Special Purpose Diodes
Light Emitting Diode:
In a forward biased diode free electrons cross junction and enter into P layer where they recombine with the holes. Each recombination radiates energy as electron falls from the higher energy level to the lower energy level. In ordinary diodes this energy is in form of heat. In the light emitting diode, this energy is in form of light.
The symbol of LED is shown in figure. Ordinary diodes are made of Ge or Si. This material blocks passage of light. LEDs are made of different materials like arsenic, gallium, and phosphorus. LEDs can radiate green, red, yellow, blue, orange or infrared. The LED's forward voltage drop is approximately 1.5V. Typical LED current is in between 10 mA to 50 mA.
Fig. 1 Fig. 2
Seven Segment Display:
Seven segment displays are taken in use to display digits and few alphabets. It comprises of seven rectangular LEDs. Each LED is called as Segment. The external resistors are used to limit currents to safe Values. It can display any letters a, b, c, d, e, f, g as shown in the figure 2.
Fig. 3
LEDs of seven-segment display are connected in the in common anode configuration or in the common cathode configuration as shown in the figure 3.
Photo diode:
When the diode is reversed biased as shown in figure 4 a reverse current flows because of minority carriers. These carriers exist because the thermal energy keeps on producing free electrons and holes. The lifetime of minority carriers is short, but while they exist they can contribute to reverse current. When light energy bombards a PN junction, it too can produce free electrons.
Fig.4
In other words, the amount of light striking the junction can control the reverse current in a diode. A photo diode is made on the same principle. It is sensitive to the light. In this diode, through a window light falls to the junction. Stronger the light, the greater will be the minority carriers and larger will be the reverse current.
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