RE: IMPATT Diode
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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 . 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.
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.