IMPATT Structure, Structure & Fabrication

Key details of the structure of different types of IMPATT diode: how the structure and fabrication affect performance and give different types of IMPATT.

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IMPATT Diode Tutorial Includes:
IMPATT diode How does an IMPATT diode work IMPATT diode structure TRAPATT diode BARITT diode

Other diodes: Diode types

The structure of an IMPATT microwave diode bears many similarities to that of an ordinary PN junction diode and also to a Schottky diode.

The differences are created in the fabrication process by altering the structure so that it can operate in its avalanche mode so that the transit time provides the negative resistance.

Obviously the structure of the IMPATT diode is optimised so that it can operate at microwave frequencies.

IMPATT diode structure basics

There is a variety of structures that are used for the IMPATT microwave diode.

The structures are all basic variations of the PN junction although often an intrinsic layer is placed between the P type and N type regions.

The IMPATT diode structures are designed for optimum operation in reverse bias so that avalanche multiplication occurs within the high field region. In most structures a Schottky barrier format is used as the injecting junction.

The most common method of fabricating an IMPATT diode is to use a vertical structure where there is vertical current flow.

Vertical structure for an IMPATT diode — IMPATT diode vertical structure

For this format of diode, the layers are generally grown epitaxially. Where very high frequency devices are to be made layers can become very thin. For these layers, techniques including MBE, molecular beam epitaxy, or MOCVD, metallo-organic chemical vapour deposition can be used.

For a typical Read diode the n-layer may be only 1 to 2 µm thick, and the intrinsic layer may be between 2 and 20µm thick. For very high frequency operation, these dimensions are reduced.

The dopants needed for the different layers may be introduced using one of a number of techniques including diffusion, ion implantation or even in-situ doping during the epitaxial growth process for a given layer.

Apart from the vertical or mesa fabrication, a horizontal structure may also be used using more traditional planar technology.

Horizontal structure for an IMPATT diode — IMPATT diode horizontal structure

IMPATT diode packaging

Packaging semiconductor devices can be a challenging process. This is particularly true for IMPATT diodes because of the microwave frequencies that are used and the power levels involved.

To enable the IMPATT devices to operate satisfactorily, the devices are mounted into packages where the heat can be transferred away from the active areas of the devices as fast as possible. To this end, the devices are often mounted in what may be termed an upside down fashion where the active layers are closest to the heat sinking provided by the package.

Often the package is coaxial in format so that the correct transmission line properties are presented to the RF signal which may be at many tens of GHz. As a result the package is often quite intricate and accordingly very expensive, especially when very high frequencies are used. As IMPATT devices are often used in waveguide cavities which provide the resonant circuit, the diodes are often mounted in packages that can be easily mounted into waveguides.

The most commonly used materials for IMPATT devices are Silicon and Gallium Arsenide, but other materials including Germanium, and Indium Phosphide or Gallium Aluminium Arsenide may also be used.