662 avalanche
TRANSCRIPT
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ECE 662
Microwave Devices
Transit-Time Diodes
February 17, 2005
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Two-Terminal Negative
Resistance Devices
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Avalanche Transit-Time Devices
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Avalanche Transit-Time Devices
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Avalanche Transit-Time Devices
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Measured Ionization
rates for electrons andholes vs reciprocal field
for Si and GaAs
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Ref: Sze
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Diode Configurations
B
mm
m
qN
EWE
W
xExW
np
nip
22
1V
ionconcentratbulkdopedlightlytheisNwhere
)1()(qN
E(x)
diode,junctionAbrupt
width)(depletionWEVisvoltagebreakdown
andconstantisEfield,Diode,
2
sB
B
s
B
mB
m
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IMPATT Mode Diodes
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IMPATT Mode Diodes
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Injected carriers therefore traverse the length wD of the drift region
During the negative half-cycle if we choose the transit time to be
oscillation period.
)()sin()(21Pand
2I
2IIII
conservedisCharge.vcircuitexternalinCurrent
)2/(for)/1(5.0/
2
0
rfdc
maxinjdcinjmax
s
tdtVtIIVP
wvfvw
rfindBdc
dW
d
W
DsDd
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;1cos2/
)2/sin(
s,IMPATT'
)cos(cos
2/
)2/sin(
;2/
)2/sin()cos(cos
d
m
m
dc
dW
W
B
rf
m
d
dm
W
W
B
rf
dc
rf
W
W
d
Dm
rfrf
V
V
for
V
V
P
P
VIP
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W
v
W
vf
v
Wnow
VVMax
V
V
ss
s
B
rf
dB
rf
W
W
W
2
74.0
2
30%to20typically,
,72.0so74.0whenoccurs
1cos
and,1
2/
)2/sin(so
smallis,efficiencybestfor
dd
dmax
d
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IMPATT Mode Diodes
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Double-Drift Region IMPATTs
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TUNNETT Mode
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BARrier Injection Transit Time Devices
(BARITTs)
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BARrier Injection Transit Time Devices
(BARITTs)
The injected carrier density increases with the ac voltage.
Then the carriers will traverse the drift region.
The injected hole pulse at 90o
and the corresponding
induced current which travels 3/4s of a cycle to reach thenegative terminal. Or w/vs = (1/f)
Note that for /2t, both the ac voltage and external
current are positive therefore ac power is dissipated in the
device.Consequently, the BARITT diodes have low power
capabilities and low efficiencies but they also have low
noise (avoiding the avalanche phenomena).
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TRApped Plasma Avalanche Triggered
Transit Time Devices (TRAPATTs)
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TRApped Plasma Avalanche Triggered
Transit Time Devices (TRAPATTs)
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Comparison of Microwave
Devices An important figure of merit for microwave
devices is power output as a function of oscillation
frequency.
Due to limitations of semiconductor materials, themaximum power of a single device at a given
frequency is limited.
Two basic limitations: Critical field, at which avalanche breakdown occurs
Saturation velocity which is the maximum attainable
velocity in semiconductors
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Power Output -1
The maximum voltage that can be appliedacross a semiconductor sample is limited by
the break down voltage.
For a uniform avalanche this is Vm = EcWwhere W is the depletion layer width
The maximum current that can be carried by
the semiconductor is also limited by the
avalanche breakdown process, because the
current in the space charge region causes an
in crease in the electric field.
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Power Output -2
W/AEIbetoallowedcurrentmaximumthefind
EE(W)setting).IW/(A)dx/(W)E(x
ischargespacethetoduefield
electricin theE(x)edisturbancThearea.theisA
densitychargespacetheiswhereA,Ithen
:regiondepletiontheacross,velocity,saturationat theirtravelelectronsthat theAssume
sscm
css
W
0ss
ssspchrg
s
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Power Output -3
1
sc
c
2s2cmm
2
m
s
ss
2
cmmm
)/f(2reactancedevicetheisX
where,X2
EIVfP
asRewritedomain.imetransit tunder the
operatedTEDfor the1andBARITTfor3/4and
IMPATTfor1/2iswhereW,/ffrequency,
imetransit ttheandAEIVP:isinputpoweron thelimituppertheTherefore
WA
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Noise - Microwave Devices
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Bip
olar
MES
FET
TUN
NEL
TED
InP
T
ED
G
aAs
BAR
ITT
IMP
ATT
Devices
Noise
Figure
in dB
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Solid-StateDevice
Power
Output vs
Frequency
ref: Sze
and
modifiedby Tian
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