Homo-junction InGaAs Band-to-band Tunneling Diodes

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Network for Computational Nanotechnology (NCN) UC Berkeley, Univ.of Illinois, Norfolk State, Northwestern, Purdue, UTEP Homo-junction InGaAs Band-to-band Tunneling Diodes Cho, Woo-Suhl [email protected]

description

Homo-junction InGaAs Band-to-band Tunneling Diodes. Cho, Woo-Suhl [email protected]. Motivation. Moore’s law and MOSFET scaling. Downscaling of Transistors**. Moore’s law*. Transistor dimensions scale to improve performance, and reduce cost per transistor - PowerPoint PPT Presentation

Transcript of Homo-junction InGaAs Band-to-band Tunneling Diodes

Page 1: Homo-junction InGaAs  Band-to-band Tunneling Diodes

Network for Computational Nanotechnology (NCN)UC Berkeley, Univ.of Illinois, Norfolk State, Northwestern, Purdue, UTEP

Homo-junction InGaAs Band-to-band Tunneling Diodes

Cho, Woo-Suhl

[email protected]

Page 2: Homo-junction InGaAs  Band-to-band Tunneling Diodes

Moore’s law and MOSFET scaling

• Transistor dimensions scale to improve performance, and reduce cost per transistor

• Increased packing density followed by Moore’s law

Moore’s law* Downscaling of Transistors**

Motivation

* http://en.wikipedia.org/wiki/Moore's_law/  ** http://www.intel.com/technology/mooreslaw/

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Dramatic Increase of Power Consumption

• CMOS microprocessors have reached the maximum power dissipation level that BJT based chips had

Motivation

* R. R. Schmidt, and B. D. Notohardjono, “High-End Server Low-Temperature Cooling”, IBM J. Res. & Dev., vol.46, No. 6, p. 739, 2002

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• New device concept or idea required

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Power consumption in MOSFETs

P IOFF VDD

Motivation

* S. Borkar, “Getting Gigascale Chips: Challenges and Opportunities in Continuing Moore’s Law”, ACM Queue, vol. 1, No. 7, p. 26, 2003

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• Downscaling of MOSFETs

- Leakage current usually fixed at IOFF=0.1μA/ μm- Increased transistor density per chip (>1 billion)

• Increase of power consumption & heat generation

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Limitations of MOSFET ScalingMotivation

log(Id)

Vg

VDD

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• Device with SS ≤ 60mV/dec is highly desired

VDD

ION ∝ (VDD −VT )η

(1≤η < 2)

• Limitations of scaling- Almost non-scalable supply voltage VDD

- Physical limit of Sub-threshold Swing (SS)

SS ≥2.3kTq;60mV

dec

ION

ION

IOFF

IOFF

VT`

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New Device Candidate: BTBT FETsMotivation

S

D

EF

EF

+Vg

BTBT FETs

• Majority carrier transport through the barrier

• Band-to-band tunneling of cold electrons

• Boltzmann tails are ignored

MOSFETs

• Minority carrier transport over the barrier

• Diffusion of hot electrons• Depends on the thermal

distribution of carriers• SS ≥ 60mV/dec limit

S

D

+Vg

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• SS ≤ 60mV/dec possible

Possible candidate to replace MOSFETs

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BTBT FET BTBT Diode

P+ drain

N+ source

Substrate

• Vertical structure- Sharp p-n interface can be more

easily fabricated

• Experimental data exist

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No Gate Bias: OFF STATE

Source

Drain

Positive gate bias: ON STATE

BTBT

+Vg

Study of BTBT DiodesMotivation

Buried Oxide

P+ N+

Gate oxide

S DI

Gate

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• Horizontal structure- Difficult to get sharp interface- Need excellent channel control

through gate contact

• Low on current

• Learn about the tunneling properties

• Test the potential of a given material as a TFET

• Test simulation model to design BTBT

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Outline

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• Approach• Basic Physics of Tunneling Diodes

- Band-to-band Tunneling- I-V Characteristic of BTBT Diodes

• InGaAs Diodes- Junction Modeling and Effects of Junction Abruptness- Solution to Increase Tunneling Currents

• Band Gap Narrowing Effect and Modeling

- Solution to Shift the Onset of Thermionic Current• Effects of Doping Variation• Excess Current• Temperature Dependence

• Summary and Future Work

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Outline

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• Approach• Basic Physics of Tunneling Diodes

- Band-to-band Tunneling- I-V Characteristic of BTBT Diodes

• InGaAs Diodes- Junction Modeling and Effects of Junction Abruptness- Solution to Increase Tunneling Currents

• Band Gap Narrowing Effect and Modeling

- Solution to Shift the Onset of Thermionic Current• Effects of Doping Variation• Excess Current• Temperature Dependence

• Summary and Future Work

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• Use full-band and atomistic quantum transport simulator based on the tight-binding model (OMEN) to model TDs- Ballistic transport using NEGF

• Reproduce and understand experimental data- Homogeneous InGaAS tunneling diodes (TDs) fabricated and

measured at Penn State, a partner in the MIND center

Simulation Approach and Objective

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10 Approach

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Outline

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• Approach• Basic Physics of Tunneling Diodes

- Band-to-band Tunneling- I-V Characteristic of BTBT Diodes

• InGaAs Diodes- Junction Modeling and Effects of Junction Abruptness- Solution to Increase Tunneling Currents

• Band Gap Narrowing Effect and Modeling

- Solution to Shift the Onset of Thermionic Current• Effects of Doping Variation• Excess Current• Temperature Dependence

• Summary and Future Work

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Band-to-band Tunneling

Pt =exp −4 2m* Eg

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3qhε⎛

⎝⎜⎜

⎠⎟⎟

Basic Physics12

• Narrow band gap- Increase tunneling probability- Material property

P+

N+

EFPEFNW

• High doping density- More degeneracy- High electric field- Small width barrier- Increase tunneling current

P+

N+

EFN

EFP

W

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Use of InGaAsBasic Physics

MaterialsEg (eV) at 300K

m*/m0

Si 1.12 1.08

Ge 0.67 0.55

InAs 0.35 0.013

In0.53Ga0.47As 0.75 0.038

• Small band gap material: Si Ge III-V (InAs)

• Indirect semiconductor Direct semiconductor

• In0.53Ga0.47As: Lattice matched to InP

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Indirect

Direct

Eg

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I-V Characteristics of BTBT DiodesBasic Physics

I

V

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IV

VV

P+

N+

EFNEFP

EV

EC

P+

N+

EFN

EFP

EV

EC

Tunneling current

IP

VP

P+

N+EFN

EFP

EV

EC

Excess current(Gap state current)

P+

N+

EFN

EFP

EV EC

Thermionic current

P+

N+

EFN

EFP

EVEC

Zenercurrent

NDR

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Outline

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• Approach• Basic Physics of Tunneling Diodes

- Band-to-band Tunneling- I-V Characteristic of BTBT Diodes

• InGaAs Diodes- Junction Modeling and Effects of Junction Abruptness- Solution to Increase Tunneling Currents

• Band Gap Narrowing Effect and Modeling

- Solution to Shift the Onset of Thermionic Current• Effects of Doping Variation• Excess Current• Temperature Dependence

• Summary and Future Work

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Fabricated device Simulated device

Device Structure and Doping ProfilePenn State: InGaAs Diode

• A InGaAs lattice matched to InP BTBT Diode

• NA=1020/cm3, ND=5×1019/cm3

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10nm

20nm

N+

x

In0.53Ga0.47As

P+

3nm

NA=8×1019

ND=4×1019

I

Measured I-V

• I-V chracteristics of BTBT diodes

Page 17: Homo-junction InGaAs  Band-to-band Tunneling Diodes

Abrupt doping Linear doping

20nm10nm

3nm

D (N+)S (P+)

NA=8×1019/cm3

ND=4×1019/cm3

x

20nm10nm

3nm

D (N+)S (P+)

ND=4×1019/cm3

x

Doping Profiles at the Junction

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NA=8×1019/cm3

0 0

Junction Modeling

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• Only Zener tunneling branch is shown

• Step junction uses Rs closer to the estimated value (20Ω)

Effect of Junction Abruptness

18 Junction Modeling

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I-V Characteristics: Experiment vs Simulation

• Step junction is used

• Zener current matched- Too low series resistance: RS=13.5Ω

vs. Estimated value: RS=20Ω

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I-V Characteristics: Experiment vs Simulation

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• Poor reproduction of forward-biased region- Low peak and valley currents- Thermionic current turns on at large

bias

• Investigate potential explanations for the observed disagreements

Junction Modeling

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Outline

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• Approach• Basic Physics of Tunneling Diodes

- Band-to-band Tunneling- I-V Characteristic of BTBT Diodes

• InGaAs Diodes- Junction Modeling and Effects of Junction Abruptness- Solution to Increase Tunneling Currents

• Band Gap Narrowing Effect and Modeling

- Solution to Shift the Onset of Thermionic Current• Effects of Doping Variation• Excess Current• Temperature Dependence

• Summary and Future Work

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Causes of BGN: High Doping Effects

• High doping level ≥ 1018/cm3

- D.O.S depends on the impurity concentration- Overlapping impurity states form an impurity band

~200meV BGN

Band Gap Narrowing21

Impurity Bands

EC

EV

ΔED

Donor Impurity Band

ΔED

E

EC

ρDOS(E)

• Random distribution of impurities- Potential fluctuation of the band edges- Impurity states tails into the forbidden gap

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BGN Calculation ModelJain-Roulston model*

Band Gap Narrowing22

• AdvantagesAdvantages1.Compact model calculated based on many-body theory2.Compute BGN as function of doping concentrations (N), and material parameters (A, B, C)3.Compute band shifts in major and minor bands separately for all materials4.No need for experimental fitting parameters

•S. C. Jain, and D. J. Roulston, Solid-State Electronics, vol. 34, No. 5, p. 453, 1990

S (P+) D (N+)

Before BGN

Eg

After BGN

Eg1 Eg2S (P+) D (N+)

ΔEV(min)

ΔEC(min)

ΔEV(maj)

ΔEC(maj)P+

N+

EF

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BGN calculation for In0.53Ga0.47As

Band Gap Narrowing23

p-In0.53Ga0.47As

ΔEg

ΔEc

ΔEV

n-In0.53Ga0.47As

ΔEg

ΔEcΔEV

NA=8e19/cm-3 ND=4e19/cm-3

• Most shift occurs at conduction band

• Not negligible shift in minor band

• Less BGN than n-type material

* S. C. Jain, J. M. McGregor, and D. J. Roulston, and P.Balk, Solid-State Electronics, vol. 35, No. 5, p. 639, 1992.* James C. Li, Marko Sokolich, Tahir Hussain, and Peter M. Asbeck, Solid-State Electronics, vol. 50, p. 1440, 2006.

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Inclusion of BGN in Tight-Binding

Band Gap Narrowing

S (P+) D (N+)

In0.53Ga0.47As before BGN

0.75eV

In1-x1Gax1As-In1-x2Gax2As after BGN

Eg1 Eg2

In1-x1Gax1AsIn1-x2Gax2As

S (P+) D (N+)

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1. Calculate new compositions of In and Ga from the reduced band gaps

2. Calculate tight-binding parameters from the empirical parameters of InAs and GaAs, and Bowing parameters

3. Shift band edges

Eg(300K ) =0.43x2 + 0.63x+ 0.36

CIn1−xGaxAs =(1−x)CInAs + xCGaAs + x(1−x)BIn1−xGaxAs

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23nm10nm

S (P+) D (N+)0.6450eV0.5804eV

In0.64Ga0.36As In0.71Ga0.29As

Page 25: Homo-junction InGaAs  Band-to-band Tunneling Diodes

1

Penn State: InGaAs DiodeThe effect of BGN

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• Closer to the experimental data: Effect of BGNCloser to the experimental data: Effect of BGN1.An increase of the series resistance2.An increase of tunneling current including the peak current3.An earlier turn-on of the thermionic current

2.2.3.3.

2.2.

1.1.

1.1.

• Discrepancies:Discrepancies:1.Mismatch in NDR region, and low valley current2.A shift of the thermionic current

Page 26: Homo-junction InGaAs  Band-to-band Tunneling Diodes

Outline

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• Approach• Basic Physics of Tunneling Diodes

- Band-to-band Tunneling- I-V Characteristic of BTBT Diodes

• InGaAs Diodes- Junction Modeling and Effects of Junction Abruptness- Solution to Increase Tunneling Currents

• Band Gap Narrowing Effect and Modeling

- Solution to Shift the Onset of Thermionic Current• Effects of Doping Variation• Excess Current• Temperature Dependence

• Summary and Future Work

Page 27: Homo-junction InGaAs  Band-to-band Tunneling Diodes

What can shift the thermionic current?

* Effect of doping variation

1. Influence of the donor concentration2. Influence of the acceptor concentration

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(1) Variation of the donor concentration ND

• Higher tunneling current for higher ND

- Increase in tunneling window ( )

• No shift of the thermionic current onset- No variation of potential barrier ( )

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P+

N+

EF

28 Effect of Doping Variation

P+

N+

EF

P+

N+

EF

NA=8e19/cm3Experiment dataND=8e19/cm3

ND=4e19/cm3

ND=2e19/cm3

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P+

N+

EF

(2) Variation of the acceptor concentration NA

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29 Effect of Doping Variation

P+

N+

EF

P+

N+

EF

• Small increase in tunneling current for higher NA

- Increase in tunneling window ( )

• Earlier turn-on of the thermionic current for lower NA

- Lowered potential barrier ( )- No strong influence 8

ND=4e19/cm3Experiment dataNA=4e19/cm3

NA=8e19/cm3

NA=1.2e20/cm3

Page 30: Homo-junction InGaAs  Band-to-band Tunneling Diodes

What can increase the valley current?

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* Excess current

1. Existence of excess current via gap states2. Influence of excess current

I

V

IV

VV

Excess current(Gap state current)

Thermionic current

Zener

NDR

Tunneling current

IP

VP

Page 31: Homo-junction InGaAs  Band-to-band Tunneling Diodes

Source of Excess Current (Ix) Excess Current

P+

N+

EFN

EFP

A

B

C

V Eg

qV

EV

EC E

Tail states

EC

EV

Conduction band

Valence band

ρDOS(E)

E

x

• Tunneling + Energy loss mechanism through gap states*• Gap States are mostly originated from the band edge tails

- A: Tails of acceptor levels extending to the forbidden gap- B: Tails of donor levels extending to the forbidden gap

* A. G. Chynoweth, W. L. Feldmann, and R. A. logan, Phys. Rev, vol. 121, p. 684, 1961

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Page 32: Homo-junction InGaAs  Band-to-band Tunneling Diodes

(1) Existence of Ix: Intrinsic I-V dataExcess Current

q

kTq

3kTσ

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• No series resistance is included• Purely thermionic current beyond the valley in the simulation data• Lower slope of the experiment data (σ≈⅓ of q/kT) at the valley confirms

the existence of Ix

• Assume that there is a dominant Ix around the valley

Page 33: Homo-junction InGaAs  Band-to-band Tunneling Diodes

Excess Current CalculationExcess Current

• Exponential nature of the excess current*

Linear increase of the currents beyond the valley

* D. K. Roy, Solid-State Electron., vol. 14, p.520, 1971

I x ≈IV exp(σ(V −VV )m−IR)

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IV =4.1×105[A / cm2 ]VV =0.765[V]

R=20(600 ×10−7 )2π[Ω⋅cm2 ]

σ =13×

qkT

Page 34: Homo-junction InGaAs  Band-to-band Tunneling Diodes

(2) The Effect of Excess Current Excess Current

* Effects of excess current (BGN is included)of excess current (BGN is included)

1. Increased current around and beyond the valley2. Closer match to the experiment results

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Effect of BGNPenn State: InGaAs Diode

V=0.95V

Efl

Efr

VP=0.35V

Efl Efr

V=-0.4V

Efl Efr

Efl

Efr

VV=0.64V

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(Ix included)

Page 36: Homo-junction InGaAs  Band-to-band Tunneling Diodes

(3) The Effect of TemperatureTemperature Dependence

* Effects of temperatureof temperature

1. 20meV more BGN occurs at room temperature2. Increase of peak and NDR region currents

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Page 37: Homo-junction InGaAs  Band-to-band Tunneling Diodes

Outline

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• Approach• Basic Physics of Tunneling Diodes

- Band-to-band Tunneling- I-V Characteristic of BTBT Diodes

• InGaAs Diodes- Junction Modeling and Effects of Junction Abruptness- Solution to Increase Tunneling Currents

• Band Gap Narrowing Effect and Modeling

- Solution to Shift the Onset of Thermionic Current• Effects of Doping Variation• Excess Current• Temperature Dependence

• Summary and Future Work

Page 38: Homo-junction InGaAs  Band-to-band Tunneling Diodes

• Investigate the performances of homogeneous InGaAs 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 (TFETs)

• 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 Penn State

• BGN provides good agreement with experimental data for tunneling currents: Zener and peak currents

• Excess current increase current around and beyond valley

• Current in NDR region is not well captured

• Solution: T-dependence, e-ph scattering

OBJECTIVE RESULTS

APPROACH

Summary

Page 39: Homo-junction InGaAs  Band-to-band Tunneling Diodes

Conclusion & Future works

• To investigate tunneling device, high doping effects such as BGN, and current via gap states should be considered

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• Electron-phonon scattering should be included to examine the effect on the increase of the current in the NDR region

• The approach can be applied to the analysis of other tunneling devices, such as the broken gap heterostructure diodes, and TFETs

• Need the verification of the approach by analyzing another fabricated device

• Exploring some other scattering mechanisms that may explain the mismatches between the experiments and simulation results

Page 40: Homo-junction InGaAs  Band-to-band Tunneling Diodes

40 Acknowledgement

Prof. Klimeck

Prof. Lundstrom and Prof. Garcia

Dr. Mathieu Luisier

All NCN Students and Group Members

Thank you!