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Lecture 17 The Bipolar Junction Transistor (I) Forward Active Regime Outline • The Bipolar Junction Transistor (BJT): – structure and basic operation • I­V 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 n­channel MOSFET …except IB is the control variable rather than VBE IC = IC(IB, VCE) + V − CE IB (a) IC� (µA) 300 IB = 2.5 µA IB = 2 µA 250 200 IB = 1.5 µA (saturation) 150 (forward active) IB = 1 µA 100 IB = 500 nA 50 −3 −2 IB = 0 (cutoff) −1 1 2 3 4 5 6 VCE (V) −4 IB = 1 µA (reverse� active) −8 IB = 2 µA (b) 6.012 Spring 2009 Lecture 17 3 Simplified one­dimensional model of intrinsic device: Emitter Base Collector n p n NdE NaB NdC IE IC - - VBE + IB + VBC x -WE-XBE -XBE 0 WB WB+XBC WB+XBC+WC 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: VCE = VBE­VBC - C VBE + VBC + B saturation VCE + VBE forward� active VBC - - cut-off reverse E 6.012 Spring 2009 Lecture 17 4 Basic Operation: forward­active regime n-Emitter p-Base n-Collector IC>0 IE<0 IB>0 VBE > 0 VBC < 0 VBE>0 ⇒ injection of electrons from the Emitter to the Base injection of holes from the Base to the Emitter VBC<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: forward­active regime • Carrier profiles in thermal equilibrium: ln po, no 1019cm­3 1017cm­3 NdE NaB 1016cm­3 NdC po no po no ni 2 NdE -WE-XBE ni 2 NaB -XBE x WB 0 ni 2 NdC WB+XBC WB+XBC+WC • Carrier profiles in forward­active regime: ln p, n NdE NaB NdC n p ni2 NdE -WE-XBE 6.012 Spring 2009 ni2 NdC ni2 NaB -XBE 0 x WB WB+XBC WB+XBC+WC Lecture 17 6 Basic Operation: forward­active regime Dominant current paths in forward active regime: n-Emitter p-Base n-Collector IC>0 IE<0 IB>0 VBE > 0 VBC < 0 IC: electron injection from Emitter to Base and collection by Collector IB: hole injection from Base to Emitter IE: IE = ­(IC+IB) 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. I­V characteristics in forward­active regime Collector current: focus on electron diffusion in base n npB(0) npB(x) JnB npB(WB)=0 ni 2 NaB WB 0 x Boundary conditions: [ ] n pB (0) = n pBoe , VBE Vth npB (WB ) = 0 Electron profile: x n pB (x) = npB (0)1− WB 6.012 Spring 2009 Lecture 17 9 Electron current density: JnB = qDn dn pB dx = −qDn n pB (0) WB Collector current scales with area of base­emitter junction AE: E B n p n Ic AE � Collector terminal current: [ ] Dn I C = −J nB AE = qAE n pBo • e WB or VBE Vth [ ] IC = IS e VBE Vth IS ≡ transistor saturation current 6.012 Spring 2009 Lecture 17 10 Base current: focus on hole injection and recombination at emitter contact. p pnE(-xBE) pnE(x) pnE(-WE-xBE)= ni2 NdE ni2 NdE -WE-xBE -xBE x Boundary conditions: pnE(−xBE ) = pnEo e [ ]; VBE Vth pnE(−WE − xBE ) = pnEo Hole profile: x+x BE + p pnE (x) = [ pnE (−x BE ) − pnEo ]• 1 + nEo W E 6.012 Spring 2009 Lecture 17 11 Hole current density: dpnE pnE (−x BE ) − pnEo J pE = −qD p = −qDp dx WE Base current scales with area of base­emitter junction AE: E B p IB AE n � [ ] I B = −J pE AE = qAE pnEo • e WE Base terminal current: I B ≈ qAE Dp WE Dp VBE Vth −1 [ ] pnEo • e VBE Vth Emitter current: ­(IB + IC) Dp Dn IE = − qAE pnEo + qAE n pBo • e WE WB 6.012 Spring 2009 Lecture 17 [ ] V BE Vth 12 Forward Active Region: Current gain IC αF = = IE 1+ 1 N aB D p WB N dE DnWE Want αF close to unity­­­> typically αF = 0.99 IB βF 1 − αF = −I E − I C = − I C = I C αF αF IC α F = = I B 1 − αF IC Dn n pBo • IC WB N dE DnWE βF = = = D IB N aB D pWB p pnEo • WE To maximize βF: • • • NdE >> NaB WE >> WB want npn, rather than pnp design because Dn > Dp 6.012 Spring 2009 Lecture 17 13 Plot of log IC and log IB vs VBE 6.012 Spring 2009 Lecture 17 14 Common­Emitter 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 IC>0 IE<0 IB>0 VBE > 0 VBC < 0 • Emitter “injects” electrons into Base, Collector “collects” electrons from Base – IC controlled by VBE, independent of VBC – (transistor effect) [ ] IC ∝ e VBE Vth • Base: injects holes into Emitter ⇒ IB IC ∝ IB 6.012 Spring 2009 Lecture 17 16 MIT OpenCourseWare http://ocw.mit.edu 6.012 Microelectronic Devices and Circuits Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.
