Transcript UNIT 4 PPT

CROSS FIELD TUBES AND
MICROWAVE
SEMICONDUCTOR
DEVICES
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BY: P. Vijaya & M. Niraja
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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.
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◦ 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
Negative resistance is achieved by creating a
delay (1800 Phase shift) between the voltage
and current.
 Delay is achieved by,
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 Delay in generating the avalanche current
multiplication
 Delay due to transit time through the material
So called Avalanche transit time (ATT)
devices
 Avalanche is generated by Carrier impact
ionization
 TT is due to the drift in the high field
domain
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Presence of P-N junctions
Diode is reverse biased
High field (potential gradient) is applied of
the order 400 KV/cm
Two modes of ATT
◦ IMPATT- Impact ionization ATT (Efficiency 5-10%)
◦ TRAPATT- Trapped plasma ATT (Efficiency 20-60%)
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Read diode is n+ p i p+ diode
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Avalanche multiplication at p region
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Intrinsic region acts as the drift space
where the generated holes must drift
toward p+
Space between n+ p junction and i p+
junction is called the space charge
region
The device operation delivers power
from the dc bias to the oscillation
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Operation:
◦ Avalanche multiplication and drift of the
high field zone
 Carrier current
I0(t) and
External
current Ie(t)
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The physical mechanism is the
interaction of impact ionization
avalanche and the transit time of
charge carriers.
So Read-type diodes are also called
IMPATT diode
Most simplest IMPATT diodes are the
basic Read diodes
Three typical Si IMPATT diodes are
shown below. Operations are similar to
Read diode
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Derived from IMPATT diode
Presence of P-N junctions
Diode is reverse biased
High current densities than normal
avalanche operation
It is
diode.
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Operation:
•Uses
•
non linear reactance or time varying reactance
Parametric term is derived from parametric excitation,
since the capacitance or inductance, which is a reactive
parameter, can be used to produce capacitive or
inductive excitation.
•Parametric
excitation is subdivided into parametric
amplification and oscillation.
Many of the essential properties of non linear energy
storage systems were described by Faraday and Lord
Rayleigh.
•
•The
first analysis of non linear capacitance was
given by Van der Ziel in 1948 which suggested that
such a device might be useful as a low noise
amplifier, since it was essentially a reactive device in
which no thermal noise is generated.
•In
1949 Landon analyzed and presented
experimental results of such circuits used as
amplifiers, converters, and oscillators.
•In the age of solid state electronics, microwave
electronics engineers thought of a solid state
microwave device to replace the noisy electron
beam amplifier.
•In
1957 Suhl proposed a microwave solid state
amplifier that used ferrite.
•The first realization of a microwave parametric
amplifier was made by Weiss in 1957 after which
the parametric amplifier was last discovered.
•At present the soild state varactor diode is the most
widely used parametric amplifier.
•Unlike microwave tubes, transistors and lasers, the
parametric diode is of reactive nature and thus
generates a very small amount of Johnson (thermal)
noise.
•Parametric
amplifier utilizes an ac rather than a dc
power supply as microwave tubes do. In this
respect, the parametric amplifier is analogous to the
quantum amplifier laser or maser in which an ac
power supply is used.
•A
reactance is defined as a circuit element that stores
and releases electromagnetic energy as opposed to a
resistance, which dissipates energy.
•If
the stored energy is predominantly in the electric
field, the reactance is said to be capacitive; inductive if
in the magnetic field.
•C
= Q/V
•If the ratio is not linear, the capacitive reactance is said
to be nonlinear. In this case it is convenient to define a
non linear capacitance as the partial derivative of charge
with respect to voltage.
i.e
C(v) = dQ/dt
dQ
dv
The analogous definition of non linear inductance is
L(i) = dΦ/di.
In the operation of parametric devices, the mixing
effects occur when voltages at two or more different
frequencies are impressed on a nonlinear reactance.
•Derived
a set of general energy relations regarding
power flowing into and out of an ideal nonlinear
reactance.
•These
relations are useful in predicting whether
power gain is possible in a parametric amplifier.
•One
signal generator and one pump generator at their
respective frequencies
, together with
associated series resistances and bandpass filters, are
applied to a nonlinear capacitance C(t).
•These
resonating circuits of filters are designed to reject
power at all frequencies other than their respective signal
frequencies.
•In the presence of two applied frequencies
an infinite number of resonant frequencies of
are generated, where m and n are any integers.
•Each
ideal.
of the resonating circuits is assumed to be
•The
power loss by the nonlinear susceptance is
negligible. That is the power entering the nonlinear
capacitor at the pump frequency is equal to the
power leaving the capacitor at the other frequencies
through the nonlinear interaction.
•Manley
and Rowe established the power relations
between the input power at the frequencies
and the output power at the other frequencies
•It
is assumed that the signal voltage vs is much
smaller than the pumping voltage vp, and the total
voltage across the nonlinear capacitance C(t) is given
by
•The
general expressionof the charge Q deposited on
the capacitor is given by
For Q to be real,
The total voltage v can be expressed as a function
of the charge Q.
A similar taylor series expression of v(Q) shows that
V to be real,
The current flowing through C(t) is the total
derivative of Q w r t time. Hence,
Where
•Since
the capacitance C(t) is assumed to be pure
reactance, the average power at the frequencies
•
is
Then conservation of power can be written
Multiply the above equation by a factor of
and
rearrangement of the resultant into two parts yield
Since
Then,
Becomes,
And is independent of ωp or ωs.
For any choice of the frequencies fp and fs, the
resonating circuit external to thatof the nonlinear
capacitance C(t) can be so adjusted that the
currents may keep all the voltage amplitudes
Unchanged.
The charges
are also unchanged, sincethey
are functions of the voltages
.
Consequently, the frequencies
arbitrarily adjusted in order to require
Eqn I can be expressed as
can be
Since
, then
Similarly,
Where
respectively.
are replaced by
The above equations are standard forms for the
Manley-Rowe power relations.
The term
indicates the realpower flowing into
or leaving the nonlinear capacitor at a frequency of
.The frequency
represents the
fundamental frequency of the pumping voltage
oscillator and the frequency
signifies the
fundamental frequency of the signal voltage
generator.
The sign convention for the power term
will
follow that power flowing into the nonlinear
capacitance or the power coming from the two
voltage generators is positive, whereas the power
leaving from the nonlinear capacitance or the power
flowing into the load resistance is negative.
Consider for instance, the case where the power
output flow is allowed at a frequency of
as shown in fig.