master degree (lm) in electronic engineering microwaves

Microwaves, a.a. 2019/20 Prof. Luca Perregrini Active devices, pag. 1

MICROWAVES

Università di Pavia, Facoltà di [email protected]

http://microwave.unipv.it/perregrini/

Prof. Luca Perregrini

Master Degree (LM) in Electronic Engineering

ACTIVE DEVICES


SUMMARY

Chapter 11


SUMMARY

• Diodes and diode circuits: Schottky diodes and detectors, PIN diodes and control circuits, varactor diodes, other diodes

• Bipolar junction transistor (BJT) and heterojunction bipolar transistor (HBT)

• Field effect transistors: MeSFET, MOSFET, HEMT

• Integration technologies: hybrid microwave integrated circuits, monolithic microwave integrated circuits

• Microwave tubes


MOTIVATIONActive devices are used for signal detection, mixing, amplification, frequencymultiplication, and switching, and as sources of RF and microwave signals.Models of diodes and transistors will be given using equivalent circuits orscattering parameters, avoiding detailed discussion of the physics.These results will be used to study some basic diode detector and controlcircuits, and (later) for the design of amplifier, mixer, and oscillator circuitsusing diodes and transistors.An overview of technologies for microwave integrated circuits (MICs) will begiven.Microwave tubes will also be presented.


INTRODUCTIONOne of the first diode used in early radio work of the nineteenth century wasthe “cat-whisker” crystal detector


INTRODUCTIONThe advent of electron tubes (early 20th century) used as detectors andamplifiers later eliminated crystal diodes in most radio systems, but crystaldiodes still were used for high frequencies.


INTRODUCTIONThe invention of the transistor in 1947 (Nobel Prize 1956 to Shockley,Bardeen, and Brattain) rapidly led to integrated circuits in 1958 (Nobel Prize2000 to Kilby).

The first high-frequencytransistor was the surface-barrier germanium transistordeveloped by Philco in 1953,capable of operating up to60 MHz (!!).


SCHOTTKY DIODESThe classical 𝑝𝑝𝑝𝑝 junction diode commonly used at low frequencies has arelatively large junction capacitance that makes it unsuitable for highfrequency application.

The Schottky barrier diode,however, relies on asemiconductor–metal junctionthat results in a much lowerjunction capacitance, allowingoperation at higher frequencies.

Commercially available microwave Schottky diodes generally use 𝑝𝑝-typegallium arsenide (GaAs) material, while lower frequency versions may use 𝑝𝑝-type silicon.Schottky diodes are often biased with a small DC forward current, but can beused without bias.


SCHOTTKY DIODESPrimary applications of Schottky diodes are in frequency conversion:

mixing (frequency shifting)

detection (demodulation of an amplitude-modulated signal)

rectification (conversion to DC)


SCHOTTKY DIODES

A junction diode can be modeled as a nonlinear resistor, with a small-signalV–I relationship expressed as

where

𝐼𝐼𝑠𝑠 = saturation current (typicallybetween 10−6 and 10−15 A)

𝛼𝛼 = 𝑞𝑞/𝑝𝑝𝑛𝑛𝑛𝑛 (approximately 1/25 mV−1 for 𝑛𝑛 = 290 K)

𝑞𝑞 = charge of electron

𝑛𝑛 = Boltzmann’s constant

𝑛𝑛 = temperature

𝑝𝑝 = ideality factor (can vary from about 1.05 for Schottkybarrier diodes to about 2.0 for point-contact silicon diode)


SCHOTTKY DIODES

Small signal approximation: the voltage is expressed as a DC bias voltage𝑉𝑉0 is and a small AC signal 𝑣𝑣 (𝑣𝑣 ≪ 𝑉𝑉0):

Since v ≪ 𝑉𝑉0, by series expansion we have

where

Therefore the current can be expressed as

The three-term approximation is adequate for most of our purposes.

𝐺𝐺𝑑𝑑 = dynamic conductance𝑅𝑅𝑗𝑗 = junction resistance


SCHOTTKY DIODESThe equivalent AC circuit model for a Schottky diode involves not only anonlinear resistance, but also reactive effects due to the structure andpackaging of the diode.

The leads or contacts of the diodepackage are modeled as a series inductance 𝐿𝐿𝑠𝑠 and shunt capacitance 𝐶𝐶𝑝𝑝.The series resistor 𝑅𝑅𝑠𝑠 accounts for contact and current-spreading resistance.The junction capacitance 𝐶𝐶𝑗𝑗 and the junction resistance 𝑅𝑅𝑗𝑗 are biasdependent.

A typical equivalent circuit for anRF diode is shown in the figure.

Parameters for a few commercially available Schottky diodes


RECTIFIERS WITH SCHOTTKY DIODESIn a rectifier application, a diode is used to convert a fraction of an RF inputsignal to DC power.Rectification is a very common function and is used for power monitors,automatic gain control circuits, and signal strength indicators.The rise of the wireless power transfer makes this function more and morepopular nowadays.

From the small signal approximation the current is

Bias current DC rectified current

Undesired terms eliminated by a low-pass filter

The diode voltage consists of a DC bias voltage and a small-signal RFvoltage


RECTIFIERS WITH SCHOTTKY DIODESA current sensitivity 𝛽𝛽𝑖𝑖 can be defined as a measure of the change in the DCoutput current for a given RF input power.

An open-circuit voltage sensitivity 𝛽𝛽𝑣𝑣 can be defined in terms of the voltagedrop across the junction resistance when the diode is open circuited:

The mean RF input power is

and the DC current due to the RF is

𝑃𝑃in =𝑣𝑣02

2𝐺𝐺𝑑𝑑

ThereforeΔ𝐼𝐼dc =

𝑣𝑣02𝐺𝐺𝑑𝑑′

4𝐺𝐺𝑑𝑑

Typical values for 𝛽𝛽𝑣𝑣 of an RF diode range from 400 to 1500 mV/mW.


DETECTORS WITH SCHOTTKY DIODESThe diode nonlinearity is used to demodulate an amplitude modulated (AM)RF carrier. The diode voltage can be expressed as

where

• 𝜔𝜔𝑚𝑚 is the modulation frequency• 𝜔𝜔0 is the RF carrier frequency (𝜔𝜔0 ≫ 𝜔𝜔𝑚𝑚)• 𝑚𝑚 is the modulation index (0 ≤ 𝑚𝑚 ≤ 1)

The diode current is

𝑖𝑖 𝑡𝑡 =


DETECTORS WITH SCHOTTKY DIODESExpanding the products we have

Demodulated signal

The desired demodulated signal at 𝜔𝜔𝑚𝑚 is easily separated from theundesired frequency components with a filter.


DETECTORS WITH SCHOTTKY DIODESThe spectrum of the current is

with relative amplitudes


DETECTORS WITH SCHOTTKY DIODESObserve that the amplitude of the current component at 𝜔𝜔𝑚𝑚 is

which is proportional to the square of the input signal voltage, and hence tothe input signal power.

𝑖𝑖𝑚𝑚 =𝑚𝑚𝑣𝑣02𝐺𝐺𝑑𝑑′

2cos𝜔𝜔𝑚𝑚𝑡𝑡

Square-law behavior is theusual operating condition fordetector diodes, but in arestricted range of inputpower.Input power too large lead tosaturation, input signal verylow will be lost in the noisefloor.

Detectors may be DC biased to provide the best sensitivity.


PIN DIODESA PIN diode contains an intrinsic (lightly doped) layer between the 𝑝𝑝 and 𝑝𝑝semiconductor layers. When reverse biased, a small series junctioncapacitance leads to a relatively high diode impedance, while a forward biascurrent removes the junction capacitance and leaves the diode in a low-impedance state.

These characteristics make the PIN diode a useful RF switching element.

p n


PIN DIODESThe equivalent circuit depends on the bias state:

PIN forward-biased

The parasitic inductance 𝐿𝐿𝑖𝑖 is typically less than 1 nH. The reverse resistance𝑅𝑅𝑟𝑟 is usually small relative to the series reactance due to the junctioncapacitance and is often ignored.

PIN reverse-biased

Typical bias:forward bias current 10– 30 mA,reverse bias voltage 10– 60 V.

Parameters for some commercial PIN diodes


SINGLE-POLE PIN DIODE SWITCHESA PIN diode can be used in either a series or a shunt configuration to form asingle-pole, single-throw RF switch:

In both cases, input power is reflected when the switch is in the OFF state.High-impedance 𝜆𝜆/4 lines are frequently used for RF blocking.An ideal switch would have zero insertion loss in the ON state, and infiniteattenuation in the OFF state.

DC blocking capacitors (short at RF) RF choke (open at RF)


SINGLE-POLE PIN DIODE SWITCHESRealistic switching elements, of course, result in some insertion loss for theON state and finite attenuation for the OFF state.With reference to the simplified switch circuits

the insertion loss can be calculated as

Series PIN switch Shunt PIN switch

=

where


SINGLE-POLE PIN DIODE SWITCHESExample: single-pole switch operating at 1.8 GHz using a Microsemi UM9605 PIN diode with 𝐶𝐶𝑗𝑗 = 0.5 pF and 𝑅𝑅𝑓𝑓 = 1.5 Ω. Assume that 𝐿𝐿𝑖𝑖 = 0.5 nH,𝑅𝑅𝑟𝑟 = 2 Ω, and 𝑍𝑍0 = 50 Ω.

Seriesconfiguration

Shuntconfiguration

The shunt configuration has the greatest difference in attenuation betweenthe ON and OFF states and has the lowest ON insertion loss.


MULTIPLE-POLE/-THROW PIN DIODE SWITCHESSeveral PINs can be combined for multiple-pole/multiple-throw. For example:

single-poledouble-throwseries

single-poledouble-throwshunt

SP3Toperating from6 to 27 GHz


PIN DIODE PHASE SHIFTERSCompared with ferrite phase shifters, diode phase shifters have theadvantages of small size, integrability with planar circuitry, and high speed.The power requirements for diode phase shifters, however, are generallygreater than those for a latching ferrite phase shifter because diodes requirecontinuous bias current, while a latching ferrite device requires only a pulsedcurrent to change its magnetic state.There are basically three types of PIN diode phase shifters: switched line,loaded line, and reflection.


SWITCHED-LINE PIN DIODE PHASE SHIFTERThe switched-line phase shifter is the most straightforward type, using twosingle-pole, double-throw switches to route the signal flow between one oftwo transmission lines of different length.

For TEM (or quasi-TEM, like microstrip) TLs, the phase shift is a linearfunction of frequency (useful feature in wideband systems).It is reciprocal, so it can be used for both receive and transmit functions.The insertion loss of the switched line phase shifter is equal to the loss of theSPDT switches plus line losses.

The differential phaseshift between the twopaths is given by


SWITCHED-LINE PIN DIODE PHASE SHIFTERLike many other types of phase shifters, the switched-line phase shifter isusually designed for discrete binary phase shifts of 𝜑𝜑 = 180°, 90°, 45°, etc.

PINs

High-impedance bias lines

Lines with different electrical lengths


LOADED-LINE PIN DIODE PHASE SHIFTERUseful for small amounts of phase shift (generally 45°, or less).A transmission line is loaded with a shuntsusceptance 𝑗𝑗𝑗𝑗.

and the transmission phase shift is

A disadvantage is the insertion loss that is inherently present due to thereflection from the shunt load.In addition, increasing 𝑏𝑏 to obtain a larger 𝜑𝜑 entails a greater insertion loss.

(positive or negative, depending on the sign of 𝑏𝑏)

The reflection andtransmission coefficients can be written as

The susceptance 𝑗𝑗 can be implemented with a lumped inductor or capacitoror with a stub, and switched between two states with an SPST diode switch.


LOADED-LINE PIN DIODE PHASE SHIFTERTo reduce the reflection, a different configuration can be adopted:

equivalent to

The ABCD matrices of the two circuits are

Equating term by term, we obtain

𝑏𝑏, 𝜃𝜃𝑒𝑒 small

The component is practically matched, even when 𝑏𝑏 is switched (𝑏𝑏 is small).


REFLECTION PIN DIODE PHASE SHIFTERUses an SPST switch to control the path length of a reflected signal. Usuallya quadrature hybrid is used to provide a two-port circuit, although other typesof hybrids, or even a circulator, could be used for this purpose.An input signal divides equallybetween the two right-hand ports ofthe hybrid. The diodes are bothbiased in the same state (forwardor reverse), so the waves reflectedfrom the two terminations will add inphase at the indicated output port.Turning the diodes ON or OFF changes the total path length for both reflectedwaves by Δ𝜑𝜑, producing a phase shift of Δ𝜑𝜑 at the output.A good input match for the reflection-type phase shifter requires that thediodes be well matched. The insertion loss is limited by the loss of the hybrid,as well as by the forward and reverse resistances of the diodes. Impedancetransformation sections can be used to improve performance in this regard.


VARACTOR DIODEPIN diode has a junction capacitance that can be switched ON or OFF withbias voltage. In the varactor this effect is enhanced by tailoring the size anddoping profile of the intrinsic layer of the diode to provide a desired junctioncapacitance versus junction voltage 𝐶𝐶(𝑉𝑉) behavior when reverse biased.

𝐶𝐶 varies smoothly with𝑉𝑉 , thus providing anelectrically adjustablereactive circuit element.

Main application is electronic frequency tuning of the local oscillator in amultichannel receiver (e.g., in cellular phones, wireless local area networkradios, TV receivers), by controlling the DC reverse bias applied to the diode.The nonlinearity of varactor diodes il also exploited in frequency multipliers.Varactor diodes are generally made from silicon for RF applications, andgallium arsenide for microwave applications.


VARACTOR DIODEIn the simplified equivalent circuit for a reverse-biased varactor diode thejunction capacitance 𝐶𝐶 is dependent on the (negative) junction bias voltage 𝑉𝑉according to

𝐶𝐶0 = junction capacitance with no bias(0.5– 2.0 pF for a typical GaAs varactor diode)

𝑉𝑉0 = 0.5 V for silicon diodes, and 1.3 V for GaAs diodes𝛾𝛾 = coefficient depending on the doping profile of the intrinsic layer of the

diode (many practical diodes have an exponent of about 𝛾𝛾 = 0.47)𝑅𝑅𝑠𝑠 = series junction and contact resistance (typically on the order of few Ω)

where

For a GaAs varactor diode the junction capacitance that varies from about0.1 to 2.0 pF as the bias voltage ranges from −20 to 0 V.Parasitic reactances due to the diode package should be included in arealistic design.


GUNN DIODEBased on the transferred electron effect discovered in1963 by J.B.Gunn (also known as the Gunn effect).

Oscillators using Gunn diodes require ahigh-Q resonant circuit or cavity, which isoften tuned mechanically. Electronic biasadjustment is limited to 1% or less,sometimes improved using varactor diodes.Extensively used in low-cost applications(e.g., traffic radars, motion detectors for dooropeners and security alarms, and test andmeasurement systems.

Use specially doped bulk GaAs or InP materials. The 𝐼𝐼–𝑉𝑉characteristic exhibits a negative differential resistance(negative slope), able to generate RF power directly froma DC source when properly biased (100 mW from 1 to200 GHz, efficiency 5 − 15%).


OTHER DIODESImpact avalanche and transit time (IMPATT) diode: structure similar to PIN,but relatively high reverse-bias voltage (70–100 V) for avalanche breakdowncurrent. It exhibits a negative resistance over a broad frequency band (10-300 GHz).Generally noisier than Gunn diodes but capable of higher powers (from 10 Wat 10 GHz to 1 W at 94 GHz), higher DC-to-RF conversion efficiencies (up to15%), and better temperature stability than Gunn diodes.Used for frequency multiplication and amplification. Thermal considerationsare a limiting factor for both CW and pulsed operation.Oscillators based on IMPATT can be mechanically or electrically tuned, buttheir AM noise level is generally higher than that of other sources.


OTHER DIODESTunnel diodes: (invented by L. Esaki in 1957) is a pn junction diode with adoping profile that allows electron tunneling through a narrow energy bandgap, leading to negative resistance at high frequencies.Tunnel diodes can be used for oscillators as well as amplifiers. The diode ismounted in a one-port reflection circuit, where the negative RF resistance ofthe device produces a reflection coefficient with a magnitude greater thanunity (amplification). Such amplifiers have been made obsolete by modernRF and microwave transistors, but tunnel diodes are still used in someapplications today.

Barrier injection transit time (BARITT) diode has a structure similar to ajunction transistor without a base contact. Like the IMPATT diode, it is atransit time device. It generally has a lower power capability than IMPATTdiode, but the advantage of lower AM noise. This makes it useful for localoscillator applications at frequencies up to 94 GHz. BARITT diodes are alsouseful for detector and mixer applications.


BIPOLAR JUNCTION TRANSISTOR (BJT)Usually made using silicon (Si), is one of the oldest and most popular activeRF devices in use today because of its low cost and good operatingperformance in terms of frequency range, power capacity, and noisecharacteristics (but not as good as that of FETs).Silicon junction transistors are useful for amplifiers up to the range of 2–10GHz, and in oscillators up to about 20 GHz.Bipolar transistors typically have very low1/𝑓𝑓-noise characteristics, making them wellsuited for oscillators with low-phase noise.Typical silicon BJT has multiple fingers forthe base and emitter electrodes. The BJT iscurrent driven, with the base currentmodulating the collector current. The upperfrequency limit of the BJT is controlledprimarily by the base length, which istypically on the order of 0.1 μm.


BIPOLAR JUNCTION TRANSISTOR (BJT)Small-signal equivalent circuit modelThe capacitor 𝐶𝐶𝑐𝑐 between the baseand collector has a relatively smallvalue and may be ignored, thusmaking 𝑆𝑆12 = 0 (unilateral device).In many cases, it is simpler to treat the transistor as a two-port network,characterized by scattering parameters.S-parameters for an NPN Silicon BJT (NEC NE 58219, 𝑉𝑉𝑐𝑐𝑒𝑒 = 5.0 V, 𝐼𝐼𝑐𝑐 = 5.0 mA, common emitter)

Relatively large mismatches (|𝑆𝑆11| & |𝑆𝑆22|)

|𝑆𝑆12| relatively small (unilateral device)

gain (|𝑆𝑆21|) drops when frequency increases


BIPOLAR JUNCTION TRANSISTOR (BJT)The equivalent circuit can be used to estimate the upper frequency limit,defined as the threshold frequency 𝑓𝑓𝑇𝑇 where the short-circuit current gain ofthe transistor is unity.If we assume an input current 𝐼𝐼𝑖𝑖𝑖𝑖at the base, and ignore the seriesbase resistance 𝑅𝑅𝑏𝑏 (typicallysmall) and the shunt resistance,𝑅𝑅𝜋𝜋 (typically large), then the voltage across the capacitor 𝐶𝐶𝜋𝜋 is 𝑉𝑉𝜋𝜋 = 𝐼𝐼𝑖𝑖𝑖𝑖/𝑗𝑗𝜔𝜔𝐶𝐶𝜋𝜋.The output short-circuit current at the collector is 𝐼𝐼𝑜𝑜𝑜𝑜𝑜𝑜 = 𝑔𝑔𝑚𝑚𝑉𝑉𝜋𝜋, so the short-circuit current gain is

The current gain decreases with frequency, and is unity at the thresholdfrequency


BIPOLAR JUNCTION TRANSISTOR (BJT)

Typical DC operating characteristicsfor a BJT. The biasing point for thetransistor depends on theapplication and type of device, withlow collector currents generallygiving the best noise figure, andhigher collector currents giving thebest power gain.

Typical bias and decoupling circuitfor a BJT in a common emitterconfiguration.


HETEROJUNCTION BIPOLAR TRANSISTOR (HBT)The operation of a heterojunction bipolar transistor (HBT) is essentially thesame as that of a BJT, but an HBT has a base-emitter junction made from acompound semiconductor material such as GaAs, indium phosphide (InP), orsilicon germanium (SiGe), often in conjunction with thin layers of othermaterials (e.g., aluminum).This structure offers much improved performance at high frequencies. SomeHBTs can operate at frequencies exceeding 100 GHz, and recentdevelopments with HBTs using SiGe have demonstrated that these devicesare useful in low-cost circuits operating at frequencies of 60 GHz or higher.

Since the HBT is similar in structureand operation to the BJT, theequivalent circuit model can beused.


HETEROJUNCTION BIPOLAR TRANSISTOR (HBT)Equivalent circuit models may have limited applicability when attempting tomodel HBTs over a range of operating conditions, so scattering parameterdata, measured for a particular bias point, may be more useful.

S-parameters for a SiGe HBT (Infineon BFP640F, 𝑉𝑉𝑐𝑐𝑒𝑒 = 2.0 V, 𝐼𝐼𝑐𝑐 = 1.2 mA, common emitter)

High levels of monolithic integration are easy and inexpensive with SiGeHBTs, so this technology is proving to be very useful for low-cost millimeterwave circuits for both defense and commercial applications.

Observe that |𝑆𝑆21| decreases much less rapidly with frequency whencompared with the BJT. The device also is seen to be approximatelyunilateral, as |𝑆𝑆12| is relatively small.


FIELD EFFECT TRANSISTORS (FET)In contrast to BJTs, FETs are monopolar, with only one carrier type (electronsfor n-channel FET, holes for p-channel FET) providing current flow throughthe device. FET has a source-to-drain characteristic similar to that of avoltage-dependent variable resistor (voltage-controlled device).FET can take many forms, including the MESFET (metal semiconductorFET), the MOSFET (metal oxide semiconductor FET), the HEMT (highelectron mobility transistor), and the PHEMT (pseudomorphic HEMT).GaAs MESFETs are among the most commonly used transistors formicrowave and millimeter wave applications, being usable at frequencies upto 60 GHz or more especially for low-noise amplifiers. Even higher operatingfrequencies can be obtained with GaAs HEMTs.Recently developed gallium nitride (GaN) HEMTs are very useful for highpower RF and microwave amplifiers.CMOS FETs are increasingly being used for RF integrated circuits, offeringhigh levels of integration at low cost and low power requirements, forcommercial wireless applications.


FIELD EFFECT TRANSISTORS (FET)The performance characteristics of some of the most popular microwavetransistors are summarized in this table:


METAL SEMICONDUCTOR FET (MESFET)GaAs MESFETs can be used at frequencies well into the millimeter waverange, with high gain and low noise figure. Often chosen for hybrid andmonolithic integrated circuits at frequencies above 10 GHz.

The maximum frequency of operation is limited by the gate length; presentFETs have gate lengths on the order of 0.2– 0.6 μm, with correspondingupper frequency limits of 100 to 50 GHz.

In operation, electrons are drawn from the source to the drain by the positive𝑉𝑉𝑑𝑑𝑠𝑠 supply voltage. An applied signal voltage on the gate then modulatesthese majority electron carriers, producing voltage amplification.

The desirable gain and noise featuresare due to the higher electron mobilityof GaAs compared to silicon, and theabsence of shot noise. The device isbiased with a drain-to-source voltage𝑉𝑉𝑑𝑑𝑠𝑠 and a gate-to-source voltage 𝑉𝑉𝑔𝑔𝑠𝑠.


METAL SEMICONDUCTOR FET (MESFET)Small-signal equivalent circuit for a microwave MESFET for a common-source configuration. Typical values:

However, scattering parameters are frequently adopted to represent thedevice, especially for high frequency modeling.

S-parameters 𝑝𝑝-channel GaAs MESFET (NEC NE76184A, 𝑉𝑉𝑑𝑑𝑠𝑠 = 3.0 V, 𝐼𝐼𝐷𝐷 = 10 mA, common source)


METAL SEMICONDUCTOR FET (MESFET)The biasing depends on the application (low noise, high gain, high power),the class of the amplifier (class A, class AB, class B), and the transistor.

DC bias voltage must be appliedto both gate and drain, withoutdisturbing the RF signal paths.

typical DC 𝐼𝐼𝑑𝑑𝑠𝑠 vs 𝑉𝑉𝑑𝑑𝑠𝑠 curves

biasing and decou-pling circuitry for a dual-polarity supply

For low-noise design, the draincurrent is generally chosen to beabout 15% of 𝐼𝐼𝑑𝑑𝑠𝑠𝑠𝑠 (the saturateddrain-to-source current). High-power circuits generally use highervalues of drain current.


METAL OXIDE SEMICONDUCTOR FET (MOSFET)Based on silicon, the MOSFET is the most common type of FET, being usedextensively in analog and digital integrated circuits.It consists of a lightly doped 𝑝𝑝 substrate,and differs from a MESFET by having athin insulating layer (SiO2) between thegate contact and the channel region.

Can be used at frequencies into the UHF range, and can provide powers ofseveral hundred watts (devices packaged in parallel).Laterally diffused MOSFETs (LDMOS) operate at low microwave frequencieswith high powers (used for high-power TX base stations at 900-1900 MHz).High-density integrated circuits typically use complementary MOS (CMOS).This technology is very mature, and has the advantages of low powerrequirements and low unit cost.The small-signal equivalent circuit for a MOSFET is the same as that of theMESFET. Scattering parameters are used for high-frequency applications.


HIGH ELECTRON MOBILITY TRANSISTORS (HEMT)An heterojunction FET made with several layers of compound semiconductormaterials (GaAlAs, GaAs, GaInAs, and similar compounds).High carrier mobility (~ twice aMESFET), and operation frequenciesabove 100 GHz.Relatively high cost due to thesophisticated fabrication techniques.

The small-signal equivalent circuit and the DC bias characteristics are thesame as that of the MESFET.

S-parameters for GaN HEMT (Cree CGH21120, 𝑉𝑉𝑑𝑑𝑑𝑑 = 328 V, 𝐼𝐼𝐷𝐷 = 500 mA, common source)


TECHNOLOGY OVERVIEW


DIAMOND TRANSISTORS (FINFET)The diamond FinFET is a breakthrough in semiconductor materials anddesign because it has much greater power performance than standardsilicon carbide or gallium nitride semiconductors, and the diamondtransistors are extremely tolerant to high power usage, extreme heat, andeven radiation.This technology is expected to have a great impact in electronics and highfrequency devices.


TECHNOLOGY OVERVIEW


THICK FILM TECHNOLOGYUsed for relatively low frequency microwave circuits (viable up to ~20 GHz).

Typically based on low cost plastic substrates.Circuits are realized by chemical attack or onmilling machining.

Lumped components (e.g., SMD) are solderedin a second step.Metallized vias can be realized.


THIN FILM TECHNOLOGYMore precise than thick film,is based on chemicaldeposition or physicaldeposition (e.g., sputtering).Typically based on ceramicsubstrates (e.g., alumina).

Allows for higher operatingfrequency circuit (up to mm andsub-mm range).Resistive layers can beembedded in the circuit with thistechnology. Other componentsand chips are soldered in asubsequent phase, leading toan hybrid microwave circuit.


MONOLITHIC MICROWAVE INTEGRATED CIRCUITSImplemented using the well-assessed and very accurateintegrated circuit technology.Can integrate both passive andactive microwave devices.Most microwave componentsrequires lengths proportional to𝜆𝜆 (e.g., matching adapters,couplers, dividers, filters, …),thus requiring larger real estatethan low frequency circuits.


RF MEMSMicroelectromechanical Systems (MEMS) usesintegrated circuit (IC) manufacturing technology.RF MEMS offer potentially various advantagesover conventional microwave devices includingimproved isolation, lower power dissipation, andreduced cost, size, and weight. To date, the RFMEMS device which has found some commercialsuccess is switches.


MICROWAVE TUBESMany types of electron tubes serve as circuit elements, functioning asrectifiers, microwave RF sources, and amplifiers in a wide range of high-power microwave and millimetre-wave applicationSpecial applications have given impetus to the development of microwavepower sources capable of generating tremendous amounts of power (up tobillions of watts).Within this category the main varieties are klystrons, magnetrons, traveling-wave tubes, gyrotrons, and free-electron lasers.


MICROWAVE TUBES: KLYSTRONThe klystron is a linear beam device. The principle of operation can beexplained in terms of a two-cavity klystron amplifier. The beam is modulatedby the buncher cavity (RF input), accelerated by the DC energy, and excitethe resonance mode of the catcher cavity (RF output). In this way, the DCaccelerating power is converted to kinetic energy and then into RF power(equal to the difference in the kinetic energy of the electrons before and afterpassing the interaction gap).The efficiency can be ashigh as 70%.Klystrons are used in TV(up to 50 kW), ground-based communications (1-20 kW). Pulsed klystronsare used in radar and inscientific and medicallinear accelerators.


MICROWAVE TUBES: MAGNETRONMagnetrons generate power at microwave frequencies for radar systems,microwave ovens, plasma screens, linear accelerators, and the creation ofplasmas used for such applications as thin-film deposition and ionic etching.Electrons are constrained by the combined effect of a radial electrostatic fieldand an axial magnetic field.

Magnetrons have awide range of outputpowers (from hundredsof W to MW).The DC-to-RF power-conversion efficiencytypically ranges from50% to 85%.


MICROWAVE TUBES: TWTUsed to amplify signals over broad bandwidths (exceeding 100%).Two main types of TWTs: a slow-wave circuit called a helix for propagatingthe RF wave for electron-RF field interaction; a series of staggered cavitiescoupled to each other for wave propagation.The electrons emitted by the catode are injected into the opening of thehelix, and transfer

Typical gains are on the order of 4 dB/cm (40 to 60 dB for helix tubes ofpractical sizes). The DC-to-RF conversion efficiency range of 50% to 75%.The helix is ideal for satellite application because of its small size andweight, high efficiency, and low RF-distortion characteristics.

kinetic energy tothe RF signal.Up to 10 kV DCvoltage may berequired.


The trend toward high power (>1 MW @ 60 GHz and 100 KW @ 200 GHz)requires vacuum electronic devices. In conventional slow-wave tubes thesize of the RF elements must be in the order of 𝜆𝜆, leading to extremely smalldimensions above 60 GHz. A different way is to allow the RF wave topropagate at essentially the speed of light, leading to dimensions > 𝜆𝜆.These tubes provides RF power up to MW at more than 100 GHz.The picture shows one major type of fast-wave electron tube: the gyrotron(also called cyclotron resonance maser).

MICROWAVE TUBES: GYROTRON

Gyrotrons and other fast-wave tubes are used inhigh-frequency (35 to 94GHz) radar applications, incommunications systems,for plasma heating in someexperimental thermonuclearfusion reactors, and in industrial materials processing.


MICROWAVE TUBES


MICROWAVE TUBES

Microwave oscillator tubes Microwave amplifier tubes

master degree (lm) in electronic engineering microwaves

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