Zetex Variable Capacitance Diodes

Neil Chadderton

Introduction

The advent of varactor diodes has madea huge impact in many areas ofelectronic design, which is only tooevident in todays consumer products.Formerly, where bulky or unreliablemechanical methods were used, thesize, reliability and excellent trackingabilities of the varactor has led tosmaller, cheaper and more elaboratecircuitry, previously impossible toattain.

Zetex manufacture an extensive rangeof Variable Capacitance diodes that areprocessed using ion-implantationtechniques to assure accurate dopinglevels, and hence produce the exactingjunction profiles necessary for highperformance devices. An overallcapacitance range of approximately 1pFto 200pF assures a broad applicationsbase, enabling designs operating fromkHz and extending into the microwaveregion. Product variability withinspecification is comparable to, or betterthan, competi tors devices; theHyperabrupt series, for example, areavailable to 5% tolerance on nominalcapacitance, due to close targetingduring fabrication. Furthermore, Zetexare capable of matching (either to devicetype or customer specifications) or

banding on capacitance parameters, asrequired. They are available in surfacemount packages that exhibit lowinductance ensuring a wide frequencyapplication, and assure environmentalendurance and mechanical reliability.

This application note gives some basicbackground information, examines theimportant parameters and specificationsfor the Zetex range of devices, andsuggests a few application circuitexamples.

Background

The varactor diode is a device that isprocessed so to capitalise on theproperties of the depletion layer of a P-Ndiode. Under reverse bias, the carriersin each region (holes in the P type andelectrons in the N type) move away fromthe junction, leaving an area that isdepleted of carriers. Thus a region thatis essentially an insulator has beencreated, and can be compared to theclassic parallel plate capacitor model.The effective width of this depletionregion increases with reverse bias, andso the capacitance decreases. Thus thedepletion layer effectively creates avoltage dependent junct ioncapacitance, that can be varied betweenthe forward conduction region and thereverse breakdown voltage.

AN 9 - 1

Applications Note 9Issue 4 July 2002

Different junction profiles can beproduced that exhibi t di f ferentCapacitance-Voltage characteristics.The Abrupt junction type for example,shows a small range of capacitance dueto its diffusion profi le, and as aconsequence of this is capable of high Qand low distort ion, whi le theHyperabrupt variety allows a largerchange in capacitance for the samerange of reverse bias. So cal ledHyper-hyperabrupt, or octave tuningvariable capacitance diodes show alarge change in capacitance for arelat ively small change in biasvoltage.This is particularly useful inbattery powered systems where theavailable bias voltage is limited.Thevaractor can be modelled as a variablecapacitance (Cjv), in series with aresistance (Rs). (Please refer to Figure 1).

The capacitance, Cjv, is dependent uponthe reverse bias voltage, the junctionarea, and the doping densities of thesemiconductor material, and can bedescribed by:

The series resistance exists as aconsequence of the remainingundepleted semiconductor resistance, acontribution due to the die substrate,and a small lead and packagecomponent, and is foremost indetermining the performance of thedevice under RF conditions.

This follows, as the quality factor, Q, isgiven by:

So, to maximise Q, Rs must beminimised. This is achieved by the useof an epitaxial structure so minimisingthe amount of high resistivity material inseries with the junction.

AN 9 - 2

Applications Note 9Issue 4 July 2002

RSCjv

Figure 1

Common Model for the Varactor Diode.

( )CjvCj0

1=

+ Vr NϕWhere:

Cj0 = Junction capacitance at 0VCjv = Junction capacitance at applied

bias voltage VrVr = Applied bias voltage� = Contact PotentialN = Power law of the junction or slope

factor.

Q2 fC R

=1

π jv s

Where:

Cjv = Junction capacitance at applied biasvoltage Vr

Rs = Series Resistancef = Frequency

NOTE: Zetex has produced a set ofSPICE models to enable designers tosimulate their circuits in SPICE, PSPICEand similar simulation packages. Themodels use a version of the abovecapacitance equation and so the modelparameters may also be of interest forother software packages. Information isalso provided to allow inclusion ofparasitic elements to the model. Thesemodels are available on request, fromany Zetex sales office.

Important Parameters

This section reviews the importantcharacteristics of varactor diodes withparticular reference to the Zetex range ofvariable capacitance diodes.

The characteristic of prime concern tothe designer is the Capacitance-Voltagerelationship, illustrated by a C-V curve,and expressed at a particular voltage byCx, where x is the bias voltage. The C-Vcurve summarises the range of usefulcapacitance, and also shows the shapeof the relationship, which may berelevant when a specific response isrequired. Figures 2a, 2b and 2c show

families of C-V curves for the 830hyperabrupt series, 930 hyper-hyperabruptseries and the 950 low voltagehyperabrupt series. Obviously, the choiceof device type depends upon theapplication, but aspects to considerinclude: the range of frequencies thecircuit must operate with, and hence anappropriate capacitance range; theavailable bias voltage; and the requiredresponse.

AN 9 - 3

Applications Note 9Issue 4 July 2002

Figure 2b

Typical Capacitance-Voltage Characteristics for

the 930 Series

931

930

932

933

934

1 10

Reverse Voltage, Vr (Volts)

1

10

100

100

110.1 10 100

10

200

836835834

833

832

830829

Figure 2a

Typical Capacitance-Voltage Characteristics for

the 830 Series

Capacitance v Reverse VoltageVR Reverse Voltage (V)

CJ

Cap

acita

nce

(pF

)

100

10

1 0.1 1 10

ZV953V2ZMDC953

ZV952V2

ZV952V2Tj = 25°Cf = 1MHz

Figure 2c

Typical Capacitance-Voltage Characteristics for

the 950 Series

The capacitance ratio, commonlyexpressed as Cx/Cy (where x and y arebias voltages), is a useful parameter thatshows how quickly the capacitancechanges with applied bias voltage. So,for an Abrupt junction device, a C2/C20figure of 2.8 may be typical, whereas aC2/C20 ratio of 6 may be expected for aHyperabrupt device. This feature of theHyperabrupt variety can be particularlyimportant when assessing devices forbattery-powered applications, wherethe bias voltage range may be limited. Inthis instance, the 930 series that featurea better than 2:1 tuning range for a 0 to6V bias may be of particular interest.

The quality factor, Q, at a particularcondition is a useful parameter inassessing the performance of a devicewith respect to tuned circuits, and theresulting loaded Q.

Zetex guarantee a minimum Q at testconditions of 50MHz, and a relativelylow VR of 3 or 4V, and ranges 100 to 450depending on device type (see ProductRange Tables).

The specified VR is very important inassessing this parameter, because aswell as the C-V dependence as detailedpreviously, a significant part of theseries resistance (Rs), is due to theremaining undepleted epitaxial layer,which is also dependant upon VR. ThisRs-VR relationship is shown in Figure 3for the ZC830, ZC833 and ZC836Hyperabrupt devices, measured atfrequencies of 470MHz, 300MHz and150MHz respectively, and also serves toillustrate the excellent performance ofZetex Variable Capacitance Diodes atVHF and UHF.

AN 9 - 4

Applications Note 9Issue 4 July 2002

600

1 10 100

Reverse Voltage VR(V)

100

200

400

300

500

700

TEMP = 25°CTYPICAL

ZC830

@ 470MHz

@ 300MHz

ZC833

@ 150MHz

ZC836

Rs

(m)

�

Figure 3

Typical RS

v VR

Relationship for ZC830 Series

Diodes

Also of interest, with respect to stability,is the temperature coeff icient ofcapacitance, as capacitance changeswith VR, and is shown for the ranges infigures 4a, and 4b.

The reverse breakdown voltage, V(BR)also has a bearing on device selection,as this parameter limits the maximumVR that may be used when biasing forminimum capacitance. Zetex variablecapacitance diodes typically possess aV(BR) of 25V for the 830 series and 12Vfor the 930, 950 series.

The maximum frequency of operation willdepend on the required capacitance andthe series resistance (and hence useful Q),that is possessed by a particular devicetype, but also of consequence are theparasitic components exhibited by thedevice package. These depend on thesize, material, and construction of thepackage. For example, the Zetex SOT-23package has a typical stray capacitance of0.08pF, and a total lead inductance of2.8nH. These low values allow a wide

frequency application, for example, theZC830 and ZC930 series, configured asseries pairs are ideal for low costmicrowave designs extending to 2.5GHzand above.

Applications

Variable capacitance diodes can be usedin any tuned circuit application wherepreviously mechanical methods wereutilised, and provide a size, cost andperformance advantage. This sectionbriefly examines some typical examplesof varactor application.

The conventional simple tuned circuit ofFigure 5a can also be effected by thevaractor version as in Figure 5b, wherecapacitor C1 isolates the DC bias. Thechoice of varactor for such a circuitdepends on the tuning range and hencecapacitance, with particular attentionbeing paid to the C-V regionapproaching 0V, as this may introducenon-linearity and poor Q. Anothersimilar approach is to use the seriesconfiguration shown in Figure 5c which,while allowing a lower apparent diode

AN 9 - 5

Applications Note 9Issue 4 July 2002

600

1 10 30

100

200

400

300

500

700Tj = 0-100°C

TYPICAL

0

pp

m/°

C

Figure 4a

Temperature Coeffcient of Capacitance v VRfor

the ZC830 Series.

1010.1

Vr -Reverse Voltage (V)

600

800

1000

1200

400

200

0

1400Tj=25 to 125° C

TYPICAL

pp

m/°

C

Figure 4b

Temperature Coefficient of Capacitance v VRfor

the ZC930 Series.

capacitance, also prevents RFrectification at low values of diode biastherefore inhibiting generation ofintermodulation products, and alsosimplifies bias requirements.

A practical front-end for an FM receiveris shown in Figure 6, with each stagebeing tuned by it ’s own diode.Multi-stage units are therefore possible

without the severe tracking errors, andthe massive size penalty inherent tomechanical mechanisms.

As the tuning is now controlled by avoltage, the inevitable inclusion of themicroprocessor and memory in manymodern receivers has allowed band-scanning and stat ion storage byproducing the control voltage

AN 9 - 6

Applications Note 9Issue 4 July 2002

470

22p

3n9

4K7

BFS17H

4n715K

47p

47K

47p

D2

10p

47K

3K322n 47p

2N3819

223n9

47p

D3

47K

10p

BFS17H

10K

2p2

22p

2K727K

3n9

10n

D1

10p

+12V

SUPPLY VOLTAGE

0-30V

I.F. OUTPUT

10.7 MHz

D1-D3 ZC823ZMV823

CONTROL VOLTAGE

AERIAL

Figure 6

Typical FM receiver Front End employing Variable Capacitance Diodes.

(c)(b)(a)

C1

DCBias DC

Bias

Figure 5

Basic Tuned Circuits.

automatically. It is noteworthy that thecontrol voltage of any system mustpossess good voltage and temperaturestability.

Perhaps the largest effect on consumerproducts due to the varactor has been thedevelopment of varactor based VHF andUHF tuners for televisions. These haveenabled solid state non-mechanicaldesigns that are smaller, more reliableand allow elaborate features such asremote control and station searching.Figure 7 shows a typical circuit of a UHFtuner incorporating varactor diodes.Stage matching is effected by the biastrimmers RV1-5, and allows adjustmentremote from the actual tuning element;the mechanical equivalent being to addpadding capacitance, or to bend thevanes on an air-spaced capacitor.

Such a tuner can, using the largecapacitance range of Hyperabruptvaractor diodes, tune the whole channelrange of bands IV and V(470MHz-850MHz).

Another common application for thevaractor is as the frequency controllingelement in a Voltage Control ledOscillator (VCO). There are manyapplications for such circuits, either asstand alone units or as part of a phaselocked loop in a frequency synthesiser.This latter method is commonly utilisedin radio telecommunications and for thetuning stages in satellite receivers.Closely allied to these are functions suchas frequency pul l ing on crystalosci l lators, narrow band FM andtemperature compensat ion offrequency within an oscillator, all ofwhich can benefit from a varactor diodebased design.

AN 9 - 7

Applications Note 9Issue 4 July 2002

D2

D4

RV3RV2

D5

RV5RV1

D1

CONTROL VOLTAGE

RV4

SUPPLY VOLTAGE

D3

SUPPLY VOLTAGE

AERIALINPUT

AGC

I.F.OUTPUT

Figure 7

Typical UHF Variable Capacitance Diode Tuner Module

D1 - D5 - ZC831 or ZMV831 orZV831V

In its basic form, and for a square waveoutput, a common method of providinga VCO is to use logic gates coupled asshown in Figure 8; this particular designgiving a 1 to 1.25MHz range. For atransistor design, a VCO can be realised

by modifying the Clapp configuration asshown in Figure 9. For this example, thefrequency can be varied over a 75 to150MHz range for a 0-30V controlvoltage.

AN 9 - 8

Applications Note 9Issue 4 July 2002

ZC834

0.15u

22K

39p

4K74K7

O/P

74C00VC

& & &

Figure 8

Basic Form of VCO using Logic Gates.

ZMV834

22K

820p

820

47p

3K3

BFS17H

390

1n

L1

47p

on 1/4" Former1

+12V

O/P

NOTE : L = 5T

VC

Figure 9

Transistor Effected VCO using Clapp Configuration.

Figure 10 shows a similar configurationfor a 1GHz VCO. Obviously at thisfrequency, circuit construction is criticaland capable of producing large tuningrange changes. For this example, thetransistor was mounted in a slot in asmall ground plane configured board,and the other components supported byshort leads. This produced a signal levelof -5dBm with a 10dB PAD into 50 ohms.The second harmonic was observed at-35dB down from the fundamental.

Other techniques using lumpedcomponents and Zetex variablecapacitance diodes are capable ofoutput at 1.5-2.5GHz, typically for tuningunits for satellite receivers.

AN 9 - 9

Applications Note 9Issue 4 July 2002

Figure 10

1GHz VCO Producing -5dBm with a 10dB PAD into 50 .

2 x ZC831or 1 x ZMDC831

33K33k 5K6

L1 22pBFR90

150

27 27

33

20K10u

VC

NOTE : L = 1 cm long1mm dia

O/P

+12V

1n

4p72p2

2p2

1

Low Phase Noise Capability

Due to the geometr ic featuresemployed in the Zetex range ofvariable capacitance diodes, theseproducts exhibit extremely low valuesof leakage current (typically less than20 pA which enables excellent lowphase noise performance within VCXOcircuitry.

The varactor diode also enablesfrequency multipliers to be producedthat exhibit very high conversionefficiency, a zero DC power requirement,and low component count. Figure 11ashows the general appearance of avaractor tripler, and consists of inputand output matching, and a trap for theunwanted second harmonic. Figure 11bshows a similar circuit for a 100-300MHz

tripler using a ZC830, and includes abandpass filtered output and a trap forthe fundamental.

AN 9 - 10

Applications Note 9Issue 4 July 2002

OUTPUTINPUT

Figure 11a

Varactor Tripler with Marching Input and Output Stages.

ZC830

Figure 11b

Varactor Tripler with Bandpass Filtered Output and a Trap for the Fundamental.

Appendix

Zetex Variable Capacitance Diode Product Range.

The tables presented within this Appendix illustrate the standard abrupt,hyperabrupt and hyper-hyperabrupt ranges available with respect to datasheetcharacteristics and package style. In addition, Zetex can also supply to competitors’type numbers and customers’ specific requirements.

AN 9 - 11

Applications Note 9Issue 4 July 2002

SOD523 SOD323 SOT23 SOT323

DUAL

SOT23

DUAL

Capacitance

VR

= 2V,

f = 1MHz

Capacitance Ratio

C2/C20

at f = 1MHz

Q

VR

= 3V

f = 50MHz

TYP (pF) Min Max Min

ZMV829A ZC829A 8.2 4.3 5.8 250

ZV829BV2 (1) ZMV829B ZC829B ZMDC829B (1) 8.2 4.3 5.8 250

ZMV830A ZC830A 10.0 4.5 6.0 300

ZV830BV2 (1) ZMV830B ZC830B ZMDC830B (1) 10.0 4.5 6.0 300

ZMV831A ZC831A 15.0 4.5 6.0 300

ZV831BV2 ZMV831B ZC831B ZMDC831B 15.0 4.5 6.0 300

ZMV832A ZC832A 22.0 5.0 6.5 200

ZV832BV2 ZMV832B ZC832B ZMDC832B 22.0 5.0 6.5 200

ZMV833A ZC833A ZDC833A 33.0 5.0 6.5 200

ZMV833B ZC833B ZMDC833B (1) 33.0 5.0 6.5 200

ZMV834A ZC834A ZDC834A 47.0 5.0 6.5 200

ZMV834B ZC834B 47.0 5.0 6.5 200

ZMV835A ZC835A 68.0 5.0 6.5 100

ZMV835B ZC835B 68.0 5.0 6.5 100

ZC836A 100.0 5.0 6.5 100

ZMV836B ZC836B 100.0 5.0 6.5 100

Note:Suffix A diodes are 10% tolerance.Suffix B diodes are 5% tolerance.(1) Planned for future release. Please contact your local Zetex sales office for moredetails.

HIGH PERFORMANCE HYPER-HYPER ABRUPT

AN 9 - 12

Applications Note 9Issue 4 July 2002

SOD523 SOT323 DUAL Capacitance

VR

= 0.5V,

f = 1MHz

Capacitance

VR

= 1.5V,

f = 1MHz

Capacitance

C0.5/C2.5

f = 1MHz

Q

VR

= 0.5V

f = 50MHz

Min (pF) Min Max Min Min

ZV950V2 ZMDC950 (1) 9.5 6.3 7.8 2.0 250

ZV951V2(1) ZMDC951 (1) 15.0 9.4 11.6 2.0 250

ZV952V2 ZMDC952 (1) 19.00 12.7 15.7 2.0 250

ZV953V2 ZMDC953 45.0 30.0 37.0 2.0 200

(1) Planned for future release. Please contact your local Zetex sales office for moredetails.

SOD523 SOT323 SOT23 Capacitance

VR

= 1V,

f=1MHz

Capacitance

VR

= 2.5V,

at f=1MHz

Capacitance

VR = 4V

f=1MHz

Q VR = 4V

f=50MHz

Min (pF) Min Max Min (pF) Min

ZV930V2(1) ZMV930 ZC930 8.70 4.30 5.50 2.90 200

ZV931V2 ZMV931 ZC931 13.50 6.50 7.80 4.00 300

ZV932V2 ZMV932 ZC932 17.00 8.50 10.50 5.50 200

ZV933V2 ZMV933 ZC933 42.00 18.00 27.00 12.00 150

ZMV933A ZC933A 42.00 20.25 24.75 12.00 150

ZMV934 ZC934 95.00 40.00 65.00 25.00 80

ZMV934A ZC934A 95.00 47.25 57.75 25.00 80

(1) Planned for future release. Please contact your local Zetex sales office for moredetails.

LOW VOLTAGE HYPER ABRUPT