# Ajantha College and School Students Science Projects materials

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#### PN junction breakdown characteristics

In this article, we are going to learn a little deep about break down characteristics of PN Junction Diode and how “reverse break down” characteristics is put into practical applications. Before getting into the details, lets have a walk through our previous articles.

1. Understanding the PN Junction - is a great article to begin your learning curve about pn junction. In this article, we have clearly explained what is a pn junction and how is it formed.

2. PN Junction Characteristics- is another great article you must read to understand behavior of a PN junction under different conditions; say forward bias and reverse bias . This article will also help you understand why a PN junction is used in rectification applications.

Now lets comes to the essence of this article. “Break down” of a diode occurs during its reverse biased condition. We all know, under reverse bias the positive terminal of battery is connected to n side and the negative terminal of battery is connected to p side. As a result, electrons will be drawn towards the terminal of n side and holes will be draw towards terminal of p side. At this condition electron hole recombination will not happen and hence minority carrier movement is absent. This is the reason a diode is not conducting under reverse bias condition.

If we keep on increasing the applied reverse voltage, the depletion width will increase accordingly. At a point which we can call as “breakdown point”, the diode will get damaged. At this point, the diode behave more like a shorted wire and hence current flows through it easily. The internal resistance of diode at this stage is approximately near zero. According to Ohms Law, V = I/R and since resistance is very very low, current increases many folds with voltage. This is the reason we get a perpendicular line shoot in VI characteristics of reverse bias.

I hope you understood why break down of a diode occurs! It is because we increase the applied reverse voltage across the diode beyond a limit. Now let’s see how “break down” occurs!

#### Fixed Voltage Regulators

These regulators provide a constant output voltage. A popular example is the 7805 IC which provides a constant 5 volts output. A fixed voltage regulator can be a positive voltage regulator or a negative voltage regulator. A positive voltage regulator provides with constant positive output voltage. All those IC’s in the 78XX series are fixed positive voltage regulators. In the IC nomenclature – 78XX ; the part XX denotes the regulated output voltage the IC is designed for. Examples:- 7805, 7806, 7809 etc.

A negative fixed voltage regulator is same as the positive fixed voltage regulator in design, construction & operation. The only difference is in the polarity of output voltages. These IC’s are designed to provide a negative output voltage. Example:- 7905, 7906 and all those IC’s in the 79XX series.

#### Zener Breakdown

When we increase the reverse voltage across the pn junction diode, what really happens is that the electric field across the diode junction increases (both internal & external). This results in a force of attraction on the negatively charged electrons at junction. This force frees electrons from its covalent bond and moves those free electrons to conduction band. When the electric field increases (with applied voltage), more and more electrons are freed from its covalent bonds. This results in drifting of electrons across the junction and electron hole recombination occurs. So a net current is developed and it increases rapidly with increase in electric field. Zener breakdown phenomena occurs in a pn junction diode with heavy doping & thin junction (means depletion layer width is very small). Zener breakdown does not result in damage of diode. Since current is only due to drifting of electrons, there is a limit to the increase in current as well.