MESFETs

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Fundamentals of Nanoelectronics
Lecture 7 MESFETs Schottky Barrier
Devices Heterojunction Transistor HEMTs
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• Outline
• MESFETs
• Schottky Barrier Devices
• Heterojunction Bipolar Transistor
• High Energy Mobility Transistors (HEMTs)

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MESFET
Metal Epitaxial
Semiconductor Field Effect Transistor

• Gate looks like Schottky diode
• Dont forward bias

Schottky diode
gate
source
drain
metal (e.g. TiAu)
ohmic
ohmic
depletion region
n (heavy)
Insulating substrate
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• MESFETs are quite similar to a JFET in
  construction and terminology
• The difference is that instead of an using a p-n
  junction for a gate, a Schottky (metal
  -semiconductor) junction is used
• MESFETs are usually constructed in compound
  semiconductor technologies lacking high quality
  surface passivation such as GaAs, InP, or SiC,
  and are faster but more expensive than
  silicon-based JFETs or MOSFETs
• Production MESFETs are operated up to
  approximately 30 GHz, and are commonly used for
  microwave frequency communications and radar

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• From a digital circuit design perspective, it is
  increasingly difficult to use MESFETs as the
  basis for digital integrated circuits as the
  scale of integration goes up, compared to CMOS
  silicon based fabrication
• The absence of an insulator under the gate
  implies that the MESFET gate should, in
  transistor mode, be biased such that the metal
  semiconductor diode is not forward biased

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MESFET
Schottky diode
gate
source
drain
metal (e.g. TiAu)
ohmic
ohmic
depletion region
Xdep(x)
a
n (heavy)
b(x)
Insulating substrate
When they touch, define VDS,sat
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HEMT High Electron Mobility Transistor
Schottky diode
gate
source
drain
metal (e.g. TiAu)
ohmic
ohmic
n-AlGaAs
tb
i-AlGaAs
d
2DEG
i-GaAs
Insulating substrate
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• A HEMT is a field effect transistor incorporating
  a junction between two materials with different
  band gaps as the channel instead of a doped
  region, as is generally the case for MOSFETs
• A commonly used material combination is GaAs with
  AlGaAs, though there is wide variation, dependent
  on the application of the device.
• Devices incorporating more indium generally show
  better high-frequency performance,
• Gallium nitride HEMTs have a greater high - power
  performance

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• In general, to allow conduction, semiconductors
  need to be doped with impurities to generate
  mobile electrons in the layer
• However, this causes electrons to slow down
  because they end up colliding with the impurities
  which were used to generate them in the first
  place
• HEMT resolves this contradiction by use of high
  mobility electrons generated using the
  heterojunction of a highly - doped wide - bandgap
  n - type donor-supply layer (such as AlGaAs) and
  a non - doped narrow - bandgap channel layer with
  no dopant impurities (such as GaAs)

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• The electrons generated in the n - type AlGaAs
  thin layer drop completely into the GaAs layer to
  form a depleted AlGaAs layer,
• The heterojunction created by different band gap
  materials forms a quantum well in the conduction
  band on the GaAs side where the electrons can
  move quickly without colliding with any
  impurities because the GaAs layer is undoped, and
  from which they cannot escape
• The effect of this is to create a very thin layer
  of highly mobile conducting electrons with very
  high concentration, giving the channel very low
  resistivity (high electron mobility)

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• This layer is called a two-dimensional electron
  gas
• Like all the other types of FETs, a voltage
  applied to the gate alters the conductivity of
  this layer
• Applications are similar to MESFETs - microwave
  and millimeter wave communications, imaging,
  radar, and radio astronomy
• Any application where high gain and low noise at
  high frequencies are required

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• HEMTs have shown current gain of gt600GHz and
  power gain to gt1THz
• Heterojunction bipolar transistors (HBTs) have
  demonstrated a current gain at frequencies over
  600 GHz
• HEMTs can be used as discrete transistors but
  more often are used in the form of an integrated
  circuit called a MMIC (Monolithic Microwave
  Integrated Circuit)
• HEMT devices are found in many types of equipment
  ranging from cellphones and DBS receivers to
  electronic warfare systems such as radar and
  radio astronomy

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Band diagram
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Current
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Integrating
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Bipolar Junction Transistor (BJT)

• Two back - to - back pn junctions
• Close enough for minority carriers to interact
  (can quickly diffuse in the base region)
• Far apart enough so that the depletion regions do
  not interact (i.e., punchthrough)

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Basic BJT Characteristics
p type base
n type collector
n type emitter
electrons
electrons
Ic
Ie
recombine
a few holes
holes
Ib
What fraction of minority carriers make it
across? Emitter injection efficiency Current
transfer ratio
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Basic BJT Characteristics
Minority carrier transit time ?t
p type base
n type collector
n type emitter
Ic
Ie
Minority carrier lifetime ?p
Ib
For efficient transistor ? 1 Base current? Gain
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BJT, npn, forward active
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BJT, npn, reverse active
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BJT, npn, saturation
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BJT, npn, cutoff
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BJT Currents
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Normal Active Schematic
p type base
n type collector
n type emitter
electrons
electrons
Ic
Ie
a few holes
Bad for base resistance. Bad for E-B capacitance.
So bad for speed.
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Normal Active Schematic
p type base
n type collector
n type emitter
electrons
electrons
Ic
Ie
recombine
a few holes
holes
Ib
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Normal active Heterojunction Bipolar Transistor
(HBT)
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
a few holes
Shockley, Kroemer Holes exponentially suppressed
if emitter is wider gap.
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• The heterojunction bipolar transistor (HBT) is an
  improvement of the bipolar junction transistor
  (BJT) that can handle signals of very high
  frequencies up to several hundred GHz
• It is common in modern ultrafast circuits, such
  as used radio - frequency (RF) systems
• The principal difference between the BJT and HBT
  is the use of differing semiconductor materials
  for the emitter and base regions, creating a
  heterojunction
• The effect is to limit the injection of holes
  into the base region, since the potential barrier
  in the valence band is so large
• Unlike BJT technology, this allows high doping to
  be used in the base, creating higher electron
  mobility while maintaining gain

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HBT
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Heterojunctions
Al Ga As
Ga As
? E C 0.77 eV ? E V 0.48 eV
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Heterojunctions
In P
In Ga As
? E C 0.26 eV ? E V 0.34 eV
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Heterojunctions
Al Sb
In As
? E C 1.35 eV ? E V - 0.13 eV
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Heterojunctions
Ga Sb
In As
? E C 0.88 eV ? E V - 0.51 eV
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Modulation Doping
Put them in the AlGaAs, Let the electrons fall
into the GaAs (Dingle 78)
Instead of putting the dopants in the GaAs.
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