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, 7.2 6.012 Spring 2009 Lecture 17 1
1. BJT: structure and basic operation metal contact to base \ / n+ polysilicon contact to n+ emitter region A' I 'field oxide n+-p - n sandwich (intrinsic npn transistor) (a) edge of n+ buried layer Bipolar Junction Transistor: excellent for analog and fkont-end communications applications. 6.012Spring 2009 Lecture 17'
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 I C = I C (I B, V CE ) + V CE I B (a) I C (µa) 300 250 I B = 2.5 µa I B = 2 µa 200 150 100 (saturation) (forward active) I B = 1.5 µa I B = 1 µa I B = 500 na 3 2 1 50 I B = 0 (cutoff) 1 2 3 4 5 6 V CE (V) I B = 1 µa I B = 2 µa (reverse active) 4 8 (b) 6.012 Spring 2009 Lecture 17 3
Simplified one dimensional model of intrinsic device: Emitter Base Collector I E n p n I C - N de N ab NdC - 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 6.012 Spring 2009 Lecture 17 4
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 6.012 Spring 2009 Lecture 17 5
Basic Operation: forward active regime Carrier profiles in thermal equilibrium: ln p o, n o NdE 10 19 cm 3 10 17 cm 3 10 16 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 6.012 Spring 2009 Lecture 17 6
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 ) 6.012 Spring 2009 Lecture 17 7
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 difhse 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 nic contact. -... -... -...-... K "...+ : po1ysiliwn:iii:i;i::iii: 6.012Spring 2000 Lecture I?
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 [ V BE ] Vth, npb (W B ) = 0 Electron profile: n pb (x) = n pb (0) 1 x W B 6.012 Spring 2009 Lecture 17 9
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 W pbo e B I C = I S e [ ] V BE V th I S transistor saturation current [ ] VBE V th 6.012 Spring 2009 Lecture 17 10
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 [ V BE ] 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 6.012 Spring 2009 Lecture 17 11
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 [ ] V BE V th [ V BE ] 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 [ ] 6.012 Spring 2009 Lecture 17 12
Forward Active Region: Current gain α F = I C I E = 1 1 + 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 α = F I B 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 6.012 Spring 2009 Lecture 17 13
Plot of log I C and log I B vs V BE 6.012 Spring 2009 Lecture 17 14
Common Emitter Output Characteristics 6.012 Spring 2009 Lecture 17 15
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 [ ] VBE V th Base: injects holes into Emitter I B I C I B 6.012 Spring 2009 Lecture 17 16
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