LECTURE No. 4.pptxEngineeringEngineering

CHAPTER 4:
TRANSFERRED ELECTRON DEVICES -
AVALANCHE TRANSIT-TIME DEVICES

4.1 Gunn Effect. Differential Negative Resistance
• Overview
• The application of two-terminal semiconductor devices at Me Freq has been increased usage
during the past decades.
• The CW, average, and peak power outputs of these devices at higher microwave frequencies are
much larger than those obtainable with the best power transistor.
• The common characteristic of all active two-terminal solid-state devices is their negative
resistance.
• The real part of their impedance is negative over a range of frequencies.
• In a positive resistance the current through the resistance and the voltage across it are in phase.
• The voltage drop across a positive resistance is positive and a power of (I2
R) is dissipated in the
resistance.

• In a negative resistance, however, the current and voltage are
out of phase by 180°.
• The voltage drop across a negative resistance is negative, and a
power of (-I2
R) is generated by the power supply associated with the
negative resistance.
• In other words, positive resistances absorb power (passive
devices),
• Whereas negative resistances generate power (active devices).

• The differences b/n microwave transistors and transferred electron devices (TEDs) are
fundamental.
• Transistors operate with either junctions or gates, but TEDs are bulk devices having
no junctions or gates.
• The majority of transistors are fabricated from elemental semiconductors, such as
silicon or germanium,
• Whereas TEDs are fabricated from compound semiconductors, such as:
• Gallium Arsenide (GaAs),
• Indium Phosphide (InP), or
• Cadmium Telluride (CdTe).
• Transistors operate with "warm" electrons whose energy is not much greater than the thermal energy
(0.026 eV at room To
) of electrons in the semiconductor,
• Whereas TEDs operate with "hot" electrons whose energy is very much greater than the
thermal energy.
• Thus, the theory and technology of transistors cannot be applied to TEDs.

GUNN-EFFECT DIODES-GaAs DIODE
 Named after J. B. Gunn, who discovered in 1963, a periodic fluctuations of current
passing through the n-type GaAs specimen, when the applied voltage exceeded a
certain critical value.
 In 1965, B. C. DeLoach, R. C. Johnston, & B. G. Cohen discovered the Impact
Ionization Avalanche Transit-time (IMPATT) mechanism in silicon,
 Which employs the avalanching & transit-time properties of the diode to generate Mw
Freq.
 In later years the Limited Space-charge-Accumulation diode (LSA diode) & the
Indium Phosphide diode (InP diode) were also successfully developed.
• These are bulk devices - Mw amplification and oscillation are derived from the bulk
negative-resistance property of uniform semiconductors rather than from the junction
negative-resistance property b/n two different semiconductors, as in tunnel diode.

Background
• The principle involved is to heat carriers in a light-mass, high-mobility sub-band with an
electric field so that the carriers can transfer to a heavy-mass, low-mobility, higher-energy
sub-band when they have a high enough temperature.
• Their theory for achieving negative differential mobility in bulk semiconductors by
transferring electrons from high-mobility energy bands to low-mobility energy bands was
taken a step further by Hilsum in 1962.
• Hilsum carefully calculated the transferred electron effect in several III -V compounds and
was the first to use the terms transferred electron amplifiers (TEAs) and oscillators (TEOs).
• He predicted accurately that a TEA bar of semi-insulating GaAs would be operated at
373°K at a field of 3200 V/cm.

• It was not until 1963 that J. B. Gunn of IBM discovered the so-called Gunn
effect from thin disks of n-type GaAs & n-type InP specimens while studying
the noise properties of semiconductors.
• Gunn did not connect-and even immediately rejected-his discoveries with
the theories of Ridley, Watkins, and Hilsum.
• In 1963 Ridley predicted that:
• The field domain is continually moving down thr the crystal, disappearing at the
anode and then reappearing at a favoured nucleating center, & starting the whole
cycle once more.

• Finally, Kroemer stated that:
• The origin of the negative differential mobility is Ridley-Watkins-Hilsum's mechanism
of electron transfer into the satellite valleys that occur in the conduction bands of
both the n-type GaAs & the n-type InP.
• And that, the properties of the Gunn effect are the current oscillations
caused by the periodic nucleation and disappearance of traveling
space-charge instability domains.
• Thus:
• “The correlation of theoretical predictions & experimental discoveries
completed the theory of Transferred Electron Devices (TEDs)”.

Gunn Effect
• A schematic diagram of a uniform n-type GaAs diode with ohmic contacts at
the end surfaces
• Gunn observed the Gunn effect in the n-type GaAs bulk diode in 1963, a
• A effect best explained by Gunn himself, who published
several papers about his observations.
• He stated in his first paper [7] that:
• “Above some critical voltage, corresponding to an electric field of 2000-4000
volts/em, the current in every specimen became a fluctuating function of
time.

 In The GaAs Specimens,
• This fluctuation took the form of a periodic oscillation superimposed upon the
pulse current. ...
• The freq of oscillation was determined mainly by the specimen, and not by the
external circuit. ...
• The period of oscillation was usually inversely proportional to the specimen
length and closely equal to the transit time of electrons b/n the electrodes,
calculated from their estimated velocity of slightly over 107
cm/s ....
• The peak pulse Mw power delivered by the GaAs specimens to a matched load
was measured.
• Value as high as 0.5 W at 1 Gcls, and 0.15 W at 3 Gcls, were found,
• Corresponding to 1-2% of the pulse input power.*”

• From Gunn's observation,
• “The carrier drift velocity is linearly increased from zero to a maximum
when the electric field is varied from zero to a threshold value”.
• When the electric field is beyond the threshold value of 3000
V/cm for the n-type GaAs,
• The drift velocity is decreased & the diode exhibits negative resistance.
• This situation is shown below.

•Drift velocity of electrons in n-type GaAs versus electric
field.

 Current Fluctuation
• The current waveform was produced by applying a voltage pulse of 16-V
amplitude and 10-ns duration to a specimen of n-type GaAs 2.5 x 10-3
cm in
length.
• The oscillation freq was 4.5 GHz. The lower trace had 2 ns/cm in the
horizontal axis and 0.23 A/cm in the vertical axis.
• The upper trace was the expanded view of the lower trace.
• Gunn found that,
• “The period of these oscillations was equal to the transit time of the electrons thro
the specimen calculated from the threshold current”.

• Current waveform of ntype GaAs reported by Gunn

• Gunn also discovered that the threshold electric field Eth varied with the
length and type of material.
• He developed an elaborate capacitive probe for plotting the electric field
distribution within a specimen of n-type GaAs of length L = 210 μm & cross-
sectional area 3.5 x 10-3 cm2
with a low-field resistance of 16 Ω.
• Current instabilities occurred at specimen voltages above 59 V, which means
that, the threshold field is:
• Eth = V/L = 59 / (210 X 10-6 X 102) = 2810 volts/cm

Differential Negative Resistance
• The fundamental concept of the Ridley-Watkins-Hilsum (RWH)
theory is the differential negative resistance developed in a bulk
solid-state Ill-V compound.
• When either a voltage (or electric field) or a current is applied to the
terminals of the sample.
• There are two modes of negative-resistance devices: voltage-
controlled & current-controlled modes shown below.

• In the voltage-controlled mode the current density can be
multivalued,
• Whereas in the current-controlled mode the voltage can be
multivalued.
• The major effect of the appearance of a differential negative-
resistance region in the current density-field curve is to render the
sample electrically unstable.
• As a result, the initially homogeneous sample becomes electrically
heterogeneous in an
attempt to reach stability.

• In the voltage-controlled negative-resistance mode, high-field domains are
formed, separating two low-field regions.
• The interfaces separating low & high-field domains lie along equipotentials;
• Thus they are in planes perpendicular to the current direction as shown in (a).
• In the current-controlled negative-
resistance mode splitting the sample
results in high-current filaments
running along the field direction as
shown in (b).
• Diagram of negative resistance

• Expressed mathematically,
• The negative resistance of the sample at a particular region is: dI / dV = dJ /dE = Negative
Resistance
•
• If an electric field Eo (or voltage Vo) is applied to the sample, i.e., the current
density Jo is generated.
• As the applied field (or voltage) is increased to E2 (or V2), the current density is
decreased to J2.
• When the field (or voltage) is decreased to E1 (or V1), the current density is
increased to J1, as shown in (a).
• Similarly, for the current controlled mode, the negative-resistance profile is shown
in (b).

•Multiple Values Of Current Density For Negative Resistance

4.2 Two-Valley Model Theory, Mw Generation, Amp’tion & Appns
Two-Valley Model Theory
• A few years before the Gunn effect was discovered, Kroemer proposed a
negative mass Mw amplifier in I958.
• According to the energy band theory of the n-type GaAs, a high-mobility lower valley
is separated by an energy of 0.36 eV from a low-mobility upper valley as shown
below.
• Two-valley Model Of
Electron Energy Versus
Wave Number For N-type
GaAs

• Data For Two Valleys In GaAs

• Data For Two-valley Semiconductors

• Electron densities in the lower and upper valleys remain the same under an equilibrium
condition. When the applied electric field is lower than the electric field of the lower valley (E < El), no
electrons will transfer to the upper valley as shown in Fig. 4….(a).
• When the applied electric field is higher than that of the lower valley and lower than that
of the upper valley (El < E < Eu), electrons will begin to transfer to the upper valley as
shown in Fig. 4…..(b).
• When the applied electric field is higher than that of the upper valley (Eu < E), all
electrons will transfer to the upper valley as shown in Fig. 4….(c).

• If electron densities in the lower and upper valleys are nc and nu, the
conductivity of the n -type GaAs is:
• Where:
• e = the electron charge
• µ =the electron mobility
• n =nc + nu is the electron density
•
•

• Worked: Example Conductivity of an n-Type GaAs Gunn Diode
• Determine The Conductivity Of The Diode.
•
• Solution
• Conductivity Is Given By:

• On the basis of the Ridley-Watkins-Hilsum theory as described earlier,
• The band structure of a semiconductor must satisfy three criteria in
order to exhibit negative resistance.
• 1. The separation energy between the bottom of the lower valley and the
bottom of the upper valley must be several times larger than the thermal
energy (about 0.026 eV) at room temperature.
• This means that:
• 2. The separation energy between the valleys must be smaller than the gap
energy between the conduction and valence bands. This means that DeltaE
< Eg.
• Otherwise the semiconductor will break down and become highly conductive before
the electrons begin to transfer to the upper valleys because hole-electron pair
formation is created.

3. Electrons in the lower valley must have high mobility, small
effective mass, and a low density of state,
• Whereas those in the upper valley must have low mobility, large effective
mass, and a high density of state.
• In other words, electron velocities (dE/ dk) must be much larger in
the lower valleys than in the upper valleys.

• The two most useful semiconductors-silicon and germanium-do not meet all these criteria.
• Some compound semiconductors, such as gallium arsenide (GaAs), indium phosphide (InP), &
cadmium telluride (CdTe) do satisfy these criteria.
• Others such as indium arsenide
(InAs), gallium phosphide (GaP),
& indium antimonide (InSb) do
not.
• Current versus field characteristic of a two-valley semiconductor.

 A mathematical analysis of differential negative resistance requires a detailed
analysis of high-field carrier transports.
• From electric field theory the magnitude of the current density in a
semiconductor is given by:
• J = qnv
where q =electric charge
• n =electron density, and
• v =average electron velocity.

Microwave Generation & Amplification
 Microwave Generation
• As described earlier, if the applied field is less than threshold the specimen
is stable.
• If, the field is greater than threshold, the sample is unstable and divides up
into two domains of different conductivity and different electric field but
the same drift velocity, shown below
• Electric field versus drift velocity,
• Showing stable and unstable regions

• At the initial formation of the accumulation layer, the field behind the layer
decreases and the field in front of it increases.
• This process continues as the layer travels from the cathode toward the anode.
• As the layer approaches the anode, the field behind it begins to increase again;
• and after the layer is collected by the anode, the field in the whole sample is higher than
threshold.
• When the high-field domain disappears at the anode,
• a new dipole field starts forming again at the cathode and the process repeats
itself.
• Since current density is proportional to the drift velocity of the electrons, a Mw
pulsed current output signal is obtained .

• The oscillation frequency of the pulsed current is given by:
• Where:
• vd is the velocity of the domain or approximately the drift velocity of the electrons
• Leff is the effective length that the domain travels.
• Experiments have shown that;
• The n-type GaAs diodes have yielded 200-W pulses at 3.05 GHz and 780-mW CW power
at 8. 7 GHz.
• Efficiencies of 29% have been obtained in pulsed operation at 3.05 GHz and 5.2% in CW
operation at 24.8 GHz.
• Predictions have been made that 250-kW pulses from a single block of n -type
GaAs are theoretically possible up to 100 GHz.
• The source generation of solid-state microwave devices has many advantages
over the vacuum tube devices they have replaced.

• Below is the latest state-of-the-art performance for GaAs and InP Gunn
diodes.
• The numbers adjacent to the data points
indicate efficiency in percent.
• Gunn diode oscillators have better noise
performance than IMPATTs.
• They are used as local oscillators for
receivers and as primary sources where
CW powers of up to 100 mW are required.
• InP Gunn diodes have higher power &
efficiency than GaAs Gunn diodes.

• When an RF signal is applied to a Gunn oscillator, amplification of the
signal occurs,
• Provided that the signal freq is low enough to allow the space charge in the
domain to readjust itself.
• There is a critical value of fL above which the device will not amplify. Below this
frequency limit the sample presents an impedance with a negative real part that can
be utilized for amplification.
• If noL becomes less than 1012
/cm2
,
• Domain formation is inhibited and the device exhibits a non-uniform field
distribution that is stable with respect to time and space.
• Such a diode can amplify signals in the vicinity of the transit-time freq & its
harmonics without oscillation.

• In contrast to the stable amplifier,
• The Gunn-effect diode must oscillate at the transit-time freq while it is
amplifying at some other freq.
• The value of noL must be larger than 1012
/cm2
in order to establish traveling domain
oscillations;
• Hence, substantially larger output power can be obtained.
• Because of the presence of high-field domains,
• This amplifier is called a Traveling Domain Amplifier (TDA).
• Gunn diodes have been used in conjunction with circulator-coupled networks in the
design of high-level wideband transferred electron amplifiers that have a voltage
gain-bandwidth product in excess of 10 dB for frequencies from 4 to about 16 GHz.

4.2 AVALANCHE TRANSIT-TIME DEVICES
Overview
•
• Avalanche transit-time diode oscillators rely on the effect of voltage
breakdown across a reverse-biased p-n junction to produce a supply of holes
and electrons.
• The tunnel diode was the first of such devices to be realized in practice.
• Its operation depends on the properties of a forward-biased p-n junction in
which both the p and n regions are heavily doped.
• The other two devices are:
• The transferred electron devices &
• The avalanche transit-time devices.

• The transferred electron devices or the Gunn oscillators operate
simply by the application of a de voltage to a bulk semiconductor.
• There are no p-n junctions in this device.
• Its frequency is a function of the load and of the natural frequency of the
circuit.
• The avalanche diode oscillator uses carrier impact ionization &
drift in the high-field region of a semiconductor junction:
• To produce a negative resistance at microwave frequencies.

• The device was originally proposed in a theoretical paper by Read in which
he analyzed the negative-resistance properties of an idealized n+p- i-p+
diode.
• Two distinct modes of avalanche oscillator have been observed. One is the
IMPATT mode, which stands for Impact Ionization Avalanche Transit-time
Operation.
• In this mode the typical dc-to-RF conversion efficiency is 5 to 10%, & freqs
are as high as 100 GHz with silicon diodes.
• The other mode is the TRAPATT mode, which represents trapped plasma
avalanche triggered transit operation.
• Its typical conversion efficiency is from 20 to 60%.

• Another type of active microwave device is the BARITT (barrier injected
transit-time) diode.
• It has long drift regions similar to those of IMPATT diodes.
• The carriers traversing the drift regions of BARITT diodes, are generated by
minority carrier injection from forward-biased junctions rather than being
extracted from the plasma of an avalanche region.
• Several different structures have been operated as BARITT diodes, such as
p-n-p, p-n-v-p, p-n-metal, and metal-n metal.
• BARITT diodes have low noise figures of 15 dB, but their bandwidth is
relatively narrow with low output power.

READ DIODE
• Physical Description
• The basic operating principle of IMPATT diodes can be most easily
understood by reference to the first proposed avalanche diode, the Read
diode [1].
• The theory of this device was presented by Read in 1958, but the first
experimental Read diode was reported by Lee et al. in 1965 [3].
• A mode of the original Read diode with a doping profile and a de electric
field distribution that exists when a large reverse bias is applied across the
diode is shown below.

The Read diode
• It is an
n+ -p-i-p+ structure,
where the superscript
plus sign denotes very
high doping and the
i or v refers to intrinsic
material.

• The device consists essentially of two regions.
• One is the thin p region at which avalanche multiplication occurs. This
region is also called the high-field region or the avalanche region.
• The other is the i or v region thro which the generated holes must drift in
moving to the p+ contact.
• This region is also called the intrinsic region or the drift region. The p
region is very thin.
• The space b/n the n+ -p junction and the i-p+ junction is called the space-
charge region.
• Similar devices can be built in the p+ -n-i-n+ structure,
• In which electrons generated from avalanche multiplication drift thro the
I region.

• The Read diode oscillator consists of an n+ -p-i-p+ diode biased in
reverse & mounted in a Mw cavity.
• The impedance of the cavity is mainly inductive & is matched to the
mainly capacitive impedance of the diode to form a resonant circuit.
• The device can produce a negative ac resistance that, in turn,
• Delivers power from the de bias to the oscillation.
• The Read diode is mounted in a microwave resonant circuit.

 Avalanche Multiplication
• When the reverse-biased voltage is well above the punch-through or
breakdown voltage,
• The space-charge region always extends from the n+ -p junction through the p and i
regions to the i-p+ junction.
• The fixed charges in the various regions are shown in (b).
• A positive charge gives a rising field in moving from left to right.
• The maximum field, which occurs at the n+ -p junction, is about several hundred
kilovolts per centimeter.
• Carriers (holes) moving in the high field near then+ -p junction acquire
energy to knock valence electrons into the conduction band, thus producing
hole-electron pairs.

• Avalanche Multiplication
• The transit time of a hole across the drift i -region L is given
• by:
• and the avalanche multiplication factor is:
•
• Where:
V = applied voltage
vb = avalanche breakdown voltage
n = 3-6 for silicon is a numerical factor depending on the doping of p+
-n or n+ -p junction.

• The breakdown voltage for a silicon p+ -n junction can be expressed as:
where Pn = resistivity
µn = electron mobility
εs = semiconductor permittivity
Emax = maximum breakdown of the electric field

• Below is the avalanche breakdown voltage as a function of impurity at a
p+ -n junction for several semiconductors.

 IMPATT Diodes
• Physical Structures
• A theoretical Read diode made of an n+ -p-i-p+ or p+ -n-i-n+ structure has been
analyzed.
• Its basic physical mechanism is the interaction of the impact ionization avalanche
and the transit time of charge carriers. Hence the Read-type diodes are called
IMPATT diodes.
• These diodes exhibit a differential negative resistance by two ffects:
1. The impact ionization avalanche effect, which causes the carrier current
lo(t) & the ac voltage to be out of phase by 90°
2. The transit-time effect, which further delays the external current l,(t)
relative to the ac voltage by 90°

• It has been confirmed that a negative resistance of the IMPATT diode can be
obtained from a junction diode with any doping profile.
• Many IMPATT diodes consist of a high doping avalanching region,
• Followed by a drift region where the field is low enough that the carriers
can traverse through it without avalanching.
• The Read diode is the basic type in the IMPATT diode family.
• The others are:
• The one-sided abrupt p-n junction,
• The linearly graded p-n junction (or double-drift region), and
• The p-i-n diode, all of which are shown in Fig. 8-2-1.

 Negative Resistance
• Small-signal analysis of a Read diode results in the following
expression for the real part of the diode terminal impedance:

• Moreover, ϴ is the transit angle, given by:
•
• and wr is the avalanche resonant frequency, defined by:
• The quantity a' is the derivative of the ionization coefficient with respect to
the electric field.
• This coefficient, the number of ionizations per centimeter produced by a single carrier,
is a sharply increasing function of the electric field.
• The variation of the negative resistance with the transit angle when w > Wr
is plotted below.

• The peak value of the negative resistance occurs near ϴ = π.
• For transit angles larger than π and approaching 3π/2, the negative
resistance of the diode decreases rapidly.
• For practical purposes, the Read-type IMPATT diodes work well only in a freq
range around the π transit angle.
• That is,
• Negative resistance
versus transit angle

• At a given freq, the maximum output power of a single diode is limited
by semiconductor materials & the attainable impedance levels in Mw
Ccts.
• For a uniform avalanche, the maximum voltage that can be applied
across the diode is given by:
• where L is the depletion length and Em is the maximum electric field.
This maximum applied voltage is limited by the breakdown voltage.
• The maximum current that can be carried by the diode is also limited by
the avalanche breakdown process,
• For the current in the space-charge region causes an increase in the electric
field.

• The maximum current is given by:
• Therefore the upper limit of the power input is given by:
• The capacitance across the space-charge region is defined as:
• Combining last 2 equations & application of 2πfT = 1 yield:
• The maximum power that can be given to the mobile carriers decreases as 1/f.
• For silicon, this electronic limit is dominant at frequencies as high as 100 GHz.
• The efficiency of the IMPATT diodes is given by:
•

• State-of-the-art Performance for GaAs and Si IMPATTs

 Worked Example for CW Output Power of an IMPATT Diode
• An IMPATT diode has the following parameters:

• TRAPATT Diodes & Their Applications
•
• The abbreviation TRAPATT stands for:
• “Trapped Plasma Avalanche Triggered Transit” mode, a mode first reported by Prager et al.
• It is a high-efficiency Mw generator capable of operating from several hundred Megahertz to
several Gigahertz.
• The basic operation of the oscillator is a semiconductor p-n junction diode reverse biased to
current densities,
• Well in excess of those encountered in normal avalanche operation.
• High-peak-power diodes are typically silicon n+ -p-p+ (or p+ -n-n+) structures with the n-type
depletion region width varying from 2.5 to 12.5 µm.
•
• The doping of the depletion region is generally such that:
• The diodes are well "punched-through" at breakdown;
• That is, the dc electric field in the depletion region just prior to breakdown is well above the
saturated drift-velocity level.

Principles of Operation
•Approximate analysis shows that,
• A high-field avalanche zone propagates through the diode and
fills the depletion layer with a dense plasma of electrons &
holes that become trapped in the low-field region behind the
zone.
•A typical voltage waveform for the TRAPATT mode of an
avalanche p+ -n-n+ diode operating with an assumed
square-wave current drive is shown below.

• I-V waveforms for
TRAPATT diode.
• At point A the electric
field is uniform throughout
the sample and its magnitude
is large but less than the value
required for avalanche breakdown.
• The current density
is expressed by:
• where Es is the semiconductor
dielectric permittivity of the diode
•

• At the instant of time at point A, the diode current is turned on.
• Since the only charge carriers present are those caused by the thermal generation,
the diode initially charges up like a linear capacitor,
• Driving the magnitude of the electric field above the breakdown voltage.
• When a sufficient number of carriers is generated,
• The particle current exceeds the external current & the electric field is depressed
throughout the depletion region,
• Causing the voltage to decrease (point B to point C)
• During this time interval,
• The electric field is sufficiently large for the avalanche to continue, & a dense plasma of
electrons & holes is created.
• As some of the electrons & holes drift out of the ends of the depletion layer,
• The field is further depressed & “Traps" the remaining plasma.

• The voltage decreases to point D. A long time is required to remove the plasma
because the total plasma charge is large compared to the charge per unit time
in the external current.
• At point E the plasma is removed, but a residual charge of electrons remains in
one end of the depletion layer and a residual charge of holes in the other end.
• As the residual charge is removed, the voltage increases from point E to point F.
• At point F, all the charge that was generated internally has been removed.
• This charge must be greater than or equal to that supplied by the external current;
• Otherwise the voltage will exceed that at point A.
• From point F to point G, the diode charges up again like a fixed capacitor.
• At point G the diode current goes to zero for half a period and the voltage remains
constant at VA until the current comes back on and the cycle repeats.

• Power Output and Efficiency
• RF power is delivered by the diode to an external load when the diode is
placed in a proper circuit with a load.
• The main function of this circuit is to match the diode effective negative
resistance to the load at the output freq,
• While reactively terminating (trapping) freqs above the oscillation frequency in order
to ensure TRAPATT operation.
• TRAPATT
Oscillator
Capabilities

BARITT DIODES
•BARITT diodes, I.e., Barrier Injected Transit-time diodes,
are the latest addition to the family of active microwave
diodes.
• They have long drift regions similar to those of IMPATT diodes.
•The carriers traversing the drift regions of BARITT diodes,
are generated by:
• Minority carrier injection from forward-biased junctions
instead of being extracted from the plasma of an avalanche
region.

• Several different structures have been operated as BARITT diodes, including:
• p-n-p,
• p-n-v-p,
• p-n-metal, and
• metal-n-metal.
• For a p-n-v-p BARITT diode, the forward-biased p-n junction emits holes into
the v region.
• These holes drift with saturation velocity through the v region and are
collected at the p contact.
• The diode exhibits a negative resistance for transit angles b/n π & 2π.

•The optimum transit angle is approximately 1.6π.
•Such diodes are much less noisy than IMPATT diodes. Noise
figures are as low
•as 15 dB at C-hand frequencies with silicon BARITT
amplifiers.
•The major disadvantages of BARITT diodes are:
• Relatively narrow bandwidth &
• Power outputs limited to a few milliwatts.

• Principles of Operation
•
• A crystal n-type silicon wafer with 11 Ω resistivity and 4 X 1014
per cubic
centimeter doping is made of a 10-JLm thin slice.
• Then the n-type silicon wafer is sandwiched b/n two PtSi Schottky barrier contacts of
about 0.1 µm thickness.
• A schematic diagram of a metal-n-metal structure is shown in (a).
• The energy-band diagram at thermal equilibrium is shown below in (b)
• Where are the barrier heights for the metal-semiconductor contacts,
respectively.
• For the PtSi-Si-PtSi structure mentioned previously, 0.85 eV.

• Energy-band Diagram
•
• The hole barrier height
for the forward-
biased contact is about
0.15 eV.
(c) shows the energy-
band diagram when a
voltage is applied.

• The mechanisms responsible for the Mw oscillations are
derived from:
1. The rapid increase of the carrier injection process caused by
the decreasing potential barrier of the forward-biased metal-
semiconductor contact
2. An apparent 3π /2 transit angle of the injected carrier that
traverses the semiconductor depletion region

• The rapid increase in terminal current with applied voltage (above 30 V) as
shown above is caused by:
• Thermionic hole injection into the semiconductor as the depletion layer
of the reverse-biased contact reaches through the entire device
thickness.
• The critical voltage is approximately given by:

• Current versus voltage
of a BARITI diode
(PtSi-Si-PtSi).
• The current-voltage
characteristics of the
silicon MSM structure
(PtSi-Si-PtSi) were
measured at at 77° K
and 300° K.

• The device parameters are L = 10
• The current increase is not due to avalanche multiplication, as is apparent
from the magnitude of the critical voltage and its negative temperature
coefficient.
• At 77°K the rapid increase is stopped at a current of about 10-5
A.
• This saturated current is expected in accordance with the thermionic
emission theory of hole injection from the forward-biased contact with a
hole barrier height of about 0.15 eV

• Worked Example of Breakdown Voltage of a BARITT Diode
• An M-Si-M BARITT diode has the following parameters:
Determine: a. the breakdown voltage; b. the breakdown electric field.
• Solution
• a. The breakdown voltage is double its critical voltage as:

LECTURE No. 4.pptxEngineeringEngineering

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LECTURE No. 4.pptxEngineeringEngineering