What is Point Contact Diode : Working & Its Applications

In 1930 during world war-II, point-contact diodes were implemented. At present, these diodes are used normally in the range of 3 to 30 gigahertz. Point contact diodes are the old microwave semiconductor devices that are used in microwave receivers and also used as detectors & receiver mixers. Nott like the PN junction diode, this diode mainly depends on the contact pressure between a semiconductor crystal & point for its operation. So, this article discusses an overview of a point contact diode – working with applications.


What is a Point Contact Diode?

A diode that can be formed by stroking a metallic-based wire through an N-type-based semiconductor to form a small contact region is known as a point-contact diode. The metal is usually silver. The metal is polished at one end to produce a sharp tip with which to make contact to the semiconductor material. The resulting structure resembles an inverted pyramid in shape, hence its name “point contact”. Point Contact Diode is more widely used diode because of its very small junction capacitance and hence a fast switching.

The point-contact diode symbol is shown below.

Point Contact Diode Symbol
Point Contact Diode Symbol

Point Contact Diode Construction

Pont contact diode is constructed with a substrate contact of an N-type semiconductor as well as cat whisker or phosphor-bronze wire. In this type of diode construction, the semiconductor used is either Ge or Si although Ge is used widely because it has high carrier mobility.

Point Contact Diode Construction
Point Contact Diode Construction

The length of the semiconductor substrate is approsimately1.25 mm^2 & its width is 0.5 millimeters thick. Here, one part of the substrate of the semiconductor is connected with the base of metal by soldering with the RF heating method The cross-sectional region of the cat whisker is connected to an N-type semiconductor however the substrate phase which is connected to the cat whisker must be reversed to that metal contact phase. Here both the anode & cathode terminals are simply connected through metallic contacts.

Point Contact Diode Circuit & Its Working

The point-contact diode circuit is shown below. The working principle of the point-contact diode circuit is whenever this diode is connected in forward bias then the current generated within the diode can be supplied throughout the cat whisker then this wire gets heated. Because of this heating, the wire experiences deformation. Therefore, a little gap is intentionally left for the wire expansion in the large current.

Point Contact Diode Circuit
Point Contact Diode Circuit

Once the wire gets heated, the semiconductor within the contact throughout the wire can also get heated. Because of this, it will melt & atoms from the cat whisker are supplied to the semiconductor crystal. So, the cat whisker works like a P-type semiconductor. As a result, a PN Junction can be formed however the region of the junction is extremely small which is called a pointed junction. The diode’s junction cannot be observed clearly due to the small size of the junction. The whole device is arranged within a ceramic or glass envelope. In addition, the supporting arrangement is given to the N-type semiconductor & cat whisker for providing mechanical power to the diode.

In this diode, both the junction & diffused capacitances are extremely small ranging from 0.1 – 1pF because the contact region between the N-type substrate & the wire is extremely small so that the charge carrier’s density near the junction is extremely low.

Point Contact Diode Vs Hot Carrier Diode

The main difference between point contact diode & hot-carrier diode includes the following.

Point Contact Diode

Hot Carrier Diode

Point contact diode is commonly called crystals. A hot carrier diode is called a Schottky barrier diode.
This diode in the detector provides good performance. As compared to the point-contact diode, its performance is low.
This diode has high output voltage at low levels. This diode has low output voltage at low levels.
This diode has a low-saturation level. This diode has a high-saturation level.

 

.This diode is used as an AM detector within a radio receiver and a video detector within a TV receiver. This diode is used in UHF & VHF mixers & detectors.
The diode includes two sections which contain a tiny N-type silicon rectangular crystal & a fine bronze-phosphor & beryllium-copper. It also includes tungsten wire (cat whisker wire).

 

This diode is a rectifying junction diode, wherever the junction is formed through a metal & a semiconductor. For semiconductor injunction, it uses either p-type or n-type silicon with different metals like silver, gold, palladium, or platinum.

V-I Characteristics

The point-contact diode V-I characteristics are shown below.

V-I Characteristics
V-I Characteristics
  • The characteristics of the point-contact diode are almost similar to the PN junction diode but they are highly dependent on temperature.
  • The following diagram shows the V-I characteristics of the point-contact diode at different temperatures like 25 degrees C and 60 degrees C.
  • This diode also has avalanche breakdown voltage.

Advantage and Disadvantages

The advantages of point contact diode include the following.

  • These diodes are perfect for low signal-based applications.
  • They have extremely low junction capacitance.
  • These are extensively used within microwave mixers & detectors.
  • These diodes have extremely low forward resistance.
  • These types of diodes have extremely low reverse resistance.

The disadvantages of point contact diode include the following.

  • The Point contact diode generates high electrical noise.
  • This diode V-I characteristics do not stay constant.
  • This diode has a low-breakdown voltage.
  • It cannot withstand very high power.
  • This diode is less reliable than the conventional diode.
  • These are extremely fragile, so vibration & shock must be avoided throughout handling.
  • They cannot resist extremely high power.

Application/ Uses

The point-contact diode applications or uses include the following.

  • The point-contact diode is mainly used in radio receivers as an amplitude modulation detector and in TV receivers as a video detector.
  • This type of diode is applicable in high-frequency circuits (10 GHZ or above).
  • This diode is used in different circuits like microwave frequency, pulse & frequency conversation.
  • It is used in rectifying elements of radio frequency signal detection.

What are the advantages of using a point contact diode over other types of semiconductors like Schottky diodes and PIN diodes?

Answer: The main advantage of using point contact diodes over other types is that they have low capacitance. This makes them suitable for high frequency applications such as radio receivers, television receivers and microwave communication systems where large values of capacitance can lead to problems with signal distortion and noise generation.

What is the difference between a P-N Junction and a Point Contact Diode?

A P-N Junction has two junctions, and it can conduct current in both directions. A point contact diode has only one junction, so it can only conduct current in one direction.

Why does the point contact diode only have one junction?

Because it is a p-n junction and the n-side is not used for current flow, it is not necessary to have an additional p-side.

Thus, this is all about an overview of a point contact diode – working with applications. In this diode, a small point junction is formed in between an N-type semiconductor crystal & metal wire. When the very small junction is formed then the diode’s junction capacitance is extremely low. So, charge storage capacity is very low, which makes it a very fast switching device. Here is a question for you, what is Schottky Diode?

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