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Solid State ILT 176

Solid State ILT 176. Chapter 3. Review. Conductors – valence shell is almost empty Insulator – valence shell is almost full Semiconductor – valence shell is half full. (4 valence electrons). Review. Two types of semiconductor material used – Germanium and Silicon

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Solid State ILT 176

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  1. Solid StateILT 176 Chapter 3

  2. Review • Conductors – valence shell is almost empty • Insulator – valence shell is almost full • Semiconductor – valence shell is half full. (4 valence electrons)

  3. Review • Two types of semiconductor material used – Germanium and Silicon • Silicon – is more abundant, cheaper and works better. • Adding impurities to pure silicon is called doping.

  4. Review • Adding certain impurities (pentavalent) to silicon makes “N type” material. • Adding other impurities (trivalent) to silicon makes “P type material” • A silicon chip doped to form a p type material on end, and n type on another will form a pn junction in the middle.

  5. Review • The junction between p and n types of material is also called a barrier or depletion region. • The diode is a nothing more than a pn device.

  6. Review • The “p” side of a diode is called the anode. • The “n” side of a diode is called the cathode. • Applying a positive voltage potential to the anode (p side) and a negative voltage potential to the cathode (n side) is called forward biasing.

  7. Review • A forward biased diode will conduct if enough potential is applied. 0.7v for Si., 0.3v for Ger. • Applying a negative voltage potential to the anode, and a positive voltage potential to the cathode, is called reverse bias.

  8. Review • A reverse biased diode will not conduct unless its’ breakdown voltage is exceeded.

  9. PN Junction Diode

  10. PN Junction Diode

  11. Diode • A nonlinear device • The graph of current vs. voltage is not a straight line • The diode voltage must exceed the barrier voltage to conduct

  12. Linearity • The volt-ampere characteristic curve for a resistor is a straight line (linear). • A diode has a non-linear characteristic curve. • The barrier potential produces a knee in the diode curve. • The knee voltage is about 0.7 volts for a silicon diode.

  13. Silicon diode volt-ampere characteristic curve

  14. Silicon diode reverse bias characteristic curve

  15. Diode Curve

  16. First Approximation • This represents the diode as being ideal. • The first approximation ignores leakage current, barrier potential and bulk resistance. • When an ideal diode is forward biased, the model is a closed switch. • When an ideal diode is reverse biased, the model is an open switch.

  17. First (ideal) Approximation

  18. Ideal Diode • When forward biased, acts like a short. • 0v across the diode. • 0 ohms of resistance for the diode. • When reverse biased, acts like an open. • Used when troubleshooting circuits to get an approximate (ballpark) value.

  19. Second Approximation • This model assumes that no diode current flows until the forward bias across the diode reaches 0.7 volts. • This model ignores the exact shape of the knee. • This model ignores the diode’s bulk resistance.

  20. Second Approximation

  21. Second Approximation • When forward biased: • 0.7v drop across Si. diode, 0.3v across Ge. Diode. • 0 ohms of resistance for the diode. Used to get more accurate circuit values, especially with smaller supply voltages. Always use this one unless told to do otherwise in lab or on test.

  22. Third Approximation • When the diode is forward biased: • 0.7v drop across Si. Diode. • Include the bulk resistance of diode in all calculations. • We will seldom ever use this one.

  23. Diode Ratings • Specified on manufacturers’ data sheets • The maximum reverse bias rating must not be exceeded. • The maximum forward current rating must not be exceeded. • The power rating of a diode is determined by its maximum current rating and the forward voltage drop at that current flow.

  24. Data Sheets • Useful to circuit designers • Useful to repair technicians • Typical entries include: • Breakdown voltage • Maximum forward current • Forward voltage drop

  25. Diode Theory • http://electronics.wisc-online.com/SSE.asp

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