Two-Terminal Negative Resistance Devices
Varactor – small pn diodes that are operated as nonlinear capacitorsIn the reverse bias region
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 >> Vb, where n = ⅓ 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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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 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.
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
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
Tunnel Diode• In classical case, particle is reflected if E<
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 dopeddepletion regions very narrow and tunneling distance is small ~ 50 to 100 Å– (1 Å =10-8 cm=10-4 m)– High dopings cause Fermi levels within allowable
bands
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.
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.
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.
Application of Negative Resistance Devices
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Negative Resistance Devices I & V, 180 out of phase I2R power absorbed, but if R –R then power generated
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.
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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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
Field-Effect TransistorsV is changed by Vgs – to change channel size {reverse bias between Source and gate to adjust channel forward bias between source andDrain for current flow (majority carrier)}
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Gain 20 – 40 dB ~10 dB
BW Several GHz Several 1/10 GHz
Power Out 0.5 to 5 W 20 W
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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
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, MESFET’s, pHEMT’s (better linearity
and efficiency)
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,
MESFET’s, GaN, HEMT’s (higher power, higher voltage and promising linearity performance)
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 MMIC’s.
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)