green transistor for 10x lower ic power ?

Green Transistor for 10X Lower IC Power ?

Chenming Hu

University of California, Berkeley

Supported by: DARPA STEEP, FCRP-MSD

Electronics Infrastructure

6/2009 Chenming Hu

the world enabled

IC chips

fabs

transistors

30nm

Buried Oxide

27nm

30nm

Buried Oxide

27nm

electronic systems

$

Expectation: ICs will be even more..

• Affordable (size reduction..): manufacturing, device physics limit,…

• Useful (speed, density..): natural human interface, bio-medical sensing…

• Usable (low power): heat management, portability, global energy conservation…

6/2009 Chenming Hu

Power Consumption Problems

1.Thermal management/package issues may limit integration density.

2. IC usage of electricity at an inflection point.

• ICs use a few % of world’s electricity today and growing exponentially.

• Power per chip is growing.

• IC units in use also growing.

3.Need to reduce IC power consumption with architecture and circuit innovations, and a low voltage transistor.

6/2009 Chenming Hu

• Because power consumption Vdd2

• and Vdd (operation voltage) scaling has slowed.

High Performance ITRS Roadmap

Technology Node

0.25 μm

0.18 μm

0.13 μm

90 nm

65 nm

45 nm

32 nm

22 nm

16 nm

Vdd 2.5 V 1.8 V 1.3 V 1.2 V 1.1 V 1.0 V 0.9 V 0.8 V 0.7 V

IC Power Consumption Rising Much Faster Than Past Trend

6/2009 Chenming Hu

Why Vdd scaling slowed

•We used to control power by scaling Vdd and maintain good speed by reducing Tox.

•But, Tox can not be reduced much more, not even with high-k dielectrics.

•But new materials will raise the mobility, μ

1.2 nm SiO2

Speed transistor current μ ( Vdd – Vt ) / Tox

6/2009 Chenming Hu

New material, e.g. Ge film on Si substrate

Industry is also funding InGaAs, InAs, and graphene MOSFET research.

Oxide

Silicon

DrainSource

Gate

3nm Ge film

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How to Reduce Power by 20X

Two steps to reduce Vdd to 0.2V for 20x power

reduction?

1. Reduce Vdd – Vt to < 0.15V with high-mobility-

channel material (Ge, III-V, graphene...), etc.

2. Reduce Vt to 50mV. But, there is the fundamental 60mV/decade turn-off limit …..

6/2009 Chenming Hu

9

Lowering Vt by 60mV increases the leakage current (power) by 10 times.

Vs

Vt

dd eVtVgIVgI

)()0(

Source: Intel Corporation

6/2009 Chenming Hu

Vt

Dra

in C

urr

ent,

ID

S (A

/ m

)

Gate Voltage, VGS (V)

0.0 0.3 0.6 0.910

-11

10-9

10-7

10-5

10-3

Lower

Vt

Ioff Limit - 60mV/decade Swing

The “fundamental” 60mV/decade Limit

Electrons go over a potential barrier. Leakage current is determined by the Boltzmann distribution or 60 mV/decade, limiting MOSFET, bipolar, graphene MOSFET…How to overcome the limit:Let electrons go through the energy barrier, not over it tunneling

106/2009 Chenming Hu

Ec

Ev

COX

VG

Source Channel Drain

Semiconductor Band-to-Band TunnelingEC

EV

6/2009 Chenming Hu

A known mechanism of leakage current since 1985.

J. Chen, P. Ko, C. Hu, IEDM 1985

Called Gate Induce Drain Leakage (GIDL) because the current depends on the gate voltage.

Basic Tunnel Transistor Structure

Some references

~100X less current than MOSFET

Need a more optimal tunneling transistor structure.

P+ DrainN+ Source P-

12

W. Reddick, G. Amaratunga, Appl. Phys. Letters, vol. 67, 1994.W. M. Reddick, et al., Appl. Phys. Lett., vol. 67(4), pp. 494-497, 1995.C. Aydin, A. Zaslavsky, et al., Appl. Phys. Lett., vol. 84(10), pp. 1780-82, 2004.WY. Choi et al., Tech. Dig. Int. Electron Device Meet, pp. 955-958, 2005.K. K Bhuwalka, et al., Jpn. J. of Appl. Phys., vol. 45(4B), pp. 3106-3109, 2006.Th. Nirschl, et al., Electron Device Letters, vol. 28(4), p. 315, 2007.

6/2009 Chenming Hu

Large field, good capacitive coupling between gate and pocket, abrupt turn-on due to over-lap of valence/conduction bands, adjustable tun-on voltage.

Green Transistor (gFET)--Simulation

13

Energy band diagram

Simulated carrier generation rates

Hole flow

Electron flow

N+ Source

P+ Pocket Gate

P+ Drain

N+ P+

Buried Oxide

P+ Pocket

P-

6/2009 Chenming Hu

C. Hu et al, 2008 VLSI-TSA, p.14, April, 2008

G

DS

gFET vs Basic Tunnel FET-simulation

* K. K Bhuwalka, et al., Jpn. J. of Appl. Phys., vol. 45(4B), pp. 3106-3109, 2006

1E-11

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

I DS

(A/u

m)

VGS (V)

EOT= 1 nm VDD=1V Lg=40nm


gFETEOT= 4.5 nm VDD=4V

Dra

in C

urre

nt, I

DS (

A/µm

)

Basic Tunnel FET *

6/2009 Chenming Hu


Simulated Id-Vd of 0.5V Ge gFET

• Good output resistance and DIBL. Lg = 40nm

0

100

200

300

400

500

600

700

800

0.0 0.1 0.2 0.3 0.4 0.5

Ids (u

A/um

)

Vgs (V)

Vgs: 0.5 V

Vgs: 0.4 V

Vgs: 0.3 V

Drain-Source Voltage, VDS (V)

Dra

in C

urre

nt, I

DS (

µA/µ

m)

EOT=0.5nm

6/2009 Chenming Hu


Vdd (Power) Scaling Path: Reduce Band Gap


6/2009 Chenming Hu

1E-11

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

0.0 0.2 0.4 0.6 0.8 1.0

Ids (A/um)Eg=0.36eV (InAs)Eg=0.69eV (Ge)Silicon

Eg=0.36eV, Vdd=0.2V, EOT=5 Å, CV/I=0.42pS

Eg=0.69eV, Vdd=0.5V, EOT=7 Å, CV/I=2.2pS

Eg=1.1eV, Vdd=1V, EOT=10 Å, CV/I=4.2pS


Dra

in C

urre

nt, I

DS (

µ/µm

)

Lg=40nm

P+ Source N+ Drain

B Substrate

Gate

AEC

EV

Gat

e O

xid

e

~~~~~~~~

~~~~~~~~

Gate

A B

Egeff

Hetero-tunneling gFET• In lieu of low Eg semiconductor, a heterojunction can provide a very small effective tunneling band gap, Egeff.

Egeff is 0.3eV for Si/Ge hetero-tunneling gFET.

6/2009 Chenming Hu

A. Bownder et al., 8th International workshop Junction Technology, Extended Abstracts , p.93, 2008. AlsoIEEE Silicon Nanoelectronics Workshop, 2008.

Compound Semiconductors

18

Egeff

•Example: InAs-AlGaSb provides tunable Egeff from positive to negative values.

• Very low voltage gFET may be possible.

• Wide choices of heterojunction materials, band engineering and strain engineering.

6/2009 Chenming Hu

Source

SiO2

Si N+ Si

Drain

P+ Ge

Gate

I D [A

/m

]

Experiment

Model

VGS [V]

Ge-Source Tunnel Transistor

S. Kim et al., VLSI Tech Symp., 2009

)/exp( ssD EBAEI

Es = |VGS+Vtunnel|/(Toxge/ox)

Vtunnel ~ 0.6VS [m

V/d

ec]

ID [A/m]

VD=0.5V

LG=5m

W=0.33m

Green’s Function Based SimulationSayeef Salahudin

• ICs use of world’s electricity is several % and growing fast.

• A low voltage transistor can slow the growth.

• Green Transistor may potentially provide orders-of-magnitude IC power reduction.

Summary

6/2009 Chenming Hu

green transistor for 10x lower ic power ?

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