Chapter 1: Diode circuits • Objective • To understand the diode operation and its equivalent circuits • To understand various parameters of diodes • Load line analysis • Diode applications in rectifiers; HWR,FWR • Diode testing • Zener diode • Diode data sheets and specifications • Diode applications in clipper circuits • Numerical Semiconductor diode Fig – a semiconductor diode symbol
Basic operation Fig – b Vi characteristics of a diode n-type versus p-type • n-type materials make the Silicon (or Germanium) atoms more negative. • p-type materials make the Silicon (or Germanium) atoms more positive. • Join n-type and p-type doped Silicon (or Germanium) to form a p-n junction. p-n junction • When the materials are joined, the negatively charged atoms of the n-type doped side are attracted to the positively charged atoms of the p-type doped side. • The electrons in the n-type material migrate across the junction to the p-type material (electron flow). • the ‘holes’ in the p-type material migrate across the junction to the n-type material (conventional current flow). • The result is the formation of a depletion layer around the junction. Depletion region
Operating conditions • No Bias • Forward Bias • Reverse Bias No bias condition • No external voltage is applied: VD = 0V and no current is flowing ID = 0A. Only a modest depletion layer exists Reverse bias condition External voltage is applied across the p-n junction in the opposite polarity of the p- and ntype materials. • This causes the depletion layer to widen. • The electrons in the n-type material are attracted towards the positive terminal and the ‘holes’ in the p-type material are attracted towards the negative terminal.
Avalanche breakdown Avalanche breakdown occurs when a high reverse voltage is applied to a diode and large electric field is created across the depletion region. The effect is dependant on the doping levels in the region of the depletion layer. Minority carriers in the depletion region associated with small leakage currents are accelerated by the field to high enough energies so that they ionise silicon atoms when they collide with them. A new hole-electron pair are created which accelerate in opposite directions causing further collisions and ionisation and avalanche breakdown Zener breakdown Breakdown occurs with heavily doped junction regions (ie. highly doped regions are better conductors). If a reverse voltage is applied and the depletion region is too narrow for avalanche breakdown (minority carriers cannot reach high enough energies over the distance traveled ) the electric field will grow. However, electrons are pulled directly from the valence band on the P side to the conduction band on the N side. This type of breakdown is not destructive if the reverse current is limited. Forward Bias Condition • External voltage is applied across the p-n junction in the same polarity of the p- and ntype materials. • The depletion layer is narrow. • The electrons from the n-type material and ‘holes’ from the p-type material have sufficient energy to cross the junction. • Actual v-i characteristics is as shown in fig below