Reading: Finish Chapter 12 Lecture #27 OUTLINE BJT small signal model BJT cutoff frequency BJT transient (switching) response Reading: Finish Chapter 12 EE130 Lecture 27, Slide 1

Small-Signal Model “hybrid-pi” BJT small signal model: Common-emitter configuration, forward-active mode: “hybrid-pi” BJT small signal model: Transconductance: EE130 Lecture 27, Slide 2

Small-Signal Model (cont.) where QF is the magnitude of minority-carrier charge stored in the base and emitter regions forward transit time EE130 Lecture 27, Slide 3

Example: Small-Signal Model Parameters A BJT is biased at IC = 1 mA and VCE = 3 V. bdc=90, tF=5 ps, and T = 300 K. Find (a) gm , (b) rp , (c) Cp . Solution: (a) (b) rp = bdc / gm = 90/0.039 = 2.3 kW c) EE130 Lecture 27, Slide 4

Cutoff Frequency, fT The cutoff frequency is defined to be the frequency (f = w/2p) at which the short-circuit a.c. current gain equals 1: EE130 Lecture 27, Slide 5

fT is commonly used as a metric for the speed of a BJT. For the full BJT equivalent circuit: fT is commonly used as a metric for the speed of a BJT. SiGe HBT by IBM To maximize fT: increase IC minimize CJ,BE, CJ,BC minimize re, rc minimize tF EE130 Lecture 27, Slide 6

Base Widening at High IC: the Kirk Effect At very high current densities (>0.5mA/mm2), base widening occurs, so QB increases. tF increases, fT decreases. Consider an npn BJT: At high current levels, the density of electrons (n  IC/qAvsat) in the collector depletion region is significant, resulting in widening of the quasi-neutral base region. As W increases, the depletion width in the collector also increases, since the charge density decreases: At very high current densities, the excess hole concentration in the collector is so high that it effectively extends the p-type base. Top to bottom : VCE = 0.5V, 0.8V, 1.5V, 3V. EE130 Lecture 27, Slide 7

Summary: BJT Small Signal Model Hybrid-pi model for the common-emitter configuration, forward-active mode: EE130 Lecture 27, Slide 8

BJT Switching - Qualitative EE130 Lecture 27, Slide 9

Turn-on transient We know: The general solution is: Initial condition: QB(0)=0. since transistor is in cutoff where IBB=VS/RS EE130 Lecture 27, Slide 10

Turn-off transient We know: The general solution is: Initial condition: QB(0)=IBBtB EE130 Lecture 27, Slide 11

Reducing tB for Faster Turn-Off The speed at which a BJT is turned off is dependent on the amount of excess minority-carrier charge stored in the base, and also the recombination lifetime tB By reducing tB, the carrier removal rate is increased Example: Add recombination centers (Au atoms) in the base EE130 Lecture 27, Slide 12

Schottky-Clamped BJT When the BJT enters the saturation mode, the Schottky diode begins to conduct and “clamps” the C-B junction voltage at a relatively low positive value.  reduced stored charge in quasi-neutral base EE130 Lecture 27, Slide 13