IMPATT Structure, Fabrication & Construction
- the IMPATT diode structure and the way it is constructed or fabricated and how this impacts its operation and characteristics.
In many respects, the IMPATT diode structure is very similar to that of many other forms of diode, and in particular the standard PN junction diode or the Schottky diode.
However the differences in its construction and fabrication mean that it is able to operate in its avalanche mode whether the transit time provides the negative resistance.
IMPATT diode construction
There is a variety of structures that are used for the IMPATT diode. All are variations of a basic PN junction and usually there is an intrinsic layer, i.e. a layer without any doping that is placed between the P type and N type regions.
The structures use a PN junction which is reverse biased so that avalanche multiplication occurs within the high field region. In most structures a Schottky barrier can be 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..
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.
IMPATT diode horizontal structure
The devices are normally used as microwave power sources and as a result, heat dissipation is a key issue. As a result 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.
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.
Electric field profiles
One major element of the structure of an IMPATT diode is the way in which the electric field profile occurs. The diagram below shows the electric field profile an also the areas of highest electric field represented by the grey areas show where the avalanche breakdown occurs.
Electric field profiles for common IMPATT diode structures
The diagram shows the main types of avalanche diode. The p+ n, i n+ diode (leftmost on the diagram) is the Read diode and the rightmost diode structure, p+ i n+ is also known as the Misawa diode.
By Ian Poole
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