5.4 Reverse-Bias Breakdown

Figure 5—19 Reverse breakdown in a p-n junction.

Figure 5—20 The Zener effect: (a) heavily doped junction at equilibrium; (b) reverse bias with electron tunneling from p to n; (c) I–V characteristic.

Figure 5—22 Variation of avalanche breakdown voltage in abrupt p+-n junctions, as a function of donor concentration on the n side, for several semiconductors. [After S.M. Sze and G. Gibbons, Applied Physics Letters, vol. 8, p. 111 (1966).]

Figure 5—23 Piecewise-linear approximations of junction diode 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.

Figure 5—26 A breakdown diode: (a) I–V characteristic; (b) application as a voltage regulator.

Chapter 7 Bipolar Junction Transistors (BJT) The First Transistor-point contact transistor (Bardeen, Brattain, and Shockley, Bell Labs, 1947) Establish a solid physical understanding of BJT operation Amplification and Switching Use p-n-p transistor as an example

7.1 Qualitative BJT Operation A BJT is essentially two back-to-back p-n junctions Two design requirements for a good p-n-p transistor: The current IE should consists mostly of holes Wb< Lp

Figure 7—1 External control of the current in a reverse-biased p-n junction: (a) optical generation; (b) junction I–V characteristics as a function of EHP generation; (c) minority carrier injection by a hypothetical device.

Figure 7—2 A p-n-p transistor: (a) schematic representation of a p-n-p device with a forward-biased emitter junction and a reverse-biased collector junction; (b) I–V characteristics of the reverse-biased n-p junction as a function of emitter current.

Figure 7—3 Summary of hole and electron flow in a p-n-p transistor with proper biasing: (1) injected holes lost to recombination in the base; (2) holes reaching the reverse-biased collector junction; (3) thermally generated electrons and holes making up the reverse saturation current of the collector junction; (4) electrons supplied by the base contact for recombination with holes; (5) electrons injected across the forward-biased emitter junction.

Current Flow in a p-n-p BJT Three mechanisms contribute to base current IB recombination in the base electron injection from n to p in the forward biased emitter junction electrons are swept into the base at the reverse biased collector junction

7.2 Amplification with BJTs Neglect saturation current at the collector and recombination in the transition regions. iC=BiEp B is the base transport factor No amplification between iB and iC

Electron injection across the emitter junction (iEn) and electron recombination in the base must be included to account for the base current iB (7-4) Neglect the collector saturation current (7-5) (7-6) Since  is near unity,  can be large for a good transistor Can iB control iC?

Example of an amplification circuit

Figure 7—4 Example of amplification in a common-emitter transistor circuit: (a) biasing circuit; (b) addition of an a-c variation of base current ib to the d-c value of IB, resulting in an a-c component ic.