This diode tutorial covers types of diode including junction diode, point contact diode, zener diode, varicap (varactor) diode, gunn diode etc.
A diode allows the current to flow in one direction and blocks the current in the other direction. It has very low resistance in the forward direction. It has very high resistance in the reverse or opposite direction. Most of the semiconductor diodes are made of silicon or germanium.
A diode will have two leads i.e. anode and cathode. Figure depicts the diode symbol. The cathode is marked by a band at one end. The band mentions the lead from where current will leave the diode device when it is forward biased i.e. anode is positive and cathode is negative.
There are several types of diode constructed based on the application of use. They are junction diode, point contact diode, zener diode, varactor diode, gunn diode etc.
Diode Identification codes
There are two identification codes for diode viz. americal code and conventional system code.
American system starts with 1N and followed by diode serial number. For example 1N4001.
In the conventional system first letter gives material(e.g. A for germanium, B for silicon) and second letter ( e.g. A for signal diode, Y for rectifier diode, Z for Zener diode) gives diode type. For example, AA119 is a germanium signal diode
Following two electrical parameters are considered while selecting diode for the design.
• IF (Average Forward Current):
• VRRM (Maximum Reverse voltage):
This values should not be exceeded under normal conditions.
For example, diode 1N4001 has IF of 1A and VRRM of about 50Volt.
Junction diode consists of a p-n junction with one lead connection to p-side(referred as anode A) and the other lead connection to the n-side(referred as cathode K). Figure-2 depicts typical characteristic curve of silicon and germanium diodes at temperature of 25 degreeC.
The junction diode is used as rectifiers in power supplies for AC to DC conversion.
The larger junction areas will have capacity to pass larger forward current.
Silicon is preferable over germanium in diode construction due to following:
• Lower reverse current(make it more efficient rectifier)
• Higher breakdown voltage
• operate at higher temperatures
Point contact diode
The figure depicts germanium point contact diode. As shown tip of gold or tungsten wire is connected with pellet of n-type germanium. p-n junction is formed when brief current is passed from it resulting into tiny p-region around the tip.
Point contact diodes are used to detect radio frequency signals due to their very low capacitance. or tiny junction area. Germanium is used as signal diode because of lower turn on voltage compare to silicon. Lower signal voltages start it conducting in the forward direction.
In a normal diode reverse bias is increased until depletion layer breaks down and diode suffers permanent damage. Zener diode is made to work in the breakdown region as long as resistor limits the current. It looks like a rectifier diode. Zener diode is used in reverse bias at its breakdown voltage. Zener diode
Varicap (Varactor) diode
Varactor diode is used to tune TV and VHF radio stations which will help lock the frequency to desired stations. This is known as "automatic frequency control" or AFC.
When reverse biased; normal junction diode behaves as capacitor due to its depletion layer acts as insulator between conductors. The conductors are p-type and n-type regions here. The greater the area of junction, thinner the depletion layer and greater the capacitance.
Most of the diodes are designed to have minimum capacitance but varactor diode is designed to have capacitance in the range from 2 to 10 pF. The value of the capacitance can be changed by varying reverse voltage which in turn changes the width of the depletion layer.
Gunn diode is made using n-type gallium arsenide sandwiched between metal electrodes. It is used in microwave oscillators.