In this article we are explaining about Impatt diode, these kinds of diodes are not like our conventional diode. Compare Impatt diode with normal diodes, they are way much more specific to their applications.

What is Impatt Diode?

The Impatt diode is a very high-power semiconductor device that also can handle microwave frequency, the technical operations and applications of this diode are more different from general-purpose diodes. The Impatt diodes have very specific applications at high power devices such as radars at communication systems.

Invention background of Impatt diode  

Before the invention of avalanche transmit-time devices, the semiconductor components such as transistors are limited by transit time and other effects to frequencies below few GHz.

The applications needed to handle gigahertz (GHz) frequencies such as, DC to microwave power conversion and also amplification at microwave frequency, these circuits are been works with Impatt diode with frequency range of (30 to 300GHz)

The frequency range and power generation capability of Impatt diodes, given an advancement to millimeter wave systems like, communication, radars, security screening, medical research and etc.

At the operation, the Impatt diode kept the junction temperature at a low level possible for a reliable operation, the silicon based Impatt diode had an upper temperature limit of 250°C.

Impatt diode full form

IMPact ionization Avalanche Transit Time diode is the full form of this semiconductor device.

Impatt diode construction

Impatt diode construction
Impatt diode construction

The construction of the Impatt diode is somewhat similar to the pin diode because of the presence of an intrinsic layer at the middle of the device.

The semiconductor layer arrangement of the Impatt diode is shown in the figure, first P, N, Intrinsic layer and N-type layer, the gallium arsenide GaAs is used to build the Impatt diode due to low noise behavior of this material.

The Impatt diode structure is designed to works with the reverse-biased operation, the avalanche multiplications process occurs within the high field region.

At this structure a Schottky barrier format is used as the injection junction, the thin layer is main for the high-frequency operation.

Materials used to made Impatt diode

The semiconductor material combinations such as Ga-si, GaAS or InP, from these combination Gallium silicon Ga-si and Gallium arsenide GaAS are the materials mainly used at Impatt diode.

Both these material based diode provide high efficiency and highest operating frequency with a least noise figure.

Most of the modern Impatt diodes are silicon based.

Impatt diode symbol

The symbolic indication of the Impatt diode is always a mystery for everyone, but the fact is that they have the normal p-n junction diode symbol.

Impatt diode diagram

Impatt diode diagram
Impatt diode diagram

 

The Impatt diode diagram put forward more information’s about the device, the purposes such as avalanche breakdown will happen at avalanche region and drift current flow due to the electric field is happened at drift region.

Working of Impatt diode

We early mentioned that the Impatt diode operation has happened at the reverse biased condition, also called avalanche breakdown condition.

Before we start to explain the working of this device, this may operate at a high voltage value gradient that is about 400kv/cm

At the reverse biased condition current increases gradually, which results in avalanche breakdown, and at reverse condition width of the depletion region increases that is an intrinsic layer.

And at the junction majority of carriers drift out at the drift region, this is because of the presence of the high electric field.

On the condition that charged carriers move with super-fast velocity, the charged carriers collide with each other with other atoms. And this results in forming electron-hole pairs, this is called the avalanche multiplication process of the Impatt diode.

So the moving charged carriers generate a heavy current inside the diode, which is also called impact ionization condition.

Impatt diode characteristics

Impatt diode characteristics
Impatt diode characteristics

These are the characteristics of the Impatt diode, which shows the current pulse actually at the cathode terminal.

The RF voltage is there at a negative peak, and voltage and current in the Impatt diode are 180˚ out of phase.

The first 90° phase shift is achieved within the avalanche region and the remaining 90° is obtained by adjusting the drift region length.

An RF negative resistance exists at the Impatt diode junction, this particular resistance factor is used at the oscillator and amplifier.

Impatt diode equivalent circuit

Impatt diode equivalent circuit
Impatt diode equivalent circuit

The figure shows the equivalent circuit of Impatt diode, the impedance of the Impatt diode is inductive La on avalanche region and Ld on drift region, which is matched with the capacitance of the diode which is Ca on avalanche region and Cd on drift region.

Both these inductance and capacitance to form a resonant circuit, such device can produce a negative ac resistance that in turns deliver from the DC bias to the oscillator.

Features of Impatt diode

  1. Operating frequency = 4GHz to 200GHz
  2. Operation principle = avalanche multiplication
  3. Output power = 1w CW and >400watt pulsed
  4. Efficiency = 3% CW and 60% pulsed below 1GHz, more powerful than GUNN diode
  5. Noise figure = 30db

Avalanche multiplication or Impatt avalanche or impact ionization

Avalanche multiplication or Impatt avalanche or impact ionization
Avalanche multiplication or Impatt avalanche or impact ionization

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  • If a free electron with a sufficient energy strikes a silicon atom, it will breaks the covalent bond of silicon and liberate an electron from the covalent bond.
  • And if the electron liberated gain energy being in an electric field and liberate other electrons from other covalent bonds, then this process will spreads very quickly as a chain reaction and produce a large number of electrons and result a large amount of current flow.
  • This is phenomenon called avalanche multiplication, Impatt avalanche or Impact ionization.

Avalanche multiplication factor

  m= 1/1- (V/Vb) n

  V= Applied voltage

  Vb = Avalanche break down voltage

  n= it is the numerical factor depending on the doping of silicon (3 to 6) is the range, for p+ n- or n+ p- junctions.

  Vb = Pn µn ES |Emax|b2

  Pn = resistivity

  µn = electron mobility

  Es = semiconductor permittivity

  Emax = maximum breakdown of the electric field

Types of Impatt diode

The three type classification of Impatt diode is based on doping profile or the arrangement of regions at the diode.

  1. Single drift region (SDR)

It is a three region Impatt diode type consist of one avalanche zone and one drift zone combined to form a p+ n n+ structure.

The single drift region (SDR) structure have a doping concentration of 1020 to 1016.

  1. Double drift region (DDR)

As like the name suggested, the ‘DDR’ Impatt diode consist of two drift layers, one electron and one hole layers on both side of the central avalanche zone, combined to form a P+ pn n+ structure.

The double drift region (DDR) structure have a doping concentration of 1020 to 1013

  1. Double avalanche region (DAR)

The DAR Impatt diode consist of two avalanche regions, the structure is like p+ ni pn+, the drift zone is sandwiched between the double avalanche region zones and the electrons and holes from the both junctions will be travel across the i (intrinsic) in opposite direction and then delivers the power.

Impatt diode in microwave

The Impatt diode is a microwave diode, which can handle 3GHz to 100GHz frequency range, thus they can the ability to generate microwaves or work with microwaves.

Advantages of Impatt diode

  • The microwave diode has a high power capability compared to other diodes.
  • Output is reliable
  • Less expensive
  • Reliable operation at high temperatures
  • Compact size

Disadvantages of Impatt diode

  • High noise figure
  • They offer a low turning range
  • High operation current
  • High spurious AM/FM noise
  • They offer high sensitivity to different operating conditions

 Impatt diode applications

  • Voltage-controlled oscillator circuit
  • Low power radar systems
  • Telecommunication receivers
  • Injection locked amplifiers
  • Cavity stability Impatt diode oscillators
  • Intrudes alarm
  • General detector using RF technology
  • Microwave oscillator
  • Parametric amplifier
  • Microwave generator
  • Telecommunication transmitters

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