1. Transferred Electron Devices
TEDs are made of compound semiconductors such as GaAs.
They exhibit periodic fluctuations of current due to negative resistance effects when a threshold voltage (about 3.4 V) is exceeded.
The negative resistance effect is due to electrons being swept from a lower valley (more mobile) region to an upper valley (less mobile) region in the conduction band.
(solid-state physics) The variation in the effective drift mobility of charge carriers in a semiconductor when significant numbers of electrons are transferred from a low-mobility valley of the conduction band in a zone to a high-mobility valley, or vice versa.
(electronics) A semiconductor device, usually a diode, that depends on internal negative resistance caused by transferred electrons in gallium arsenide or indium phosphide at high electric fields; transit time is minimized, permitting oscillation at frequencies up to several hundred megahertz.
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2. Gunn Diode The Gunn diode is a transferred electron device that
can be used in microwave oscillators or one-port
reflection amplifiers. Its basic structure is shown
below. N-, the active region, is sandwiched between
two heavily doped N+ regions. Electrons from the l
cathode (K) drifts to
the anode (A) in bunched
formation called domains.
Note that there is no p-n
junction. K N- A
Metallic
Electrode N+
Metallic
Electrode

3. Gunn Operating Modes
Stable Amplification (SA) Mode: diode behaves as an amplifier due to negative resistance effect.
Transit Time (TT) Mode: operating frequency, fo = vd / l where vd is the domain velocity, and l is the effective length. Output power < 2 W, and frequency is between 1 GHz to 18 GHz.
Limited Space-Charge (LSA) Mode: requires a high-Q resonant cavity; operating frequency up to 100 GHz and pulsed output power > 100 W.

4. Gunn Diode Circuit and Applications
Resonant
Cavity
Tuning
Screw
The resonant cavity
is shocked excited by
current pulses from
the Gunn diode and
the RF energy is
coupled via the iris
to the waveguide.
Iris
Diode V
Gunn diode applications: microwave source for
receiver local oscillator, police radars, and
microwave communication links.
H. Chan; Mohawk College

5. IMPATT IMPact ionization Avalanche Transit Time
They are made from in Si, GaAs and InP.
It can generate higher powers above 30 GHz
It provides negative resistance using phase shift between current through the device and the applied voltage. (>900)

6. n+ -P -I - p+ It consists of two regions:
IMPATT
n+ -P -I - p+ It consists of two regions:
It consists of two regions:
1) P+ N region, at which avalanche multiplication occurs, and
2) i (intrinsic) region, through which generated holes must drift in moving to  N+ contact.

7. In all the structures (Si, GaAs and InP) there exists two regions
Avalanche region: in this region avalanche multiplication takes, doping concentration and field intensity are high.
Drift region: in this region avalanche multiplication does not take place, doping concentration and field levels are low.
Depletion region is AR plus DR.
Employs ‘impact ionization’ and ‘transit time’ properties of semiconductor structures to get negative resistance at microwave frequencies.

8. device consists of a reverse-biased P-N junction (operating in avalanche breakdown) and a drift zone.
Carriers are injected into the drift zone from the
junction in avalanche breakdown where they drifted at saturated velocity.
The constant value of the saturated velocity provides a linear relationship between the device length and
current delay.
It is, therefore, possible to determine a length that will result in negative resistance within a certain frequency band.

9. avalanche
When the PN junction diode is reverse-biased, then current does not flow.
However, when the reverse voltage exceeds a certain value, the junction breaks down and current flows with only slight increase of voltage.
This breakdown is caused by avalanche multiplication of electrons and holes in the space charge region of the junction.

10. Avalanche Transit-Time Devices
If reverse-bias potential exceeds a certain threshold, the diode breaks down.
Energetic carriers collide with bound electrons to create more hole-electron pairs.
This multiplies to cause a rapid increase in reverse current.
The onset of avalanche current and its drift across the diode is out of phase with the applied voltage thus producing a negative resistance phenomenon.

11. Avalanche breakdown
negative resistance occurs from the delay, which cause current to lag behind voltage by half cycle time region phenomenon occurs due to:
Avalanche time delay caused by ‘finite buildup time of the
avalanche current.
Other is transit time delay by the finite time for the carriers to cross drift r

12. IMPATT Diode - A single-drift structure of an IMPATT (impact
avalanche transit time) diode is shown below: - + P+ N N+ l
Avalanche
Region
Drift Region
Operating frequency:
where vd = drift
velocity

13. features
The current build-up and the transit time for the current pulse to cross the drift region cause a 180o phase delay between V and I; thus, negative R.
IMPATT diodes typically operate in the 3 to 6 GHz region but higher frequencies are possible.
They must operate in conjunction with an external high-Q resonant circuit.
They have relatively high output power (>100 W pulsed) but are very noisy and not very efficient.

14. Microwave Transistors
Silicon BJTs and GaAsFETs are most widely used.
BJT useful for amplification up to about 6 MHz.
MesFET (metal semiconductor FET) and HEMT (high electron mobility transistor) are operable beyond 60 GHz.
FETs have higher input impedance, better efficiency and more frequency stable than BJTs.

15. Energy band diagram for GaAS
Gunn diodes are manufactured using III–V compound semiconductors, such as GaAs or Pin (Phosphorus-Indium).
Gunn diodes are manufactured in three layers, with an N-doped layer embedded between two, thinner, N+-doped layers
As the bias voltage is increased
from a low value, the electrons acquire a higher energy and their velocities increase. When
the field strength reaches a threshold, Eth, electron collisions become sufficiently frequent
(due to their enhanced velocities) that their predilection to drift in the electric field becomes significantly impaired; in short, their mobility is reduced. This corresponds to electrons in
the lower energy valley being transferred to the higher energy valleys.
Energy band diagram for GaAS

16. Gunn diode basics
The Gunn diode is a unique component - even though it is called a diode, it does not contain a PN diode junction. The Gunn diode or transferred electron device can be termed a diode because it does have two electrodes. It depends upon the bulk material properties rather than that of a PN junction. The Gunn diode operation depends on the fact that it has a voltage controlled negative resistance.

17. Gunn diode construction
Gunn diodes are fabricated from a single piece of n-type semiconductor. The most common materials are gallium Arsenide, GaAs and Indium Phosphide, InP.
The device is simply an n-type bar with n+ contacts. It is necessary to use n-type material because the transferred electron effect is only applicable to electrons and not holes found in a p-type material.
Gunn diode symbol for circuit diagrams
The Gunn diode symbol used in circuit diagrams varies. Often a standard diode is seen in the diagram, however this form of Gunn diode symbol does not indicate the fact that the Gunn diode is not a PN junction. Instead another symbol showing two filled in triangles with points touching is used as shown below.

19. Gunn Effect When electric field i –
mobility of electrons decrease as the electric field is increased in the material reaches a threshold level
-results in thereby
producing negative resistance.
A two-terminal device made from such a material can produce
microwave oscillations, the frequency of which is primarily determined by the characteristics of the
specimen of the material and not by any external circuit.

20. Gunn Diode
GaAs, electrons can exist in a high-mass low velocity state as well
as their normal low-mass high-velocity state and they can be forced into the high-mass state by a
steady electric field of sufficient strength. In this state they form clusters or domains which cross the
field at a constant rate causing current to flow as a series of pulses.
The frequency of the current pulses so generated
depends on the transit time through the n-layer and hence on its thickness.

21. Gunn diode is a so-called transferred electron device
Gunn diode is a so-called transferred electron device. Electrons are transferred from one valley in the conduction band to another valley.

23. If the diode is mounted in a
suitably tuned cavity resonator, the current pulses cause oscillation by shock excitation and r.f. power
up to 1 W at frequencies between 10 and 30 GHz is obtainable.

24. Gunn Device Slab of N-type GaAs (gallium arsenide)
Sometimes called Gunn diode but has no junctions
Has a negative-resistance region where drift velocity decreases with increased voltage
This causes a concentration of free electrons called a domain

26. Transit-time Mode
Domains move through the GaAs till they reach the positive terminal
When domain reaches positive terminal it disappears and a new domain forms
Pulse of current flows when domain disappears
Period of pulses = transit time in device

28. Gunn Oscillator Frequency
T=d/v
T = period of oscillation
d = thickness of device
v = drift velocity, about 1  105 m/s
f = 1/T

29. IMPATT Diode IMPATT stands for Impact Avalanche And Transit Time
Operates in reverse-breakdown (avalanche) region
Applied voltage causes momentary breakdown once per cycle
This starts a pulse of current moving through the device
Frequency depends on device thickness

30. Gunn Diodes Gunn diodes are used as transferred electron oscillators (TEO) by using the negative resistance property of bulk Gallium Arsenide. The figure below shows the electron velocity in GaAs as a function of the applied electric field. Greater than about an electric field of 3.2 KV/cm, the electrons in N type GaAs move from a high-mobility, lowenergy valley to another valley where the mobility is lower. Consequently, the net electron velocity is lower. This negative resistance is used for generation of microwave power. Reference: