Lecture 17 The Bipolar Junction Transistor (I) Forward Active Regime

Transcription

1 Lecture 17 The Bipolar Junction Transistor (I) Forward Active Regime Outline The Bipolar Junction Transistor (BJT): structure and basic operation I-V characteristics in forward active regime Reading Assignment: Howe and Sodini; Chapter 7, Sections 7.1, Spring 2007 Lecture 17 1

2 1. BJT: structure and basic operation metal contact to base n + polysilicon contact to n + emitter region p-type base metal contact to collector A p + n n + A' n + buried layer n + buried layer field oxide - p - n sandwich (intrinsic npn transistor) n + (a) p-type substrate (base) (emitter) edge of n + buried layer field oxide A A' p + p n + n + emitter area, A E (intrinsic npn transistor) (b) (collector) Bipolar Junction Transistor: excellent for analog and front-end communications applications Spring 2007 Lecture 17 2

3 NPN BJT Collector Characteristics Similar to test circuit as for an n-channel MOSFET except I B is the control variable rather than V BE Spring 2007 Lecture 17 3

4 Simplified one-dimensional model of intrinsic device: Emitter Base Collector I E n p n I C - N de N ab N dc - V BE + I B + V BC -W E -X BE -X BE 0 W B W B +X BC W B +X BC +W C x BJT=two neighboring pn junctions back-to-back Close enough for minority carriers to interact can diffuse quickly through the base Far apart enough for depletion regions not to interact prevent punchthrough Regions of operation: V BE V CE = V BE -V BC B + + V BC C - + V CE forward active saturation V BC V BE - E - cut-off reverse Spring 2007 Lecture 17 4

5 Basic Operation: forward-active regime n-emitter p-base n-collector I E <0 I C >0 I B >0 V BE > 0 V BC < 0 V BE >0 injection of electrons from the Emitter to the Base injection of holes from the Base to the Emitter V BC <0 extraction of electrons from the Base to the Collector extraction of holes from the Collector to the Base Transistor Effect : electrons injected from the Emitter to the Base, extracted by the Collector Spring 2007 Lecture 17 5

6 Basic Operation: forward-active regime Carrier profiles in thermal equilibrium: ln p o, n o NdE cm cm cm -3 NaB p o n o NdC ni 2 NdE n o ni 2 NaB x -W E -X BE W B +X BC +W C p o -X BE 0 W B W B +X BC ni 2 NdC Carrier profiles in forward-active regime: ln p, n NdE NaB NdC n ni 2 NdE -W E -X BE p ni 2 NaB -X BE 0 W B W B +X BC ni 2 NdC W B +X BC +W C x Spring 2007 Lecture 17 6

7 Basic Operation: forward-active regime Dominant current paths in forward active regime: n-emitter p-base n-collector I E <0 I C >0 I B >0 V BE > 0 V BC < 0 I C : electron injection from Emitter to Base and collection by Collector I B : hole injection from Base to Emitter I E :I E = -(I C +I B ) Spring 2007 Lecture 17 7

8 The Flux Picture - Forward Active Region The width of the electron flux stream is greater than the hole flux stream. The electrons are supplied by the emitter contact injected across the base-emitter SCR and diffuse across the base Electric field in the base-collector SCR extracts electrons into the collector. Holes are supplied by the base contact and diffuse across the emitter. The reverse injected holes recombine at the emitter ohmic contact. ;;; n + polysilicon hole diffusion flux n + emitter electron p-type base ;; ;;;; diffusion n-type collector n + buried layer ;;; majority hole flux from base contact hole diffusion flux (a) n + polysilicon majority electrons electron diffusion ;; ;;;;;; n + emitter p-type base ;;;;;;;;;;;; ;;;;;;;;;;; ;;;;;;;;;; ;;;;;;;; ;;;;;;;;; ;;;;;;; n-type collector ;;;;; n + buried layer majority electron flux to coll. contact (minimum resistance path is through the n + buried layer) ;;; ; (b) Spring 2007 Lecture 17 8

9 2. I-V characteristics in forward-active regime Collector current: focus on electron diffusion in base n n pb (0) n pb (x) J nb n i 2 N ab 0 W B n pb (W B )=0 x Boundary conditions: n pb (0) = n pbo e [ VBE] V th, npb (W B ) = 0 Electron profile: n pb (x) = n pb (0) 1 x W B Spring 2007 Lecture 17 9

10 Electron current density: J nb = qd n dn pb dx = qd n n pb (0) W B Collector current scales with area of base-emitter junction A E : B E p n n I c A E Collector terminal current: or D I C = J nb A E = qa n E n pbo e W B I C = I S e [ ] V BE V th I S transistor saturation current [ ] V BE V th Spring 2007 Lecture 17 10

11 Base current: focus on hole injection and recombination at emitter contact. p ne (-x BE ) p p ne (x) p ne (-W E -x BE )= n i 2 N de n i 2 N de -W E -x BE -x BE x Boundary conditions: p ne ( x BE ) = p neo e [] VBE V th ; pne ( W E x BE ) = p neo Hole profile: p ne (x) = [ p ne ( x BE ) p neo ] 1 + x + x BE + p neo W E Spring 2007 Lecture 17 11

12 Hole current density: J pe = qd p dp ne dx = qd p p ne ( x BE ) p neo W E Base current scales with area of base-emitter junction A E : B E p I B A E Base terminal current: D p I B = J pe A E = qa E p W neo e E D p I B qa E p W neo e E Emitter current: -(I B + I C ) n [ ] VBE V th [ VBE] V th 1 D p D I E = qa E p neo + qa n E n pbo W E W B e [ ] VBE V th Spring 2007 Lecture 17 12

13 Forward Active Region: Current gain α F = I C I E = N ab D p W B N de D n W E Want α F close to unity---> typically α F = 0.99 I B = I E I C = I C 1 α I α C = I F C F β F = I C I B = α F 1 α F α F β F = I C I B = To maximize β F : n pbo D n W B p neo D p W E = N ded n W E N ab D p W B N de >> N ab W E >> W B want npn, rather than pnp design because D n > D p Spring 2007 Lecture 17 13

14 Plot of log I C and log I B vs V BE Spring 2007 Lecture 17 14

15 Common-Emitter Output Characteristics Spring 2007 Lecture 17 15

16 What did we learn today? Summary of Key Concepts npn BJT in forward active regime: n-emitter p-base n-collector I E <0 I C >0 I B >0 V BE > 0 V BC < 0 Emitter injects electrons into Base, Collector collects electrons from Base I C controlled by V BE, independent of V BC (transistor effect) I C e [ ] V BE Vth Base: injects holes into Emitter I B I C I B Spring 2007 Lecture 17 16