Nanotransistors

1
Nanotransistors

• Using Carbon Nanotubes

2
What is a Transistor ?

• Building block of electronics
• They are switches that control current
• There are two main types used today
• Bipolar Junction Transistor (BJT)
• Metal-Oxide Semiconductor Field Effect Transistor
  (MOSFET)

3
What is a Transistor ?
Basic Idea

• Both types of transistors require three terminals
  
• one for the input of the switch
• one for the output of the switch
• one to control the switch
• Both are built using p-type and n-type
  semiconductors

4
What is a Transistor ?
Semiconductors ?

• Valance band full like an insulator
• Conduction band close to valence band similar to
  a metal
• Electrons can jump to conduction band unlike an
  insulator

5
What is a Transistor ?

• Silicons resistivity is similar to an insulator
• Doping is used to reduce resistivity
• N-type donates electrons (donor)
• Produces extra electrons
• P-type accept electrons (acceptors)
• Produces holes
• Equation
• r resistivity
• m electron mass
• e electron mass
• n of Charge Carriers
• t mean time between collisions

6
What is a Transistor ?
BJT
NPN

• Two types of BJTs
• NPN
• PNP

PNP
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What is a Transistor ?
BJT - NPN

• Electrons injecting into Emitter terminal
• Diffuse to Base
• Composed of p-type semiconductor
• Electrons see potential on collector is higher
  and diffuse to it
• Some electrons combine with extra holes in the
  p-type
• Base current is needed to compensate for lost
  holes

• Holes injected into Base
• Move Emitter side
• Move towards lower potential
• Holes do not flow to Collector
• CBJ is reversed bias

8
What is a Transistor ?
BJT - PNP

• Holes injecting into Emitter terminal
• Diffuse to Base
• Composed of n-type semiconductor
• Holes see potential on collector is lower and
  diffuse to it
• Some holes combine with extra electrons in the
  n-type
• Base current is needed to compensate for lost
  electrons

• Electrons injected into Base
• Move to Emitter side
• Move towards higher potential
• Electrons do not flow to Collector
• CBJ is reversed bias

9
What is a Transistor ?
Deriving I -V Equations

• np(0) highest electron concentration
• pn(0) highest hole concentration
• pn0 thermal equilibrium of holes

10
What is a Transistor ?
Deriving I -V Equations Collector Current

• NA - doping concentration of the base
• AE - crosssectional area of the BEJ
• q - magnitude of electron charge
• Dn - electron diffusivity in base
• W - effective width of base

11
What is a Transistor ?
Deriving I -V Equations Base Current

• iB1 - due to the holes injected from base into
  emitter
• iB2 - due to holes supplied by external circuit
• Dp - hole diffusivity in the emitter
• Lp - hole diffusion length in the emitter
• ND - doping concentration of the emitter
• Qn - electrons form the emitter
• tb - average amount of electrons being destroyed

12
What is a Transistor ?
13
What is a Transistor ?
Deriving I -V Equations Emitter Current

• Looking at node 1
• Emitter current must equal the sum of the Base
  and Collector currents
• Plug in relationship of iC and iB to get
  relation emitter and collector currents

14
What is a Transistor ?
Deriving I -V Equations Summary

• For pnp transistor replace vBE with vEB
• because of change in currents direction

15
What is a Transistor ?
MOSFET

• Uses an electric field to control the current
• Hence the part of the name Field Effect
  Transistor (FET).
• Unlike BJTs only one type of carrier (electrons
  or holes) is used to conduct the current
• depending on the type of MOSFET (NMOS or PMOS)
• give MOSFETs there other name the unipolar
  transistor

16
What is a Transistor ?
NMOS - Construction

• Fabricated on a p-type substrate
• a single crystal silicon wafer
• provides physical support for both the transistor
  and the rest of the Integrated Circuit (IC)
• Two heavily doped n-type regions created inside
  the substrate
• indicated as n source (S) and n drain (D)

17
What is a Transistor ?
NMOS - Construction

• A layer of silicon dioxide (SiO2) is grown over
  the surface of the substrate
• thin (0.02 to 0.1 µm)
• covering region between source and drain
• excellent electrical insulator
• Metal is deposited on all three terminals to form
  an electrode
• Body connection required to suck up any extra
  charge
• Not needed for every transistor
• usually 1 for every 5

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What is a Transistor ?
NMOS - Construction

• All pn junctions are kept at reverse bias to
  prevent a short form power to ground
• A pn junction forms a diode
• connected in series
• block any current from flowing form the drain to
  the source
• path between the drain and source have a very
  high resistance on the order of 1012 ?

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What is a Transistor ?
NMOS - Operation

• Appling a positive voltage to gate
• repels holes deep into the substrate
• leaves electrons that would have normally have
  been killed by the holes
• some electrons are pulled from the n-wells,
  towards the gate
• Forms a channel for the electrons to travel
  through

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What is a Transistor ?
NMOS - Operation

• Applying small voltage on Drain causes current
  iD (drain current) to form
• equals the source current (iS) since that is the
  only current coming into the source
• forms because free electrons exist inside induced
  n-channel
• current is only being produced by one type of
  carrier, the electron
• note no gate current because no direct connect
  available

21
What is a Transistor ?
NMOS - Operation

• Increasing vDS , causes the n-channel to form a
  more trapezoidal shape.
• resistance also increases
• drain end of channel depth approaches zero
• when zero then channel is said to be pinched off
• no longer an effect on channel
• transistor has entered saturation mode
• Steady state for a digital circuit

22
What is a Transistor ?
NMOS - Current Voltage Triode region

• Assume vGS gt Vt
• Transistor acts ohmic

23
What is a Transistor ?
NMOS - Current Voltage
Triode Region

• dx - infinitesimal portion of length of the
  channel at a point x
• v(x) - voltage at that point
• Cox - capacitance per unit of the parallel plate
  capacitor formed by gate and channel
• v(x)-Vt - voltage between gate and x
• must be greater than Vt
• eox - permittivity of silicon oxide
• tox - thickness of oxide layer
• mn - electron mobility in the channel

24
What is a Transistor ?
NMOS - Current Voltage
Triode Region

• Obtain drift current by multiplying charge per
  unit length by drift velocity
• i must be constant throughout the channel
• since no other source of current
• i must be negative of drain-to-source current

25
What is a Transistor ?
NMOS - Current Voltage
Saturation Region

• Substitute the condition for saturation into the
  triode equation

26
What is a Transistor ?
PMOS

• PMOS is similar to NMOS expect that ns are
  replaced with p
• Transistor sits in an n well that is dug out of
  the p-type substrate
• Electrons are repelled when a voltage is applied
• Holes are free to create a p type channel
• Holes are the carriers of the current
• All equation for the NMOS apply to PMOS with
  following exception

- Triode Condition
- Saturation Condition
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Future Problems of CMOS

• Cant go smaller than atomic distances
• Current manufacturing technology cant resolve
  atomic distances
• Quantum effects become important i.e. Quantum
  tunneling
• Classical effect become unmanageable like
  inductance and crosstalk.

28
What are Carbon Nanotubes ?

• CNT
• Thin tubes constructed of carbon arranged in a
  hexagonal ring structure
• Diameter are typically 1.4 nanometers
• Constructed out of carbon

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What makes CNTs good ?

• Easily withstand high current densities of
  billion A/cm2
• Made to be conductors or semiconductors
• Circumference is inversely proportional to the
  band gap
• No clean room required to make them
• Nanoscale diameter
• Strength

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How to make metallic and semiconducting CNTs

• Naturally metallic
• Twist to make semiconducting

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How to make metallic and semiconducting CNTs

• Saturation point after about 1.5 or 1.6
  degrees/section

32
Can we separate metallic and semiconducting CNTs
Constructive Destruction

• Deposit ropes of stuck together metallic and
  semiconducting nanotubes on a silicon-oxide
  wafer.
• Project a lithographic mask onto the wafer to
  form electrodes (metal pads) over the nanotubes.
  These electrodes act as a switch to turn the
  semiconducting nanotubes on and off.
• Using the silicon wafer itself as an electrode,
  "switch-off" the semiconducting nanotubes, which
  essentially blocks any current from traveling
  through them.
• The metal nanotubes are left unprotected and an
  appropriate voltage is applied to the wafer,
  destroying only the metallic nanotubes, since the
  semiconducting nanotubes are now insulated.
• The result a dense array of unharmed, working
  semiconducting nanotube transistors that can be
  used to build logic circuits like those found in
  computer chips.

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I dont want to destroy my metallic CNTs !

• Small kinks in CNTs create different conduction
  environments
• kinks can be caused by some pentagon or heptagon
  structures among an otherwise hexagonal
  arrangement of carbon atoms
• One side is a conductor and the other is a
  semiconductor
• Nanowires are now connected to Nanotransistors.

34
Nanotransistors from CNTs

• Draped the CNT over two electrodes that sit on
  top of SiO2
• Gate is set either on or under SiO2

35
Nanotransistors from CNTs

• Gate isolated from CNT
• The CNT is the channel)
• Just like a MOSFET
• Operation similar to MOSFET
• Carbon NanoTube Field Effect Transistor or CNTFET
  for short
• N-CNTFETS and P-CNTFETS
• P-CNTFETs made naturally
• N-CNTFETS made by annealing in a vacuum at 200o
  for 10 hrs or doping the nanotube with an
  electropositive element such as potassium

36
Nanotransistors from CNTs
So which is better?

• Clearly annealing process is best for most
  systems.
• Curves are similar in shape to MOSFETs

37
Single Electron Transistor

• When cooled to 4 degrees Kelvin CNTFETS change
  behavior to a Single-Electron Transistors (SETs)
• Advantages of SETs are reduction in current thus
  reducing power consumption
• Operate with only one electron therefore
  switching times are shorter
• More research is needed to fully exploit
• Expensive to cool

38
Nanocircuits

• The simplest circuit element is the inverter
• Inverters the incoming signal
• Has a gain greater that 1 to dive other parts of
  the circuit
• Constructed by having a pfet in series with an
  nfet
• Input is shorted together
• Output is taken in the middle where pfets drain
  meets nfets drain

39
Nanocircuits
Build an Inverter
Annealing Process

• NCNTFET is covered with PMMA to seal it in a low
  oxygen environment (10-2 Torr)
• Expose to a 10-3 Torr of oxygen for 3 min.
• Unprotected NCNTFET turn back into PCNTFET
• Requires separate CNTs

40
Nanocircuits
Build an Inverter
Doping Process

• Covered by PMMA
• Window opened using electron beam lithography
• Potassium used to n-dope half CNT
• Doping adjusted so thresholds of p and n-CNTFETs
  overlap
• Requires one CNT

41
Nanocircuits
Build an Inverter
Doping
Annealing
Comparing

• All three similar
• Doping produced best curve
• Sharpest
• Thus Fastest

Current CMOS ?
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Conclusion

• CNTFET are a great solution to keep Moores law
  going
• Nanotransistors would have a faster switching
  time then the current technology
• nanotransistors could achieve terahertz
• todays computers operate around two gigahertz
• Power consumption would be significantly lower
• Do not require a clean room