EE105 - Fall 2006 Microelectronic Devices and Circuits

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1 EE105 - Fall 2006 Microelectronic Devices and Circuits Prof. Jan M. Rabaey Lecture 21: Bipolar Junction Transistor Administrative Midterm Th 6:30-8pm in Sibley Auditorium Covering everything up to, but NOT INCLUDING multi-stage amplifiers Review session Tonight 6-7:30pm in 003 Leconte. No lab this week. Homework 9 due on We by 5pm. Solutions will be posted by 6pm. Pick up graded homeworks in 253 Cory (up to and including homework 7) 2 1

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6 Bipolar Junction Transistors (BJT) Reading: Ideal BJT Structure I C + I E Collector (N) Base (P) Emitter (N) + I B V BE + V CE I E Emitter (P) Base (N) Collector (P) I B V EB + V EC I C NPN or PNP sandwich (Two back-to-back diodes) How does current flow? Base is very thin. A good BJT satisfies the following I C >> I I B C I E I C I e S qv BE kt 12 6

7 Actual BJT Cross Section Vertical npn sandwich (pnp is usually a lateral structure) n+ buried layout is a low resistance contact to collector Base width determined by vertical distance between emitter diffusion and base diffusion 13 BJT Layout Emitter area most important layout parameter 14 7

8 BJT Schematic Symbol I C = β I B + I B V BE I + V CE C I E I e S qv BE kt V B V C V E Collector current is linearly controlled by base current Collector current is an exponential function of the baseemitter voltage 15 Operations Modes 16 8

9 BJT Collector Characteristic Ground emitter Fix V CE Drive base with fixed current I B Measure the collector current 17 Forward Active Operation Carrier Concentration Depletion Regions E B C n b (0) p c0 p e0 n b0 0 W x W B 18 9

10 Current Components E B C I E 1 I C 2 3 x I B electrons holes 19 Reverse Active Carrier Concentration E B C n b (W) p e0 n b (0) n b0 0 W p c0 x W B 20 10

11 Saturation Mode Carrier Concentration n b (0) E B C Q A n b (W) p e0 QS n b0 0 W p c0 x W B 21 Cutoff Carrier Concentration E B C p e0 nb (0) n b0 n b (W) 0 W p c0 x W B 22 11

12 Collector Characteristics (I B ) Saturation Region (Low Output Resistance) Breakdown Reverse Active (Bad Transistor) Linear Increase Forward Active Region (Very High Output Resistance) 23 Base-Emitter Voltage Control Saturation Region (Low Output Resistance) ~0.3V Breakdown Reverse Active (Bad Transistor) Exponential Increase Forward Active Region (High Output Resistance) 24 12

13 V CB V BE + Transistor Action + > 0 > 0 Collector (n) Base (p) Emitter (n+) e recombination e h h h Base-emitter junction is forward biased and collector-base junction is reverse biased Electrons emitted into base much more than holes since the doping of emitter is much higher Magic: Most electrons cross the base junction and are swept into collector Why? Base width much smaller than diffusion length. Base-collector junction pulls electrons into collector 25 Forward Active Operation Carrier Concentration Depletion Regions E B C n b (0) p c0 p e0 n b0 0 W x W B 26 13

14 27 Diffusion Currents Minority carriers in base form a uniform diffusion current. Since emitter doping is higher, this current swamps out the current portion due to the minority carriers injected from base 28 14

15 BJT Currents Collector current is nearly identical to the (magnitude) of the emitter current define KCL: DC Current Gain: I = α I C F E IE = IC + IB I = α I = α ( I + I ) C F E F B C α =.999 F F I = α I = β I 1 α C B F B F β F α F.999 = = = α.001 F 29 Origin of α F Base-emitter junction: some reverse injection of holes into the emitter base current isn t zero E B C Some electrons lost due to recombination Typical: α.99 βf 100 F 30 15

16 Current Components E B C I E 1 I C 2 3 x I B electrons holes 31 Collector Current Diffusion of electrons across base results in dn diff p qdnn pb0 Jn = qdn = e dx WB qv BE kt I S qdnnpb0 AE = WB I C = I e S qv BE kt 32 16

17 Base Current Diffusion of holes across emitter results in qv BE diff dp qd ne ppne0 kt Jp = qdp = e 1 dx WE I B qv qd BE ppne0 AE kt = e 1 WE 33 Current Gain β F qdnn pboae I W C B D n n pb0 W E = = = I qd B ppneoae D p pne0 WB WE Minimize base width 2 ni n N N = = p N pb0 A, B D, E 2 ne0 ni NA, B DE, Maximize doping in emitter 34 17

18 Ebers-Moll Equations Derivation: Write emitter and collector currents in terms of internal currents at two junctions VBE / Vth / ( 1 VBC Vth ) α ( 1 ) I = I e + I e E ES R CS VBE / Vth / ( 1 VBC Vth ) ( 1 ) I = α I e I e C F ES CS α I = α I F ES R CS 35 Ebers-Moll Equivalent Circuit Building blocks: diodes and I-controlled I sources 36 18

19 Forward Active Region B-C junction is not forward-biased I R is very small Typical Values: V BE = V CE > Simplified Ebers-Moll Forward-Active Case: V = 0.7 IC = βf IB BE B I B Saturation: both diodes are forward-biases batteries I C C E V BE = B 0.7 I B I C C V CE = E