EECS 105 Fall 2003 Lecture 14 Bipolar Junction

EECS 105 Fall 2003, Lecture 14: Bipolar Junction Transistors Prof. Niknejad Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Lecture Outline l l l l Department of EECS Diode Small Signal Model Diode Charge Storage (6. 4. 4) Diode Circuits The BJT (7. 1) BJT Physics (7. 2) BJT Ebers-Moll Equations (7. 3) BJT Small-Signal Model University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Diode Small Signal Model l The I-V relation of a diode can be linearized Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Diode Capacitance l l We have already seen that a reverse biased diode acts like a capacitor since the depletion region grows and shrinks in response to the applied field. the capacitance in forward bias is given by But another charge storage mechanism comes into play in forward bias Minority carriers injected into p and n regions “stay” in each region for a while On average additional charge is stored in diode Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Charge Storage Extra charge Stored in diode l l l Increasing forward bias increases minority charge density By charge neutrality, the source voltage must supply equal and opposite charge A detailed analysis yields: Time to cross junction (or minority carrier lifetime) Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Ideal BJT Structure Collector (N) Base (P) Emitter (N) l l l Emitter (P) Base (N) Collector (P) NPN or PNP sandwich (Two back-to-back diodes) How does current flow? Base is very thin. A good BJT satisfies the following Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Actual BJT Cross Section l l l 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 Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad BJT Layout l l Emitter area most important layout parameter Multi-finger device also possible for reduced base resistance Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad BJT Schematic Symbol l l Collector current is control by base current linearly Collector is controlled by base-emitter voltage exponentially Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad BJT Collector Characteristic l l Ground emitter Fix VCE Drive base with fixed current IB Measure the collector current Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Collector Characteristics (IB) Saturation Region (Low Output Resistance) Breakdown Linear Increase Reverse Active (Crappy Transistor) Forward Active Region (Very High Output Resistance) Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Base-Emitter Voltage Control Saturation Region (Low Output Resistance) ~0. 3 V Breakdown Exponential Increase Reverse Active (Crappy Transistor) Forward Active Region (High Output Resistance) Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Transistor Action recombination Collector (n) e Base (p) h h h e Emitter (n+) l l 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 Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Diffusion Currents l 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 Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad BJT Currents Collector current is nearly identical to the (magnitude) of the emitter current … define Kirchhoff: DC Current Gain: Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Origin of αF Base-emitter junction: some reverse injection of holes into the emitter base current isn’t zero Some electrons lost due to recombination Typical: Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Collector Current Diffusion of electrons across base results in Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Base Current Diffusion of holes across emitter results in Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Current Gain Minimize base width Maximize doping in emitter Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Ebers-Moll Equations Exp. 6: measure E-M parameters Derivation: Write emitter and collector currents in terms of internal currents at two junctions Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Ebers-Moll Equivalent Circuit Building blocks: diodes and I-controlled I sources Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Forward Active Region B-C junction is not forward-biased IR is very small Typical Values: Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Simplified Ebers-Moll Forward-Active Case: B C E Saturation: both diodes are forward-biases batteries B C E Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Analogy from MOSFET s. s. model: Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Transconductance gm l The transconductance is analogous to diode conductance Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Transconductance (cont) l Forward-active large-signal current: • Differentiating and evaluating at Q = (VBE, VCE ) Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Comparison with MOSFET l Typical bias point: drain/coll. current = 100 A; Select (W/L) = 8/1, n. Cox = 100 A/V 2 BJT: l MOSFET: l Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad BJT Base Currents Unlike MOSFET, there is a DC current into the base terminal of a bipolar transistor: To find the change in base current due to change in base-emitter voltage: Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Small Signal Current Gain Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Input Resistance rπ In practice, the DC current gain F and the small-signal current gain o are both highly variable (+/- 25%) Typical bias point: DC collector current = 100 A Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Output Resistance ro Why does current increase slightly with increasing v. CE? Collector (n) Base (p) Emitter (n+) Model: math is a mess, so introduce the Early voltage Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Graphical Interpretation of ro slope~1/ro slope Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad BJT Small-Signal Model Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad BJT Capacitances Base-charging capacitance Cb: due to minority carrier charge storage (mostly electrons in the base) Base-emitter depletion capacitance: Cj. E= 1. 4 Cj. Eo Total B-E capacitance: C = Cj. E + Cb Department of EECS University of California, Berkeley

EECS 105 Fall 2003, Lecture 14 Prof. A. Niknejad Complete Small-Signal Model Department of EECS University of California, Berkeley