Chapter 5 Lecture - Part 3

The es are carried across w carriers a multiplicative

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Unformatted text preview: gh kinetic energy (from large electric fields fields – These electrons collide with and strip electrons These from lattice atoms, yielding additional charge carriers (e’s and h’s). The e’s are carried across W carriers – A multiplicative effect is realized Avalanche Breakdown Avalanche • Avalanche: – – Collisions yield EHP’s If the collisions also occur in W, the multiplicative If effect increases significantly effect – If the e’s from the EHP undergo collisions that If yield more EHP’s, and so on an so on, avalanche occurs occurs • Reverse biasing generates the required electric Reverse fields fields Rectifiers Rectifiers • Definition: Definition: – An ideal diode with an a-c input signal An – Current only flows when the diode has a forward Current bias bias • Only the positive portion of the a-c signal passes Only through the junction through • The output voltage is a ½ rectified sine wave • Qualifications for good rectifiers – Bandgap: The larger the better • ni is usually lower for wide bandgap materials due to the larger gap (low EHP generation rates) larger • These materials are more stable electrically at higher These temperatures (higher powers) temperatures Piecewise-linear Approximations of Junction Diode Piecewise-linear Characteristics: (a) the ideal diode; (b) ideal diode with an offset voltage; (c) ideal diode with an offset voltage and a resistance to account for slope in the forward characteristic. Rectifiers Rectifiers – Doping concentration • Doping influences such things as avalanche breakdown Doping voltage, contact potentials, and series resistance voltage, • A high resistivity region is needed on one side of the high junction in order to increase Vbr junction – This region should not be too resistive This – Resistance is also dependent on geometry » Area (increase) and length (reduce) – Increasing the area of a junction increases the risk of Increasing breakdown due to larger numbers of incorporated defects breakdown Punch-through Punch-through Definition: a situation where W extends completely through a Definition: lightly doped region. It is especially relevant for short, lightly doped regions doped A p+-n-n+ junction diode: (a) device configuration; (b) zero-bias condition; (c) reverse-biased to punch-through. Results in Vbr that is lower than would be predicted under “normal” conditions Breakdown Diodes Breakdown • Some diodes are designed to operate in Some breakdown mode breakdown – Set to operate at a specific breakdown voltage – Called breakdown diode or Zener diode • Characteristics – – – The breakdown voltage should be sharp The breakdown current should be voltage independent Breakdown mechanism: avalanche (despite the name) • Uses – Voltage regulators – Reference diodes Breakdown Diodes A breakdown diode: (a) I–V characteristic; (a) (b) application as a voltage regulator. (b)...
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