1. Contact Resistance Modeling in HEMT Devices
M. Salmani-Jelodar, S. H. Park, H.-H. Park, S. Steiger, G. Klimeck
Network for Computational Nanotechnology (NCN), Purdue University
Towards III-V MOSFET
Why III-V HEMTs?
Towards realistic contact modeling
Explore effective parts for resistances in contact-to-channel region.
Acknowledgement: Robert Chau, Intel
Research
Channel doping
S/D doping
Strained channel
New gate dielectrics:
HfO2 and Al2O3
Device geometries
Channel materials
High-k dielectrics and metal gates
III-V channel
devices
Low-power & high-speed
III-V: Extraordinary electron transport properties and high injection velocities
HEMTs: Similar structure to MOSFETs except high-κ dielectric layer
Excellent to Test Performances of III-V material (strain, band gap, effective masses, band-to-band tunneling, contacts, …) without interface defects
Short Gate Length HEMTs are Introduced by del Alamo’s Group at MIT
Channel region
Contact 1
Contact 2
Lead 1
channel
Lead 2
Contact-to-channel
region
channel
channel
Device Pie
Regular compact model features:
Uses a virtual source and drain.
Need to fit I-V characteristic with series resistances RS and RD.
Simulation Domain
2007: 40nm
2008: 30nm
Simulation domain of compact model
(IEDM 2009, N. Kharche et al.)
D.H. Kim et al., EDL 29, 830 (2008)
Contact resistance of HEMT device
Contact resistance of HEMT device
2D simulations setting
Objective:
Guide III-V InAs experimental device design through simulation
Challenge:
2D geometries, and confinement
New materials, strain, disorder
Gate leakage
Contacts – scattering, disorder, and curved shape.
Approach:
NEMO5 quantum simulator
Quantum transport simulations using realistic geometries
Includes phonon scattering
Parallel computing
region of interest
40nm
90nm
In53Ga47As
Virtual Source
In52Al48As
InP
InAs
virtual drain
2D sim.
domain
Si δ-doping
Source
Drain
N+ Cap
In0.53Ga0.47As
N+ Cap
In0.53Ga0.47As
25nm
Hetero-structures represented in 2D
Ohmic contacts for virtual source/drain
NEGF/Poisson self-consistent simulation
Intra- and inter-valley phonon scattering mechanisms
VDS = 0~0.15V for experimental VDD = 0.5V
 Considered the channel and series resistances measured experimentally
In0.52Al0.48As
In0.52Al0.48As
InP
InP
In0.52Al0.48As
Gate
In0.52Al0.48As
In0.52Al0.48As
In0.53Ga0.47As
InAs
In0.53Ga0.47As
In0.52Al0.48As
Contact Pad
Rpad
35nm
N+ Cap
InGaAs
Rcap
15 nm
In0.52Al0.48As Y
6 nm
InP etch stop
Source spacing = 2 μm*
Series resistance = 240 Ωμm
In0.52Al0.48As
Rbarrier X
11 nm
2 nm
In0.53Ga0.47As
Rside
InAs
5 nm
Virtual Drain
3 nm
In0.53Ga0.47As
In0.52Al0.48As
*D.-H. Kim, J. D. A. del Alamo, IEEE Trans. Elec. Dev. 57, 1504 (2010)
500nm
2D simulation results: electron density and current flow
2D simulation results:
electron density spectrum
Methodology
Real-space non-equilibrium Green’s function (NEGF) formalism with single-band effective-mass basis
Self-consistent Born approx. for phonon self-energy functions1
Bulk phonon parameters based on deformation potential theory2
Limitations of phonon model : local in real and k spaces
--- Conduction Band
--- Electron Density
Electron density spectrum
Electron density profile
Electron flux vectors
δ-doped Layer EF EF
n+ cap
InGaAs
InAlAs
InP
InAs
( /cm3)
InAlAs barrier
0 nm
100 nm
Plot Line
( a.u.)
n+ cap
channel
Corner effect
Preliminary results with single-band effective-mass model
Electrons are thermalized at source/drain regions due to electron-phonon interactions
Electrons pass the barrier
NEMO5 simulator:
Atomistic tight-binding / effective-mass basis
Self-consistent NEGF-Poisson Solver
4-level MPI parallelization
Thick InAlAs barrier is the main element of resistance
[1] S. Jin et al., JAP 99, (2006)
[2] M. Lundstrom, Fundamentals of carrier transport (Cambridge Univ. Press)
2D simulation results:
current and resistance
Summary
Future work EF
Quantum transport modeling of the contact-to-channel region
Achievements: - 2D L-shaped simulation domain
- Phonon scattering
- Resistive behavior
Limitations: - Parabolic effective-mass model  inaccurate for high energy electrons
- Scattering model not fully calibrated
Experimental resistance and model are at the same order of magnitude
The InAlAs barrier plays the main role in the series resistance
Current density spectrum at the source/drain contacts
 thermal injection + tunneling
Include nonparabolic band structure effects
Improving phonon scattering model
– calibrate against experimental mobility models
Include alloy disorder effects, impurity / doping disorder
Surface roughness effects
Extend spatial region of the device sections
Predict higher performance HEMT devices
Explore more realistic modeling including:
Process variation
Dopant and surface randomness with atomistic simulations
More elaborate phonon scattering models
Preliminary model – parabolic effective-mass model
conduction band too low
 working on nonparabolic band model
Series resistance vs. applied bias
 resistive characteristic