ECE 662Microwave Devices

Microwave Materials,

Diodes and TransistorsFebruary 3, 2005

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.

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 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.

Crossed Waveguide Frequency Multiplier Ref. Golio

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

p-n junction

Tunnel Diode- abrupt junctions of heavily doped p & n

semiconductor material p~n~1019

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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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

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.

Solid-State Device Power

Output vs Frequency

ref: Sze and

modifiedby Tian

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

pnp transistor with all leads grounded

ref. Sze

pnp transistor in the active mode of operation

ref. Sze

Various current components in a p-n-p transistor under the active mode of operation. ref. Sze

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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

fmax ~ 8 GHz ~ 3 GHz

Pfmax2 ~ 3x1020 WHz2 ~ 2x1020 WHz2

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)