15EC407

RF and Microwave Engineering

MICROWAVE TUBES
 OUTLINE
1. Introduction of Microwaves
2. High frequency limitation of
conventional tubes
3. Types of Microwave tubes
4. Reflex klystron-Mechanism of operation
5. Mode of oscillation
6. Power output and Efficiency

2
 Electromagnetic Spectrum

3
 Microwaves
 Microwaves are the electromagnetic waves
 wavelengths ranging from few cm to mm
 frequencies ranging from 1GHz to 1000 GHz (1 GHz = 10 9 Hz)
 Advantage
 Power requirement is very less compared to LF signals
 Larger Bandwidth : The band width of microwaves is larger than the low
frequency signals - more information can be transmitted using single
carrier
 For example, the microwaves extending from  = 1 cm -  = 10 cm
(i.e) from 30,000 MHz – 3000 MHz, this region has a bandwidth of
27,000 MHz.

 Improved directive properties: ability to use high gain directive antennas,

any EM wave can be focused in a specified direction (Just as the focusing of
light rays with lenses or reflectors)
 Less Fading effect and more reliable

4
 Applications Of Microwaves
 Microwaves have a wide range of applications in
modern technology, which are listed below

1. Telecommunication: Intercontinental Telephone

and TV, space communication (Earth – to – space
and space – to – Earth), telemetry communication
link for railways etc.
2. Radars: detect aircraft, track / guide supersonic
missiles, observe and track weather patterns, air
traffic control (ATC), burglar alarms, garage door
openers, police speed detectors etc.

5
 3.Commercial and industrial applications
 Microwave oven
 Drying machines – textile, food and paper industry for
drying clothes, potato chips, printed matters etc.
 Food process industry – Precooling / cooking,
pasteurization / sterility, hat frozen / refrigerated precooled
meats, roasting of food grains / beans.
 Rubber industry / plastics / chemical / forest product
industries
 Mining / public works, breaking rocks, tunnel boring,
drying / breaking up concrete, breaking up coal seams,
curing of cement.
 Drying inks / drying textiles, drying / sterilizing grains,
drying / sterilizing pharmaceuticals, leather, tobacco,
power transmission.
 Biomedical Applications ( diagnostic / therapeutic ) –
diathermy for localized superficial heating, deep
electromagnetic heating for treatment of cancer,
hyperthermia ( local, regional or whole body for cancer
therapy).

6
 Other Applications
4. Identifying objects or personnel by non – contact
method.

5. Light generated charge carriers in a microwave

semiconductor make it possible to create a whole
new world of microwave devices, fast jitter free
switches, phase shifters, HF generators, etc.

7
 High Frequency Limitations of
Conventional Tubes

8
 Contd..

 Conventional tubes fails to operate above 1 GHz.Reasons:

 Inter electrode capacitance:
 Vacuum has a dielectric constant of 1. As the elements of the triodes are made of metal and
are separated by a dielectric, capacitance exists between them. This capacitance is
interelectrode capacitance.
 The capacitance between the plate and grid is Cpg.
 The grid-to-cathode capacitance is Cgk.
 The total capacitance across the tube is Cpk.
 Now, we know that the capacitive reactance is given by
 X=1/2Лfc
 So as the input frequency increases, the effective grid to cathode impedance decreases due to decrease
in reactance of interelectrode capacitance. At higher frequencies (greater than 100MHz) it becomes so
small that signal is short circuited with the tube. Also, gain of the device reduces significantly.

9
 Lead inductance effect:
 Lead or stray inductance are effectively in parallel within the device with the inter
electrode capacitance. Inductive reactance is given by:
 XL=2 π f L
 As the frequency increases, the effective reactance of the circuit also increases.
 This effect raise the frequency limit to the device.
 The inductance of cathode lead is common to both grid and plate circuits. This provides a
path for degenerative feedback which reduces the overall efficiency of the circuit.

10
  Transit time
 Transit time is the time required for electrons to travel from the cathode to the plate. At
low frequency, the transit time is very negligible. But, however at higher frequencies,
transit time becomes an appreciable portion of a signal cycle which results in decrease in
efficiency of device.
 4) Gain bandwidth product
 Gain bandwidth product is independent of frequency. So for a given tube higher gain can
be only obtained at the expense of narrower bandwidth.
 5) Skin effect
 This effect is introduced at higher frequencies. Due to it, the current flows from the
small sectional area to the surface of the device. Also at higher frequencies, resistance of
conductor increases due to which the there are losses.
 R=ρl( √f)
 6) Dielectric loss
 Dielectric material is generally different silicon plastic encapsulation materials used in
microwave devices. At higher frequencies the losses due to these materials are also
prominent.

11
 Solution to this problem??

is
Microwave tubes

12
 Microwave Tubes
Klystron
Traveling Wave Tube
Magnetron

13
 REFLEX KLYSTRON

14
 Reflex Klystron oscillator

15
 Mechanism of operation

 A reflex klystron consists of an electron gun, an accelerating

grid, a single re-entrant cavity and a repeller plate
 Electrons are emitted from cathode’K’ is accelerated by the
grid ‘G’ and passes through the cavity anode A to the repeller
space
 Due to DC energy, RF noise is generated in the cavity
 Electrons passing through cavity gap experiences
Velocity Modulation

16
 Mechanism of operation
 The electrons
 ‘a’ which encountered the positive half cycle of the RF field in
the cavity gap d will be accelerated,
‘b’ which encountered zero RF field will pass with unchanged
original velocity, and
‘c’ which encountered the negative half cycle will be
decelerated on entering the repeller space.
 All these velocity modulated electrons will be repelled back to
the cavity by the repeller due to the negative potential.

17
 Mechanism of operation
 The repeller distance L and the repeller voltage can be
adjusted to receive all the electrons at a same time on the
positive peak of the cavity RF cycle.
 Thus the velocity modulated electrons are bunched together
and lose their kinetic energy when they encounter
the positive peak of the cavity RF field.
 This loss of energy is transferred to the cavity to conserve
total power.
 When power delivered by the electrons is equal to the power
loss in the cavity- Microwave oscillation is started

18
 Mode of Oscillation
 These bunched electrons deliver maximum power at any instant of
positive peak of RF cycle
 If T is the time period at the resonant frequency, to is the
time taken by the reference electron to travel in the repeller space
between entering the repeller space and returning to the cavity at
positive peak voltage on formation of the bunch
Then, to = (n + ¾)T = NT
Where N = n + ¾, n = 0,1,2,3…….
N – mode of oscillation
The Power output of lowest mode?
is Maximum

19
 Analysis of Reflex Klystron:

 Velocity Modulation
 Transit time
 Density Modulation and beam current
 Power output
 Efficiency

20
 Velocity modulation

Basic assumptions:
 Cavity grid and repeller plate are parallel and large
 No RF field is excited in repeller space
 No electron interception by the cavity anode grid
 No debunching action in the cavity space
 V1<<V0

21
 Performance Characteristics
1. Frequency: 2– 200 GHz
2. Power: 10 mW – 2.5 W
3. Theoretical efficiency : 22.78 %
4. Practical efficiency : 10 % - 20 %
5. Tuning range : 5 GHz at 2 W – 30 GHz at 10
mW

22
 Applications

 The reflex klystrons are used in

1. Radar receivers
2. Local oscillator in microwave receivers
3. Portable microwave links
4. Pump oscillator in parametric amplifier

23
 Biological effects of microwaves
 A part of radiofrequency (RF) radiation, which covers 0.5
MHz to 300 GHz range produces adverse biological effects.

24
 Ionizing radiation and non-ionizing radiation
 Ionization is a process - electrons are stripped from atoms
and molecules and this can produce molecular changes that
can lead to damage in biological tissue, including effects on
DNA, the genetic material.

 The energy levels associated with RF and microwave

radiations are not great enough to cause the ionization of
atoms and molecules, therefore, it is a type of non-ionizing
radiation.

25
 Non ionizing radiation
 Microwave energy is non-ionizing electromagnetic radiation.
 Ionizing radiation messes up molecules, non-ionizing
radiation merely heats them.
 In general, it does not have sufficient energy to kick an
electron off an atom thus producing charged particle in a
body and cause biological damage.
 The only proven harmful effect from exposure to microwave
(or RF) radiation is thermal.
 RF radiation can enter deep into the body and heat human
organs.

26
 Effect of microwaves in human body
 The blood vessels are dilating and the blood flow increases
substantially as the thermoregulatory mechanism is
activated in order to keep the body temperature constant.

 With rising body temperature the metabolic rate rises,

which may lead to Stress-Adaptation-Fatigue Syndrome.

27
 Effects produced by the electromagnetic waves at
different frequency level
 Above 10 GHz (3 cm wavelength or less) heating occurs
mainly in the outer skin surface.
 From 3 GHz to 10 GHz (10 cm to 3 cm) the penetration is
deeper and heating higher
 From 150 MHz to about 1 GHz (200 cm to 25 cm
wavelength), penetration is even deeper and because of high
absorption, deep body heating can occur.
 Any part of the body that cannot dissipate heat efficiently or is
heat sensitive may be damaged by microwave radiation of
sufficient power.

28
 Effects of Microwave energy
Power
level Long-term effect Remarks
(mW /cm2) on human body
0.01 Nothing
0.1 Nothing
1 Nothing
5 Nothing Accepted standard for microwave
oven leakage
10 Nothing Accepted standard for maximum
continuous exposure to radiated
emissions (cell phones, etc.)
30 You can feel heat
100 Cataracts can be Summer sunlight is at this level
produced
1000 Pain is induced
29
 The GOOD NEWS is... with Microwave radiation
Boil water
Cook meat
Fry eggs

The BAD NEWS is...

with Microwave radiation

Your head and brain heat up significantly when you talk

on your cell phone or cordless phone.

30
 Statistics shows that you are now exposed to electromagnetic
radiation daily, 100 million times greater than your grandparents.
So….

AVOID FREQUENT USE OF

CELL PHONES!!!

31
 Multicavity Klystron

32
 Application
 As power output tubes
1. in UHF TV transmitters
2. in troposphere scatter transmitters
3. satellite communication ground station
4. radar transmitters
 As power oscillator (5 – 50 GHz), if used as a
klystron oscillator

33
 TRAVELING-WAVE TUBE (TWT)

34
Travelling Wave Tube
 Introduction:
 The traveling-wave tube (TWT) was invented in 1944 by
Kompfner.
 The Traveling-Wave Tube (TWT) is an amplifier of microwave
energy.
 It accomplishes this through the interaction of an electron beam
and an RF circuit known as a slow wave structure.
 TWT are commonly used as amplifiers in satellite transponders,
where the input signal is very weak and the output needs to be
high power.
 TWT transmitters are used extensively in radar systems,
particularly in airborne fire-control radar systems, and in
electronic warfare and self-protection systems.
Difference between TWT & Klystron:
 In the case of the TWT, the microwave circuit is non-resonant.
 The interaction of electron beam and RF field in the TWT is
continuous over the entire length of the circuit, but the interaction
in the klystron occurs only at the gaps of a few resonant cavities.
 The wave in the TWT is a propagating wave; the wave in the
klystron is not.
 In the coupled-cavity TWT there is a coupling effect between the
cavities, whereas each cavity in the klystron operates
independently.
Types of TWT:
 Helix Travelling wave Tube:
 How it works:

 A helix traveling-wave tube consists of an electron

beam and a slow-wave structure. The electron beam is
focused by a constant magnetic field along the
electron beam and the slow-wave structure.
 The commonly used slow-wave structure is a helical
coil with a concentric conducting cylinder
 The electron beam can be accelerated only to velocities
that are about a fraction of the velocity of light.
 A slow-wave structure must be incorporated in the
microwave devices so that the phase velocity of the
microwave signal can keep pace with that of the electron
beam for effective interactions.
 It can be shown that the ratio of the phase velocity vp
along the pitch to the light velocity along the coil is given
by:

 Where c = 3 x 108 m/s is the velocity of light in free

space
 p = helix pitch
 d = diameter of the helix
 ψ = pitch angle
 The TWT contains an electron gun which produces and
then accelerates an electron beam along the axis of the tube.
 The surrounding magnet provides a magnetic field along the axis
of the tube to focus the electrons into a tight beam.
 The helix, at the center of the tube, is a coiled wire that provides a
low-impedance transmission line for the RF energy within the tube.
 The RF input and output are coupled onto and removed from the
helix by waveguide directional couplers that have no physical
connection to the helix.
 The attenuator prevents any reflected waves from traveling back
down the helix.
 The applied signal propagates around the turns of the helix and
produces an electric field at the center of the helix, directed
along the helix axis.
 When the electrons enter the helix tube, an interaction takes
place between the moving axial electric field and the moving
electrons.
 This interaction causes the signal wave on the helix to be
amplified.
 The characteristics of the Traveling-wave tube are:
1. Frequency range: 3 GHz and higher
2. Bandwidth: about 0.8 GHz
3. Efficiency: 20 to 40%
4. Power output: up to 10 kW
5. Average Power gain: up to 60 dB
Types of TWT:
 Coupled Cavity Travelling wave Tube:
 The Coupled-cavity TWT uses a slow wave structure of a series
of cavities coupled to one another.
 The resonant cavities are coupled together with a transmission
line.
 The electron beam is velocity modulated by an RF input signal
at the first resonant cavity.
 This RF energy travels along the cavities and induces RF
voltages in each subsequent cavity.
 If the spacing of the cavities is correctly adjusted, the voltages at
each cavity induced by the modulated beam are in phase and
travel along the transmission line to the output, with an additive
effect, so that the output power is much greater than the power
input.
 Microwave Crossed Field
Tubes
1-Classification of M-type tubes is shown in the figure below.
2- we shall study in detail magnetron as commonly used M- type
tube.
 Magnetron oscillator

 Magnetrons provide microwave oscillations

of very high frequency.

Types of magnetrons

1. Negative resistance type

2. Cyclotron frequency type
3. Cavity type

48
 Description of types of magnetron
Negative resistance Magnetrons
 Make use of negative resistance between two anode
segments but have low efficiency and are useful only at
low frequencies (< 500 MHz).
Cyclotron frequency Magnetrons
 Depend upon synchronization between an alternating
component of electric and periodic oscillation of electrons
in a direction parallel to this field.
 Useful only for frequencies greater than 100 MHz.
Cavity Magnetrons
 Depend upon the interaction of electrons with a rotating
electromagnetic field of constant angular velocity.
 Provide oscillations of very high peak power and hence
are useful in radar applications
49
 Cavity Magnetrons

Fig (i) Major elements in the Magnetron oscillator

50
Cavity Magnetron
51
Anode Assembly

52
 Construction

 Each cavity in the anode acts as an inductor having only

one turn and the slot connecting the cavity and the
interaction space acts as a capacitor.
 These two form a parallel resonant circuit and its resonant
frequency depends on the value of L of the cavity and the
C of the slot.
 The frequency of the microwaves generated by the
magnetron oscillator depends on the frequency of the RF
oscillations existing in the resonant cavities.

53
 Description
 Magnetron is a cross field device as the electric field
between the anode and the cathode is radial whereas the
magnetic field produced by a permanent magnet is axial.
 A high DC potential can be applied between the cathode
and anode which produces the radial electric field.
 Depending on the relative strengths of the electric and
magnetic fields, the electrons emitted from the cathode and
moving towards the anode will traverse through the
interaction space as shown in Fig. (iii).
 In the absence of magnetic field (B = 0), the electron travel
straight from the cathode to the anode due to the radial
electric field force acting on it, Fig (iii) a.
54
Cavity Magnetrons

55
 Fig (ii) Cross sectional view of the anode
assembly

56 PH0101 Unit 2 Lecture 5

PH0101 Unit 2 Lecture 5 57
PH0101 Unit 2 Lecture 5 58
 Description
 If the magnetic field strength is increased slightly, the lateral
force bending the path of the electron as given by the path
‘b’ in Fig. (iii).
 The radius of the path is given by, If the strength of the
magnetic field is made sufficiently high then the electrons
can be prevented from reaching the anode as indicated
path ‘c’ in Fig. (iii)),
 The magnetic field required to return electrons back to the
cathode just grazing the surface of the anode is called the
critical magnetic field (Bc) or the cut off magnetic field.
 If the magnetic field is larger than the critical field (B > Bc),
the electron experiences a greater rotational force and may
return back to the cathode quite faster.

59 PH0101 Unit 2 Lecture 5

 Fig (iii) Electron trajectories in
the presence of crossed
electric and magnetic fields
(a) no magnetic field
(b) small magnetic field
(c) Magnetic field = Bc
(d) Excessive magnetic
field

60 PH0101 Unit 2 Lecture 5

 Working
Fig (iv) Possible trajectory of electrons from cathode to anode
in an eight cavity magnetron operating in  mode

61 PH0101 Unit 2 Lecture 5

 Working
 The RF Oscillations of transient nature produced when
the HT is switched on, are sufficient to produce the
oscillations in the cavities, these oscillations are
maintained in the cavities reentrant feedback which
results in the production of microwaves.
 Reentrant feedback takes place as a result of interaction
of the electrons with the electric field of the RF
oscillations existing in the cavities.
 The cavity oscillations produce electric fields which fringe
out into the interaction space from the slots in the anode
structure, as shown in Fig (iv).
 Energy is transferred from the radial dc field to the RF
field by the interaction of the electrons with the fringing RF
field.

62 PH0101 Unit 2 Lecture 5

PH0101 Unit 2 Lecture 5 63
 Working
 Due to the oscillations in the cavities, the either sides of the
slots (which acts as a capacitor) becomes alternatively
positive and negative and hence the directions of the
electric field across the slot also reverse its sign
alternatively.
 At any instant the anode close to the spiraling electron
goes positive, the electrons gets retarded and this is
because; the electron has to move in the RF field, existing
close to the slot, from positive side to the negative side of
the slot.
 In this process, the electron loses energy and transfer an
equal amount of energy to the RF field which retard the
spiraling electron.
 On return to the previous orbit the electron may reach the
adjacent section or a section farther away and transfer
energy to the RF field if that part of the anode goes positive
atPH0101
that instant.
64 Unit 2 Lecture 5
 Working
 This electron travels in a longest path from cathode to the
anode as indicated by ‘a’ in Fig (iv), transferring the
energy to the RF field are called as favoured electrons and
are responsible for bunching effect and give up most of its
energy before it finally terminates on the anode surface.
 An electron ‘b’ is accelerated by the RF field and instead
of imparting energy to the oscillations, takes energy from
oscillations resulting in increased velocity, such electrons
are called unfavoured electrons which do not participate in
the bunching process and cause back heating.
 Every time an electron approaches the anode “in phase”
with the RF signal, it completes a cycle. This corresponds
to a phase shift 2.
 For a dominant mode, the adjacent poles have a phase
difference of  radians, this called the  - mode.
65 PH0101 Unit 2 Lecture 5
 Fig (v) Bunching of electrons in
multicavity magnetron

66 PH0101 Unit 2 Lecture 5

 Working
 At any particular instant, one set of alternate poles goes
positive and the remaining set of alternate poles goes
negative due to the RF oscillations in the cavities.
 AS the electron approaches the anode, one set of
alternate poles accelerates the electrons and turns back
the electrons quickly to the cathode and the other set
alternate poles retard the electrons, thereby transferring
the energy from electrons to the RF signal.
 This process results in the bunching of electrons, the
mechanism by which electron bunches are formed and
by which electrons are kept in synchronism with the RF
field is called phase focussing effect. electrons with the
fringing RF field.

67 PH0101 Unit 2 Lecture 5

 Working
 The number of bunches depends on the number of cavities
in the magnetron and the mode of oscillations, in an eight
cavity magnetron oscillating with  - mode, the electrons
are bunched in four groups as shown in Fig (v).
 Two identical resonant cavities will resonate at two
frequencies when they are coupled together; this is due to
the effect of mutual coupling.
 Commonly separating the pi mode from adjacent modes is
by a method called strapping. The straps consist of either
circular or rectangular cross section connected to alternate
segments of the anode block.

68 PH0101 Unit 2 Lecture 5

 Performance Characteristics
1. Power output: In excess of 250 kW ( Pulsed
Mode), 10 mW (UHF band), 2 mW (X band),
8 kW (at 95 GHz)
2. Frequency: 500 MHz – 12 GHz
3. Duty cycle: 0.1 %
4. Efficiency: 40 % - 70 %

69 PH0101 Unit 2 Lecture 5

 Applications of Magnetron

1. Pulsed radar is the single most important

application with large pulse powers.
2. Voltage tunable magnetrons are used in sweep
oscillators in telemetry and in missile
applications.
3. Fixed frequency, CW magnetrons are used for
industrial heating and microwave ovens.

70 PH0101 Unit 2 Lecture 5

 Microwave Transistors

71
 Properties of important semiconducors

72
 Application of microwave solid state devices

73
  The microwave transistor is a nonlinear device is
 It is similar to that of the low-frequency device, but requirements for
 dimensions, process control, heat sinking, and packaging are much more severe.

74
 Physical structure

75
  For high-frequency applications, the n p n structure
is preferred.

76
 Operation modes of an npn transistors

77
IMPATT DIODE
A wide variety of solid state diodes and transistor
have been developed for microwave use.

 IMPact ionization Avalanche Transit-Time

 Function as microwave oscillator.
 Used to produce carrier signal for microwave transmission
system.
 IMPATT can operate from a few GHz to a few hundred
GHz
 IMPACT IONIZATION
If a free electron with sufficient energy strikes a silicon atom, it
can break the covalent bond of silicon and liberate an
electron from the covalent bond.
If the electron liberated gains energy by being in an electric
field and liberates other electrons from other covalent bonds
then this process can cascade very quickly into a chain
reaction producing a large number of electrons and a large
current flow.
This phenomenon is called impact avalanche.

79
 AVALANCHE MULTIPLICATION
 When the reverse bias voltage is above the breakdown
voltage, the space charge region always extends from n+ -p
junction to the i –p+ junction through the p and the i
regions.

80
 IMPATT DIODE Operation
 The diode is operated in reverse bias near
breakdown, and both the N and N- regions are
completely depleted
 Because of the difference in doping between the
"drift region" and "avalanche region", the
electric field is highly peaked in the
avalanche region and nearly flat in drift
region.
 In operation, avalanche breakdown occurs at
the point of highest electric field, and this
generates a large number of hole-electron pairs by
impact ionization.
 The holes are swept into the cathode, but the
electrons travel across the drift region toward
anode.
Figure 1: Impatt Diode Operation
 IMPATT DIODE Operation

Figure 2: The Build Up Of Microwave Oscillation.

As they drift, they induce image charges on the anode, giving rise
to a displacement current in the external circuit that is 180° out of
phase with the nearly sinusoidal voltage waveform
Figure 2 shows the buildup of microwave oscillations in the diode
current and voltage when the diode is embedded in a resonant
cavity and biased at breakdown
IMPATT DIODE Operation

Figure 3: Close Up Current And Voltage.

Figure 3 shows a close-up of the current and voltage
waveforms after oscillations have stabilized. It is clear from
Fig. 3 that the current is 180° out of phase with the voltage
This represents a NEGATIVE AC RESISTANCE
 The external current Ie(t) because of the moving holes is
delayed by 90 relative to the pulsed Io(t).
Since the carrier current Io(t) is delayed by one quarter cycle
or 90 relative to the ac voltage, Ie(t) is then delayed by 180
relative to the voltage.
Hence negative conductance occurs and the diode can be
used for microwave oscillation and amplification.
85
 Diode Mounting Procedure
and Precautions
 The IMPATT diode has a negative resistance from DC
through microwave frequencies. Consequently, it is prone
to oscillate at low frequencies, with the lead inductance
from bias circuit connections. The voltage due to bias circuit
oscillations may be large enough to burn the device out if
adequate precautions are not observed. It is prudent practice to
suppress the bias circuit oscillation.
Diode Mounting Procedure
and Precautions
 Adequate heatsink must be provided for the diode to
operate properly. These IMPATT diodes have been designed
to operate in the precollection mode. As the diode is tuned
up from a low operating current from a constant current source,
it will be noticed that at the onset of precollection mode, the
diode voltage falls down..
Diode Mounting Procedure
and Precautions
 The power output will increase by several dBs. with a slight
shift in the operating frequency. When the circuit is detuned
in such a fashion that the diode falls out of the precollection
mode, the diode voltage will increase. The power
dissipation will increase as the power output falls
down. If the diode is not adequately heatsink, the
diode may burn out
IMPATT DIODE Applications

 A main advantage is their high power capability.

These diodes are used in a variety of applications from low
power radar systems to alarms.
 Nevertheless these diodes make excellent microwave
generators for many applications. An alternating signal
is generated simply by applying a DC supply when a
suitable tuned circuit is applied. The output is reliable
and relatively high when compared to other forms of
diode.
IMPATT DIODE Applications
 In view of its high levels of phase noise it is used in
transmitters more frequently than as a local oscillator
in receivers where the phase noise performance is
generally more important.
IMPATT DIODE Applications
 The following products are available as examples of IMPATT
diodes application:

1) Cavity Stabilized IMPATT diode Oscillators CIDO series.

2) Pulsed IMPATT Power Sources IPSP series.

3) IMPATT Active Frequency Multipliers IAFM series.

4) Pulsed and CW IMPATT Injection-Locked Amplifiers IILAP and IILA

series.

5) Voltage Controlled IMPATT Oscillators VCIO series.

92
 Structure:

93
94
95
96
 Advantages

 Applications:

97
Transferred Electron Devices (TED)
 TED’s are semiconductor devices with no junctions and
gates.

 They are fabricated from compound semiconductors like

GaAs, InP, CdTe etc.

 TED’s operate with hot electrons whose energy is much

greater than the thermal energy.
Gunn Diode

 Invented by J.B Gunn

Gunn Effect:
 Above some critical voltage (Corresponding to Electric field of
2k-4k V/cm) the current passing through n-type GaAs becomes a
periodic fluctuating function of time.
 Frequency of oscillation is determined mainly by the specimen,
not by the external circuit.
 Period of oscillation is inversely proportional to the specimen
length and is equal to the transit time of electrons between the
electrodes
 The current waveform was produced by applying a voltage pulse
of 16V and 10ns duration to an n-type GaAs of 2.5 x 10-3 cm
length.The oscillation frequency was 4.5Ghz
RWH Theory
 Explanation for Gunn Effect:
Ridley –Watkins – Hilsum (RWH) Theory

 Two concepts related with RWH Theory.

 Differential negative resistance
 Two valley model
Differential negative resistance
 Fundamental concept of RWH Theory.
 Developed in bulk solid state III-V compound when a voltage
is applied
 Differential negative resistance make the sample electrically
unstable.
Two valley
model
theory
 Data for two valleys in GaAs
Electron transfer mechanism
 Conductivity of n-type GaAs:

 e = Electron charge
 μ = Electron mobility
 = Electron density in the lower valley
 = Electron density in the upper valley
 is the electron density
 According to RWH theory, in order to exhibit negative resistance
the energy band structure of semiconductor should satisfy

 The energy difference between two valleys must be several times

larger than the thermal energy (KT ~ 0.0259eV)
 The energy difference between the valleys must be smaller than the
bandgap energy (Eg)
 Electron in lower valley must have a higher mobility and smaller
effective mass than that of in upper valley

 Possessed by GaAs, InP, CdTe etc

Formation of high field domain
 In GaAs, at electric
fields exceeding the
critical value of Ec ≈
3.2 kV/cm the
differential mobility is
–ve.

 When the field

exceeds Ec and
further increases, the
electron drift velocity
decreases.
 Modes of Operation
 Gunn Oscillation Mode:
◦ (f x L) = 107 cm/s and (n x L) > 1012 /cm2
◦ Cyclic formation of High field domain
 Stable Amplification Mode
◦ (f x L) = 107 cm/s and 1011/cm2 < (n x L) >1012/cm2
 LSA Oscillation Mode
◦ (f x L) >107 cm/s and 2 x 104 < (n/f) > 2 X105/cm2
 Bias-circuit
◦ (f x L) is small. L is very small. When E=Eth current falls as Gunn
oscillation begins, leads to oscillation in bias circuit (1KHz to 100MHz)
 Gunn Oscillation Mode
 Condition for successful domain drift:
Transit time (L/vs) > Electric relaxation time

 Frequency of oscillation = vdom/Leff.

 Gunn diode with a resistive circuit -> Voltage change across diode is
constant-> Period of oscillation is the time required for the domain
to drift from the cathode to anode. Not suitable for microwave
applications because of low efficiency.
 Gunn diode with a resonant circuit has high efficiency.
 There are three domain modes for Gunn oscillation modes.
1. Transit time domain mode, (Gunn mode)
2. Delayed domain mode

 Here domain is collected while

 New domain cannot form until E rises above threshold again.
,
 Also called inhibited mode.
 Efficiency: 20%
3. Quenched domain mode:

 If bias field drops below Es, domain collapses before it reaches

anode.
 When the bias field swings above Eth, a new domain starts and
process repeats.
 Frequency of oscillation is determined by resonant circuit.
 Efficiency : 13%
 Limited Space charge Accumulation Mode (LSA)

Most Important mode for Gunn oscillator.

Domain is not allowed to form.
Efficiency : 20%
Gunn Characteristics
 Power: 1W (Between 4HHz and 16GHz)
 Gain Bandwidth product : >10dB
 Average gain : 1 – 12 dB
 Noise figure : 15 dB
Applications of Gunn Diode
 In radar transmitters
 Air traffic control (ATC) and Industrial Telemetry
 Broadband linear amplifier
 Fast combinational and sequential logic circuit
 Low and medium power oscillators in microwave receivers
 As pump sources
135 PH0101 Unit 2 Lecture 5