HO3_214W09_BJTtech_2pp

HO3_214W09_BJTtech_2pp - Handout#3 EE 214 Winter 2009...

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B. Murmann, B. Wooley EE214 Winter 2008-09 1 Bipolar Junction Transistor IC Devices and Technology B. Murmann and B. A. Wooley Stanford University Handout #3 EE 214 Winter 2009 B. Murmann, B. Wooley EE214 Winter 2008-09 2 MOS and Bipolar Transistors Drain Gate Source Channel Charge Gate-controlled “resistor” Both are 3-terminal “ charge control ” devices in which the current flow between two terminals is modulated (controlled) from the third terminal The control mechanism is charge – minority carriers in a BJT and majority carriers in a MOSFET MOS Source Drain Emitter Collector Base-controlled vertical “current source” Bipolar Emitter Base Collector Base Charge
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B. Murmann, B. Wooley EE214 Winter 2008-09 3 Log I vs V – MOS and Bipolar Log 10 I (I D or I C ) Drive Voltage (V GS or V BE ) slope = q/kT sub-Threshold power-law ! ( V GS -V t m V t MOS BJT (Larger slope implies larger g m ) V BE (on) ! e V (kT/q) " # $ $ % & ! e V mkT/q " # $ $ % & g m ! ! I out ! V in " drive B. Murmann, B. Wooley EE214 Winter 2008-09 4 Common Emitter/Source I-V Characteristics Also called the “saturation” region Bipolar MOS saturation region forward- active region active region Triode (or ohmic) region
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B. Murmann, B. Wooley EE214 Winter 2008-09 5 Why Bipolar? Gain – Maximum g m ( = qI C /kT) over a very large current range • MOS g m is lower and depends on lateral channel dimensions – r o remains high ! Large intrinsic gain (g m r o ) • MOS intrinsic gain does not scale well Speed – Generally faster than MOS • Large current in smaller area than MOS – Has been easier to build narrow bipolar base widths than short channels I/O – Easier to drive low impedances B. Murmann, B. Wooley EE214 Winter 2008-09 6 Small-Signal BJT and MOS Models C gs C gd g m v 1 r o + v 1 MOS r " C " C μ g m v 1 r o r c r b + v 1 Bipolar Small signal models are basically the same except for r " , which models base current. Ohmic resistances r b and r c can be important in BJT circuits
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B. Murmann, B. Wooley EE214 Winter 2008-09 7 NPN Bipolar Junction Transistor “long-base” diode hole profile in emitter << “short-base” diode electron profile in base Planar BJT structure forward biased reverse biased B. Murmann, B. Wooley EE214 Winter 2008-09 8 One-Sided, Short-Base Diode I-V Boundary Conditions : Minority carriers on each side ~exp(V D /V T ) Expression for J S has W p in the denominator (instead of L p ) Shape of minority carrier profile on “short base” side is straight line instead of exponential decay J S only includes term from “lightly-doped” side J n = qD n dn p (x) dx J S = qD n n po W p J = J S e qV D /kT ! 1 " # $ % & 1/e n + p -x n 0 x p W p Minority carrier (injected) electrons in neutral p region L n
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B. Murmann, B. Wooley EE214 Winter 2008-09 9 Bipolar Junction Transistor This depletion edge (reverse biased) collects the injected electrons.
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This note was uploaded on 08/13/2009 for the course EE EE214 taught by Professor Borismurmann during the Winter '08 term at Stratford.

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HO3_214W09_BJTtech_2pp - Handout#3 EE 214 Winter 2009...

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