Transport5 - High Electric Fields - NDR & Gunn Effect

8/9/2019 Transport5 - High Electric Fields - NDR & Gunn Effect

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NDR & The Gunn Effect



• For direct bandgap materials, like GaAs: vd

vs. E peaks before saturation & decreases

again, after which it finally saturates.• Because of this peak, there are regions in the

vd vs. E relationship that have:

(dvd/dE) < 0(for high enough E

!his effect is called

“Negative Differentia Resistance!or “Negative Differentia "#$iit%!

or “Negative Differentia #nductivit%!



GaAs 'andstructure



"

• #s the field increases, the

electron drift velocity ingallium arsenide reaches a

peak and then decreases.

• #s the E$field increases, the

energy of the electron

increases & the electron can be

scattered int# the uer

vae%, where the density ofstates effective mass is 0. *0



Transferred Eectr#n Devices (Gunn Di#de)

E(GaAs) + 0.,- e

Effective Mass increases

upon transfer under bias



Negative Differentia Resistance (NDR)



Gunn Di#de

%an operate around ' point to get an oscillator



Gunn Effect)B (*an +unn discovered the +unn$effect in February

-/. 0e observed random noise$like oscillations when

biasing n$type +a#s samples above a certain threshold.0e also found that the resistance of the samples dropped at

even higher biasing conditions, indicating a region of negative

differential resistance.



1aterials which have the +unn 2ffect,

such as +a#s, *n3, +a, must be direct

bandgap materials that have more thanone valley in the conduction band & the

effective mass & the density of states in

the upper valley(s must be higher than

in the main valley.ε ε ε

k k k

εth 4 ε 4 ε sat

ε 5 ε sat ε 4 ε

th

Gunn Effect





Gunn Effect



• *t is important to note that the sample had to be biased in the

' region to produce a +unn$domain. 6nce a domain has

formed, the electric field in the rest of the sample falls below

the ' region & will therefore inhibit the formation of asecond +unn$domain. #s soon as the domain is absorbed by

the anode contact region, the average electric field in the

sample rises & domain formation can again take place.

• !he successive formation & drift of +unn$domains throughthe sample leads to ac current oscillations observed at the

contacts. *n this mode of operation, called the +unn$mode,

the fre7uency of the oscillations is dictated primarily by the

distance the domains have to travel before being annihilatedat the anode. !his is roughly the length of the active region

of the sample, 8. !he value of the dc bias will also affect the

drift velocity of the domain, & conse7uently the fre7uency.



+unn diodes are semiconductor

diodes that form a cheap andeasy method of producing

relatively low power radio

signals at microwave

fre7uencies. +unn diodes are aform of semiconductor

component able to operate at

fre7uencies from a few +igahert9

up to fre7uencies in the !09region. #s such they are used in a

wide variety of units re7uiring

low power F signals.

Gunn Di#des



tead%tate Resuts

0 1 2 3 4 5 6 7

x 105

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2x 10

5

Electric Field [V/m]

D r i f t V e l o c i t y [ m / s ]

0 1 2 3 4 5 6 7

x 105

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Electric Field [V/m]

C o n d u c t i o n B a

n d V a l l e y O c c u p a n c y

gamma valley occupancy

L valley occupancy

X valley occupancy

Gunn Effect

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Transport5 - High Electric Fields - NDR & Gunn Effect