L38: BJT High-Frequency Characteristics

6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-1 Lecture 38 - Bipolar Junction Transistor (cont.) May 9, 2007 Contents: 1. Non-ideal effects in BJT in FAR Reading material: del Alamo, Ch. 11, §11.5 (§§11.5.1, 11.5.3, 11.5.4, 11.5.5 but only qualitatively) Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-2 Key questions • Why are the output characteristics of a BJT in FAR not perfectly flat? • What is the maximum voltage that the collector of a BJT can sustain in FAR? What are the key design issues for the break­ down voltage? • Why does the performance of a BJT degrade at high collector current? • How does one model the base resistance in a BJT? Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-3 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 1. Non-ideal effects in BJT � Base-width modulation VBE and VBC affect xBE and xBC , respectively ⇒ WB = f (VBE , VBC ). log N log N NE NE NB NB NC xBE xBC NC x thermal equilibrium (VBE=VBC=0) xBE xBC x forward active (VBE>0, VBC<0) • Early effect: impact of VBC on WB |VBC | ↑⇒ xBC ↑⇒ WB ↓⇒ IC ↑, IB unchanged • Reverse Early effect: impact of VBE on WB VBE ↑⇒ xBE ↓⇒ WB ↑⇒ smaller IC than ideal ⇒ βF ↓, IB unchanged Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-4 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 � Early effect: impact of VBC on WB ⇒ IC = f (VBC ) n nB(0) nB(x) IC nB(WB)=0 ni2 NB WB(VBC) 0 x VBC more negative ⇒ WB (VBC ) ↓⇒ IC (VBC ) ↑ To capture first-order impact, linearize WB (VBC ): WB (VBC )  WB (VBC = 0)(1 + VBC ) VA VA is Early voltage: VA = qNB WBo Cjco Impact on IC : IC (VBC )  IC (VBC = 0) VBC  I (V = 0)(1 − ) C BC VA 1 + VVBC A Also, since IB unchanged: βF (VBC ) = βF (VBC = 0) VBC  β (V = 0)(1 − ) F BC VA 1 + VVBC A Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-5 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Manifestation of Early effect in output characteristics: IC IB IB=0 -VA 0 VCE Main consequence of Early effect: finite slope in output characteris­ tics in FAR: output conductance. Cjc B C + vbe Cπ+Cje gmvbe gπ go - E with go given by: go = IC VA Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-6 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 � Avalanche breakdown Sudden rise in IC for large reverse VCB . Key issue: breakdown voltage depends on terminal configuration: IC VCB VCE IE=0 IB=0 0 0 BVCEO BVCBO VCE Experimental observation: BVCEO < BVCBO Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-7 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 • Common-base configuration: breakdown as in isolated p-n junction n-Emitter p-Base n-Collector IC IE=0 VBC < 0 ⇒ breakdown when M → ∞ • Common-emitter configuration: BE and BC junctions interact in a positive feedback loop n-Emitter p-Base n-Collector IC IB=0 VCE With IB = 0 (or fixed), holes generated in B-C junction must be injected into E ⇒ B-E goes into forward bias ⇒ IC ↑. Breakdown occurs at finite M and enhanced by high βF : Mcrit  1 + 1 βF Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-8 Key dependencies: • NC ↑⇒ BVCEO ↓, BVCBO ↓ • βF ↑⇒ BVCEO ↓ but BVCBO unchanged ⇒ don’t want more βF than necessary!. Yamaguchi, T., et. al. "Process and Device Characterization for a 30-GHz fT Submicrometer Double Poly-Si Bipolar Technology Using BF2 -implanted Base with Rapid Thermal Process." IEEE Transactions on Electron Devices 40, no. 8 (1993): 1484-1495. Copyright 1993 IEEE. Used with permission. Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-9 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 � High collector current effects As IC ↑, electron velocity in collector ↑. But, there is a limit: vsat. Then, as IC approaches: ICK = qAE NC vsat the electrostatics of the collector are profoundly modified ⇒ transis­ tor performance degrades: log fT βF ideal βFo ideal 1 2πτF fTpk actual actual 1 IC2 log IC ICpk log IC To first order: IC2  ICpk  ICK Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-10 Main origin: formation of current-induced base inside collector SCR ⇒ effective quasi-neutral base width ↑ ⇒ βF ↓ ⇒ new delay com­ ponent ⇒ fT pk ↓ Key design issue: NC NC ↑ ⇒ ICK ↑ ⇒ fT pk ↑ Experiments [Crabbé IEDM 1993]: Si base SiGe base Key trade-off: Crabbe, E.F., et. al. "Vertical Profile Optimization of Very High Frequency Epitaxial Si and SiGe-base Bipolar Transistors." Technical Digest of the International Electron Devices Meeting, Washington, DC, December 5-8, 1993. Piscataway, NJ: Institute of Electrical and Electronics Engineers, 1993, pp. 83-86. ISBN: 9780780314504. Copyright 1993 IEEE. Used with permission. NC ↑ ⇒ BV ↓ Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-11 � Base resistance Very important parasitic for digital and analog applications. SE B E LE B n+ RBext p RBint n C Two components to RB : • external: associated with ohmic contact to base and lateral flow through extrinsic base • internal: associated with intrinsic base RB = RBext + RBint Important issues: 1. RBint is distributed: how does it scale? 2. Non-linear behavior of RBint for high IB Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-12 1. Low-current resistance (in absence of debiasing) Define lateral resistance of intrinsic base: Rblat = Rshb SE LE With: Rshb = 1 qμB NB WB Clearly, RBint < Rblat because not all IB flows across entire intrinsic base. But, what is proportionality constant? Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-13 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 • BJT with one base contact: SE B LE E n+ p IB JhE(y) IB AE 0 0 SE y 0 SE y iB(y) IB 0 Assume small ohmic drop along base ⇒ iB (x) uniformly distributed: iB (x) = IB (1 − x ) SE RBint obtained by examining power dissipation in base: pB = IB2 RBint = � S E 0 i2B (x) 1 SE Rshb dx = IB2 Rshb LE 3 LE Then: 1 1 SE = Rblat RBint = Rshb 3 LE 3 Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-14 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 • BJT with two base contacts: SE B LE E n+ p IB 2 IB 2 JhE(y) IB AE 0 0 SE y SE y iB(y) IB 2 0 0 SE 2 Now: IB 2x (1 − ) 2 SE IB 2x iB (x) = ( − 1) 2 SE iB (x) = for 0 ≤ x ≤ for SE 2 SE ≤ x ≤ SE 2 Intrinsic base resistance: RBint = 1 Rblat 12 This is a quarter of the design with one-base contact. Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-15 Total base resistance is: RB = RBext + RBint To get small RB : • minimize RBext ⇒ self-aligned process • minimize RBint ⇒ – use two base contacts ⇒ takes more space – increase WB ⇒ degrades frequency response – increase NB ⇒ hard, costly, Cje ↑, BVEBO ↓ – decrease SE (scaling!) ⇒ costly Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-16 Useful observation: trade-off between RB and IC Remember: qVBE qAE n2i DB qVBE IC = IS exp = exp kT NB WB kT Then: JC qVBE = q 2n2i DB μB exp Rshb kT rather independent of NB or doping profile. [from Tang TED 27, 563 (1980)] Tang, D. D. "Heavy Doping Effects in p-n-p Bipolar Transistors." IEEE Transactions on Electron Devices 27, no. 3 (1980): 563-570. Copyright 1980 IEEE. Used with permission. Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-17 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 2) Non-linear behavior of RB More accurate equivalent circuit model of distributed RB : SE B E B n+ RBext LE p RBint jB(x) IB AE 0 0 SE x SE x iB(x) IB 2 0 0 SE 2 Non-linear B-E diode current ⇒ non-uniform current distribution across base: • base resistance ”debiases” center of base • current ”crowds” at edges of base Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. Lecture 38-18 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Analytical model possible (but not pretty). General behavior of RBint: RBint 1 R blat 12 0 log IB For negligible debiasing to occur: IB Rblat < 0.2 kT q RBin drops by 50% at: IB Rblat  6 kT q Overall behavior of RB : RB 1 R +R blat Bext 12 RBext 0 log IB Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-19 Consequences of base debiasing: • RB depends on IB ⇒ RB depends on VBE and IC Weng, J., J. Holz, and T. F. Meister. "New Method to Determine the Base Resistance of Bipolar Transistors." IEEE Electron Device Letters 13, no. 3 (1992): 158-160. Copyright 1992 IEEE. Used with permission. • For high current, RBint → 0 ⇒ RB  RBext and independent of SE . Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-20 • Non-linear behavior of RBint: small-signal resistance = large-signal resistance Ohmic drop in RB : VB = RB IB Then, small-signal base resistance: rB = dVB dRB = RB + IB dIB dIB Since IB ↑⇒ RB ↓: rB ≤ RB RBint, rBint 1 R blat 12 rBint RBint 0 log IB Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-21 Consequences of RB : • RB has no direct impact on fT , but has indirect impact through constraint in base design. Actually, fT,pk does not appear correlated to RB (fT,pk dominated by NC ): • rB crucially important to noise • rB Cjc important time constant in high-frequency operation of BJT Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-22 Key conclusions • Early effect: modulation of WB by VBC ⇒ output conductance in output characteristics. go = IC VA • Base-width modulation effects affect IS and βF , but not IB . • In avalanche breakdown in a BJT: BVCEO < BVCBO because of transistor current gain. • βF ↑⇒ BVCEO ↓ w/o affecting BVCBO. • Key device design issue in avalanche breakdown: NC ↑⇒ BV s ↓ • At high collector current, velocity saturation of electrons in col­ lector ⇒ performance degrades: βF ↓, fT ↓ • Maximum current: ICpk  qAE NC vsat • Two components in RB : – RBext: associated with ohmic contact and transport through extrinsic base Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-23 – RBint: associated with intrinsic base • RBint is a distributed resistance ⇒ – RBint < Rblat – RBint is non linear in IB . • Consequence of non-linearity of RBint at high IB : small-signal resistance different from large-signal resistance. • Three technological approaches to reduce RB : – use two base contacts – use self-aligned process – decrease SE • Fundamental trade-off between IC and Rshb. Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].