Lecture 27 OUTLINE BJT small signal model BJT

Lecture #27 OUTLINE • BJT small signal model • BJT cutoff frequency • BJT transient (switching) response Reading: Finish Chapter 12 1 Spring 2007 EE 130 Lecture 27, Slide 1

Small-Signal Model Common-emitter configuration, forward-active mode: “hybrid-pi” BJT small signal model: Transconductance: 2 Spring 2007 EE 130 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 3 Spring 2007 EE 130 Lecture 27, Slide 3

Example: Small-Signal Model Parameters A BJT is biased at IC = 1 m. A and VCE = 3 V. bdc=90, t. F=5 ps, and T = 300 K. Find (a) gm , (b) rp , (c) Cp. Solution: (a) (b) rp = bdc / gm = 90/0. 039 = 2. 3 k. W c) 4 Spring 2007 EE 130 Lecture 27, Slide 4

Cutoff Frequency, f. T The cutoff frequency is defined to be the frequency (f = w/2 p) at which the short-circuit a. c. current gain equals 1: 5 Spring 2007 EE 130 Lecture 27, Slide 5

For the full BJT equivalent circuit: f. T is commonly used as a metric for the speed of a BJT. Si. Ge HBT by IBM To maximize f. T: – increase IC – minimize CJ, BE, CJ, BC – minimize re, rc – minimize t. F 6 Spring 2007 EE 130 Lecture 27, Slide 6

Base Widening at High IC: the Kirk Effect • At very high current densities (>0. 5 m. A/mm 2), base widening occurs, so QB increases. ® t. F increases, f. T decreases. Top to bottom : VCE = 0. 5 V, 0. 8 V, 1. 5 V, 3 V. Consider an npn BJT: At high current levels, the density of electrons (n IC/q. Avsat) 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. 7 Spring 2007 EE 130 Lecture 27, Slide 7

Summary: BJT Small Signal Model Hybrid-pi model for the common-emitter configuration, forward-active mode: 8 Spring 2007 EE 130 Lecture 27, Slide 8

BJT Switching - Qualitative 9 Spring 2007 EE 130 Lecture 27, Slide 9

Turn-on transient • We know: where IBB=VS/RS • The general solution is: • Initial condition: QB(0)=0. since transistor is in cutoff 10 Spring 2007 EE 130 Lecture 27, Slide 10

Turn-off transient • We know: • The general solution is: • Initial condition: QB(0)=IBBt. B 11 Spring 2007 EE 130 Lecture 27, Slide 11

Reducing t. B 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 t. B – By reducing t. B, the carrier removal rate is increased Example: Add recombination centers (Au atoms) in the base 12 Spring 2007 EE 130 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 13 Spring 2007 EE 130 Lecture 27, Slide 13