Lecture Text PN Junctions

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Intrinsic Semiconductors:
 
Semiconductor Material has a resistance between that of a conductor and that of an insulator.  In the Intrinsic semiconductor the amount of charge carriers is characteristic of the material itself.  Thermal excitation causes electrons to be raised from the valance band  to the conduction band.   Note that this is different than the case of a metal conductor, which always has electrons in the conduction band.  The intrinsic semiconductor has an equal number of electrons and holes.  When an electron in an intrinsic semiconductor jumps into the conduction band a hole is formed in the valance band.  An increase in temperature causes an increase in conductivity of the semiconductor by exciting more electrons into the conduction band and thus creating more holes.  Holes and electrons are the charge carriers in a semiconductor.  The positive holes move in the direction of the E field and the negative electrons flow in the opposite direction of the E field. 

Impurity Semiconductors: 

Silicon and Germanium semiconductors can have impurities added to make them more conductive.  N-type material is used to dope the Silicon crystal to increase its conductivity by increasing the availability of free electrons.  Adding P-type material decreases the availability of electrons and increases availability of holes.  The surface created by bonding P-type and N-type together is called a PN junction.  A small contact potential (.3 volts for Germanium and .6 volts for Silicon) keeps the holes and the electrons separated and controls the conduction of the PN junction.  When a voltage across the PN junction forces the electrons and the holes to move towards the junction, than conduction occurs.   Conduction consists of electrons and holes  moving towards the junction, where electrons fill the holes.  When the polarity of the voltage across junction is reversed, than no current flows, but electrons and holes are pulled away from each other a very short distance.  This small separation of charge at the junction is referred to as the depletion zone.  When junction is reversed biased no current flows, When the depletion zone area increases, a  corresponding decrease in the capacitance of PN junction occurs.  Thus, a reversed biased PN junction can be used as a variable capacitor.  The ability of a PN junction to conduct only in one direction makes a PN junction a rectifier. 

Semiconductor Diodes:

A semiconductor diode does not have to have a PN junction.  A diode is simply a semiconductor with two leads attached.  The PN junction diode can be simple rectifier, Zener Diode, Varactor Diode, or other types of junction diode.  Non Junction diodes are manufactured for there intrinsic semiconductor bulk properties, such as sensitivity to temperature or light.

Semiconductor Diode Rectifier circuit:

The schematic of a full wave rectifier is at top of the page.  The rectifier circuit output is displayed on the oscilloscope below it.  The filter capacitor is varied from one to a thousand micro-farads, and the ripple voltage varies inversely to rise in capacitance.  When the capacitance is one micro-farad the output waveform  appears as an unfiltered rectified positive DC voltage.  Note that the rectified output voltage has two positive peaks per cycle of AC input.  One peak of what is called pulsating DC occurs at the positive peak of the AC input, and the other peak of the pulsating DC occurs at the negative peak of the AC input voltage.  Current flows through CR1 for the first half cycle of the input voltage and through CR2 during the last half of the input cycle.  The rectifier circuit is called a Full Wave rectifier, because current flows during both the positive and negative cycles of the input waveform. 

 

 

 

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