Lec16

Lec16 - Subject_15: Outline Applications of pn Junctions *...

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Unformatted text preview: Subject_15: Outline Applications of pn Junctions * General comments * Discrete circuit applications Signal diodes, Zener diodes, varactors * Optoelectronic applications Photodetectors, LEDs, solar cells General Comments The technological APPLICATIONS of pn junctions are almost LIMITLESS since this device forms the elementary BUILDING BLOCK of almost all other semiconductor electronic devices * pn junctions are commonly found in DIODES , TRANSISTORS , SOLAR CELLS , LEDs , and INTEGRATED CIRCUITS In all of these more complicated structures/systems pn junctions are typically the critical sites that DOMINATE electronic action p+ or n+ Si p+ or n+ Si OXIDE LAYER n- or p-TYPE Si GATE SOURCE DRAIN pn JUNCTIONS FORM THE CRITICAL COMPONENTS OF A NUMBER OF DIFFERENT SEMICONDUCTOR DEVICES SHOWN TOP IS A BIPOLAR JUNCTION TRANSISTOR (BJT) THAT CONSISTS OF p-n-p (OR ALTERNATIVELY n-p-n) REGIONS AND WHICH LOOKS LIKE TWO BACK-TO-BACK pn JUNCTIONS (WHOSE DEPLETION REGIONS ARE CIRCLED) AT BOTTOM IS A METAL-OXIDE-SEMICONDUCTOR FIELD-EFFECT TRANSISTOR (MOSFET) WHICH CONSISTS OF HEAVILY-DOPED n- (OR ALTERNATIVELY p-) TYPE CONTACTS THAT ARE IMPLANTED INTO A p- (OR n-) TYPE SUBSTRATE LIKE THE BJT THIS DEVICE THEREFORE ALSO LOOKS LIKE TWO BACK-TO-BACK pn JUNCTIONS (CIRCLED) Discrete Circuit Applications In addition to their important role as component elements in more complicated devices pn junctions also have a number of applications in their OWN RIGHT * Variations in host semiconductor and doping allow use of pn junctions as SIGNAL DIODES , RECTIFYING DIODES , ZENER DIODES (voltage protection diodes for power supplies), and VARACTORS (amplifying and switching diodes) * SIGNAL DIODES utilize the ability of pn junctions to exhibit RECTIFICATION to allow them to function as discrete DIODE elements in electrical circuits VERY SMALL REVERSE CURRENT HERE FORWARD CURRENT ONSET I-V CHARACTERISTIC FOR AN IDEAL pn JUNCTION FOR SIGNAL DIODES IT IS DESIRED THAT THE REVERSE-BIAS CURRENT SHOULD BE AS SMALL AS POSSIBLE AND THE DIODE SHOULD ALSO NOT EXHIBIT REVERSE-BREAKDOWN FOR THE RANGE OF BIAS OF INTEREST UNDER FORWARD BIAS IT IS DESIRED THAT THE SLOPE OF THE I-V CURVE SHOULD BE AS LARGE AS POSSIBLE SO THAT THE FORWARD-BIAS CURRENT EXHIBITS LITTLE VOLTAGE DEPENDENCE IT IS ALSO DESIRED THAT THE ONSET OF CURRENT FLOW UNDER FORWARD BIAS SHOULD OCCUR AS CLOSE TO ZERO VOLTAGE AS POSSIBLE (i.e. SMALL BUILT-IN VOLTAGE) Some further DESIGN FACTORS associated with SIGNAL DIODES...
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This note was uploaded on 02/28/2011 for the course EE 2 taught by Professor Vis during the Winter '07 term at UCLA.

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Lec16 - Subject_15: Outline Applications of pn Junctions *...

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