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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