Slide 1

Tunneling Devices


Slide 2

Motivation
• 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


Slide 3

Obligatory Moore’s Law
human brain in 2012?
Reference

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


Slide 4

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


Slide 5

Tunneling
• 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


Slide 6

(Esaki) Tunnel Diode (TD)
• Simplest tunneling device
• Heavily-doped pn junction

EF

– 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
EV


Slide 7

Band-to-Band Tunneling in a
Tunnel Diode (c)
(e)

I

(b)

(d)

V

EC
(a)

EV
EF

(a)

(b)

(c)

(d)

(e)


Slide 8

Figures of Merit
Peak current
100 kA/cm2

Peak-to-Valley Ratio (PVR)

I

V


Slide 9

Bi-stable Configuration
V

I

D1
X
D2
X1

X2 V


Slide 10

TD Differential Comparator
V
CC

D3
X

M3

D4

CK

M4

D1

D2

VOUT

VOUT
VIN

RL

M 1 M2

I1

VIN

RL
I2

ITAIL

-VEE


Slide 11

Direct vs. Indirect Tunneling

Direct

Indirect

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


Slide 12

Tunnel Current Expressions
 m*1/ 2 EG3 / 2
T  exp 2 2eF


E 

3

Jt 

1/ 2

q m*

2

4 2eF
3

Va

4 2  E g
2


 2 Et 
 exp 

E 



1/ 2

*

m EG

exp(

- 4 2m *E g
3q

3/ 2

)