Download Gunn Diode

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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.
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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
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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
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Explanation for Gunn Effect:
Ridley – Watkins – Hilsum (RWH) Theory
Two concepts related with RWH Theory.
◦ Differential negative resistance
◦ Two valley model
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Fundamental concept of RWH Theory.
Developed in bulk solid state III-V compound
when a voltage is applied
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Differential negative resistance make the
sample electrically unstable.
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Data for two valleys in GaAs
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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
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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
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Possessed by GaAs, InP, CdTe etc
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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.
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Gunn Oscillation Mode:
◦ (f x L) = 107 cm/s and (n x L) > 1012 /cm2
◦ Cyclic formation of High field domain
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Stable Amplification Mode
◦ (f x L) = 107 cm/s and 1011/cm2 < (n x L) >1012/cm2
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LSA Oscillation Mode
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Bias-circuit
◦ (f x L) >107 cm/s and 2 x 104 < (n/f) > 2 X105/cm2
◦ (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)
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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)
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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%
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Limited Space charge Accumulation Mode
(LSA)
Most Important mode for Gunn oscillator.
Domain is not allowed to form.
Efficiency : 20%
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Power: 1W (Between 4HHz and 16GHz)
Gain Bandwidth product : >10dB
Average gain : 1 – 12 dB
Noise figure : 15 dB
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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