diodogunn

CONTENTS

1 DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1 Package Styles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Gunn Oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 PRINCIPLE OF OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1 Gunn Diode Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Modes of Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Conventional Gunn Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.4 Graded-gap Gunn Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1 Oscillator Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2 Performance Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.3 Mounting and Heat Sink Considerations . . . . . . . . . . . . . . . . . . . . . 15

4 PERFORMANCE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . 154.1 Limiting Conditions of Use . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.2 Typical Performance Curves . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

# 2000 Marconi Applied Technologies LimitedThis work must not be copied in whole or in part without the prior written permission of Marconi Applied Technologies Limited

Marconi Applied Technologies

Gunn Diodes Application Notes

#2000 Marconi Applied Technologies Limited A1A-Gunn Diodes AN1 Issue 2, June 2000

Marconi Applied Technologies Limited, Carholme Road, Lincoln LN1 1SF England Telephone: +44 (0)1522 526352 Facsimile: +44 (0)1522 545140

e-mail: [email protected] Internet: www.marconitech.com Holding Company: Marconi p.l.c.

Marconi Applied Technologies Inc. 4 Westchester Plaza, PO Box 1482, Elmsford, NY10523-1482 USA Telephone: (914) 592-6050 Facsimile: (914) 592-5148 e-mail: [email protected]

1 DESCRIPTIONThe Gunn diode is the best known and most readily availabledevice in the family of transferred electron devices (TED). Theyare employed as DC to microwave converters using thenegative resistance characteristics of bulk Gallium Arsenide(GaAs) and only require a standard, low impedance, constantvoltage power supply, thereby eliminating complex circuitry.

The DC1200 series of GaAs Gunn diodes is designed foroperation at fixed frequency (determined by the oscillatorcavity) within a specified band under CW or pulsed conditions.Both conventional and hot electron injected (graded-gap) Gunndiode designs are available (see Section 2). All devices featurelow FM and AM noise characteristics and their widebandnegative resistance enables them to be used in voltagecontrolled and mechanically tuned oscillators. The hot electroninjected Gunn diodes offer the additional advantages of:

* higher fundamental frequency operation,

* increased efficiency,

* improved temperature stability,

* improved turn-on characteristic and

* reduced FM sideband noise.

Devices are available in either polarity, but are generallysupplied as negative heat sink.

1.1 Package StylesSeveral standard package styles are offered covering thefrequency range 4 to 110 GHz and designed to suit differentapplications (see Fig. 1.1). Custom packaging requirements willalso be considered upon request.

The following package limitations need to be taken intoconsideration:

Thermal Resistance

The lowest thermal resistance is offered by the screw-basedpackages (outlines 40, 86 and 106). For high volume, lowerpower applications where oscillator assembly time is moreimportant, non-threaded packages such as outline 00 will bepreferred (refer to Mounting and Heat Sink Considerations -Section 3.3).

Frequency Range

The equivalent circuit diagram of a packaged Gunn diode isshown in Fig. 1.2. The magnitude of the parasitic impedancesattributed to the package element reduces with the packagesize (004864106). Consequently, the larger, more robustpackage styles are normally specified for operation at lowerfrequencies and the smaller, low parasitic impedance packagesare recommended for the higher frequency applications.

Magnetically Tuned Circuits

For applications involving magnetically (YIG) tuned oscillators,Gunn diodes can be supplied in Kovar-free packages.

5.3/5.9

1.52/1.63 1.70/1.85 1.52/1.63 13.00/3.15

11.52/1.63

11.98/2.08

"

"

"

"

"

"

"

"

"

"

"

"

"

"

00

0.22/0.28

0.75/0.85

0.53/0.63

12.45/2.555.28 MAX

3-48 UNC 2A THREAD

12.95/3.00

12.05/2.15

"

"

"

"

"

"

"

"

""

"

"

"

"

40

Fig. 1.1 Standard Gunn Diode Package Outlines (continued on page 3)

Gunn Diodes AN1, page 2 #2000 Marconi Applied Technologies

1.2 Gunn OscillatorsMarconi Applied Technologies designs and manufactures arange of oscillator products using Marconi Gunn diodes.Standard products include a range of fixed frequency andmechanically or electronically tuned oscillators. Enquiries arewelcomed for oscillator products that are not in the standardrange.

2 PRINCIPLE OF OPERATION

2.1 Gunn Diode TheoryIn order to understand the nature of the transferred electroneffect exhibited by Gunn diodes, it is necessary to consider theelectron drift velocity versus electric field (or current versusvoltage) relationship for GaAs (see Fig. 2.1).

Below the threshold field, Eth, of approximately 0.32 V/mm, thedevice acts as a passive resistance. However, above Eth theelectron velocity (current) decreases as the field (voltage)increases producing a region of negative differential mobility,NDM (resistance, NDR). This is the essential feature that leadsto current instabilities and Gunn oscillations in an active deviceand is due to the special conductance band structure of directband gap semiconductors such as Gallium Arsenide (see Fig.2.2).

86

"

"

"

"

"

"

"

"

"

4.0/4.5

0.63/0.78

0.46/0.56

3-48 UNC 2A THREAD

11.22/1.32

12.49/2.59

"

"

"

"

"

"

"

"

"

"

"

3.5/3.7

0.47/0.57

0.31/0.47

10.72/0.90

3-48 UNC 2A THREAD

12.89/3.00

"

"

106

7504

CP = PACKAGE CAPACITANCELP = BOND LEAD INDUCTANCERS = SERIES RESISTANCE7RG = GUNN DIODE RESISTANCECG = GUNN DIODE CAPACITANCE

PACKAGE ELEMENTDIODE ELEMENT

7RGCG

RS LP

CP

" "

Fig. 1.2 Simple Equivalent Circuit for a Packaged GunnDiode

"

"0.32V/mm

7486

Eth

EH

ELECTRIC FIELD E (VOLTAGE)

DRIFTVEL

OCTYv(CURREN

T)

Fig. 2.1 The Electric Field Dependent Average DriftVelocity for N-type GaAs

#2000 Marconi Applied Technologies Gunn Diodes AN1, page 3

The energy-momentum relationship contains two conductionband energy levels, G and L (also known as valleys) with thefollowing properties:

* In the lower G valley, electrons exhibit a small effectivemass and very high mobility, m1.

* In the satellite L valley, electrons exhibit a large effectivemass and very low mobility, m2.

* The two valleys are separated by a small energy gap, DE,of approximately 0.31 eV.

In equilibrium at room temperature most electrons reside nearthe bottom of the lower G valley. Because of their high mobility(* 8000 cm2V-1s-1), they can readily be accelerated in a strongelectric field to energies in the order of the G-L intervalley

separation, DE. Electrons are then able to scatter into thesatellite L valley, resulting in a decrease in the average electronmobility, m, as given below:

m = (n1.m1 + n2.m2) / (n1 + n2)

where n1 = electron density in G valleyn2 = electron density in L valley

Above the high field, EH, most electrons reside in the L valleyand the device behaves as a passive resistance (of greatermagnitude) once again.

In a practical Gunn diode, electrons are accelerated from thecathode by the prevailing electric field. When they haveacquired sufficient energy, they begin to scatter into the lowmobility satellite valley and slow down. This charge fluctuation

"

"

"

"

"

"

"

Eg = 1.42 eV

LOWER GVALLEY

SATELLITE LVALLEY

T = 300K

SATELLITE XVALLEY

DE = 0.31 eV

72

71

0

1

2

3

4

7487

ENERGY(eV)

CONDUCTION BAND

"

"G XL [111] [100]

VALENCE BAND

Fig. 2.2 The Band Structure of GaAs at 300 K


results in a localised increase in the electric field and acorresponding decrease (to below the threshold value)elsewhere within the sample. Electrons ahead of the region stillexhibit high mobility and move away from the chargefluctuation causing a depletion of carriers. Electrons behindthe charge fluctuation, are also moving faster and accumulatebehind the depleted region causing a dipole (or high field)domain (see Fig. 2.3), which grows in amplitude.

A fall in current is associated with the domain growth due to itsreduced mobility. The domain propagates through the sampleat a constant velocity (*1 x 107 cm.s71 for GaAs) until itreaches the anode where it collapses. As the domain collapses,the electric field outside the domain rises and the current in theexternal circuit increases until the threshold field is reachedagain and a new domain is formed.

For a given doping density there will be a minimum devicelength that will support a domain due to the finite time requiredfor domain growth. If this time is longer than that required forthe domain to traverse the sample, then domain formation willnot occur. Similarly, if the doping density is too high, then thecurrent (and hence the temperature) in the semiconductorbecomes too great and the life expectancy of the device isreduced. In practice, the product of doping density, n, anddevice length, l, is maintained between the following limits:

1 x 1012 cm72 4 n.l 4 2 x 1012 cm72

"

"

"

"

0CATHODE

0CATHODE

SPACECHARGE

ELECTRIC

FIELD

ANODE

Eth

EH

EL

ANODE

7488ACCUMULATION LAYER

DEPLETION LAYER

"

"

DISTANCE

DISTANCE

Fig. 2.3 The Band Structure of GaAs at 300 K


2.2 Modes of Oscillation2.2.1 Transit Time Mode

This is the basic diode oscillation mode and is independent ofthe external circuit. Current peaks are obtained when a domainis quenched at the anode, after which another is nucleated nearthe cathode. The frequency is determined by the domain transittime, Tt:

Tt = l/vD

where l = length of devicevD = domain velocity (* 1 x 107 cm.s-1)

so frequency, ft :

ft = 1/Tt = vD/l

This mode of operation was first reported by (and is namedafter) J. B. Gunn in 1963. Its main characteristics are (see Fig.2.4):

* The total electric field across the device at any time isabove the threshold value.

* The current waveform consists of narrow spikes, indicatinga high harmonic content and low efficiency at fundamentalfrequency.

* The RF field across the device is small, indicating lowimpedance.

* The transit time frequency is a strong function of operatingvoltage and temperature.

Therefore the transit time mode has poor stability andefficiency.

7489

"

"

"

Iv

Ip

E0

Tt

t

E

t

"

I

"

"

Eth

Fig. 2.4 Current and Field Waveforms for Transit Time Mode


2.2.2 Delayed Domain Mode

Fig. 2.5 shows the basic voltage and current waveforms for thismode of operation. Unlike the transit time mode, the totalelectric field across the device drops below the threshold value,Eth, during part of the RF cycle, such that nucleation of a newdomain is delayed.

As soon as the field rises above Eth, a domain nucleates at thecathode and travels across the device. As the field swingsbelow Eth, the domain arrives at the anode and decays itscharge. A new domain cannot form at the cathode until thefield rises above Eth again. This delay time between extinctionand creation of domains modifies the operating frequency, fd,to:

fd = 1/(Tt + Td)

where Tt = transit timeTd = delay time

The transit time is a fixed quantity for a given device, but thedelay time is a function of the RF voltage, which is determinedby an external circuit. It follows that the operating frequency isalways below the transit time frequency.

The important characteristics of this mode of operation are:

* The total electric field across the device is below thethreshold field, Eth, over a part of the RF cycle.

* The current waveform consists of broad spikes, indicatinga low harmonic content and higher efficiency atfundamental frequency.

* The RF field across the device is large, indicating highimpedance.

* The operating frequency is determined mainly by theresonant frequency of the external circuit and can be madevery stable. A device can also be used over a muchbroader bandwidth below the transit time frequency.

This mode of operation is therefore most commonly used in themajority of commercial applications.

7490

"

"

"

Iv

Ip

E0

t

E

"

I

TtTd"

"

"

t

Eth

Fig. 2.5 Current and Field Waveforms for Delayed Domain Mode


2.3 Conventional Gunn DiodeA conventional Gunn diode generally consists of three layers; arelatively low doped transit region sandwiched between twohighly doped contact regions, forming an n+- n -+ structure(see Fig. 2.6). The device is defined basically by fiveparameters:

n The doping concentration in the transit region of theGallium Arsenide.

l The thickness of the transit region.

R0 The low field resistance of the diode measured close tothe origin.

Ith Threshold current. This is the maximum current throughthe device and can exceed the operating current by asmuch as 50%.NB. The power supply must be capable of passingthrough this point.

Vth Threshold voltage. The voltage at the currentmaximum.

The ratio 7RG / R0 is an important circuit design parameter. Vth

and Ith are independent of the external circuit and may bemeasured with the diode in any good heat sink. Because Vth ismeasured on the flat part of the curve, the measuring techniquemust be carefully specified to avoid error.

The frequency of the Gunn diode is determined primarily by thetransit region length, l. However, a portion of this region is usedto accelerate the electrons from the cathode until they havesufficient energy to enter the low mobility state and does notsupport domain formation. This ‘dead zone’ may be as much as0.25 mm in a transit region length of 1.5 mm (for a millimetricdiode) and, because it acts as a parasitic resistance, results inreduced efficiency. For conventional Gunn diodes there is arapid fall-off in power at frequencies above 60 GHz, where theless efficient second harmonic component of the power has tobe utilised.

The starting voltage must be well below the required operatingvoltage (typically Vop = 3 x Vth), particularly when lowtemperature operation is required, as the starting voltage riseswith falling temperature. This can be controlled to some extentby the correct choice of n and l, but severely restricts the use ofconventional Gunn diodes at temperatures below about 725 8C.The turn-on voltage, Von (the voltage above threshold at whichcoherent RF power is obtained), increases to the point where itequals the peak power voltage Vpk (see Fig. 2.7a). This forcesthe diode to be operated at a higher voltage with correspondingloss in power, reduced efficiency, poor FM sideband noise andthe increased possibility of device failure.

2.4 Graded-gap Gunn DiodeThe limitations of the conventional Gunn diode (describedearlier) can be overcome by injecting high energy, ‘hotelectrons’, into the transit region. The concept is to introduceelectrons into a region so that the temperature, which describestheir energy distribution, is much greater than that of thesemiconductor lattice. In the graded-gap Gunn diode, electronsare injected into the transit region with an energy equal to thatof the G-L intervalley separation so that stable domains willform very near to the cathode and move across the transitregion as soon as the field is high enough to sustainaccumulation and propagation. The dead zone is effectivelyeliminated and the transit length is fixed and independent ofbias. Therefore, coherent power can be generated over a widerange of operating voltages (see Fig. 2.7b).

7491

"

Ith

Vth

R0

V

"

I

7492

OHMICCONTACT

TRANSIT REGION OHMICCONTACT

SUBSTRATE

n

n++ n++

l

"

DOPING

DENSITY

"

"

Fig. 2.6 Structural Schematic and Current-Voltage Relationship for a Conventional Gunn Diode


There are a variety of possible structures which can be used forhot electron injection. To produce microwave power with aslittle bias dependence as possible, the optimum injector shapehas a slowly increasing potential with an abrupt drop back tothe transit region value. This is best realised with a graded-gapinjector (see Fig. 2.8). Simulations have shown that a thin n+

layer between the injector and the transit region is critical forcontrolling the electric field, while retaining the hot electronproperties. Fig. 2.8b shows the n+ spike depleted giving aninjection energy of DE.

An additional benefit from using a graded-gap Gunn diode isthe much improved temperature stability. This is due to theelectron temperature being set by the injection energy, typicallyequivalent to 2000 K. Changes in the substrate temperature inthe 130 8C range usually required for military specifications, etc,are relatively small in comparison.

The structural schematic and current-voltage relationship for agraded-gap Gunn diode are shown in Fig. 2.9. Under forwardbias, the peak current is much less defined due to the action ofthe cathode injector (i.e. significant numbers of electronsalready reside in the upper valley when entering the transitregion) and the gradient of the curve beyond threshold isshallower.

7493

" "

Von Vpk

VOLTAGE (V)

POWER(m

W)

0

25

50

0

1 2 3 4 5 6

a)

"

Von

VOLTAGE (V)

POWER(m

W)

0

25

50

0

1 2 3 4 5 6

"

Vpk

b)

Fig. 2.7 Typical Turn-on Characteristic of: a) Conventional Gunn Diode and b) Graded-gap Gunn Diode


7494

"e7a)

b)

n+ SPIKEBIAS = 0

CONTACT INJECTOR TRANSIT REGION CONTACT

"

"

""

" "

"

n+ n+ n+n7

GRADEDGAP

BIAS = V

"

"

*DE

"

"

V

Fig. 2.8 A Schematic Diagram of a Hot Electron Injector Gunn Diode under a) Zero Bias, and b) Forward Bias V.

7495

1 2 3 40

0

1

BIAS (V)

FORWARD

REVERSE

CURRENT(A

)

7496

OHMICCONTACT

TRANSIT REGIONGRADEDAlGaAs

OHMICCONTACT

SUBSTRATE

n

n++

n+

n++

l

"

DOPING

DENSITY

"

"

Fig. 2.9 Structural Schematic and Current-Voltage Relationship for a Graded-gap Gunn Diode


Graded-gap Gunn diodes have demonstrated room temperatureperformance of up to 80 mW and 2.4% efficiency at 90 GHz;the best results achieved for mm-wave GaAs devices at thisfrequency. 50 - 60 mW at 94 GHz, with efficiencies of 1.6%, isreproducibly achieved (see Table 2.1). FM sideband noise isbetter than 780 dBc/Hz, 100 kHz from carrier, equal to thatobtained from the best conventional devices. Significantlythese devices exhibit a turn-on voltage very close to threshold(see Figure 2.7b), which allows coherent oscillations aroundpeak power over the full military specification temperaturerange. Further evidence of the improved temperature stabilitycan be seen from the power, frequency and peak powervariations across this temperature range, generally a factor oftwo or more below that exhibited by devices without hotelectron injection. This is an added bonus for VCO designerswho can then utilise a larger bandwidth, because there is nolonger need to compensate for frequency drift withtemperature.

Freq(GHz)

Temp(8C)

Von

(V)Vop

(V)Iop

(mA)Power(mW)

Eff(%)

Noise *(dBc/Hz)

90 740 3.9 4.9 680 58 1.75

90 +25 3.2 4.7 660 50 1.6 786

90 +80 3.1 4.8 640 42 1.4

94 +25 3.0 4.5 600 60 2.0 788

60 +25 3.0 4.5 600 120 5.0 790

35 +25 3.0 4.5 1300 350 5.5 795

* 100kHz offset frequency

Table 2.1 Typical Performance of Graded-gap GaAs Gunn

Diodes

3 APPLICATIONSGunn diodes are reliable, relatively easy to install and the loweroutput power levels fall well below the safety exposure limits.They are ideally suited for use in low noise sources such as localoscillators, locking oscillators, low and medium powertransmitter applications and motion detection systems. Higherpower varieties can be used in phase-locked oscillators or asreflection amplifiers in point-to-point communication links andtelemetry systems.

Microwave sources have the advantages over ultrasonicdetectors of size and beamwidth, and over optical systems ofworking in dusty and adverse environments. The low voltagerequirements of Gunn oscillators mean that battery or regulatedmains supplies may be used, (battery drain can be reduced byusing low current devices or by operation in a pulsed mode).However, microwaves are reflected from metal surfaces andpartially reflected from many others e.g. brick, Tarmac andconcrete, and they are attenuated by oxygen, water or watervapour. Figure 3.1 shows attenuation effects in the frequencyrange of interest.

7497

FREQUENCY (GHz)

WAVELENGTH (mm)

IEEE RADAR LETTERBANDS

ITU REGION 2 RADIOLOCATION BANDS

ATTENUATION

(dB/km)

10

10 3002201501009570603015

100

1.0

0.1

0.01

0.00135102030 2 1

"

"

"

"

"

"

"

22 GHz

SEA LEVEL

4 km

X Ku K Ka mm

H20

H2002

118 GHz

183 GHz60 GHz

"

02

Fig. 3.1 Atmospheric attenuation at millimetre wavelengths with IEEE radar letterbands and ITU radiolocation bands forregion 2 (after Altshuler et al)


The range of application of Gunn sensors for industrial andcommercial use is extensive and the following is only a brief list:

Collision avoidance radar

Vehicle ABS

Traffic analyser sensors

‘Blind spot’ car radar

Pedestrian safety systems

Elapsed distance meters

Automatic identification

Presence/absence indicators

Movement sensors

Distance measurements

Slow-speed sensors

Level sensors

Traffic signal actuators

Proximity movement detectors

Door opening sensors

Barrier operation

Process control devices (object counting)

Intruder/burglar alarms

Perimeter protection

Train derailment sensors

Contactless vibration transducers

Rotational speed tachometers

Linear distance indicators

Moisture content measurement

Table 3.1 indicates the most commonly used Gunn diode typesby application.

APPLICATION

Fixed Frequency CW Broadband CW Pulsed

Low Power High Power

Low Frequency High Frequency

Local Oscillators:RadarFast tunable ECMDiode noise measurement

*

*

*

Telecommunications:TransmittersLow noise oscillators

Point-to-point links

*

*

*

Control devices:Railway crossingsTraffic controlVehicle ABSDoor openers

*

*

*

* *

Motion detectors:Speed controlRadar detectorsIntruder alarmsShoreline navigation

*

*

*

*

*

Transmitters:Radar transpondersMissile beacons

*

*

Radio link exciters *

Injection locked amplifiers *

Paramp pump sources *

Instrumentation *

Table 3.1 Gunn Diode Selection Chart


3.1 Oscillator DesignThe natural frequency of oscillation of the Gunn diode can bealtered to some extent by the external circuit. The formation orpremature collapse of a domain within a cycle can becontrolled, and hence the power, frequency and efficiencyadjusted. Control of the frequency by external means ratherthan by device parameters only is essential for a stableoscillator. The external cavity used to resonate the devicenegative impedance can be coaxial, waveguide, microstrip, YIGcrystal, etc, depending on the required application.

Generally, coaxial and microstrip circuits offer low Q valueswith poor noise and stability performance, resulting in thefrequency of oscillation changing with load and environmentalvariations. YIG crystal tuned circuits are usually too expensivefor commercial and industrial applications and therefore themost common cavity is waveguide. Usually the Gunn diode ismounted on a post structure between the waveguide walls,either lg/2 from an iris or lg/2 from a short circuit (see Fig.3.2). Some alteration is necessary to set the exact frequency toallow for diode and package parasitics and manufacturingtolerances. Tuning screws of either metal or dielectric are usedto modify the cavity resonant frequency. Power outputvariations are achieved by adjusting the coupling betweendiode and load using variations in post size or tuning screws.A more detailed schematic of a packaged Gunn diode mountedin a radial disc, microwave cavity and its equivalent circuitdiagram are given in Figs. 3.3 and 3.4.

"

"

"

RF

*lg/2

"

RF IRISDIODE DIODE

"

"

*lg/2

7498

Fig. 3.2 Basic Microwave Cavity Design

"

"

"

"

"

v

BACKSHORT

7499

GUNN DIODE

BIAS CHOKEFILTER

WAVEGUIDE

RADIAL DISC

Fig. 3.3 The Packaged Gunn Diode Mounted in aMicrowave Cavity (approximately half scale).

7500

-RG

LB

CS

BACKSHORT

CP CD

DISC

LPLOAD

WAVEGUIDE

BIASCHOKEFILTER

Fig. 3.4 An Equivalent Circuit Diagram for the Gunn Diode, Package, Disc and Cavity


Most simple Gunn oscillators are designed empirically using afew ground rules and a lot of experience. The designs have totolerate batch to batch variations in diode parameters and havethe required stability. They must not present the diode with anRF impedance such that the device takes DC power butproduces no RF output, thus leading to a possible turn-on orswitch-on failure.

A recommended Gunn diode driver circuit is shown in Fig. 3.5

3.2 Performance Considerations3.2.1 Matching

The Gunn diode is a current generator and needs matching intothe circuit for optimum output power, noise, smooth tuningand power-temperature variation. Small differences may existeven between cavities which are nominally identical. Testing inthe customer’s own cavity design is always the preferredsolution.

The impedance matching problem becomes increasingly criticalas the cavity Q goes up. The impedance of the diode isdetermined to the first order by the thickness, doping andoperating voltage. However, second order trimming can beobtained by varying the voltage to alter the domain width, andtherefore the domain capacitance, by field control. The voltageat which maximum power is obtained is cavity dependent. Ifthe diode is not well matched to the cavity, there may be aconflict between the voltage for correct match and the voltageat which the diode would want to operate in a low Q cavity.Trimming the diode parameters, therefore, becomes morecritical as Q increases and the tolerances on devicespecifications become correspondingly tighter. It follows that itis more difficult to design a diode to work in a high Q cavitywithout having access to the cavity.

3.2.2 Starting Voltage

The conventional Gunn diode is a broadband negativeresistance device and random noise is required to start it.Starting becomes more difficult at low temperature and in highQ cavities. High Q operation also means higher voltageoperation for reliable starting. These limitations can beovercome (at higher frequencies) by the use of a graded-gap,current injected Gunn diode.

3.2.3 Operating Current

Increasing the operating current increases the output power.However, the maximum safe operating current for anencapsulated device is limited by the corresponding increase inthe transit region temperature (9250 8C). This, in turn, isdetermined by the thermal resistance of the diode itself and bythe effectiveness of the package heat sink (see Section 3.3).The lower limit of current is more difficult to define because theseries resistance of the ohmic contact increases non-linearly asthe diode area decreases.

3.2.4 Output Power

The output power is determined primarily by the diode area andthe doping level. The highest power/efficiency is achievedusing an integral heat sink (IHS) construction. This has theadvantage of a thick, gold plated heat sink formed directly ontothe epitaxial region and a minimal thickness of remainingsubstrate, thereby reducing the parasitic resistance. Furtherimprovement in output power can be obtained at higherfrequencies by the use of a graded-gap, current injected Gunndiode.

The upper limit on power handling is mainly determined by thedissipation of DC power through the package heat sink and thefact that the diode impedance becomes low and is difficult tomatch.

3.2.5 FM Noise and dF/dT

FM noise measurements are given as noise to carrier ratios (N/C) versus carrier offset modulation frequency, fm. Thefrequency deviation, Df, is calculated from:

N/C = 20.log (Df.H2 / fm)

where values of (N/C) are given in dBc/Hz.

In the graded-gap Gunn diode, the variation in position at whichdomain formation occurs is much less than in a conventionaldevice. Associated with this is a reduction in the variance of thefirst intervalley scattering effect for electrons entering thecathode region and hence a reduction in noise.

The factors controlling FM noise and thefrequency-temperature coefficient are complex and areresolved by work involving both the diode and oscillatordesign. The equivalent circuit of both the diode and the cavityform a total circuit concept and there is a trade-off between theparameters in each component. Oscillators and diodes need tobe designed together to achieve given second order aspects ofthe specification.

Second order effects such as noise, chirp, dF/dT, etc., bear nodirect relationship to the primary characteristics of frequencyand power. The secondary parameters can show wide variationbetween diodes, which in all other primary respects areidentical.

7501

R

Z1 Z2 C1

VS

IR

IB

IZ

VZ1 VZ2 VG0

G1

IR = IB + IZIB = Isat / hfeR = (VS 7 VZ1) / IRVZ1 = VG0 + 0.7 V

C1 = 0.1 mFG1 = Gunn diode

Z1, Z2 = Zener diodeVZ2 = VG max

"

"

"

"

Fig. 3.5 A Recommended Gunn Diode Drive Circuit


3.2.6 Pulse Diodes

These diodes are even more cavity dependent than CW diodes.

Whereas a conventional CW diode requires as low a seriesresistance as possible, this is not true in pulse diodes. Theconstant re-nucleation of domains at the start of each pulserequires closer control of the electric field at the cathode.Various methods of achieving this are available, includingadding series resistance with a disc or by partially alloying thecontacts to provide some non-ohmic behaviour. Alternatively, agraded-gap, current injected Gunn device may be used.

Frequency change during the pulse (chirp) is also affected bythese measures as is the dF/dT with which it correlates.

Pulse diode noise is normally defined as the degradation in theFourier sin(x)/x display of the RF power pulse from the idealrectangular pulse values. The factors that control pulse noiseand chirp are complex and the solution in any particular case isarrived at by modifications both to the diode processing andoscillator design.

3.2.7 Special Operating Conditions

In all cases where the optimum unique frequency and powerdesign parameters have to be modified to achieve some specialcondition of low or high temperature operation, restrictedvoltage or current consumption, or broadband tuning, thensome loss of optimum performance results. There is always atrade-off to be made to meet a special operating condition.

3.2.8 Low Harmonic Diodes

Diodes can be designed to meet low harmonic generation limits(if required by local regulations) by modifying the materialspecification. The extent to which the harmonics can bereduced, however, is limited by the cavity design and dependson the measurement technique.

3.2.9 Broadband Operation

Diodes can be specially designed by modifying the materialproperties and controlling the encapsulation parasitics to give abroadband tuning performance in a carefully defined cavity.This is achieved at the expense of output power and efficiency.

3.2.10 High Temperature Operation

Marconi Applied Technologies’ standard range of Gunn diodescan be used up to heat sink temperatures of 85 8C. At highertemperatures it is necessary to reduce the transit region dopinglevel in order to ensure long life. This means that for the samecurrent flowing through the device a larger die is used andgreater cooling is achieved by the increased area of contactwith the heat sink and the larger surface area for radiation. Thisaffects other properties of the diode and may require furthermodifications to be made to the specification.

3.3 Mounting and Heat Sink ConsiderationsThe increase in temperature between the diode heat sink andthe semiconductor transit region is defined by:

DT = Ry (Pin 7 Pout)

The thermal drop between the ambient and the diode heat sinkmust also be taken into account to avoid exceeding themaximum transit region temperature of *250 8C. The transitregion temperature may be computed as follows:

Ttr = Tamb + DTcase + DT

= Tamb + DTcase + Ry (Pin 7 Pout)

where: Ttr = transit region temperature (4250 8C).Tamb = ambient temperature.DTcase = temperature difference between the diodeheat sink and ambient at the operating power.Ry = diode thermal resistance.

In well designed packages, the temperature difference, DTcase,is usually less than 30 8C for an input power of about 15 W.

4 PERFORMANCE CHARACTERISTICS

4.1 Limiting Conditions of UseTemperature:operating (for standard types) . . . . 740 to +85 8Cstorage . . . . . . . . . . . . 755 to +150 8C

Operating voltage . . . . each type is individually rated.

Application of a bias voltage in excess of this value may lead todegradation in performance.


4.2 Typical Performance Curves

0 1 2VTH 3 4 5Vop

0

0.5

1.0

1.5

BIAS VOLTAGE

a)

7502

b)

CURRENT(A

)

ITH

0 1 2VTH 3 4 5Vop

0

0.5

1.0

1.5

BIAS VOLTAGE

CURRENT(A

)

ITH

Fig. 4.1 Typical DC Characteristic a) Standard Gunn Diode, b) Graded-gap Gunn Diode

740 720 0 20 40 60 80

7300

7200

7100

0

100

200

300

100

200

300

CASE TEMPERATURE T (8C)

OUTPUTPOWERPO(m

W)

FREQUENCYCHANGEDf(M

Hz)


OUTPUTPOWERPO(m

W)

FREQUENCYCHANGEDf(M

Hz)


OUTPUTPOWERPO(m

W)

FREQUENCYCHANGEDf(M

Hz)


OUTPUTPOWERPO(m

W)

FREQUENCYCHANGEDf(M

Hz)

7503

740 720 0 20 40 60 807120

0

80

280

Df

PO

90

100

110

55

50

45

40

35

740 720 0 20 40 60 80

7120

780

740

0

40

Df

Df

PO200

100

0

7100

8060402007207400

10

20

30

Df

PO

PO

a) b)

c) d)

Fig. 4.2 Variation of Power and Frequency with Temperature for:a) DC1276H operating in the fundamental mode;b) DC1277G-T operating in the fundamental mode;c) DC1279D operating in the second harmonic mode;d) DC1279F-T operating in the second harmonic mode


30 1001

10

Zeff

100

10

100

Pout

1000

FREQUENCY (GHz)

POWER(m

W)

PERCENTAGE(%

)

Z EFFICIENCY

Pout

Fig. 4.3 Variation of power and frequency with tempera-ture for DC1276 and DC1277 series.

30 1001

10

Zeff

100

10

100

Pout

1000

FREQUENCY (GHz)

POWER(m

W)

PERCENTAGE(%

)

Z EFFICIENCY (1)

Z EFFICIENCY (2)

Pout (1)

Pout (2)

Fig. 4.4 Variation of power and efficiency for: 1) DC1278and DC1279 series; 2) DC1279F-T series


8 9 10 11 1274

73

72

71

0

+1

720

710

0

+10

+20

+30

OPERATING VOLTAGE (V)

CHANGEIN

FREQUENCY(M

Hz)

CHANGEIN

POWER(dB)

POWER

FREQUENCY

Fig. 4.5 Variation of output power and frequency with operating voltage

8 9 10 11 1273

72

71

0

+1

+2

+3

0

5

10

15

OPERATING VOLTAGE (V)

FREQUENCY(G

Hz)

CHANGEIN

OUTPUTPOWER(dB)

VOLTAGE

POWER

Fig. 4.6 Effect of voltage tracking on the variation of output power with frequency on a wideband tunable waveguideoscillator.

Vsat Vop

Iop

Isat

GUNN VOLTAGE

GUNN

CURRENT

Fig. 4.7 DC Gunn characteristics

720 710 0 +10 +20 +30 +40 +50 +60 +7073

72

71

0

+1

730

720

710

0

+10

TEMPERATURE (8C)

CHANGEIN

FREQUENCY(M

Hz)

CHANGEIN

POWER(dB)

FREQUENCY

POWER

Fig. 4.8 Variation of frequency and power with tempera-ture.


5 BIBLIOGRAPHYMicrowave Oscillation of Current in III-V Semiconductors.Gunn J.B.Solid State Commun., 1, 88 (1963).

Theory of the Gunn Effect.Kroemer H.Proc. IEEE, 52, 1736 (1964).

The Gunn Effect.Hobson G.S.Clarendon, Oxford, 1974.

Theory of Stable Domain Propagation in the Gunn Effect.Butcher P.N.Phys. Lett., 19, 546 (1965).

Transferred Electron Amplifiers and Oscillators for MicrowaveApplication.Sterzer F.Proc. IEEE, 59, 1155 (1971).

Hot Electron Microwave Generators.Carroll J.E.Arnold, 1970.

Fundamental Mode graded-gap Gunn Diode Operation at 77and 84 GHz.Dale I., Stephens J.R.P., Bird J.Conf. Proc. Microwaves ‘94, .

Advances in Hot Electron Injector Gunn diodes.Spooner H., Couch N.R.GEC J. of Res., 7, 34 (1989).

Hot Electron Injection by Graded AlXGa1-XAs.Long A.P., Beton P.H., Kelly M.J., Kerr T.M.Electronics Letters, 22, 130 (1986).

Hot Electron Injection : Concept to Product.Couch N.R., Kelly M.J.Physics World, 2, 37 (1989).

Printed in England#2000 Marconi Applied Technologies Gunn Diodes AN1, page 19

Whilst Marconi Applied Technologies has taken care to ensure the accuracy of the information contained herein it accepts no responsibility for the consequences of any

use thereof and also reserves the right to change the specification of goods without notice. Marconi Applied Technologies accepts no liability beyond that set out in its

standard conditions of sale in respect of infringement of third party patents arising from the use of tubes or other devices in accordance with information contained herein.

diodogunn

Documents