Semiconductor Diodes Notes: Breakdown Mechanism

2. Breakdown Mechanism (i) Avalanche breakdown:- When the reverse voltage is increased, the election electric field across the junction increases and the es are accelerated The high energetic electrons, by collision , break covalent bonds and more elections are again produced This causes is again knock out es by collision. This chain reaction takes place and an avalanche of electrons are produced This cumulatore phenomenon is called avalanche breakdown The junction breaks down for a certain reverse Voltage The magnitude of avalanche breakdown voltage increases with rise in temperature. 11) Lener Breakdown:- When external field is very instense, the electric field directly cause covalent bonds to break and a large no: of electron-hole - pairs are produced This cause a sudden ruse in current and the junction breakdown This happens when the width of depletion layer is small. for this two regions of the p-n junction must be heavily doped. This breakdown ig called zener breakdown can be explained on the basis of zener effect. During year breakdown the es are not created by colliscon, but by breaking covalent bonds under the applied reverse bias.

Lener Breakdown Avalanche breakdown. 1. The process in which the es The process of applying high move across the barrier voltage and increasing the from the valence band free es or electric current of p-type material to the in semiconductors and insulating conduction band of n-type materials is called an material is known as avalanche breakdown zener breakdown 2. This is observed in zener This is observed in zener diodes having a zener diode having a zener breakdown voltage, , below breakdown voltage greater 61 than 6v/8v. 3 The valence electrons are The valence elections are pulled into conduction due pushed to conduction due to the high electric field in to the energy imported by the narrow depletion regun accelerated electrons, which gain their velocty due to their colligion with other ahoms 4. The increase in temperature The increase in temperature decreases the break down increases the breakdown voltage. Voltage 5 oft occurs in deodes that It occurs in diodes that are highly doped are lighty doped Dynamic resistance ( dd Y ac resistance (rac) : Raho of a small change in voltage corresponding to change in current Does not obey ohm's law. Resistance offered by the junction depends upon the applied voltage. rd = Sv 8I

- 3 Zener Diode Symbol : It operates in reverse biasing It is a properly doped crystal diode which has sharp breakdown voltage American suentist Lener gave the sahs factory explanation of breakdown. used as voltage stabilizer. y- I characteristics of Lener diode. I f If breakdown occurs at a voltage less thes 6V,it is due to Leney VR Vz V2K effect B Izk Vf If 1/2 > 6V, avalenche effect If diode operates b/w 5V $ 8V, breakdown P Iz cg due to both Lang and avalanche effect Main characteristics :- close 1. Zener diode gr properly cloped 30 that it has a sharp breakdown voltage compaired to ordinary diode . Zener diode is always reverse biased 2. The reverse Voltage applied to the zener diode corresponding to the point B in characteristics curve apotion

at which the function breakdown is called the break down voltage V ZK 3. The reverse current corresponding to the break down voltage is called the breakdown current I K 4. The reverse voltage V2 corresponding to any point P on the linear portion of the reverse characterstics curve is called the zener voltage V2* 5 The raho of a small change in zener voltage (dV2) to the corresponding small change in the zener current (dIz) is called the zener dynamic resistance rz 6. The zener diode will not be burnt just because it is in the breakdown region 7 The voltage beyond which the reverse current increases rapidly is called turn over vollage. 8. Zener diode is used as voltage regulator wheras the ordinary diode g used for rectification. Application: Lener diode as voltage Regulator I R I, L Vi commond input voltage > ot VR Ix I circut current 1 I + I Vi N/2 R, Vo I, Load current L 1/2 Z - Iz Zener current Voltage regulator is an electronic circuit that provides a stable DC voltage independent of the load current, temperature and AC line voltage variations

A Zener diode of break down voltage Vz is reverse connected to an input voltage source Vi across Q load resistance R and series resistance R. The Voltage across the zener will reas remain Steady at its break down Voltage V for all the values of zener current 72 as long as the current remains in the break down region Hence a regulated Dc output voltage V= V/2 is obtained across RL. whenever the input voltage remains within a minimum and maximum voltage Basically there are 2 type of regulations :- a) Line regulation: In this type of regulation, Series resistance and load resistance are fixed, only sput only input voltage is changing Output voltage remains the same as long as the input voltage is maintained above a minimum value b) Load Regulation : In In this type of regulation, input voltage g fired and load resistance cs verying Output volt remains same, as long as load resistance is maintained above a minimum value 4 Rectification { Application of ordinary diode? The process in which ac voltage is converted into a dc voltage is called rech Fication The circuit used for this conversion yr known as rectifier.