6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-1 Lecture 35 - Bipolar Junction Transistor (cont.) November 27, 2002 Contents: 1. Current-voltage characteristics of ideal BJT (cont.) Reading material: del Alamo, Ch. 11, 11.2 (11.2.1)
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-2 Key questions How does the BJT operate in other regimes? How does a complete model for the ideal BJT look like?
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-3 1. Current-voltage characteristics of ideal BJT (cont.) Forward-active regime (V BE > 0, V BC < 0) Summary of key results: W B << L B n-emitter p-base n-collector I E <0 I C >0 I B >0 V BE > 0 V BC < 0 I C = I S exp qv BE I B = I S (exp qv BE 1) I E = I C I B = I S exp qv BE I S (exp qv BE 1)
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-4 Current gain I C I B n 2 i D B N B W B n 2 i D E N E W E = N ED B W E N B D E W B To maximize : N E N B W E W B (for manufacturing reasons, W E W B ) want npn, rather than pnp because this way D B >D E hard to control if is high enough (> 50), circuit techniques effectively compensate for this.
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-5
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-6 Equivalent circuit model C B I S exp qv BE I S (exp qv BE -1) E I C = I S exp qv BE I B = I S (exp qv BE 1) I E = I C I B = I S exp qv BE I S (exp qv BE 1)
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-7 Energy band diagram E(n) B(p) C(n) thermal equilibrium E C E V E F forward active E C E fh E fe qv BE E V qv BC Summary of minority carrier profiles (not to scale) p E n B pc peo n Bo p Co x -W E -x BE -x BE 0 W B W B +x BC W B +x BC +W C
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-8 Reverse regime (V BE < 0, V BC > 0) I E : electron injection from C to B, collection into E I B : hole injection from B to C, recombination in C n-emitter p-base n-collector I E >0 I C <0 I B >0 V BE < 0 V BC > 0 Minority carrier profiles (not to scale): p E n B pc n Bo peo p Co x -W E -x BE -x BE 0 W B W B +x BC W B +x BC +W C
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-9 Current equations (just like FAR, but role of collector and emitter reversed): I E I B I C = I S exp qv BC = I S (exp qv BC β R 1) = I E I B = I S exp qv BC I S β R (exp qv BC 1) Equivalent-circuit model representation: C I S (exp β R qv BC -1) B I S exp qv BC E
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-10 Prefactor in I E expression is I S : emitter current scales with A E. B E B B E B I E A E I B A C C C But, I B scales roughly as A C : downward component scales as A C upward component scales as A C A E A C Hence, β R 0.1 5.
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-11 Energy band diagram: E(n) B(p) C(n) thermal equilibrium E C E V E F E fe qv BC reverse E C qv BE E fh E V
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-12 Cut-off regime (V BE < 0, V BC < 0) I E : hole generation in E, extraction into B I C : hole generation in C, extraction into B n-emitter p-base n-collector I E >0 I C >0 I B <0 V BE < 0 V BC < 0 Minority carrier profiles (not to scale): p E n B pc p Co peo n Bo x -W E -x BE -x BE 0 W B W B +x BC W B +x BC +W C
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-13 Current equations: I E = I S I B I C = I S I S β R = I S β R These are tiny leakage currents ( 10 12 A) Equivalent-circuit model representation: C B I S β R both reverse biased I S E
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-14 Energy band diagram E(n) B(p) C(n) thermal equilibrium E C E V E F cut-off E C E V qv BE E fe E fh qv BC
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-15 Saturation regime (V BE > 0, V BC > 0) I C,I E : balance of electron injection from E/C into B I B : hole injection into E/C, recombination in E/C, respectively n-emitter p-base n-collector I E I C I B <0 V BE > 0 V BC > 0 Minority carrier profiles (not to scale): p E n B pc p Co peo n Bo x -W E -x BE -x BE 0 W B W B +x BC W B +x BC +W C
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-16 Current equations: superposition of forward active + reverse: I C I B I E = I S (exp qv BE = I S (exp qv BE = I S (exp qv BE exp qv BC ) I S β R (exp qv BC 1) 1) + I S β R (exp qv BC 1) 1) I S(exp qv BE exp qv BC ) I C and I E can have either sign, depending on relative magnitude of V BE and V BC and and β R. Equivalent circuit model representation (Non-Linear Hybrid-π Model): C I S β R (exp qv BC -1) B qv I S (exp BE qv - exp BC ) I S (exp qv BE -1) E Complete model has only three parameters: I S,, and β R.
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-17 Energy band diagram: E(n) B(p) C(n) thermal equilibrium E C E V E F saturation E C E V E fe E fh qv BE qv BC In saturation, collector and base flooded with excess minority carriers takes lots of time to get transistor out of saturation.
6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 35-18 Key conclusions In FAR, current gain maximized if N E N B. hard to control precisely: if big enough (> 50), circuit techniques can compensate for variations in. BJT design optimized for operation in forward-active regime operation in inverse regime is poor: β R. In saturation, BJT flooded with minority carriers takes time to get BJT out of saturation. Hybrid-π model: equivalent circuit description of BJT in all regimes: C I S β R (exp qv BC -1) B qv I S (exp BE qv - exp BC ) I S (exp qv BE -1) E Only three parameters needed to describe behavior of BJT in four regimes: I S,, and β R.