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ECE 662 Microwave Devices

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Tunneling time is very short permitting its use well into the millimeter region ... Tunneling time short - mm waves. Low-power applications ... – PowerPoint PPT presentation

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Title: ECE 662 Microwave Devices


1
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

14
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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