University:
Massachusetts Institute of Technology
Course:
6.012 | Microelectronic Devices and Circuits
Academic year:

2024
Views:

359

Pages:

17
Author:

Marin Schmitt

Lecture 17 - The Bipolar Junction Transistor (I) Forward Active Regime November 8, 2005 Contents: 1. BJT: structure and basic operation 2. I-V characteristics in forward active regime Reading assignment: Howe and Sodini, Ch. 7, §§7.1, 7.2 Announcements: Quiz 2: 11/16, 7:30-9:30 PM, open book, must bring calculator; lectures #10-18. Key questions • What does a bipolar junction transistor look like? • How does a bipolar junction transistor operate? • What are the leading dependencies of the terminal currents of a BJT in the forward active regime? 1. BJT: structure and basic operation base contact emitter contact base contact collector contact n+ emitter n+ plug p base n collector "intrinsic" BJT n+ buried layer p substrate base-collector junction collector-substrate junction emitter-stripe length base-emitter junction (area AE) emitter-stripe width Uniqueness of BJT: high-current drivability per input capacitance ⇒ fast ⇒ excellent for analog and front-end communications applications. Simpliﬁed one-dimensional 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 neighbouring pn junctions back-to-back: • close enough for minority carriers to interact (can diﬀuse quickly through base) • far apart enough for depletion regions not to interact (prevent ”punchthrough”) Regimes of operation: collector VBE IC - + VBC IB + saturation VCE base + VBE forward active VBC - IE emitter cut-off reverse Basic operation in forward-active regime: n-Emitter p-Base n-Collector IC>0 IE<0 IB>0 VBE > 0 VBC < 0 VBE > 0 ⇒ injection of electrons from E to B injection of holes from B to E VBC < 0 ⇒ extraction of electrons from B to C extraction of holes from C to B Transistor eﬀect: electrons injected from E to B, extracted by C! • Carrier proﬁles in thermal equilibrium: log po, no NdE NaB NdC po no po no ni2 NdE ni2 NaB -WE-XBE -XBE 0 ni2 NdC x WB WB+XBC WB+XBC+WC • Carrier proﬁles in forward-active regime: log p, n NdE NaB NdC n p ni2 NdC ni2 NdE -WE-XBE ni2 NaB -XBE 0 x WB WB+XBC WB+XBC+WC 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 E to B and collection into C IB : hole injection from B to E IE = −IC − IB Key dependencies (choose one): √ qVBE /kT IC on VBE : e , 1/ VBE , none, other √ qVBC /kT IC on VBC : e , 1/ VBC , none, other √ qVBE /kT IB on VBE : e , 1/ VBE , none, other √ qVBC /kT IB on VBC : e , 1/ VBC , none, other IC on IB : exponential, quadratic, none, other In forward-active regime: • VBE controls IC (”transisto ”transistorr eﬀect”) eﬀect” ion”) • IC independent of VBC (”isolat ”isolation” • price to pay for control: IB Comparison with MOSFET: feature controlling terminal common terminal controlled terminal functional dependence of controlled current DC current in controlling terminal ideal MOSFET ideal BJT in saturation in FAR gate base source emitter drain collector quadratic exponential 0 exponential Figure of merit for BJT: -common-emitter current gain: βF = IC IB (want big enough, 100) 2. I-V characteristics in forward active regime 2 Collector current: focus on electron diﬀusion in base n npB(0) npB(x) JnB npB(WB)=0 ni2 NaB WB 0 x Boundary conditions: qVBE , npB (0) = npBo exp kT npB (WB ) = 0 Electron proﬁle: npB (x) = npB (0)(1 − x ) WB Electron current density: JnB dnpB npB (0) = −qDn = qDn dx WB Collector current scales with area of base-emitter junction AE : B E B IC AE C Collector terminal current: qVBE Dn IC = −JnB AE = qAE npBo exp WB kT or IC = IS exp qVBE kT IS ≡ collector saturation current [A] 2 Base current: focus on hole injection and recombination in emitter p pnE(-xBE) pnE(x) pnE(-WE-xBE)= ni2 NdE ni2 NdE -WE-xBE -xBE x Boundary conditions: pnE (−xBE ) = pnEo exp qVBE , pnE (−WE −xBE ) = pnEo kT Hole proﬁle: pnE (x) = [pnE (−xBE ) − pnEo ](1 + x + xBE ) + pnEo WE Hole current density: JpE dpnE pnE (−xBE ) − pnEo = −qDp = −qDp dx WE Base current scales with area of base-emitter junction AE : B E B IB AE C Base terminal current: qVBE Dp IB = −JpE AE = qAE − 1) pnEo(exp WE kT Then: IB = For VBE qVBE IS − 1) (exp βF kT kT : q IC IB βF Gummel plot: semilog plot of IC and IB vs. VBE : log IC, IB IC IB 60 mV/dec at 300 K IS IS βF VBC<0 VBE 2 Current gain: D IC npBo WBn NdE DnWE βF = = Dp = IB pnEo W NaB DpWB E To maximize βF : • NdE NaB • WE WB • want npn npn,, rather than pnp design because Dn > Dp State-of-the-art IC BJT’s today: IC ∼ 0.1−1 mA, βF ∼ 50 − 300 βF hard to control in manufacturing environment ⇒ need circuit techniques that are insensitive to variations in βF βF dependence on IC : βF log IC Gummel plot of BJT (VCE = 3 V ): Key conclusions 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) qVBE IC ∝ exp kT • Base injects holes into Emitter ⇒ IB IB ∝ IC