MW&OC UNIT-2 PPT -M-TYPE TUBES.pptx INTRODUCTION

UNIT-2
M-TUBE TUBES
&
MICROWAVE SOLID STATE
DEVICES
IV YEAR I SEM
Academic year 2025-26

Introduction -Cross-Coupled Tubes (Magnetron Oscillator)

Mechanism of oscillations in Magnetron-
• The magnetron requires an external magnetic field with flux
lines parallel to the axis of cathode.
• This field is provided by a permanent magnet or
electromagnet.
• The dc magnetic field is perpendicular to the dc electric field
between the cathode and anode.
• Because of the cross-field between the cathode and anode,
the electrons emitted from the cathode are affected by the
cross-field to move in curved paths.
• If the dc magnetic field is strong enough, the electrons will
not arrive in the anode but return back to the cathode.
-Cross field Effect - Magnetrons

CROSS FIELD - ELECTRIC FIELD AND MAGNETIC FIED

CYLINDRICAL TRAVELLING WAVE MAGNETRON

MAGNETRON-STRUCTURE & WORKING PRINCIPLE OPERATION

Operation- From fig (b).
Path ‘a’- If there is no magnetic field present; the electron
would be drawn directly towards the anode in accordance with
path ‘a’.
Path ‘b’- As the electron travels with a velocity the axial magnetic
field exerts a force on it. When the magnetic field is weak the
electron path is deflected as path ‘b’.
Path ‘c’- However, when the intensity of the magnetic field is
sufficiently great, the electrons are turned back towards the
cathode without ever reaching the anode accordance with path ‘c’.
path ‘d’- The magnetic field is great ,which is just able to return
the electrons back to the cathode before reaching the anode, is
termed the cut-off field as shown in path ‘d’.
Thus when the magnetic field exceeds the cut-off value, then in
the absence of oscillations all the emitted electrons return to
the cathode and the plate current is zero.
Operation

-Mode Oscillations
Let the cavity magnetron has 8 cavities, by which it
supports varieties of modes depending upon the phase
difference between fields in two adjacent cavities. Boundary
conditions are satisfied when total phase shift around the eight
cavities is multiplied by 2 radians. However, the
most important mode for magnetron operation is one where
in the phase shift between the fields of adjacent cavities is 
radians. This is known as -Mode.

-Mode Magnetron Oscillations- Structure and principle of operation

Hull cut- off voltage & Hartee condition
11
• In a cylindrical magnetron, several re-entrant cavities are
connected to the gaps.
• Thus it is also called as Cavity Magnetron. Assume the
radius of cathode is ‘a’ and anode is ‘b’.
• The dc voltage V0 is applied between the cathode and anode.
When the dc voltage and the magnetic flux (i.e. which is in the
+ve z-direction) are adjusted properly, the electrons will follow
parabolic path in the presence of cross field.
• These parabolic paths are formed in the cathode-anode
space under the combined force of both electric and magnetic
fields which are perpendicular to each other.

Hull cut- off Magnetic field (Boc) Derivation or Hull cut
off voltage (Voc)

MW&OC UNIT-2 PPT -M-TYPE TUBES.pptx INTRODUCTION

Hence Hull’s cut-off voltage is
 Bc = [1/b{1-
(a2/b2)}].(8mV0/e)………….(ix)
Above equation is called the Hull’s cut-
off magnetic field equation.
Since b>>a, so a2/b2 may be neglected, equation
(viii)
becomes
Bc = (1/b).(8mV0/e)

If B>Bc for a given V0, the electron grazes or will not reach the
anode, it means anode current is zero. For the Hull’s cut-off
voltage Vc, if Bc = B then V0=Vc and thus cut-off voltage for a
given B is found from the equation (viii),
2 2 2 2 2
Vc = (eB0 b /8m)(1- a /b ) ………………(x)
Where, e/m = 1.759  1011 ,C/kg
If V0<Vc for given B, the electrons will
not reach the anode. Equation (x) is referred to
as Hull’s cut-off voltage equation

Microwave Solid State Devices
 Microwave tunnel diode.
 Transferred electron devices.
 Gunn-effect diodes.
 RWH theory, modes of operations.
 Avalanche transit time devices.
 IMPATT diode.
 TRAPATT diode.
 BARITT diode.
 Pin diodes.
 Varactor diodes.
 Crystal detectors.

 The common characteristics of all active two terminal solid
state devices is their NEGATIVE RESISTANCE.
 Example of solid state devices are Tunnel diode, GaAS diode
etc.
 In other words=>
 Positive Resistance semiconductor devices-Absorbs power
 Negative Resistance semiconductor devices-Generates
power

Introduction
• Special electronics effects encountered at microwave
frequencies severely limit the usefulness of transistors in
most circuit applications.
• Using vacuum tubes for low power applications become
impractical.
• Need for small sized microwave devices has caused extensive
research in this area
• This research has produced solid-state devices with higher and
higher frequency ranges.
• The new solid state microwave devices are predominantly
active, two terminal diodes, such as tunnel diodes,
varactors, transferred-electron devices, and avalanche
transit-time diodes

• Tunnel diode is a pn junction with extremely heavy doping on both
side.
• Tunnel diodes are useful to many circuit applications in microwave
amplification and oscillation because of low cost, light weight,
high speed, low noise and high peak to valley current ratio.
• Tunnel diode exhibits the negative resistance.
• Because of thin junction and short transit time, tunnel diode is
also
useful for fast switching.

TUNNEL DIODE
• Tunnel diode was discovered by a Japanese scientist named Esaki
in 1958.
• The tunnel diode is a pn junction with a extremely heavy doping
on the both sides of the junction and an abrupt transition from
the p-side to n-side.
• The tunnel diode are heavily doped pn junction that have a
negative resistance over a portion of its VI characteristics as
shown in Fig
• It changes from negative to positive again. Between the points, the
Device exhibits negative resistance

VI characteristics and symbol of tunnel diode

VI characteristic of tunnel diode
If input voltage is above peak voltage Vp the current in device
decreases upto valley voltage Vv
In Negative resistance semiconductor devices, the voltage and
current are out of phase by 180 degrees in negative
resistance region .
The voltage drop across Negative resistance is negative and
a power of –I2R will be generated by device in this
region

Comparison between Tunnel Diode and Normal p-n
Junction

TRANSFERRED ELECTRON DEVICES.
⦿ Transferred electron devices (TED’s) are bulk semiconductor
devices having no junction.
⦿ TED’s are fabricated from compound semiconductor such as
GaAs (gallium arsenide),i.e mixing of III&V group elements
⦿ TED’s operate with hot electrons whose energy is very
much
greater than the thermal energy.
⦿ when applied voltage exceeds a critical value (2000-4000
v/cm),there will be transfer of electrons from LV to UV . Due
to this the device shows Negative resistance characterticts
⦿ Due to transfer of electrons from lower valley to upper
valley above critical value of electric field the device
is called as Transferred electron devices (TEDs)

TWO VALLEY THEORY (RWH THEORY)
Ridley-watkins-hilsum theory
The two-valley model, however, there are two regions in the
conduction band in which charge carriers (electrons) can exist.
These regions are called valleys and are designated the upper
valley and lower valley. According to the RWH theory, electrons in
the lower valley have low effective mass (0.068) and
consequently a high mobility (8000 cm2/V-s). In the upper valley,
which is separated from lower valley by potential of 0.36 eV,
electrons have a much higher effective mass (1.2) and
lower mobility (180 cm2/V- s) than in the lower valley.
 Basic mechanism involved in the operation of bulk n-type GaAs
devices is the transfer of electrons from lower conduction valley to
the upper conduction valley. Electron density thus in lower valley
and upper valley remain the same under equilibrium conduction.

Two valley model.
When E < El
When the applied electric field is lower than electric field of the L-
valley, no electrons will transfer to the Upper valley.
When El < E < Eu
When applied electric field is (El < E < Eu),electron will begin to
transfer to the upper valley
When E> Eu
When the applied electric field is higher than that of
uppervalley E>Eu
all electrons will transfer to the upper valley
.

Transfer of electron
densities.

Transfer of electron densities

GUNN EFFECT –GaAs DIODE
 GaAs diode is a compound semiconductor material which is formed by
mixing of III&V group.
 Gunn diode operate with hot electrons, whose energy is greater than
thermal energy.
 Gunn effect diodes are named after J.B.Gunn ,who in 1963 discovered a
Periodic fluctuations of current passing through n-type GaAs
specimen when the applied voltages exceeds a certain critical
value (2000-4000 v/cm)
 Electric field across cathode and anode exceeds critical value (2000-
4000v/cm) he observed periodic fluctuations of current passing through
GaAs diode which is Gunn Effect
 The frequency of oscillations was determined by specimen and not by
the
external circuit.
 The period of oscillations was dependent on specimen length and is
closely
equal to transit time of electrons between electrodes.

Characteristics of Gunn Diode
36

Drift velocity Vs Electric field

Equivalent Circuit of Gunn Diode
Domain Mode:

Gunn Oscillator Mode
Gunn considered several circuit configurations
for describing the behaviour of oscillations
in GaAs devices.
When the circuit is mainly resistive or
the voltage across the diode is constant,
This mode is not usually used in microwave
oscillations. For this mode, fL = 107 cm/s
and n 0L = 1012/cm2. Negative
conductivity
devices are usually operated in resonant
circuits, such as high Q
resonant microwave cavities.
When the diode is in a resonant circuit,
the frequency can be
tuned to a range of about an octave without

Gunn Diode Oscillator
A Gunn diode oscillator can be designed by mounting the diode
inside a waveguide cavity formed by a short-circuit termination at
one end and by an iris at other end diode is mounted at the centre
perpendicular to the broad wall where the electric field
component is maximum under the dominant TE 10 mode. The
intrinsic frequency f0 of the oscillation depends on the electron
drift velocity vd due to high field domain through the effective
length L

Disadvantages of Gunn Diode
Gunn diode is very much temperature dependent i.e., a
frequency shift of 0.5 to 3 MHz per °C.
By proper design this frequency shift can be reduced to 50 kHz
for a range of - 40°C to 70°C.
Other disadvantages of Gunn diode is, the power output of the
Gunn diode is limited by difficulty of heat dissipation from the
small chip.
Gunn diode is very much temperature dependent.

Applications of Gunn Diode
Gunn diode can be used as an amplifier and as an oscillator. The
applications of Gunn diode are
1. In broadband linear amplifier.
2. In radar transmitters.
3. Used in transponders for air traffic control.
4. In fast combinational and sequential logic circuit.
5. In low and medium power oscillators in microwave receivers.

Comparison Between Microwave Transistors
and TED’s
Microwave transistors
1.Operate with junction or gates.
2.Fabricated from elemental semiconductors such as Si or Ge.
3. Operate with warm electrons whose energy is not much
greater than their thermal energy (0.026 eV at room
temperature).
TED’s
Operate with bulk devices having no junctions and gates.
Fabricated from compound GaAs, CdTe or InP.
Operate with hot electrons whose energy is very much greater
than the thermal energy

AVALANCHE TRANSIT TIME DEVICES
In 1958, Read at Bell Telephone Laboratories proposed that the
delay between voltage and current in an avalanche, together with
transit time through the material, could make a microwave diode
exhibit negative resistance such devices are called Avalanche
transit time devices.
The prominent members of this family include the IMPATT and
TRAPATT diode.
1.IMPATT (Impact Ionization Avalanche Transit Time) diode as the
name suggests, utilizes impact ionization for carrier generation.
2.TRAPATT (Trapped Plasma Avalanche Triggered Transit Time)
diode is derived from the IMPATT with some modifications in the
doping profiles so as to achieve higher pulsed microwave powers
At better efficiency values.

IMPATT DIODE
The IMPATT diode or IMPact Avalanche Transit time diode is an
RF semiconductor device that is used for generating microwave
radio frequency signal, with the ability to operate at frequencies
between about 3 to 100 GHz or more, one of the main
disvantages is the relatively high power capability of the IMPATT
diode.
IMPATT Structures
There is a variety of structures that are used for the IMPATT diode
like p+nin+ or n+pip+ read evice, p+nn+, and p+in+ diode, all are
variations of a basic pn junction
IMPATT diode is semiconductor device which generate microwave
signal from 3 to 100 GHz.
In IMPATT diode, negative resistance effect phenomenon is taken
Into account

Operation of IMPATT
A cross-section of p+n n+ IMPATT diode structure is shown in Fig.
7.47. Note that it is a diode,the junction being between p+ and
then n layer.
An extremely high voltage gradient is applied in reverse bias to
resulting in very high current
the IMPATT diode, of the order of 400 kV/cm, eventually
.

Operation of IMPATT
Such a high potential gradient, back-biasing the diode, causes a
flow of minority carriers across the junction. If it is now assumed
that oscillations exist.
Now we may consider the effect of a positive swing of the RF
voltage superimposed on top of the high DC voltage

Equivalent Circuit of IMPATT
A simplified equivalent circuit for IMPATT diode chip is shown in
Fig.
Typically negative resistance varies between -0.7 Ω and -2 Ω, and
capacitance ranges from 0.2 to 0.6 pF.
where
Rd = Diode negative resistance consisting of the series lead resistance
Rs and the negative resistance - Rj due to impact avalanche process.
Cj= Junction capacitance.
Lp= Package lead inductance.
Cp= Package lead capacitance.

Advantages of IMPATT Diode
 IMPATT
diodes
are at present the most powerful solid-state
microwave power sources. Some of the major advantages of
IMPATT diode are:
1. Higher operating range are obtain (up to 100 GHz).
2. Above about 20 GHz, the IMPATT diode produces a higher CW
power output per unit than any other semiconductor device.
Higher operating range (up to 100 GHz) can be obtained
from
IMPATT diode.

Disadvantage of IMPATT Diode
The major disadvantages of IMPATT diode are:
1. Since DC power is drawn due to induced electron current
in the external circuit, IMPATT diode has low efficiency (RF
power output/DC input power).
2.Tend to be noisy due primarily to the avalanche process and to
the high level of operating current. A typical noise figure is 30 dB
which is worse than
3. Tuning is difficult as
that of
compare
to
Gunn
Gunn
diode.
diode.
4. To run an IMPATT diode, a relatively high voltage is required.

Applications of IMPATT Diode
IMPATT diodes are used as microwave oscillators such as:
1. Used in final power stage of solid state microwave
transmitter for communication purpose.
2. Used in transmitter of TV system.
3. Used in FDM/TDM system.
4. Used in microwave source in laboratory for
measurement purpose.
Photograph of a IMPATT diode is shown in Fig. 7.52.

Comparison between Gunn diode and Impatt
diode

TRAPATT DIODE
The TRAPATT (Trapped Plasma Avalanche Triggered Transit)
diodes are manufactured from Si or GaAs, and have p+nn+ (or
n+pp+) configuration
TRAPATT diode is derived from IMPATT diode.
In TRAPATT diodes the doping level between the junction and
anode changes gradually.

Operation
A TRAPATT in operation is placed in a high resonant cavity and
black biased to avalanche threshold. When the RF
oscillations begin,
they build up extremely rapidly due to the resonant structure thus
taking the voltage across the diode to a value much above the
avalanche threshold

Operation
The avalanche zone velocity vz is given by
where,
J is current density
q is the charge of electron
NA is doping
concentration.
Thus the avalanche zone or avalanche shock front will quickly
sweep across most of the diode, the electrons and holes will drift
at velocities determined by the low-field mobilities, and the transit
time of the carriers is given by
where,
vs = Saturated carrier drift velocity
L = Length of the specimen.

Advantages& Disadvantages of TRAPATT
Diode
The major advantages of TRAPATT are:
1. Low power dissipation
2. High efficiency (up to 60%)
3. TRAPATT is a pulse device capable of operating at much
larger pulse powers.
Disadvantages of TRAPATT Diode
The major disadvantages of TRAPATT are:
4. High noise figure (60 dB) limits its use as an amplifier.
5.It generates strong harmonics due to the short duration current
pulse.
3. Operating frequency is limited below 10 GHz.

Comparison Between the TRAPATT Diode and
IMPATT Diode

MW&OC UNIT-2 PPT -M-TYPE TUBES.pptx INTRODUCTION

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