value
Cins
22
tbody
3nm
tins
2.82 nm
Nbody
P type,10 '/cm·j for XOT 3 l9 N type, 2xlO cm· for
It is the thickness of the channel. Ino.3Gao.7Sb and Thickness of Hf02 gate oxide,EOT = 0.5nm Channel region laid right bellow the gate oxide. Rest of the channel region,i,e region where
Nbo,," is not used. This is the extension of
5nm
channel from that covered by gate oxide. Channel region covered
15nm
Lo
by gate oxide.
4.3eV and 4.0889 eV
�lIetal
For XOI and JLXOI FETs respectively
B. NEGF theory
The NEGF method provides a rigorous formalism for quantum transport in nanoscale devices [16]. In this quantum modeling, Schrodinger equation is solved using open boundary condition in the source and drain region (through which electron can come into or leave from the region under study). And it is solved self-consistently with the Poisson's equations to obtain the energy level, charge and current distribution [17]. NEGF are solved using two approaches namely real space and mode space method. Simulation in real space method is computationally very much costly. In case of the device where quantum confinement is strong (double gate MOS or XOI FET) the mode space approach can be used. The Hamiltonian in these device structures is separated in longitudinal and transverse directions; as a result, it greatly reduces the computational burden. Moreover, in the devices where the cross-section does not change, the sub-bands are not quantum mechanically coupled to each other; the transport equations become essentially ID for each sub-band that leads to use the uncoupled mode space approach [18]. The transformation of a real space effective mass Hamiltonian Ho to a mode �ace is done by taking a matrix element between mtl1 and n wave functions of kth and lth slices
H��kl
=
