Part -3 Diode characteristics Prepared by- Sharmila Majumdar

Characteristic curve of p-n junction According to the fig we need a junction diode, millimetre for measuring current, voltmeter for measuring voltage, power supply variable resistance etc.

I-V curve / diode curve When the diode is forward biased, there is no significant current until the diode voltage is greater than the barrier potential. On the other hand, when the diode is reverse biased, there is almost no reverse current until the diode voltage reaches the breakdown voltage. Then, the avalanche produces a large reverse current, destroying the diode. Actual and theoretical I–V characteristics of a typical semiconductor diode

Knee voltage In the forward region, the voltage at which the current starts to increase rapidly is called the knee voltage of the diode. The knee voltage equals the barrier potential. Also called zener voltage or breakdown voltage. Vk ≈ 0.7V (for silicon diode) Vk ≈ 0.3V (for Germanium diode)

Power dissipation PD = VDID The power rating is the maximum power the diode can safely dissipate without shortening its life or degrading its properties. In symbol, the definition is: Pmax = VmaxImax where Vmax is the voltage corresponding to Imax

Is there any device that acts like an ideal diode? Yes, an ordinary switch has zero resistance when closed and infinite resistance when open. Therefore, an ideal diode acts like a switch that closes when forward biased and opens when reverse biased. Ideal diode Reverse bias Forward bias

LED Light-emitting Diode Charge carriers recombination takes place at the p–n junction as electron crosses from the n-side and recombines with holes on the p-side. When the junction is forward-biased the free electron is in the conduction band and is at a higher energy level than the hole located at valence band. The recombination process involves radiation of energy in the form of photons. If the semiconductor material is transparent, the light will be emitted and the junction becomes a light source, i.e., a light-emitting diode (LED). LEDs are p–n junctions that can emit spontaneous radiation in ultraviolet, visible, or infrared regions.

LED Advantages of LEDs Low operating voltage, current and power consumption make LEDs compatible with electronic drive circuits. LEDs exhibit high resistance to mechanical shock and vibration and allow them to be used in severe environment conditions. LEDs ensure a longer operating life line, thereby improving the overall reliability and lowering the maintenance costs of equipment. LEDs have low inherent noise levels and also high immunity to externally generated noise. Limitations of LEDs Temperature dependence of radiant output power and wavelength. Sensitivity to damages by over voltage or over current. Theoretical overall efficiency is not achieved except in special cooled or pulsed conditions.

Uses of LED By using 7 segment LED display we can display 0 to 9 any digit in digital clock or calculator. 7 switches for 7 LEDs and 1 more switch DP for decimal point. N. B. Figure has given to the class.

Assignment According the reference book Example 3-1 : Finding forward bias or reverse bias. Example 3-2 : finding power dissipation. Example 3-3 & 3-4: finding load voltage and load current.