1. Full-band Simulations of Band-to-Band Tunneling Diodes Woo-Suhl Cho, Mathieu Luisier and Gerhard Klimeck Purdue University
Investigate the performance of homogeneous InGaAs and broken gap GaSb-InAs III-V band-to-band-tunneling (BTBT) diodes
Study the tunneling properties of a given material and its potential as a BTBT Field-Effect Transistors (FETs)
Use full-band and atomistic quantum transport solver based on tight-binding to simulate BTBT diodes
Coherent tunneling (no e-ph)
Compare the simulation results to experimental data from Notre Dame and Penn State
Good agreement with experimental data for the Zener tunneling branch
Poor agreement in the negative differential resistance (NDR) regime: peak, valley, and thermionic currents not well captured
Solution: band gap narrowing and e-ph scattering
Proper modeling of band gap narrowing as function of doping concentrations
Verification of thermionic current with drift-diffusion solver
Solve convergence problem for GaSb-InAs broken gap diodes
OBJECTIVE
RESULTS
APPROACH
ONGOING WORKS
III-V Band-to-band tunneling (BTBT) diodes
Fabricate and measure tunneling currents in 1-D TD (Notre Dame (A) , Penn State (B) and MIND Partners)
Homogeneous material: (A) InGaAs
Broken gap heterostructure: (B) AlGaSb-InAs
Use full band and atomistic quantum transport simulator based on the tight-binding model (OMEN)
Solve NEGF using recursive Green’s function algorithm
Reproduce experimental data
(A)
(B)
Approach
Physical Models
Device Engineering
Efficient Parallel Computing
3D Quantum Transport Solver
Accurate Representation of the Semiconductor Properties
Atomistic Description of Devices
Ballistic and Dissipative
Explore, Understand, Explain, Optimize Novel Designs
Predict Device Performances
Predict Eventual Deficiencies Before Fabrication
Accelerate Simulation Time
Investigate New Phenomena at the Nanometer Scale
Move Hero Experiments to a Day-to-Day Basis
GAA NW
Electron
Density
Id-Vgs
Parallelization
Scheme
OMEN
Multidisciplinary Effort: PHYS - EE - HPC 4
Simulation Structure
Band Diagram
In0.53Ga0.47As
20nm
10nm
3nm
D (N+)
S (P+) x EF
0.75eV P+ N+
InGaAs lattice matched to InP
NA_S=8×1019/cm3, ND_S=106/cm3
ND_D=4×1019/m3, NA_D=106/m3
Heavily doped P-N
Overlap between CB & VB
Possibility of tunneling
1D TD: Homogeneous material 5
Motivation
BTBT FET
BTBT Diode
Buried Oxide P+ N+
Gate oxide S D I
Gate
P+ drain
N+ source
Substrate
Promising device
No low limit on the SS
Low power consumption
Horizontal structure
Difficult to get sharp interface
Need excellent channel control through gate
Vertical structure
No need for sharp interface or gate control
Good to learn about the tunneling properties
Good to test the potential of a given material as a TFET
Experimental data exist 1 2 3
Only Zener tunneling branch is shown
Better match to experimental data with step-like junction 2
1.8
1.6
1.4
1.2 1
0.8
0.6
0.4
0.2
V [V]
X106
I [A/cm2]
In0.53Ga0.47As TD
Penn State (S. Datta)
Influence of p-n junction profile
Comparison to Experimental Data 6
Step junction is used
Zener current matched
Poor reproduction of NDR region
low peak and valley currents
No electron-phonon scattering
valley current cannot be matched
Investigate potential explanations for the observed misbehavior
Complete I-V Characteristics: Simulation vs Experiment 7
Higher peak current, no change in valley current
No shift of the thermionic current
EF-EC of drain ( ) varies due to donor doping
No change for ( ) region P+ N+ EF
(1) Variation of the donor concentration ND 8
Small increase of peak current, no change for valley
Shift of the thermionic current turn-on
EV-EF of source ( ) varies due to acceptor doping
Change for ( ) region P+ N+ EF
(2) Variation of the acceptor concentration NA 9
Study the effect of BGN through smaller band-gap material
In0.53Ga0.47As with Eg= (eV) vs In0.75Ga0.25As with Eg= (eV)
Increase of peak and valley tunneling current + shift of thermionic current branch
(3) Band Gap Narrowing (BGN) 10
Modeling
Compute CB and VB shift as function of doping concentrations
Model TD as heterostructure with 2 different materials
Goals: increase of peak current and shift of thermionic current
20nm
10nm S D x
In0.53Ga0.47As after BGN
0.75eV
In0.53Ga0.47As before BGN
Solution: accurate modeling of BGN 11
OMEN: Quantum transport simulator based on tight-binding model
PADRE: A device simulator using drift-diffusion
Corrected values of NV and NC based on the results of OMEN used
Shows ideal IV curve for a PN diode and where the thermionic current starts
More thermionic current shift with PADRE 12 10 8 6 4 2 -2
Voltage [V]
X 105
I [A/cm2]
Comparison to drift-diffusion simulations
Simulation Structure
Band Diagram
GaSb (P+)
InAs(N+) EF
0.751eV
0.37eV S D
InAs (N+)
25nm
50nm
2nm x
NA_S=1019/cm3, ND_S=106/cm3
ND_D=2×1018/cm3, NA_D=106/cm3
Lattice matched
a= nm at 300K
Broken gap
High tunneling current
1D TD: Heterostructure with broken gap 13
Poisson Convergence Problem
Accurate modeling of BGN in InGaAs TD
BGN can be calculated from Jain-Roulston model
Verification of thermionic current turn-on
No tunneling required direct comparison to drift-diffusion possible
Convergence problem with Poisson equation in broken gap heterostructure
Simplest solution: fictitious scattering through imaginary potential (parameter sensitivity?)
Ongoing work 15
Tunneling current through Broken gap material
Problem with hole accumulation on the p-side
Electron-phonon scattering needed to fill these states
Cannot fill the states
InAs (N+) D
GaSb (P+) S e 14