& Ha#F

K. Giridhar

rf , :w:,;s ard Radar Microwave Diodes - Transferred Electron Devices

: i - , :upersCript: : . ; ! :S appl ied:;i .i-- the thinrc":: :enefatedl i : . : l , l l lafd p+

i space-charge:-: ;:! ri e cavity,

:-, : .rpedance9:"-po\\ 'erby

region with a constant velocity 'Do' of about 107cm/sec for silicon read diode. The field remainsconstant at about 5 KV/cm throughout the space charge region. The transit-time for holes indrift region of length L is given by

,=LDd

The avalanche multiplication factor is given by

. . . . . (3.6)

I

1- (v/vo)"

applied voltage

avalanche breakdown voltage

. . . . . (3.7)

where

M

V=V.=

D

----*I

-__-* JI--l

3{::t- than the- t - - : - . -h the

^

: - -:.rJtion as: : - ih is highr , : :o the p+

where . . . . . (3.8)

n = varies from 3 to 6 for silicon which depends on doping of n*pjunctionpn = resistivity of semiconductor

Fn = electron mobilitye. = dielectric constant of semiconductor

E_u* = maximum electric field intensity at breakdown

3.8 IMPATT DIODEImpatt diodes are manufactured having different forms such as n*pip*, p+nin*, p*nn*

abrupt junction and p* i n* diode configuration. The material used for manufaiture of thesediodes are either Germanium, Silicon, Gallium Arsenide (GaAs) or Indium phosphide (In p).Out of these materials, highest efficiency, higher operating frequency and lowernoise is obtainedwith GaAs' But the disadvantage with GaAs is complex fabrication process and hence highercost. Figure 3.10 shows a reverse biased n* pi p* diode with electric field variation, dopingconcentration versus distance plot, the microwave voltage swing and the cuffent variation.Principle of operation

When a reverse bias voltage exceeding the breakdown voltage is applied, a high electricfield appears across the n+ p junction. This high field intensity imparts sufficient energy to theholes and also to valence electrons to raise themselves into the conduction band. This resultsin avalanche multiplication of hole-electron pairs. With suitable doping profile design, it ispossible to make electric field to have a very sharp peak in the close uicinity of the junctionresulting in "impact avalanche multiplication". This is a cumulative protess resulting inrapid increase ofcarrier density. To prevent the diode from burning, a constant bias source isused to maintain average cuffent at safe limit Io. The diode current is contributed by theconduction electrons which move to the n* region ind the associated holes which drift throush

r \ / r - Pn l tn €. lE-"^l tI v, l -'D ' 2

.._-*

B*

i-space charge region to the p* region, under the influence of a lower but steady electric field'

The drift time is given bYt " "1(: 'q)

'o=%

where Vo is the drift velocity of the holes and 'l' is the length of the drift region'

VoltageSwing(v*)

Dopingconcentration

i jg*,"-al Elecron current I"(t)i ,11

16r,12412a1

tIIII

ElectricField

+

II

""1",

Microwaves and Radar llicrowave Dio&lii

thesteadyfidinfluence ofQvery small, cdavalanche. D{and the osciQfield, the hob4zeto.

During {circuit whichicorresPonrlintphase shift Hresistance fciangle 0 givell

Combii

nl

FigureSsimplified q

:3{

The rer{high power{

'{&

CONSTRT'{

-IhId I

l r -- - -{ E

\ -tr\TNNIf\\U*1 RNNI\

\ t\\\\\ I\\\T

\. N\\\t

tTnmrrt]TTIJIII.4

Figrrre 3.10 : IMPATT diode doping prohle and operation

MECHANISM OF OSCILLATIONS

Noise consists of various frequency components at varying amplitudes' wlgn a biased

di"d;;;";;Jil;" a resonatolthen any noise voltage spike can trigger oscillations at a

frequency equal to the resonant frequency ofth" t"tonut* Th"t" oscillations are sustained by

t|es and Radar Microwave Diodes - Transferred Electron Devices

lec-.nc field.

. . . . . ( : .q)

fhen a biasedcillations at al sustained by

the steady field and a.c. field. The diode swings into and out of avalanche conditions under theinfluence of that reverse bias steady field and the a.c. field. Due to the drift time of holes beingvery small, carriers drift to the end contacts before the a.c. voltage swings the diode out of theavalanche. Due to building up of oscillations, the a.c. field takes energy from the applied biasand the oscillations at microwave frequencies are sustained across the diode. Due to this a.c.field, the hole current grows exponentially to a maximum and again decay exponentially tozefo.

During this hole drifting process, a constant electron current is induced in the externalcircuit which starts flowing when hole current reaches its peak and continues for half cycle (t)corresponding to negative swing of the a.c. voltage as shown in figure 3.10. Thus a 1800phase shift between the external current and a.c. microwave voltage provides a negativeresistance for sustained oscillations. The maximum nesative resistance occurs at drift transitangle 0 given by

0 = (oto=?r

Combining equations (3.9) and (3.10), we get

. . . . . (3.10)

o/vo

=t

2nf lUo-

= tt

. . . . . (3.11)

The resonator is usually tuned to this frequency so that the IMPATT diodes provide ahigh power continuous wave (CW) and pulsed microwave signals.

CONSTRUCTION AND EQUIVALENT CIRCUIT

t =Y;t

Goldribbon

Copperstud

Alumina

GaAschip

Figure 3.11 : (a) IMPATT diode construction (b) Equivalent circuit of IMPATT diode

Figure 3.11 (a) shows the constructional details of IMPATT diode and figure 3.11 (b), thesimplified equivalent circuit of IMPAIT diode. The resistance R, is the combined resistance

Multi-sectionQuarter-waveTransformer

Microwaves and Radar

of series resistance and the diode negative resistance. The capacitance C,represents the junctioncapacitance, Lo the package lead inductance and Co the package capa"citance.

The diode mount is so designed that by controlling the package lead inductance the totalreactance of the entire circuit is made zeto atresonance. At resonance, the power dissipated inthe positive rcsistance is compensated by the power in the a.c. field. For achieving this conditionthe total resistance must be zero.

The diode chip impedance is given by

.... (3.r2)

. . . . (3.13)

. . . . (3.14)

_., . ; : i l

Eramplr 3

Solution :

" F:- :

i - - : , r .L

1Z.=- lR. l+ ^

J , L, j toCi

If the load impedance

then

Equating real and imaginary parts

Rr = l\lI

xL=oq

Zr=Rr+jX,Zr = -Zi

R,+jX, = l \ l -#=l \ l+- !

and

The resistance R, is dependent on both bias and signal currents. Hence for a given biasand load impedance, dustained oscillations are obtained when lR;l = RL. The power dissipatedin the load is determined from the peak RF current. Figure 3.11 (b) shows a multisectionquarterwave transformer for impedance matching between diode circuit and load. Thistransformer is required because the total negative resistance in the circuit will be very low ofthe order of a few ohms

Example 3.2 : An IMPATT diode has a drift length of 2 pm. If the basic semi conductormaterial is silicon for which the drift velocity is 107 cm/sec, determine (a) the drift time of thecarriers, (b) the nominal frequency of the IMPAT:I diode.

Solut ion:Given, Va=107cm/sec, l=2l tm, (a) to-? @)f=?(a) The drift time to of the carriers is given by

I 2x10-6i = -=-'d - V l0-7x10-2

Na=2x10- l lsec

(b) The nominal frequency is given by

! : - i - - -

|

r - vo -I - - -

2t2

f = 25G}Jz

equation (3.11) as

10-2x2x10-6

107 x

F:?1at'es and RadarLlicrowave Diodes - Transferred Electron Devices 225

tar.- thejunction

nlc:mce the totaln:: Cissipated inm,E -,his condition

.... (3.r2)

. . . . (3.13)

.. . . (3.r4)

t":r . eiven biasn,;;r ,Jissipatedr ; ::l'.:ltisectionr::: ioad. This! !e i ery' low of

*e:l conductor&:: rime of the

Example 3.3 : An IMPATT diode has the following parameters:Carrier drift velocity Vo = 2 x I07 cm/sec

Drift region length / = 10 pmMaximum operating voltage = 100 volts

Maximum operating bias current = 60 m Amps.Junction capacitance C: = 0.42 pF

Package lead inductance Lo = 0.6 nHPackage capacitance C, = 0.25 pF

RF peak.uo"ni = 700 m Amps.Diode Junction Resistance \

= -Z.S OFind (a) Nominal frequency

(b) resonant freqlrency of oscillations(c) efficiency

Solution:(a) From equation (3.11), the nominal frequency is given by

r=3-?xlg?x10]=roGHz21 2xl0x10-o

(b) At nominal frequency of 10 GHz, we haveolo = 2nf Lr=2nx 10 x 10e x 0.6 x l0-e

= 37.7 {2

r r lCpRL = rCol \ l=Znf Cel \ l= 2nx 10 x 10e x 0.25 x IVtz x2.5= 0.039f)

Since to LD >> tD Co R' the exact resonant frequency depends on the combination of L_C,givenbf

v L ' P

" , - I'- zrltrn

I

2", , [ f f if = 10.03 GHz

(c) The output rms power is calculated as

/ RF peat< current \2P^ = | ---- /^ - | (load resistance)

' \ '12)

210

Microwaves and Radar

.D

'0 -

The d'c' input power = Pin = (Maximum operating voltage) x (maximum operating bias current)= (100) (60 x 10r)= 6 watts

Eff iciencyn = 3 xt{|Tor in

0.612sx 100 %o

Vo rl = 10.21 VoApplications of IMPATT Diodes

(i) Used in the final power stage of solid state microwave transmitters for communicationpurpose.(ii) Used in rhe transmirter of TV system.

(iii) Used in FDM/TDM sysrems.(iv) used as a microwave source in raboratory for measurement pulposes.

3.9 TRAPATT DIODESsilicon is usually used for the manufacture of TRAPATT diodes and have a configurationof p* nn* as shown in figure 3.r2. The p-N junction is reverse biased beyond the breakdownregion' so that the current density is larger. This decreases the electric field in the space chargeregion and increases the carrier trans=it time. Due to this, the frequency of operation getslowered to less than 10 GHz. But the efficiency gets increased due to lorv power dissipation.Inside a co-axial resonator, the TRAPATT diode is normally mounted at a point wheremaximum RF voltage swing is obtained. when the combined dc bias and RF voltage exceedsbreakdown voltage, avalanche occurs and a plasma of holes and electrons are generated whichgets trapped. when the external circuit cuffent fl9*,r, the voltage rises and,n",rupp"l!ffi;gets released producing current pulse across the drift space. ttre totat transit time is the sum ofthe drift time and the delay introduced by th3 release of the trapped plasma. Due to this longertransit time' the operating frequency is limited to 10 GHz. Because the currentpulse is associatedwith low voltage, the power dissipation is row resulting rniigh", efficiency.

The disadvantages of TRAPATT are high noise figure unJg"ne.ation of strong harmonicsdue to short duration of the cuffent pulse.

,rrr"ffilt*T diode finds application in s-band pulsed transmiters for pulsed array radar

I i ^ / - . ^ ^ - -|

- J i tat=

- - r

( too x 1o-, ) 't-----F-- |\J2)

0.6125 watts

(2.5) since R, = lRf

3.10

I3A i , es and Radar Microwave Diodes - Transferred Electron Devlces227

:.9 :tas cunent)

lf--llunication

:r:::-guratione ::33kdowni:;,e ChargeE:.1:tOn gets: : - : : rpat iOn.

:'- -nt where:3:: eKceeds3r;t3d rvhichl:eJ plasma:,i :h3 SUm ofr rhis longer:-. a-ssociated

I harmonics

array radar

Figure 3.12 : TRApATT diode

3.10 BARITT DEVIES (BARRIER INJECTIONTRANSITTTME DEVICES)BARITT devices are an improved version of IMpATT devices. IMpArr devices employimpact ionization techniques *tti"h is too noisy. Hence in order to achieve low noise figures,impact ionization is avoided in BARRITT devices. rt. -l""r1y

injection is provided bypunch-through of the intermediate region. (depletion ,egion;. The process is tasically oflower noise than impact ionization *rpr"]9F f"^t @;;j..ri"n i" an IMpATT. The negative

;:;HA:s obtained on account or iire drift of the injeciJJ r,ot"s to the collector end of the

Figure 3'13 shows the construction of a BARITT device consisting of emitter, base,intermediate or drift or depleted region and coilector. An "rr.ntiul

requirement for the BARITTdevice is therefore that the int"ti-t"oiaie drift region ue .ntLty depleted to cause punch-

ilr:"lr-"T the emitter-base junction *itttout causing avalanche breakdown of the base-collecror

Voltage, V

Cunent, I

Forward Bias

Figure 3.13 : BARITT Diode and electric field distribution.

228

Microwaves and Badar

r

Characr' - : :

The efficiencv of BARITTs are lower than IMPATTs but the noise performance is better.ilffi,liiT:;rTi,"Ii:,:,*:.;;G"4";;l*,15 dB gain have been obtained. BARrrrs arepri mari I v u s ed for amp I i fi ers rath er ,n ;, **ilfi :# J:"#,:T:.""?i?:Tf; ,lf+ilffi:disadvantages are low power output & rerativery narrow bandwidth.3.11 VARACTOR DIODE

varactors (variable-capacitor) have nonJinearity of capacitance which is fast enough tofollow microwaves' varactor dioo" ir u *.iconductor di"dJ; which the junction capacitancecan be varied as a function of reverse uotrug" of the di;;. i;rres in this non_linear elementwill be. almost negligible. Varactor iioJ., are used as harmoIow noise ampliriers (parametric ampririers), purse genera,t"l'i,,?lilll"||;li"llil,llltli;widely used devices oi at -i.ror"uuJ.".nr.onou.tor

devices.Operation

Any semiconductor diode has a junction capacitance varying with reverse bias. If such adiode has microwave characteristicsin"n it becomes a varactor diode.with a reverse bias' the junction is depleted of mobile carries resurting in a capacitancei'e'' the diode behaves as a capacitance wiih tne3un.tion u"rrng u. a dierectric between thetwo conducting materials' The width of the o"pr"tion iut". rn increases with reverse bias\o^'n"

capacitance decreases ut dr" r"u"rse bias in.reo."s as shown in figure 3.14(b)

l"'t.j o

;,,l' tn" avalanche region is never used as it is Iikery to destroy the device.

Co -+ the junction

nce underno bias condition.

(a) (b)

Figure 3.14 : (a) I-V characteristics (b) Capacitance variation with bias

Fl .

' , . .*,

- , : - " j

- : _-__--*t r_| : , l

i^ : - -r - i i - ] - t .

i^1- I

:

, ! : - - ,

Equir aI

n _i :

: i .

rPl r l -

_:

- . - - .J _

JLiL- i -_--

(cl

(c) Biasing of p-n junctionVaractor Construction

, th. diode encapsulation contains electrical leadsand a low loss ceramic case as shown in figure 3.15.

attached to the semiconductor wafer

Saturated

n:\i: , as and Radar Microwave Diodes - Transferred Electron Devices

:-:",--e is better." E

-RITTS are

:-:. The major

i : : ) : enough tO:: : -3pacitance---:::.1r element

-:- : lnversion,r.-_: They are

e: ' . I fsucha

: . ::lacitance:: : : i r t 'een the'.:: :: ' ' efSe biaS-- : -r : 3. 14(b)

: ; - - -e.

p-n junction

,:-iuctor wafer

Gold platedmolybdenum

stud

Gold-plated 5.3 mmwlre

Ceramic Tube

Figure 3.15 : Varactor construction

Diffused junction mesa silicon diodes are widely used at microwave frequencies. They

are capable of handling large power and large reverse breakdown voltages. They have relative

independence of ambient temperature and low noise. Frequency limit of Si range upto 25

GHz. Varactors made of gallium arsenide (GaAs) have high operating frequency (over 90

GHz) and better functioning at the lowest temperature. However the manufacturing techniques

are easier for silicon.

Characteristics and Requirements

Varactors are normally used between the reverse saturation point to a point just above

the avalanche region. The capacitance variation and the reverse voltage swing are limited to

between the operating region mentioned above.

Equivalent Circuit of Varactor Diode

The equivalent circuit of the semiconductor wafer is shown in figure 3.16(a) consisting

of,

T

C, e Junction capacitance (function of applied voltage)

n., -

Junction resistance (function of applied bias)

& R, -+ Series resistance including bulk resistance of the-

wafer and resistance of ohmic electrical leads(function of applied bias)

At microwave frequencies, junction resistanceR, (= 10 MQ) is neglected as compared to the capacitivere!actance.

Encapsulation of the varactor add parasitic resistancesand reactances to the semiconductor wafer.

RJ

Figure 3.16(a) : WaferEquivalent circuit

230 Microwaves and Radar

C"J

cr+

\ ->

Capacitance of ceramic caseFringing capacitance

Lead inductance

Microwane I

T"herpeali a.: $sigral trEl

One 1Omti :hrutrequen{lr

uf f -u l r

rf f I

Figure 3.16(b) : Equivalent Circuit andEncapsulated Varactor Diode

The parasitics should be kept as low as possible. The equivalent circuit depends on thetype of encapsulation and mounting make. For many applications, there shouid be a largecapacitance variation, small value of minimum capacitance and series resistance R".

The cut off frequency at a specified bias (V) is given by

\ is neglected as \ tt #

at microwave frequencies.I

" ( r )t." =

[z"np"Jf" for silicon diodes range upto 250 GHzand for Gallium Arsenide diodes upto 900 GHz.Operation is normally limited to f./10 125 GHzfor Si and 90 GHz for GaAsl. Frequency

of operation beyond (f"/10) leads to increase in R,, decrease in efficiency and increase innoise.

3.12 PARAMETRIC AMPLIFIERSThe parametric amplifier is an amplifier using a device whose reactance is varied to

produce amplification. Varactor diode is the most widely used active element in a parametricamplifier. It is a low noise amplifier because no resistance is involved in the amplifyingprocess. There will be no thermal noise, as the active element used involved is reactive(capacitive). Amplification is obtained if the reactance is varied electronically in somepredetermined fashion.

Due to the advantage of low noise amplification, parametric amplifiers are extensivelyused in systems such as long range radars, satellite ground stations, radio telescopes, artificialsatellites, microwave ground communication stations, radio astronomy etc.Basic Parametric Amplifier

A conventional amplifier uses a variable resistance and a d.c. power supply. For aparametric amplifier, a variable reactance and an ac power supply are needed.

Pumping signal at frequency fn and a small amplitude signal at frequency { are appliedsimultaneously to the device (varacior). The pump source supplies energy to the jignal (at thesignal frequency) resulting in amplification. This occurs at the active device where thecapacitive reactance varies at the pump frequency.

u€..:s and RadarMicrowave Diodes - Transferred Electron Devices

{

>.1 Kt/ \.+ iat

--r- (- ,

I

- - f - r

--+ Signal input voltage

together

-*Pumping voltage

Plates Apart

-) Output voltage buildup

Parametric amplifi cationwith square wave pumping.

r!,{ Circuit andcldu Diode

ftnds on them :e a largeER..

. . (3.15)

r: 900 GHz.

l. FrequencyI ;ncrease in

i; i aried tor Psrametric: rnplifyingI rs reactivellr in some

e.\rensivelye:. artificial

ppiy. For a

are appliedignal (at the$'here the

Figure 3.17 : parametric Amprification with Square wave pumping

The voltage across the varactor is increased by the pumping signal at each signal voltagepeak as shown above i'e., energy is taken from the pu-p ro*"" and added to the signal at thesignal frequency' With an input circuit and load connected, amplification results.' One port non-degenerate amplifier is the most commonly used parametric amplifier.only three frequencies areinvolvea - tn" pump, the signal and the idler frequencies. If pumpf."q11:v is fo, rhe signal frequency is r., itren ior", rrJqu"nff i, f, _ fo _ t

^If f. = f , then it is called Degenerate amplifier andif f, * f , then it is non-degenerate amplifier.

Figure 3.18 : Simplified Basic Amplifier Circuit

Microwaves and Radar

L, C, -+ tuned circuit at signal frequency fL, C, -+ tuned circuit at idler frequency f, (Rump frequency tuned circuit is not shown).The output can be taken at idler frequency f,.Gain is possible with this type of amplifier. Because the pump source gives more energy

to the tank circuit than it takes out on an averase.

. . . . . . (3.16)

. . . . . (3.17)

. . . . . (3.18)

l,'l:crou;a,.'e I :-t

Equ.'"::t,L- ^^, , - -t i lC PLt\1

- .

: :

rnto th: : - : -

. ] ' : . . : I

: : -J U3:. -- .1-- . . . - - -

- i1!J,- :*, . . . . ' : .- J jg r :

" , - : - - a

1T

- r

In non-degenerate type, usually f, > f, resulting in gain. The idler circuit permits energyto be taken from the pump source. This energy is converted into signal frequency and idlerfrequency energy and amplified output can be obtained at either frequency.

MANLEY-ROWE RELATIONS

For the determination of maximum gain of the parametric amplifier, a set of powerconservation relations known as (6Manley-Rowet' relations are quite useful. Figure 3.19 shows

Figure 3.19 : Illustrating Manley-Rowe Relations

two sinusoidal signals fo and { applied across a lossless time varying non-linear capacitanceC, (tl. et the output of this varying capacitance, harmonics of the two frequencies fo and f aregenerated.

These harmonics are separated using band-pass filters having very narrow bandwidth.The power at these harmonic frequencies are dissipated in the respective resistive loads asshown in figure 3.19.

From the law of conservation of energy, we have

Gain=*=T

i i -nt ' " - =o^u-nt|onf,

+ mfo

in f f iP*

,3 "Z- "t;;4 = o

where P_n = average power at the output frequencies * l("f. + mfe)i

tes and Radar

oot shown).

llore energy

-_. . . . (3.16)

rrnits energy11'and idler

rt of powerr3.19 shows

a

o

r capacitancerf andf areps

r bandwidth.dve loads as

. . . . . (3.17)

. . . . . (3.18)

Microwave Diodes - Transferred Electron Devices

Equation (3.I7)relaticjns are called"Manley-Rowe" power conservation unuurronr. Whenthe power is supplied by the two generators, then P,nn is positive. In this case, power will flowinto the non-linear capacitance. If it is the other way, then P.n is negative.

Figure 3.20 : Illustrating output power flow at only sum frequency

As an example, let us consider the case when the power output flow is allowed at the sumfrequency f" * t only as shown in figure 3.20, with all the remaining harmonics being opencircuited. With the above restructions, the quantities 'm' and 'n' can take on falues -1, 0 and1 each respectively. With these values of 'm' and 'n', equations (3.17) anO (l.tS; become

?.4ft =o,&0, &'and ,, +fri = 0 ....(3.20)

where Po, = Power supplied by generator vs at freqdency !P,d = Power supplied by generator vo at frequency fo

P,, = Power output flowing from the non-linear capacitance into theresistiveload at sum frequency fo + fr.

The powers Pn, and P,o are considered positive, whereas P,, is considered negative.

.'. The power gain defined as the power output from the non-linear capacitor deliveredto the load at sum frequency to that power received by it at a frequency f, is given by

P,, f" + f"Go =

#=T (formodulator)

Thus the power gain is the ratio of output to input frequency. This type of parametric

device is called "sum-frequency parametric amplifier" or "up-converter".On the other hand, if the signal frequency is fo + { and output frequency is {, then

c. = #

(fordemodulator)

. . . . . (3.1e)

. . . . . (3.2r)

, )..... (3.22)

z

t

Silicon is the semiconductor normally usedb::uu:: of its power handling capability and itoffers high resistivity for the inirinsic regi,on. But,now-a-days Gallium Arsenide (GaAs) is ilso beingused. As shown in figure 3.21, metal layers areattachedfor contact purposes. Its main appiicationsare in microwave switching and moduiation.

PIN diode acts as a more or less ordinary diode at frequencies upto about 100 MHz. Athigh frequencies, it ceases to rectify and then acts as a uuriufi. resistance with an equivalentcircuit shown in figure (3'22)and a iesistance-voltage characteristics as shown in figure (3.23).In the equivalent cilcujt, Lo and co represent the package inductance and capacitancerespectively' R. is the bulk seiliconduttor layer and contact rEsistance. R, and c, represent therespective junction resistance and capacitance of the intrinsic layer. wtien the'bias is variedon the PIN diode, its microwau" ,.rirtun"e.R changes from a typical value of 6 Ka undernegative bias to perhaps 5 Q when the bias is fositiv"e u"rrro*n in figure (3.23).Thus, if thediode is mounted across a 50 c2 co-axial line, it ril noi rig"ificuntty load this line when it isback-biased' so that the power flow will not be interfered iuith. Ho*"ver, if the diode is nowforward biased, its resistance drops significantly to 5e, so thai most of the power is reflectedand hardly any is transmitted; th; di;e is acring as a switch.

Microwaves and Radar

This type of parametric device will now be called '6parametric down-conyerter,, andthe power gain becomes power attenuation.

3.13 PIN DIODE AND tTS APPLICATIONSThe PIN diode is a p-type, intrinsic, n-type diode consisting of a narrow layer of p-typesemiconductor and u narro*luyer of n-type semiconductor, with a thicker region of intrinsic

[Jr:tl l?ltlv n-doped semiconductoi material sandwiched between ttre,ir as shown in

N- typesilicon

Intrinsic silicon(slightly N-doped)P+ypesil icon

Metallic contact

Figure 3.21 : P-I-N Diode construction

Figwe3.22: FIN Diode equivalent circuit

+v

Figure 3.23 : Resistance variation with bias voltage

Itkngrorl

hfrJr'estr dnUS;:g fi:roc. mpnaauemry

I

rcl

Idee!in the OFFON state rSimilar!1', IBecause ddiode can Icannot md

Forrrbehave as oshown in d

APPLICATION OF PIN DIODE AS SINGLE-POLE SWITCHA PIN diode can be used in either a series or.a shunt configuration to form a single-pole,single-throw RF switch. These circuits are shown in figure z i+ioand (b) with bias networks.

Metallic contact

rves and Radar Microwave Diodes - Transferred Electron Devices

rverterrt and

D''er of p-typem of intrinsicas shown in

E

I

L siliconf !,i{oped)

I

nr(rrrgti6n

l(D MHz. AtIl equivalentfigure (3.23).I capacitancenpresent the[as is varied:6 KA underlThus, if theie when it isdiode is nowr is reflected

b voltage

rsingle-pole,ls networks.

In the series configuration of figure 3.24(a),the switch is ON when the diode is forwardbiased and OFF when it is reverse biased. But, in shunt configuration of figure 3.24(b),forwardbiasing the diode"cuts-off'thetransmission and reverse biasing the diode ensures transmissionfrom input to output. The DC blocks should have a very low impedance at RF operatingfrequency and RF choke inductors should have very high RF impedance.

BIAS

Figure 3.24: Single-pole PIN diode switches(a) Series conliguration (b) Shunt configuration.

Ideally, a switch should have zero insertion loss in the ON state and infinite attenuationin the OFF state. Realistic switching elements, of course, result in some insertion loss for theON state and finite attenuation for the OFF state due to non-zero forward bias resistance.Similarly, for reverse bias shunt capacitor is not infinite & non-zero insertion loss results.Because of the large breakdown voltage (= 500 volts) compared to an ordinary diode, pINdiode can be biased at high negative region so that large a.c. signal, superimposed on d.c.cannot make the device forward biased.

Forward Bias: When the PIN diode is forward biased, the capacitors C-and C, almostbehave as open circuits so that the equivalent circuit of figure (3.22) can now de simplified asshown in the figure (3.25) where Rris the total forward resistance of the PIN diode given by

(a)

BIAS

Figure 3.25 : Simplilied equivalent circuit for forr+ard biased PIN diode.

Rr = Rr+R.'. The diode impedance Zoof the PIN diode is given by

Zd - Zr= Rr+jrol,o \ ....;. (3.24)

Reverse bias: When the PIN diode is reverse biased, the capacitance of the intrinsiclayer C, becomes significant and R. will be the equivalent reverse resistance and the siniiplifiedequivalent circuit for reverse biased PIN diode can be constructed as shown in nguret[1.26;.The diode impedance zoof the PIN diode under reverse bias, is then given by .,

Figure 3.26 : Simplified equivalent circuit for reverse biased pIN diode.

...... (3.2s)zolz,=.R,* j [ r t r -#)

t . % = incident power applied to load directly when switch is absent.

lt:.". Po a (Vo)2 and P, o .(Vr)' where Vo is the load voltage without switch and V,-

actual load voltage when switch is present. With this, equation (3.26) becomes

(v \'' .IL = l0log*l #' I- - \ vt-

' /

Insertion Loss: The insertiaon loss is given by .-'.

Insertion loss indB = fL = rO ros.^[L)- ' ' [Pt J

where Pr_ = actual power delivered to the load.

IL=

...... (3.26)

...... (3.27)

. . . . . . (3.28)

Indiodc"Aswz-

o,Ot*r( ; )

Refeming to the shunt configuration of figure 3.24 (b),let the line be terminated is Zo sothat the impedance looking towards load at AA'is Zo.Theimpedance looking towards loadsidealong with the switch ar BB'ts then given by

,7 .7

-7-LdLoe7L7

; : : r ,Tddlr . fo . .

. \

nr/es and Radar Microwave Diodes - Transferred Electron Devices 237

...... (3.23)

. . . . . . (3.24)

I the intrinsiche sirnplifiedEgure'(3.26).

...... (3.2s)

. . . . . . (3.26)

is absent.ritch and V,

. . . . . . (3.27)

rted is Zo sords loadside

. . . . . . (3.28)

where

.'. The transmission coefficient T is given by

T = l*K= l*2"-Z'Z"+Zo

, - 22,-=

22'Z"+Zo

Using equation (3.28) in (3.30), we ger

/ \.'[ ZoZo )- l - l.F _ _(2. +2" ) 2zdz"

L = ___=r_=_

toto *Z 2ZoZ"+Zo2

Zd+Zo ' ""

or T= 2zo

2Zo +2"

But, from definition of transmission coefficient T, we have

T-vt- Zzu'-V"-2Zo+2,

Substituting equarion (3.32) in (3.27),we ger

n- = zot.g,,(+*)0"\ - -d . /

...... (3.2e)

...... (3.30)

. . . . . . (3.31)

..... (3.32)

. . . . . (3.33)

In equations (3.32) and (3.33), it is assumed that there is no loss in the line and in thediode' Equation (3.32) can be represented by an equivalent circuit as shown in figure (3.27).As seen from the figure (3.27),the diode impedance zoisinshunt with the load iripedance ofzo

t 'Y' Z"

Figure 3'27 : Equivalent circuit representing equation (3.32) for shunt configuration.

238 Microwaves and Radar

Erampie

- i ' i lun: , :n:

"!

Figure 3.28 : Equivalent circuit for series configuration.

Similarly, for the series configuration of figure 3.24 (a), an equivalent circuit can beconstructed as shown in figure 3.28 where the diode impedance Zo is in series with thecharacteristic impedanceZo..Carrying out a similar analysis for the series circuit, one canobtain the insertion loss as

IL= . . . . . (3.34)

PIN DIODE AS SPDT SWITCH

Single-pole double throw (SPDT) action can be obtained by using a pair of PIN diodeseither in series configuration or in shunt configuration as shown in figure 3.29(a) and (b)respectively

In the series configuration of figure 3.29(a), when D, is forward biased and D, reversebiased, connection is established between RF input and output I and no output at OUTPUT II.When the biasing condition is reversed (D, reverse biased and D, forward biased), connectionis established between RF input and output II.

Pin diodes

Figure 3.29 : Circuits for SPDT PlN-diode Switches(a) Series configuration (b) Shunt conliguration

In the shunt configuration of figure 3.29(b), when D, is forward biased, it becomes shortcircuited throwing an open circuit at RF input line junction due to (1./4) section. Do is reversebiased so that it becomes open circuit (high impedance state) and connection is establishedbetween RF input and output II. When D, is reverse biased and Do forward biased, connectionis established between RF input and output I.

zots,,(4*)dB

[ .

,t" - ,,-,

: . . . j

DiL

- i - - : i

: .-.1

: i:l'l

<1 rl

RF

res and Radar

ircuit can bedes with theruit, one can

.... . (3.34)

f PIN diodesD(a) and (b)

d D, reverseOUTPUT II.f6connection

-omes short

D. is reverser establishedlconnection

Microwave Diodes - Translerred Electron Devices

Example 3.1 : The forward resistance R, of a shunt mounted PIN diode is 0.12 e and thecapacitance of the intrinsic layer is 0.025 pF. The shunt mounted PIN diode switch is connectedto a transmission line of characteristic impedanceZo=50 O. At a frequency of 2.5 GHz,determine the insertion loss under reverse bias and isolation under forward bias conditions.Solution : Given Rr = 0.12 O, Cj = 0.025 pF, Zo= 50 gt.

From equation (3.33), the insertion ross under reverse bias is given by

[ l= 2o,og,ol+*)*1

<oC; undet reverse bias

I- 2nf C,

(zn) (z.s x loe) (o.ozs x 1o-r2)= 2.546 KQ

Insertion loss = 20loe,^ [z x z's+o x ro31so-.|0"

- ' " L 2x2.546x10' I

When the PIN diode is forward biased, then Zo = R, = 0.12 C) and the same equation(3.33) is used which gives isolation in dB given by

Isotation = 20 toe.^ lz x o-tz + sol

" 'u L 2x0.tZ lIsolation = 46.42 dB

3.14 SCHOTTKY BARRIER DIODESchottky barrierdiode is a sophisticated version of the point-contact silicon crystal diode,

wherein the metal-semiconductor junction so formed is a surface rather than a point contact.The advantage of schottky diode over point contact crystal diode is the elimination of minoritycarrier flow in the reverse-biased condition of the diode. Due to this elimination of holes,there is no delay due to hole-electron recombination (which is present in junction diodes) andhence the operation is faster. Because of larger contact area of rectifying contact (refer figure3.30 (a)), compared to crystal diode, the forward resistance is lower as also noise. Noisefigures as low as 3dB have been obtained with these diodes. Just like crystal diodes, theschottky diodes are also used in detection and mixing

where Zo=

240

Gold orAluminium

Rectifyingsurface contact Gold or

Aluminium/

13. widl-1. Exg15. Exd16" \lid

3.16 Pt1" In al

cririinuir

t_ -{G10P tcriti

3. -{R Isiliqcerri

4" ..\x n

fi!icror|art

DefiE!

Microwaves and Radar

(a) ft)

Figure 3.30 : (a) Schottky barrier diode (b) Its equivarent circuitThe construction of schottky diode is illustrated in figure 3.30(a). The diode consists ofn* silicon substrate uponrvhich a thin layer of silicon of 2lo3 micron thickness is epitaxiallygrown' Then a thin insulating layer of silicon dioxide is grown thermally. After opening awindow through masking process' a metal-semiconductoijunction is formed by depositing

metal over SiOr. . ' 'v ' ' rvv vJ s

Figure 3'30 (b) shows the equivalent circuit of the schottky diode which is almost identicalwith that of crystal diode. s.'vol

3.1s QUESTTONS1' what is "GUNN EFFECT"? with a neat diagram explain the constructrual details of aGUNN diode.2. Explain the different modes of operation of Gunn diodes.3. Give a brief account of RWH theory.4. with a neat diagram exprain the operation of Gunn diode oscilrator.5' what are avalanche transit-time devices? How are they different from transferred electrondevices?

6. with neat diagrams explain the construction and operation of READ diode.7. with neat diagrams explain the construction and operation of IMpATT diode.8' with neat diagrams explain the construction and operation of TRApATT diode.9. with neat diagrams explain the construction and operation of BARITT diode.

10' with neat diagrams explain the construction and operation of varactor diode.11. Explain the operation of a basic parametric amplifier with square wave pumping.12' what are MANLEY-ROWE relations? How are they useful in understanding parametric

amplifiers?

*{. Thc Itud::"mfl!iefreil

6 Afu"nFFl

rrld--:nlllClfl

l *a,es and Radar Microwave Diodes - Transferred Electron Devices

13. with neat diagrarns explain the construction and operation of pIN diode.14. Explain rhe operation of single-pole switch using pIN diode.15. Explain the operation of a SPDT switch using pIN diode.16' with neat diagrams explain the construction and operation of Schottky barrier diode.

3.16 PROBLEMS

1' In a Gunn diode oscillator, the electron drift velocity u,as fbund to be 10s cm/sec. and thecritical field for GaAs was 3 KV/cm. If the effective length / is 10 pm, determine theintrinsic frequency fo and the critical voltage that can be applied to the diode.2' A GaAs Gunn diode oscillator operates at 8 GHz with drift velocity of electrgns being106 cm/sec' Determine the effective length of the active region. Also find the requirecl

critical voltage for oscilrations if the criticar field is 3 KV/cm.3' An IMPATT diode has a drift length of 2.5 pm. If the basic semi-conductor material js

silicon for which the clrift velocity is 5 x 106 cm./sec, determine (a) the drift time ol tirecarriers (b) the nominar frequency of the IMPATT diode.4. An IMPATT diode has the following parameters

Diode junction resistance = R = _2 f,)Junction capacitance = C: = 0.2 pFBreakdown voltage = Vuo = g0 volts

Package lead inductance = Lo = 0.55 nHPackage capacitance = Co = 0.3 pF

Bias current = 80 m AmpsDetermine (a) the exact resonant frequency

(b) the average output power(c) the average input power(d) the efficiency

5' The forward resistance \ of a shunt mounted PIN diode is 0. i5 a and the capacitance ofthe intrinsic layer is 0.03 pF. The shunt mounted PIIrI diode switch is connected to atransmission line of characteristic imped ance Zo = 55 e. At a frequency of 3 GHz,determine the insertion loss underreverse bias and isolation underforward bias conditions.

6' A shunt mounted PIN diode switch is connected to a transmission line of characteristicimpedance zn= 60,Q. At a frequency of 2 GHz, the insertion loss and isolation werefound to be 0'12 dB and 48 dB resiectively. Find the forward resistance R, and thecapacitance C.

z1 |

4

:c: ,-onsists ofs rs epitaxially:::i opening ai'" depositing

::,i st identical

.1 details of a

ered electron

-- . : -; -

- l

-

. i -

l -q_l- .

ri:rping.

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