EE40 Lec 17 PN Junctions Prof Nathan Cheung Prof 10 27 2009 Reading Chapter 10 of Hambley Basic Device Physics Handout optional EE40 Fall 2009 Slide 1 Prof Cheung PN Junctions Semiconductor Physics y of p pn junctions for reference only Diode Current and Equation Solar Cells Cells Photo Detectors Detectors Zener Diodes Load Line Analysis EE40 Fall 2009 Slide 2 Prof Cheung The Periodic Table III IV V EE40 Fall 2009 Slide 3 Prof Cheung 4 nearest neighbors unit cell length 5 43 1s 2s 2p orbitals filled by 10 electrons 3s 3p orbitals filled by 4 electrons 5 1022 atoms cm3 The Si Crystal The Si Atom diamond cubic structure EE40 Fall 2009 Slide 4 Prof Cheung Pure Si is not very conductive electron l t Bottom of conduction band Energy gap 1 12 eV hole Top of valence band n electron conc p hole conc ni EE40 Fall 2009 Slide 5 Prof Cheung 5 Shockley s Parking Garage Analogy for Conduction in Si Two story parking garage on a hill If the lower floor is full and top one is empty empty no traffic is possible Analog of an insulator All electrons are locked up EE40 Fall 2009 Slide 6 Prof Cheung Doping By substituting a Si atom with a special impurity atom Column V or Column III element a conduction electron or hole is created Acceptors B Al Ga In Donors P As Sb Dopant concentrations typically range from 1014 cm 3 to 1020 cm 3 EE40 Fall 2009 Slide 7 Prof Cheung Semiconductor with both acceptors and donors h 4 ki has kinds d off charge h carriers i Hole Electron I i d Ionized Donor Ionized Acceptor EE40 Fall 2009 Mobile Charge Carriers they contribute to current flow with electric field is applied applied Immobile Charges they DO NOT contribute to current flow with electric field is applied However they affect the l l electric local l i field fi ld Slide 8 Prof Cheung 8 Charge Neutrality Condition Valid for homogeneously doped semiconductor at thermal equilibrium Even NA is not equal to ND microscopic volume surrounding any position x has zero net charge Si atom neutral Ionized Donor Ionized Acceptor Hole Electron Electrons and holes created by Si atoms with conc ni EE40 Fall 2009 Slide 9 Prof Cheung 9 Shockley s Parking Garage Analogy for Conduction in Si Two story parking garage on a hill If one car is moved upstairs upstairs it can move AND THE HOLE ON THE LOWER FLOOR CAN MOVE Conduction is possible Analog to warmed up semiconductor Some get free and leave holes behind electrons g EE40 Fall 2009 Slide 10 Prof Cheung Shockley s Parking Garage Analogy for Conduction in Si Two story parking garage on a hill If an extra car is donated donated to the upper floor floor it can move move Conduction is possible Analog to N type semiconductor An electron donor is added to the crystal creating free electrons EE40 Fall 2009 Slide 11 Prof Cheung Shockley ss Parking Garage Analogy for Conduction in Si Shockley Two story parking garage on a hill If a car is removed from the lower floor floor it leaves a HOLE which can move Conduction is possible Analog to P type semiconductor Acceptors are added to the crystal g bonding g electrons creating g free holes consuming EE40 Fall 2009 Slide 12 Prof Cheung Summary of n and p type silicon Pure silicon is an insulator At high temperatures it conducts weakly If we add an impurity with extra electrons e g arsenic phosphorus p p these extra electrons are set free and we have a pretty good conductor n type silicon If we add an impurity with a deficit of electrons e g boron then bonding electrons are missing holes and the resulting holes can move around again a pretty good conductor p type silicon Now what is really interesting is when we join n type and p type silicon that is make a pn junction It has interesting electrical properties ti EE40 Fall 2009 Slide 13 Prof Cheung Junctions of n and p type Regions What happens to the electrons and holes when n and p regions are brought into contact aluminum aluminum wire n EE40 Fall 2009 p Slide 14 Prof Cheung The pn Junction Diode Schematic diagram p type p type net acceptor concentration NA n type n type net donor concentration ND cross sectional area AD Physical structure example an For simplicity assume that the doping profile changes abruptly at the junction junction EE40 Fall 2009 Circuit symbol ID VD ID metall SiO2 SiO2 VD p type Si n type Si metal Slide 15 Prof Cheung Depletion Region Approximation When the junction is first formed mobile carriers diffuse across the junction due to the concentration gradients Holes diffuse from the p side to the n side side leaving behind negatively charged immobile acceptor ions Electrons El t diff diffuse ffrom th the n side id to t the th p side id leaving behind positively charged immobile donor ions acceptor ions p donor ions n A region depleted of mobile carriers is formed at the junction The space charge due to immobile ions in the depletion region establishes an electric field that opposes carrier diffusion EE40 Fall 2009 Slide 16 Prof Cheung Charge Density Distribution and Electric Field Unbalanced Charge is created in the depletion region acceptor ions p quasi neutral p region donor ions n depletion region quasi neutral n region charge density C cm3 distance Build in electric field EE40 Fall 2009 Slide 17 Prof Cheung Effect of Applied Voltage VD p n The quasi neutral p and n regions have low resistivity whereas the depletion region has high resistivity Thus when an external voltage VD is applied across the diode diode almost all of this voltage is dropped across the depletion region Think of a voltage divider circuit If VD 0 forward bias depletion charge reduced If VD 0 reverse bias depletion charge increased EE40 Fall 2009 Slide 18 Prof Cheung EE40 Fall 2009 Slide 19 Prof Cheung EE40 Fall 2009 Slide 20 20 Prof Cheung Diode Physical Behavior and Equation Schematic Device N P type type Symbol I V V Quantitative I V characteristics Qualitative I V characteristics I V positive easyy conduction I I 0 e qV kT 1 V V negative no conduction EE40 Fall 2009 I In which kT q is 0 026V and IO is a constant t t depending d di on di diode d area Typical values 10 12 to 10 16 A A non ideality factor n times kT q is often included Slide 21 Prof Cheung The pn Junction I vs V Equation I V characteristic of PN junctions In EECS 105 130 and other courses you will learn why the I vs V relationship for PN junctions is of the form I I 0 eqV kT 1 where I0 is a constant proportional to junction area and depending on doping p g in P and N regions g q electronic charge g 1 6 10 19 k is Boltzman constant
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