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Post: #1

.ppt  Impatt-Diode.ppt (Size: 1,009.5 KB / Downloads: 269)
Presented By:Name:- SABYASACHI SABAT
Stream:- ETE


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.
The pn junction in the avalanche breakdown condition exhibits negative resistance characteristics in the microwave frequency range.
Since the negative resistance is based upon avalanche multiplication and transit-time effect of carriers, the device has been called the IMPATT (Impact Avalanche Transit-Time) Diode.
Device Structure

The original suggestion for a microwave device employing transit-time effect was made by W. T. Read and involved an n+-p-i-p+ structure such as that shown in figure. This device operates by injecting carriers into the drift region and is called an IMPATT diode.
The device consists essentially of two regions:
1) the n+p region, at which avalanche multiplication occurs, and
2) the i (essentially intrinsic) region, through which generated holes must drift in moving to p+ contact.

Principle of operation

IMPATT diode
IMPATT I-V Characteristics
Small Signal model
Some IMPATT Circuits
Device structure is based on the doping profile. The three basic types of Impatt diodes are:-
1. Single drift region (SDR) - The SDR diode consists of a single avalanche zone and a single drift zone with p+nn+ structure.
2. Double drift region (DDR) – A DDR diode has a p+pnn+ structure that consist of two drift layers, one for electrons and other for holes on either side of the central avalanche zone.
3. Double avalanche region (DAR) – The DAR diode has a p+nipn+ structure that consist of one drift zone sandwiched between two avalanche zones. The electrons and holes from the two junctions travel across the central i-region in opposite directions and deliver power.
These diodes make excellent microwave generators for many applications like:-
1. Parametric amplifier,
2. Parametric up converter,
3. Parametric down converter,
4. Negative resistance parametric amplifier.
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

IMPact Ionization Transit Time
IMPATT devices can be used for oscillator and amplifier applications
They can be fabricated with Si, GaAs, and InP
Can be used up 400 GHz.
Noisy oscillator
In general, IMPATTs have 10 dB higher AM noise than that of Gunn diodes
IMPATT diode is not suitable for use as a local oscillator in a receiver.
Post: #2

.docx  IMPATT.docx (Size: 758.82 KB / Downloads: 67)

IMPATT (IMPact Avalanche ionization and Transit Time) diodes are principal active elements for use in millimetric pulsed-mode generators. Semiconductor structures suitable for fabrication of continuos-mode
IMPATT diodes have been well known for a long time [1- 2]. They have been utilized successfully in many
applications in microwave engineering. The possibilities of using the same structures for pulsed-mode microwave
generators are very interesting because the pulsed-mode IMPATT-diode generators can successfully operate at high current densities without deterioration of reliability. The cross section of the pulsed-mode IMPATT diode
may be larger than that of continuous-mode diodes. Therefore, a pulsed-mode oscillator can provide a larger
output power. Considering that the increase of the output power of millimetric generators is one of the main
problems of microwave electronics it is important to optimize the diode's active layer to obtain the generator's maximum output power. One of the main problems in the operation of high-power
IMPATT-diode pulsed-mode generator is the large variation of the diode's admittance during the pulse. This
variation is significant during each current pulse due to the temperature change in the diode's semiconductor
structure. Therefore, diffusion coefficients, ionization rates and charge mobility experience large variations during the pulse. These changes strongly affect the amplitude and phase of the first harmonic of the diode's avalanche
current. Therefore, the admittance value also changes. This results in the instability of the generator's output
power and frequency within each generated microwave pulse.

Pulsed-mode IMPATT diodes that are utilized in microwave electronics are, most frequently, single drift
and double-drift structures similar to continuous-mode ones [1-5].

where N is the concentration of donors and acceptors and l is the length of the diode
active layer. In this type of diode, the electrical field is strongly distorted when the avalanche current density is
sufficiently high. This large space charge density is one of the main reasons for the sharp electrical field gradient
along the charge drift path. Because of this field gradient, the space charge avalanche ruins itself and consequently
the optimum phase relations degrade between the microwave potential and the current. This factor is
especially important when the IMPATT diode is fed at the maximum current density, which is exactly the case
for pulsed-mode operation. The idea to utilize a complex doping profile semiconductor structure for a microwave diode was originally proposed in the first analysis of IMPATT diodes by Read [6]. This proposed ideal structure has
never been realized till now. However, modern semiconductor technology provides new possibilities for
the fabrication of sub micron semiconductor structures with complex doping profiles. This stimulates the search
for IMPATT-diode special structure optimization for pulsed-mode operation.
The proposed new type of IMPATT diode doping profile is shown in Fig. 1 by the curve 2. This type of
semiconductor structure can be named a quasi-Read-type structure. This type of doping profile provides a
concentration of the electrical field within the p-n junction. This measure helps to decrease the destruction
of the avalanche space charge and therefore permits an improvement of the phase stability between the diode
current and voltage. Historically, many analytical and numerical models have been developed for the various operational modes of IMPATT diodes [1, 6-13]. However, they are not
adequate for very high current density values and different temperature distributions inside the structure,
which is exactly the case for the pulsed-mode IMPATTdiode oscillator. For this reason, we have developed a
new complex numerical model of the IMPATT diode that is composed of the advanced thermal model and the
modified local-field model. The thermal model provides the exact theoretical temperature distribution along the
diode active region.

Device structure

The IMPATT diode family includes many different junctions and metal semiconductor devices. The first IMPATT oscillation was obtained from a simple silicon p-n junction diode biased into a reverse avalanche break down and mounted in a microwave cavity. Because of the strong dependence of the ionization coefficient on the electric field, most of the electron–hole pairs are generated in the high field region. The generated electron immediately moves into the N region, while the generated holes drift across the P region. The time required for the hole to reach the contact constitutes the transit time delay.
The original proposal for a microwave device of the IMPATT type was made by Read and involved a structure. The Read diode consists of two regions (i) The Avalanche region (a region with relatively high doping and high field) in which avalanche multiplication occurs and (ii) the drift region (a region with essentially intrinsic doping and constant field) in which the generated holes drift towards the contact. A similar device can be built with the configuration in which electrons generated from the avalanche multiplication drift through the intrinsic region.
An IMPATT diode generally is mounted in a microwave package. The diode is mounted with its high–field region close to a copper heat sink so that the heat generated at the diode junction can be readily dissipated. Similar microwave packages are used to house other microwave devices.

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