Tunneling Devices

MotivationMotivation Scaling: some proposed tunneling field effect transistor (TFET) designs do not suffer from short channel effectsScaling: 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 MOSFETsPower Dissipation: TFETs can beat the 60 mV/decade sub-threshold swing of MOSFETs Design Flexibility: Circuits can be made with fewer devicesDesign Flexibility: Circuits can be made with fewer devices Scaling: some proposed tunneling field effect transistor (TFET) designs do not suffer from short channel effectsScaling: 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 MOSFETsPower Dissipation: TFETs can beat the 60 mV/decade sub-threshold swing of MOSFETs Design Flexibility: Circuits can be made with fewer devicesDesign Flexibility: Circuits can be made with fewer devices

Obligatory Moore’s Law Reference human brain in 2012?

What’s so great about a tunneling device? Lower sub-threshold swing can allow for lower operating voltages to be usedLower 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.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 powerLeads to chips that consume less power Lower sub-threshold swing can allow for lower operating voltages to be usedLower 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.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 powerLeads to chips that consume less power

TunnelingTunneling Tunneling is a quantum mechanical phenomenon with no analog in classical physicsTunneling 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 soOccurs 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 physicsTunneling 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 soOccurs when an electron passes through a potential barrier without having enough energy to do so

(Esaki) Tunnel Diode (TD) Simplest tunneling deviceSimplest tunneling device Heavily-doped pn junctionHeavily-doped pn junction –Leads to overlap of conduction and valence bands Carriers are able to tunnel inter-bandCarriers are able to tunnel inter-band Tunneling goes exponentially with tunneling distanceTunneling goes exponentially with tunneling distance –Requires junction to be abrupt Simplest tunneling deviceSimplest tunneling device Heavily-doped pn junctionHeavily-doped pn junction –Leads to overlap of conduction and valence bands Carriers are able to tunnel inter-bandCarriers are able to tunnel inter-band Tunneling goes exponentially with tunneling distanceTunneling goes exponentially with tunneling distance –Requires junction to be abrupt ECEC EVEV EFEF

Band-to-Band Tunneling in a Tunnel Diode ECEC EVEV EFEF I V (a) (b) (c) (d) (e) (a) (b) (c) (d) (e)

Figures of Merit I V Peak current 100 kA/cm 2 Peak-to-Valley Ratio (PVR)

Bi-stable Configuration I V D2D2 D1D1 X V X1X1 X2X2

TD Differential Comparator M1M1 M2M2 I TAIL  V EE V CC M4M4 M3M3 V OUT RLRL I1I1 I2I2 RLRL CK V IN D1D1 D3D3 D2D2 D4D4 X

Direct vs. Indirect Tunneling Direct Indirect Indirect materials require phonons to tunnel, thus reducing the probability of a tunneling event

Tunnel Current Expressions