Microwave Sources Gunn Diode

8/11/2019 Microwave Sources Gunn Diode

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Microwave SourcesSemiconductor Sources

Reference:

Samuel Y. Liao

S. M. Sze

Prof. D. Kannadassan,

School of Electronics Engineering

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Semiconductor Microwave SourcesLow Power microwave generators

Transferred Electron Devices (TEDs) Gunn diode

LSA diode

InP diode

CdTe diode

Avalanche Transit Time Devices (ATTDs) Read diode

IMPATT diode

TRAPATT diode

BARITT diode

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Mini-review on semiconductor Physics

Atomic Structure

Lattice structure: Inter atomic distance

Energy band Theory

a

Indirect bandgap

Direct bandgap

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Carrier Transport

Mobility

Mini-review on semiconductor Physics

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Transferred Electron Devices (TEDs)

At some situations, the electrons (carriers) shall transfer from one region (x-y-z) or

momentum (k) space to another. This effect is referred as Transferredelectron effect.Transferred electron effect is generally happening when the electron is

moving/transferring between various levels of conduction and valance bands. Its

happening in most of the Direct bandgap devices. The most popular TED is Gunn

diode.

If the transfer is happening in region, it is specially called realspace transfer electron

effect. With this effect, most important device is Realspace transfer diode

Momentum 5


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Gunn Diode

Gunn diode, is the first kind of Transferred Electron Device, is the most

important microwave source: useful in local oscillator. It can able to generate

Microwaves from 1GHz to 300GHz (0.3THz).

In 1963, J. B. Gunn of IBM discovered a periodic fluctuations in current for the

given DC voltage across a n-type GaAs specimen. The periodic oscillation are in

microwave frequencies and its inversely proportional to length of the specimen.

This is called GunnEffect. It also happens in many direct bandgap semiconductormaterials like InP, GaAsP, InGaAs, GaN, InGaN, InAs..

cathode

AnodeGaAs

Bulk

5ns

0.22ns~4GHz

Source: J. B. Gunn, IBM, 1963 6


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Gunn DiodeDC characteristics

When we sweep the voltage across the n-

type GaAs specimen (of length L),

The current increases initially, it reaches

maximum at voltage VP (Peak Potential).

Immediately, it starts to decrease till voltage VV

(valley voltage). It increases after that.

The region where current decrease for

increase in voltage (dV/dI = -ve) is called

Negative differential resistance (NDR)

region.

This NDR region is responsible for

microwave oscillations. The explanation for

the effect is called Transferred Electron

Effect.

Electric field [KV/cm]

Driftvelocity

NDR

3KV/cm

[In Our Lab]

NDR

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Ridley WatkinsHilsum (RWH) theory

OR

Two Valley Theory

(of Transferred Electron Effect) GaAs has two valleys at itsconduction band, called Lower

valley and Upper velley.

The lower valley is narrow and slope is higher than

upper valley

Due to this , at lower valley, since

denominator is higher so effective mass of electron is low. So

itsmobility or drift velocity is very high. But at upper valley,

mobility is very low since slope is very low, since:

2

2

dk

Ed

2

2*

dk

Edm

*0 me

n

Valley Effective mass (m*) Mobility (cm2/V.s)

Lower 0.068 8000

Upper 1.2 180 8


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DC characteristics and Two valley theory

Till Vp, the electrons at valence band moves to lower valley of conductions band,

thus it has high mobility. After Vp, the electrons at lower valley, the electrons acquire more potential energy

and move to upper valley where the mobility is low, thus current decrease.

After VV, the current increases, as electrons return to lower valley.

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Use of NDR region for microwave oscillation

Consider: A gunn diode is biased in NDR region, usually in voltage average

between peak velleypoints. So equivalently, its a resistor with negative sign.

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-R ZL

LIN

2

VP

DC

VVV

Always;1

IN

L

LIN

ZR

ZR

For any values of R and ZL

Gunn mount


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High Filed Domain Theory

The oscillations in TED, i.e. Gunn diode,

also explained by high field domain

theory.

When the diode is biased in NRD region,

respective electric fieldE0> ET

At time T=0, at low fields, the

accumulation of charge (electrons) is

encouraged near cathode. Thisaccumulated charges attracts and forms

another charge cloud (positively charged),

called Dipole

As time increases, the accumulation of

electron increase and forms high amplitude

of dipole. This forms a high field and itdrifts. At a time, it reaches anode and

drains the dipole. This dipole forms again

and this effect lead to oscillations

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Dipole

0

1

fv

LT

d

C A


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0

1

fv

LT

d

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Mode of Operation

It has been observed that the Gunn diode can oscillate without any

resonance circuit, so called Self Oscillating Diode. However, usingresonance circuit the tunnability and high power delivery is possible.

Based on the connection mode, the operations of Gunn diode changes,

such operations are classified as Mode

Modes of operation:

Gunn Oscillation Modes

Transit time domain mode

Delayed domain mode

Quenched domain mode

Limited Space charge Accumulation (LSA) Mode

Stable Amplification mode and Bias-circuit oscillation mode

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Mode, frequency and doping-length product

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Source: Samuel Y. Liao


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Gunn Oscillation Mode It is a self-Oscillation mode, where the doping concentration (n0) and active region

length (L) of device decide the oscillation frequency. The high field domain and

dipole space charge accumulation, and transition of both, forms the oscillation. Thismode occurs whenLn0>10

12.

Transit time domain mode: During dipole movement, the high field will move

entire length of device and produces the oscillation, therefore the transit time

decides the time period of oscillation, so called Transit time domain mode.

However, maximum of 10% efficiency with sine-wave form output, since the

domain formation and transition take maximum energy. Therefore with and withoutresonators, the Gunn diode oscillates

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Quenched domain mode: When the

resonant circuits operating frequency is

higher than transit time frequency of Gunn

diode (fR > f0), the high field is quenched

before it reaches anode, almost at middle

of diode. In other words, the oscillation

begins without the domain reaches the

anode, and process repeats again within the

transit time. Thus efficiency increases upto

13 to 15%.

Delayed domain mode: By optimizing the

device length and doping, transit time can

be adjusted so that the field will be lesser

than threshold. So the next oscillation isdelayed, and results almost square

waveform with high efficiency of 20%.

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M. Shaw et al, 1979

-R ZLf02

VPDC

VVV

fR


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Limited Space charge accumulation mode

This mode can be enabled by taking n0L < 1013(cm-2).

Thus the accumulation of electron near cathode (without its respective positive

region, or no dipole formation) makes the field as like a step function. During

oscillation, the domain size increases against high field region and move towards

anode. This process repeats many times within transit time of device, thus

frequency of operation and efficiency increases (more than 20%)

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Stable Amplification mode

The negative resistance region of DC characteristics has been utilized in this mode.

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-R ZL

LIN

2

VPDC

VVV

Always;1

IN

L

LIN

ZRZR


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Mode of operation

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Microwave Sources Gunn Diode

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