ECE 662 Microwave Devices

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ECE 662Microwave Devices

• Microwave Materials,
• Diodes and Transistors
• February 3, 2005

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Two-Terminal Negative Resistance Devices
Varactor small pn diodes that are operated as
nonlinear capacitors In the reverse bias region
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Application of Negative Resistance Devices
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Varactor

• Varactor Variable reactor
• Use of voltage-variable properties (such as
  capacitance) of reversed-biased p-n junctions
• Reverse biased depletion capacitance is given by
  Cj (Vb VR)-n or Cj (VR)-n for VR gtgt Vb,
  where n ? for a linearly graded junction and n
  ½ for an abrupt junction.
• Can further increase the voltage sensitivity by
  using a hyperabrupt junction having an exponent n
  greater than ½.

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Charge Depletion Regions
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Charge Depletion Regions
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Varactor

• Present applications mostly for harmonic
  generation at millimeter and sub millimeter wave
  frequencies and tuning elements in various
  microwave applications.
• A common varactor is the reversed biased Schottky
  diode.
• Advantages low loss and low noise.
• Produces only odd harmonics when a sinusoidal
  signal is applied, so a frequency tripler can be
  realized without any second harmonic.

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Varactor
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Varactor Frequency Multipliers

• Provide LO power to sensitive millimeter and
  sub-millimeter wavelengths receivers.
• Schottky doublers can deliver 55 mW at 174 GHz
• Heterostructure Barrier Varactor Diodes acting as
  triplers deliver about 9mW at 248 GHz.

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Crossed Waveguide Frequency Multiplier Ref. Golio
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Varactor Devices

• Lower frequencies reversed biased semiconductor
  abrupt p-n juction diodes made from GaAs or Si.
• Higher frequencies Schottky diodes
  (metal-semiconductor junction diodes
• High frequencies and power handling
  heterostructure barrier varactor several
  barriers stacked epitaxially

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Tunnel Diode

• To achieve microwave capability
• Device dimensions must be reduced
• Parasitic capacitance and resistance must be
  minimized.
• Tunnel diode
• Associated with a quantum tunneling phenomenon
• Tunneling time is very short permitting its use
  well into the millimeter region
• Used for low power microwave application
• Local oscillator, detectors, mixers, frequency
  locking circuit
• Low cost, light weight, high speed, low-power
  operation, low noise

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Tunnel Diode

• In classical case, particle is reflected if Elt
  potential barrier height of V0
• In quantum case particle has a finite probability
  to transmit or tunnel the potential barrier
• Single p-n junction which has both p n sides
  heavily doped?depletion regions very narrow and
  tunneling distance is small 50 to 100 Å
• (1 Å 10-8 cm10-4 ?m)
• High dopings cause Fermi levels within allowable
  bands

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p-n junction
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Tunnel Diode- abrupt junctions of heavily doped
p n semiconductor material pn1019
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Tunnel Diode

• 1) For zero bias - electrons tunneled through
  narrow barrier at equal rates in each direction.
  Net current zero.
• 2) Small forward bias - electrons at bottom of
  conductor band on n side are are raised to energy
  levels corresponding to unoccupied energy levels
  on the p side. Therefore, tunneling current in
  forward direction with increases with bias.

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Tunnel Diode

• 3) For still larger bias, more and more electrons
  are raised to levels lying opposite the forbidden
  band on p side to which to which no tunneling is
  possible therefore the current reduces with
  increasing bias.
• 4) As bias increases further, the current remains
  small until minority carrier injection similar to
  conventional diodes predominates.

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Tunnel Diode

• 5) with reverse as an increasing number of
  electrons on the p side find themselves opposited
  allowed and empty levels in the conduction band
  on the n side therefore tunneling increases
  rapidly with increasing bias.

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Application of Negative Resistance Devices
Note negative resistance
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Tunnel Diode
Note that small changes in ?VB result in large
changes in i hence VRL
Negative Resistance Devices I V, 180? out
of phase I2R ?power absorbed, but if R ? R
then power generated
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Summary of Tunnel Diode

• Quantum Tunneling Phenomena
• Tunneling time short - mm waves
• Low-power applications
• n-p sides so heavily doped that the fermi levels
  lie within the conduction and valence bands
• Good for extreme speed
• Rate of tunneling can change as fast as energy
  levels can be shifted
• Devices such as transistors give more power, but
  traditionally have suffered in speed due to rate
  of diffusion of charge changing.

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Solid-State Device Power Output vs Frequencyref
Sze and modifiedby Tian
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Transistors

• Bipolar (Homojunction)
• Inexpensive, durable, integrative, relatively
  high gain
• Bipolar (Heterojunction)
• High speed switching
• Field Effect Transistors
• Junction
• MESFET, MOSFET, High Electron Mobility (HEMT)
• Av as well as Qc, better efficiency, lower noise
  figure, higher speed, high input impedance

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pnp transistor with all leads grounded ref.
Sze
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pnp transistor in the active mode of
operation ref. Sze
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Various current components in a p-n-p transistor
under the active mode of operation. ref. Sze
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Bipolar Transistor Gain (f)
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Bipolar Transistor Gain (f) cont.
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Bipolar Transistor High Freq.
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Bipolar Transistor Gain (f)
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Field-Effect Transistors

• Advantages
• 1) Voltage gain and current gain (simultaneously)
• 2) Higher efficiency compared to bipolar
• 3) Lower Noise Figure
• 4) Higher fmax and consequently higher operating
  frequency
• 5) High input resistance, up to several Meg

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Field-Effect Transistors
V is changed by Vgs to change channel size
reverse bias between Source and gate to adjust
channel forward bias between source and Drain for
current flow (majority carrier)
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Field-Effect Transistors
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Field-Effect Transistors

• To get larger output powers use larger gate
  widths
• 1W / 1 mm gate width
• Single gate width 250 to 500 ?m
• Use multiple gates (12) to increase power

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Technology Alternatives - 1 Ref MPD, Nov 2002,
Amcom Communications

• Material technologies (GaAs, Si, SiGe)
• Process technologies (Epitaxy, Implant)
• Device technologies (BJT, HBT, MESFET, HEMT)
• Power levels less than 1 W
• BJT, HBT (use single polarity supply and offer
  cost advantages at these power levels)
• GaAs, MESFETs, pHEMTs (better linearity and
  efficiency)

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Technology Alternatives - 2 Ref MPD, Nov 2002,
Amcom Communications

• High power levels above 10 W
• Si LDMOS (attractive at frequencies below 2 GHz)
• Wide band gap devices such as SiC, MESFETs, GaN,
  HEMTs (higher power, higher voltage and
  promising linearity performance)

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Terrestrial wireless systems Ref MPD, Nov 2002,
Amcom Communications

• Broadband internet access operate in the
  frequency range of 1 6 GHz.
• Low cost subscriber units less than 1 W transmit
  power SiGe, GaAs HBT, MESFET and pHEMT MMICs.
• Higher power (2-10 W) GaAs FTE, pHEMT (optimize
  RF power output and best linearity performance
  over the specific band of interest while keeping
  the cost low)

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