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racte ''
Cha ristiss'
andApplications
of DiodeDetectors
Ron Pran
Rf & Mlcrowave
fYleasurement
Symposlum
ancl
Exhlbltlon
ftE FF&TJJ
Applications Detectors
of
AbsolutePowerMeasurements
Relative
PowerMeasurements
Leveling
Loops
Systems
Monitoring
Pulsed Measurements
RF
The diode detectoris a very common elementin microwave
measurement setupsand findswide
application systems.
in semiconduoordiodesare usedfor measuring absotutepower. Networkcharac-
terizationoften employsdiodesto measure relativepower,aiJteuet;ng loops,system
pulsedRFmeasurements other applications monitors,and
are which rreq*nily employdiode detecrors.
The system performance one obtainsis a funoion or trrl aeteaor d;ode,its associared
circuit'the circuitrywhich processes dereoed signal, RFmatching
the and the environmental conditionsover which
the system must operate.Tireparameters which .;;rJ i;;;"rtor,,.,.n.u of a deteoor are presentin atl
isimportant
to understand rtey inrliaa sothartr'eaes;lalfrt". p"rrormance
how
fr?[tj||lT:j."
Diodes microwave
for detectotr
semiconductor
interface "rnffi
Theyare majorig carrierdeviceswhich minimize
storedcharge
Pointcontactand schottky(hot carrier)
diodesare
the mostcommon typesof detectori
r
E-
\
PSi
Pointcontact
MFTAL
'7g High Barrier
Schottky
N EPI
N+si
METAL
P IPI
A@ LowBarrier
Schonky
P+Si
conventional PN junaion diodes exhibit storedchargeeffeos which limit
their operaringfrequency.
Currentflow in a metal-semiconducror junoion is due irimarily to maiorirycarriers, the
so effeo of
chargestorage minimized.Thismakei devices
is suchasthe poinr contao diode o, the Schonky
junction performasefficientreoifiersat microwave
frequencies.
For manyyears, only way to fabricate requiredmetalsemiconductor
the the interface wasto employa
point contactsystem. Thistype of constructionyieldeddiodeswith a wide variation unit to unir
in
performance and the delicateassembly subjea to damage
is from excessive power, mechanicalshock,
or temperaturecycling. -ut more rugged strucrureevolvedby depositingthe 6ariier meral
on an epitaxially I dlreJv
grown layer'ordinary schottkydiodeshavea hijh potenri.iu.rii.i.na
r.quir. ui"i----
currentto achieve sensitivities
eguivalent point contactdiodes.
to Ttremost recentdevelopmeniii "
detectordiodesis the formationof a Schonky barrieron P type siliconwhich hasa barrierheighrabout
% that of ordinary schonky diodes.The low barrier height resultsin delecrors
which are electrically
similarto point contact devicesbur much more ruggedl
Allthree typesof diodes are found in current xF-produas. The K and R 4224
derectorsuse poinr
contactdiodes,and achieve sensitivity 40 GHz tuhichis quite similarto lower
a at tr.lr"n.y point
contactdetectors.The 11664A Amplitude Analyzerdetectorsuse biasedSchonkydejces ln
conjunclion with an AM modulated sourceto achievea -50 dBm sensitivity,
and the g4s4Apower
sensoremploysa low barrier Schottkydetector and providesCW power measurement
-70 dBm. ..p.'bility to
Diode Equation
-)
lr
le- vi J
| = ts[exp(Vilvr)
-U
I = diode current
Vj : junctionvoftage
Vt: "thermal voltage,,
Vt : nKT/q ls: reverse
saturationcurrent
K = Boltzmann constant Determined by,
T = Absolute temperature
g = Electron Junctionarea
charge materials
n = idealityfactor
temperature
( 1< n < 2 )
ls changesby 2:1 for temperaturechangeof about
20 degreesc
At low signallevels,
all.three typesof diodes.closely obey the equationfor the idealdiode, and rhis
will be the startingpoint for rne Jiscuision deteoor aoion.
of ihe equationrelares currenrthrough
rhe
the diode to the vohagea.ppearing across junoion. ri" fr,.r.oerisricsof the diode
the
the so-called "thermal voliage",vi."Jii" are reffeoed in
reverse sarurationcurrenrrs.At a giventemperature wirl
be differentfor varioustypei of diodes, Vt
and this.is
,"trua"Juv t-he uarueof the idealityfactorn which has
a valuein the rangeof 1 io 2. The predominanr
faoor which ierer,',,ines;";;;;;;istics
is the valueof the reverse saturation of the diode
currentls.Thiscurrent is a function of devicearea,
form the iunoion' and temper.rrt". materialsusedto
iiu i",np.r.,rr" dependence very importantbecause
is
p"'io'',n." mostof
..i u" related vaiiationsrs,
to in which
.l,r"i", Lyapp,ori,,.'arery
;liJI:?i"#:rTH a
Effect Reverse
of Saturation
Current
I in microamps V in millivohs
A-low barrierSchortky
ls = 25 microamps
B-Point contao diode
ls = 5 microamps
C-High barrierSchottky
ls = 2.5nanoamp
D-High barrierSchonky
wirh 25 microampbias
By examining the lV curvesfor the varioustypesof detectordiodes,rhe influenceof the reverse
saturation currenl can be seen.curve A is typiialof a low barrierschonky
is contrasted with is = x microamps. This
against point contactdevice(curveB) whosels = 5 microamps.
a
high.barrier curuet i, to..a rypical
schottkyhavingan ls = 2.5nanoamps. diodesdescribed
The by curves and B show
A
significant current flow.atviry fow junctionvoltages and this is the type of tv charaaeristic
desirable a low levefdeleoor. Similar
for which is
behavioican be obtainedwitn the hif;;;;", schonkyby
applyinga biascurrentto.shiftthe operating point to a regionsimilarro that .f?; 1o."barrier
contact device' The penaltyfor doing this ,itie dc offserit or poinr
i.n is produced. g;asedderedors are either
usedfor detectingsignallevels aboui -s d?-- in dc coupledsystems are usedin ac
or coupled
systems which eliminares effea of rhe offset.
the
Squarelaw Detection
Simplified
Analysis
Vp COS(coT)
-1J
| = ls[exp(vjNtl = ls[VjA/t t/2(yjNt)2 . . . . 1
+ +
Assume: = Vp cos(
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