Crystal Diode Circuit Working and Applications

The microcontroller based projects or other electronics and electrical projects are designed by using some basic components in electrical and electronics, which are classified as elements. The elements that store or dissipate energy called as Passive elements, and the elements that provides  or suply controlled-energy- flow are called as Active elements. These basic elements include electrical Resistors, Inductors, different types of Diodes including Crystal diodes, Gunn diodes, Peltier diodes, Zener diodes, Tunnel diodes, Varactor diodes, etc. Transformers, Capacitors, Semiconductors, Transistors, Thyristors, Integrated circuits, Optoelectronic devices, Vacuum tubes, Sensors, Memristor, Transducers, Detectors, Antennas and, so on. In this article, we are going to discuss about the most frequently used component Crystal diode.

Crystal Diode

The semiconductor diode or P-N junction diode is a two-terminal device that allows current to flow only in one direction and blocks the flow of current in another direction. These two terminals are anode and cathode. If anode voltage is greater than the cathode voltage, then the diode starts conduction.Crystal diode is also called as a Cat’s-whisker diode or Point-contact diode or Crystals. These diodes microwave-semiconductor devices were developed during the World War II to be used in the microwave receivers and detectors.

Crystal Diode Circuit Working

The operation of the crystal diode depends on the pressure of contact between the semiconductor crystal and point. It consists of two sections– a small rectangular crystal of N-type silicon with one section, and a fine beryllium-copper, bronze-phosphor and tungsten wire called as Cat whisker wire thatpresses against the crystal to form another section. To form P-type region around the crystal, a large current is passed to the silicon crystal from the cat whisker during the manufacture of the crystal diode or point-contact diode. Hence, a PN-junction is formed and it behaves similar to normal PN-junction.

But, the characteristics of crystal diode is different from the characteristics of the PN-junction diode. In the forward bias condition, the resistance of the point contact diode is high compared to the general PN-junction diode. In the reverse bias condition, in case of point contact diode, the flow of current through the diode is not as independent of the voltage applied to the crystal as it is in case of the junction diode. The capacitance between the cat whisker and the crystal is less compared to the junction diode capacitance between both sides of the diode. Thus, the reactancedue to capacitance is high and at high frequency a very small capacitive current flows in the circuit.

In general, we know that theP-N junction diode or semiconductor diode conducts when the anode voltage is greater than the cathode voltage. The circuit can be realized in three ways: approximate model, simplified model and ideal model. The crystal diode circuit working for each model is shown below. If we apply a forward voltage Vf, then the characteristics of thediode as Vf vs If are shown in the figure.

Approximate Model

The approximate model of the crystal diode circuit consists of series connected ideal diode, forward resistance Rf and potential barrier Vo. The actual diode has to overcome the potential barrier Vo and internal drop VfRf. The voltage drop appears across the diode due to current If flowing through the internal resistance Rf.

The diode starts conduction only if the applied forward voltage Vf surpasses the potential barrier voltage Vo.

Simplified Model

In this model, the internal resistance Rf is not considered. Hence, the equivalent circuit consists of potential barrier Vo only. For diode circuit analysis, this model is most frequently used.

Ideal Model

In this model, both the internal resistance Rf and potential barrier Vo are not considered. In fact, practically there are no ideal diodes and it is assumed that there are ideal diodes for some diode circuit analysis.

Crystal Diode Applications

These diodes are used in many applications like crystal radio receiver. In this article, the most frequently used crystal diode applications such as crystal diode rectifier and crystal diode detector are mentioned below.

Crystal Diode Rectifier

The German physicist Ferdinand Braun while studying the characteristics of crystals conducting electricity and electrolytes in 1874, discovered the rectification effect at the contact point of the metals and some crystalline materials. When the highest purity materials were unavailable, the invented lead sulfide based point contact rectifier.

The crystal diode can be used as a rectifier to convert the AC into DC. As it conducts only in one direction and blocks the current flow in the reverse direction as similar to the normal diode- it can be used to design the half wave, full wave and bridge rectifier circuits.

Crystal Diode Detector

In 1900s, it is primarily used in a crystal radio set as a signal detector. Crystal surface makes contact with the fine metallic probe. Thus, point contact diode got a descriptive name as a cat whisker detector. These are obsolete and consists of a thin, sharpened metal wire acting as an anode and semiconductor crystal acting as the cathode. This anode thin metal wire called as the cat’s whisker wire is pressed against the cathode crystal. These crystal diode detectors were developed in the early 1900s and used in finding the hot spot on the semiconductor material crystal cathode which is manually adjusted for the best radio wave detection.

These were primarily developed by using mineral crystals galena or a piece of coal in 1906 but most of the recent diodes are being developed using silicon, selenium and germanium. As this diode allows current flow only in one direction, thus the DC voltage is provided by the  rectified carrier signal to drive headphones. In 1946, Sylvania pioneered the use of germanium for the first time in commercial crystal diode 1N34.

First of all, the sensitive spot needs to be identified by searching the entire surface which can be lost soon because of its vibration. So, to make the whole surface as sensitive and to avoid searching sensitive spot, this mineral was replaced with N-doped semiconductor.

The scientist G. W. Pickard in 1906 perfected this device by producing a localized P-type region within the semiconductor using pointed metal contact. To make it electrically and mechanically stable, entire point contact diode was encapsulated in a cylindrical body by fixing a metal point in place. Even though there are many diodes such as junction diodes and modern semiconductors, still these crystal diodes are being used as microwave frequency detectors due to their low capacitance.

We hope after reading this article you might have got a brief idea about the crystal diode. For any technical help on this topic and also about electrical and electronic projects, you can post your ideas, comments and suggestions to encourage other readers to improve their knowledge.

Photo Credits:

• Germanium Crystal Diode by circuitstoday
• Crystal Diode Detector by historywebsite