Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control model Base transit time Reading: Finish Chapter 11, 12.2

Spring 2007EE130 Lecture 26, Slide 2 Narrow base n+ poly-Si emitter Self-aligned p+ poly-Si base contacts Lightly-doped collector Heavily-doped epitaxial subcollector Shallow trenches and deep trenches filled with SiO 2 for electrical isolation Modern BJT Structure

Spring 2007EE130 Lecture 26, Slide 3  dc is larger for a poly-Si emitter BJT as compared with an all-crystalline emitter BJT, due to reduced dp E (x)/dx at the edge of the emitter depletion region Polycrystalline-Silicon (Poly-Si) Emitter Continuity of hole current in emitter

Spring 2007EE130 Lecture 26, Slide 4 Emitter Gummel Number w/ Poly-Si Emitter For a uniformly doped emitter, where S p  D Epoly /W Epoly is the surface recombination velocity

Spring 2007EE130 Lecture 26, Slide 5 Emitter Band Gap Narrowing To achieve large  dc, N E is typically very large, so that band gap narrowing (Lecture 8, Slide 5) is significant.  E GE is negligible for N E < 1E18/cm 3 N = cm -3 :  E G = 35 meV N = cm -3 :  E G = 75 meV

Spring 2007EE130 Lecture 26, Slide 6 Narrow Band Gap (SiGe) Base To improve  dc, we can increase n iB by using a base material (Si 1-x Ge x ) that has a smaller band gap for x = 0.2,  E GB is 0.1eV Note that this allows a large  dc to be achieved with large N B (even >N E ), which is advantageous for reducing base resistance increasing Early voltage (V A )

Spring 2007EE130 Lecture 26, Slide 7 If D B = 3D E, W E = 3W B, N B = cm -3, and n iB 2 = n i 2, find  dc for (a) N E = cm -3, (b) N E = cm -3, and (c) N E = cm -3 and a Si 1-x Ge x base with  E gB = 60 meV (a) At N E = cm -3,  E gE  35 meV (b) At N E = cm -3,  E gE  meV: (c) EXAMPLE: Emitter Band Gap Narrowing

Spring 2007EE130 Lecture 26, Slide 8 Charge Control Model A PNP BJT biased in the forward-active mode has excess minority-carrier charge Q B stored in the quasi-neutral base: In steady state,

Spring 2007EE130 Lecture 26, Slide 9 time required for minority carriers to diffuse across the base sets the switching speed limit of the transistor Base Transit Time,  t

Spring 2007EE130 Lecture 26, Slide 10 Relationship between  B and  t The time required for one minority carrier to recombine in the base is much longer than the time it takes for a minority carrier to cross the quasi-neutral base region.

Spring 2007EE130 Lecture 26, Slide 11 The base transit time can be reduced by building into the base an electric field that aids the flow of minority carriers. Fixed E gB, N B decreases from emitter end to collector end. Fixed N B, E gB decreases from emitter end to collector end. - EBC - E B C EcEc dx dE q C 1  E EcEc EvEv EvEv EfEf EfEf Drift Transistor: Built-in Base Field

Spring 2007EE130 Lecture 26, Slide 12 EXAMPLE: Drift Transistor Given an npn BJT with W=0.1  m and N B =10 17 cm -3 (  n =800cm 2 /V  s), find  t and estimate the base electric field required to reduce  t