Tunneling DevicesTunneling Devices

MotivationMotivation

• Scaling: some proposed tunneling field effect transistor (TFET) designs do not suffer from short channel effects

• Power Dissipation: TFETs can beat the 60 mV/decade sub-threshold swing of MOSFETs

• Design Flexibility: Circuits can be made with fewer devices

• Scaling: some proposed tunneling field effect transistor (TFET) designs do not suffer from short channel effects

• Power Dissipation: TFETs can beat the 60 mV/decade sub-threshold swing of MOSFETs

• Design Flexibility: Circuits can be made with fewer devices

Obligatory Moore’s Law ReferenceObligatory Moore’s Law Reference

http://www.intel.com/research/silicon/mooreslaw.htm

human brain in 2012?

What’s so great about a tunneling device?What’s so great about a tunneling device?• Lower sub-threshold swing can allow for

lower operating voltages to be used• Negative differential resistance (NDR)

properties can be exploited to create simpler designs for bi-stable circuits, differential comparators, oscillators, etc.

• Leads to chips that consume less power

• Lower sub-threshold swing can allow for lower operating voltages to be used

• Negative differential resistance (NDR) properties can be exploited to create simpler designs for bi-stable circuits, differential comparators, oscillators, etc.

• Leads to chips that consume less power

TunnelingTunneling

• Tunneling is a quantum mechanical phenomenon with no analog in classical physics

• Occurs when an electron passes through a potential barrier without having enough energy to do so

• Tunneling is a quantum mechanical phenomenon with no analog in classical physics

• Occurs when an electron passes through a potential barrier without having enough energy to do so

(Esaki) Tunnel Diode (TD)(Esaki) Tunnel Diode (TD)

• Simplest tunneling device• Heavily-doped pn junction

– Leads to overlap of conduction and valence bands

• Carriers are able to tunnel inter-band• Tunneling goes exponentially with

tunneling distance– Requires junction to be abrupt

• Simplest tunneling device• Heavily-doped pn junction

– Leads to overlap of conduction and valence bands

• Carriers are able to tunnel inter-band• Tunneling goes exponentially with

tunneling distance– Requires junction to be abrupt

EC

EVEF

Band-to-Band Tunneling in a Tunnel DiodeBand-to-Band Tunneling in a Tunnel Diode

EC

EVEF

I

V

(a)

(b)

(c)

(d)

(e)

(a) (b) (c) (d) (e)

Figures of MeritFigures of Merit

I

V

Peak current100 kA/cm2

Peak-to-Valley Ratio (PVR)

Bi-stable Configuration

I

V

D2

D1

X

V

X1 X2

TD Differential ComparatorTD Differential Comparator

M1 M2

ITAIL

-VEE

VCC

M4M3

VOUT

RL

I1 I2

RL

VOUT

CK

VIN VIN

D1

D3

D2

D4

X

Direct vs. Indirect TunnelingDirect vs. Indirect Tunneling

Direct Indirect

Indirect materials require phonons to tunnel, thus reducing the probability of a tunneling event

Tunnel Current ExpressionsTunnel Current Expressions

E

E

eF

EmT tG 2

exp22

exp2/32/1*

GEm

eFE

*3

24

)3

*24exp(

24

*2/3

2/122

2/13

q

Em

E

VmqJ g

g

at