30/10/2022, 14:50 Avalanche Transit Time Devices

Avalanche Transit Time Devices

The process of having a delay between voltage and current, in avalanche together with transit
time, through the material is said to be Negative resistance. The devices that helps to make a
diode exhibit this property are called as Avalanche transit time devices.

The examples of the devices that come under this category are IMPATT, TRAPATT and BARITT
diodes. Let us take a look at each of them, in detail.

IMPATT Diode
This is a high-power semiconductor diode, used in high frequency microwave applications. The
full form IMPATT is IMPact ionization Avalanche Transit Time diode.

A voltage gradient when applied to the IMPATT diode, results in a high current. A normal diode
will eventually breakdown by this. However, IMPATT diode is developed to withstand all this. A
high potential gradient is applied to back bias the diode and hence minority carriers flow across
the junction.

Application of a RF AC voltage if superimposed on a high DC voltage, the increased velocity of

holes and electrons results in additional holes and electrons by thrashing them out of the crystal
structure by Impact ionization. If the original DC field applied was at the threshold of developing
this situation, then it leads to the avalanche current multiplication and this process continues.
This can be understood by the following figure.

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Due to this effect, the current pulse takes a phase shift of 90°. However, instead of being there, it
moves towards cathode due to the reverse bias applied. The time taken for the pulse to reach
cathode depends upon the thickness of n+ layer, which is adjusted to make it 90° phase shift.
Now, a dynamic RF negative resistance is proved to exist. Hence, IMPATT diode acts both as an
oscillator and an amplifier.

The following figure shows the constructional details of an IMPATT diode.

The efficiency of IMPATT diode is represented as

P ac Va Ia
η= [P ]
=
dc Vd [ Id ]

Where,

P ac = AC power

P dc = DC power

Va & Ia = AC voltage & current

Vd & Id = DC voltage & current

Disadvantages
Following are the disadvantages of IMPATT diode.

It is noisy as avalanche is a noisy process

Tuning range is not as good as in Gunn diodes

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Applications
Following are the applications of IMPATT diode.

Microwave oscillator
Microwave generators
Modulated output oscillator
Receiver local oscillator
Negative resistance amplifications
Intrusion alarm networks highQIMPATT

Police radar highQIMPATT

Low power microwave transmitter highQIMPATT

FM telecom transmitter lowQIMPATT

CW Doppler radar transmitter lowQIMPATT

TRAPATT Diode
The full form of TRAPATT diode is TRApped Plasma Avalanche Triggered Transit diode. A
microwave generator which operates between hundreds of MHz to GHz. These are high peak
power diodes usually n+- p-p+ or p+-n-n+ structures with n-type depletion region, width varying
from 2.5 to 1.25 µm. The following figure depicts this.

The electrons and holes trapped in low field region behind the zone, are made to fill the depletion
region in the diode. This is done by a high field avalanche region which propagates through the
diode.

The following figure shows a graph in which AB shows charging, BC shows plasma formation,
DE shows plasma extraction, EF shows residual extraction, and FG shows charging.

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Let us see what happens at each of the points.

A: The voltage at point A is not sufficient for the avalanche breakdown to occur. At A, charge
carriers due to thermal generation results in charging of the diode like a linear capacitance.

A-B: At this point, the magnitude of the electric field increases. When a sufficient number of
carriers are generated, the electric field is depressed throughout the depletion region
causing the voltage to decrease from B to C.

C: This charge helps the avalanche to continue and a dense plasma of electrons and holes
is created. The field is further depressed so as not to let the electrons or holes out of the
depletion layer, and traps the remaining plasma.

D: The voltage decreases at point D. A long time is required to clear the plasma as the total
plasma charge is large compared to the charge per unit time in the external current.

E: At point E, the plasma is removed. Residual charges of holes and electrons remain each
at one end of the deflection layer.

E to F: The voltage increases as the residual charge is removed.

F: At point F, all the charge generated internally is removed.

F to G: The diode charges like a capacitor.

G: At point G, the diode current comes to zero for half a period. The voltage remains
constant as shown in the graph above. This state continues until the current comes back on
and the cycle repeats.

The avalanche zone velocity Vs is represented as

dx J
Vs = =
dt qNA

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Where

J = Current density

q = Electron charge 1.6 x 10-19

NA = Doping concentration

The avalanche zone will quickly sweep across most of the diode and the transit time of the
carriers is represented as

L
τs =
Vs

Where

Vs = Saturated carrier drift velocity

L = Length of the specimen

The transit time calculated here is the time between the injection and the collection. The repeated
action increases the output to make it an amplifier, whereas a microwave low pass filter
connected in shunt with the circuit can make it work as an oscillator.

Applications
There are many applications of this diode.

Low power Doppler radars

Local oscillator for radars
Microwave beacon landing system
Radio altimeter
Phased array radar, etc.

BARITT Diode
The full form of BARITT Diode is BARrier Injection Transit Time diode. These are the latest
invention in this family. Though these diodes have long drift regions like IMPATT diodes, the
carrier injection in BARITT diodes is caused by forward biased junctions, but not from the plasma
of an avalanche region as in them.

In IMPATT diodes, the carrier injection is quite noisy due to the impact ionization. In BARITT
diodes, to avoid the noise, carrier injection is provided by punch through of the depletion region.

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The negative resistance in a BARITT diode is obtained on account of the drift of the injected
holes to the collector end of the diode, made of p-type material.

The following figure shows the constructional details of a BARITT diode.

For a m-n-m BARITT diode, Ps-Si Schottky barrier contacts metals with n-type Si wafer in

between. A rapid increase in current with applied voltage above30v is due to the thermionic
hole injection into the semiconductor.

The critical voltage (Vc) depends on the doping constant (N) , length of the semiconductor

(L) and the semiconductor dielectric permittivity (ϵS) represented as

qNL2
2ϵS
Vc =

Monolithic Microwave Integrated Circuit MMIC

Microwave ICs are the best alternative to conventional waveguide or coaxial circuits, as they are
low in weight, small in size, highly reliable and reproducible. The basic materials used for
monolithic microwave integrated circuits are −

Substrate material
Conductor material
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Dielectric films
Resistive films

These are so chosen to have ideal characteristics and high efficiency. The substrate on which
circuit elements are fabricated is important as the dielectric constant of the material should be
high with low dissipation factor, along with other ideal characteristics. The substrate materials
used are GaAs, Ferrite/garnet, Aluminum, beryllium, glass and rutile.

The conductor material is so chosen to have high conductivity, low temperature coefficient of
resistance, good adhesion to substrate and etching, etc. Aluminum, copper, gold, and silver are
mainly used as conductor materials. The dielectric materials and resistive materials are so
chosen to have low loss and good stability.

Fabrication Technology
In hybrid integrated circuits, the semiconductor devices and passive circuit elements are formed
on a dielectric substrate. The passive circuits are either distributed or lumped elements, or a
combination of both.

Hybrid integrated circuits are of two types.

Hybrid IC
Miniature Hybrid IC

In both the above processes, Hybrid IC uses the distributed circuit elements that are fabricated
on IC using a single layer metallization technique, whereas Miniature hybrid IC uses multi-level
elements.

Most analog circuits use meso-isolation technology to isolate active n-type areas used for FETs
and diodes. Planar circuits are fabricated by implanting ions into semi-insulating substrate, and to
provide isolation the areas are masked off.

"Via hole" technology is used to connect the source with source electrodes connected to the
ground, in a GaAs FET, which is shown in the following figure.

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There are many applications of MMICs.

Military communication
Radar
ECM
Phased array antenna systems
Spread spectrum and TDMA systems

They are cost-effective and also used in many domestic consumer applications such as DTH,
telecom and instrumentation, etc.

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