Point Contact Diode is different from the normal P-N diode. A point contact diode is formed by touching a metallic wire with an N-type semiconductor to form a small area of contact. This form a small point junction. It is widely used because such a small point junction possesses a small value of junction capacitance. Thus, the charge storage at the junction is low. Due to this, the switching ability of the diode is much better than a conventional diode.
Construction of Point Contact Diode
Point 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.
The physical configuration of the semiconductor substrate is about 1.25 mm square and its thickness is 0.5 mm thick. One phase of the semiconductor substrate is connected to the metal base by the technique of radio frequency heating.
Working of Point Contact Diode
Whenever this diode is connected in forwarding 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.
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 capacitance are extremely small ranging from 0.1 – 1pF because the contact region between the N-type substrate & the wire is extremely small so the charge carrier’s density near the junction is extremely low.
Applications of Point Contact Diode
It is used as a video detector in a television receiver.
It is used in microwave frequency mixer circuits.
It is used in nonlinear i.e. rectifying elements of R.F. (radio frequency) signal detection.
It is used as an AM (amplitude modulation) detector in a radio receiver.
It is used in frequency conversation circuits.
It is used in high-frequency circuits of the order of 10 GHz or more.
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