1. Spring 2007EE130 Lecture 27, Slide 1 Lecture #27 OUTLINE BJT small signal model BJT cutoff frequency BJT transient (switching) response Reading: Finish Chapter 12

2. Spring 2007EE130 Lecture 27, Slide 2 Transconductance: Small-Signal Model Common-emitter configuration, forward-active mode: “hybrid-pi” BJT small signal model:

3. Spring 2007EE130 Lecture 27, Slide 3 Small-Signal Model (cont.) where Q F is the magnitude of minority-carrier charge stored in the base and emitter regions forward transit time

4. Spring 2007EE130 Lecture 27, Slide 4 A BJT is biased at I C = 1 mA and V CE = 3 V.  dc =90,  F =5 ps, and T = 300 K. Find (a) g m, (b) r , (c) C . Solution: (a) (b) r  =  dc / g m = 90/0.039 = 2.3 k  c) Example: Small-Signal Model Parameters

5. Spring 2007EE130 Lecture 27, Slide 5 The cutoff frequency is defined to be the frequency (f =  /2  ) at which the short-circuit a.c. current gain equals 1: Cutoff Frequency, f T

6. Spring 2007EE130 Lecture 27, Slide 6 f T is commonly used as a metric for the speed of a BJT. SiGe HBT by IBM For the full BJT equivalent circuit: To maximize f T : –increase I C –minimize C J,BE, C J,BC –minimize r e, r c –minimize  F

7. Spring 2007EE130 Lecture 27, Slide 7 At very high current densities (>0.5mA/  m 2 ), base widening occurs, so Q B increases.  F increases, f T decreases. Top to bottom : V CE = 0.5V, 0.8V, 1.5V, 3V. Base Widening at High I C : the Kirk Effect Consider an npn BJT: At high current levels, the density of electrons (n  I C /qAv sat ) 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.

8. Spring 2007EE130 Lecture 27, Slide 8 Summary: BJT Small Signal Model Hybrid-pi model for the common-emitter configuration, forward-active mode:

9. Spring 2007EE130 Lecture 27, Slide 9 BJT Switching - Qualitative

10. Spring 2007EE130 Lecture 27, Slide 10 Turn-on transient We know: The general solution is: Initial condition: Q B (0)=0. since transistor is in cutoff where I BB =V S /R S

11. Spring 2007EE130 Lecture 27, Slide 11 Turn-off transient We know: The general solution is: Initial condition: Q B (0)=I BB  B

12. Spring 2007EE130 Lecture 27, Slide 12 Reducing  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  B –By reducing  B, the carrier removal rate is increased Example: Add recombination centers (Au atoms) in the base

13. Spring 2007EE130 Lecture 27, Slide 13 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