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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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