6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-1 Lecture 18 - The ipolar Junction Transistor (II) ontents: 1. Regimes of operation. Regimes of Operation November 10, 2005 2. Large-signal equivalent circuit model. 3. Output characteristics. Reading assignment: Howe and Sodini, h. 7, 7.3, 7.4 Announcements: Quiz 2: 11/16, 7:30-9:30 PM, open book, must bring calculator; lectures #10-18.
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-2 Key questions What other regimes of operation are there for the JT? What is unique about each regime? How do equivalent circuit models for the JT look like?
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-3 1. Regimes of operation V + + V - + V forward active saturation V V - - cut-off reverse forward active: device has good isolation and high gain; most useful regime; saturation: device has no isolation and is flooded with minority carriers takes time to get out of saturation; avoid reverse: poor gain; not useful; cut-off: negligible current: nearly an open circuit; useful.
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-4 Forward-active regime: V > 0, V < 0 n-mitter p-ase n-ollector I <0 I >0 I >0 V > 0 V < 0 Minority carrier profiles (not to scale): emitter base collector p n n p pn pno n po p no x -W -X -X 0 W W +X W +X +W
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-5 mitter injects electrons into base, collector collects electrons from base: I = I S exp qv kt ase injects holes into emitter, recombine at emitter contact: mitter current: I = I S β F (exp qv kt 1) I = I I = I S exp qv kt I S β F (exp qv kt 1) State-of-the-art I JT s today: I β F 50 300. 0.1 1 ma, β F hard to control tightly circuit design techniques required to be insensitive to variations in β F.
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-6 Reverse regime: V < 0, V > 0 n-mitter p-ase n-ollector I >0 I <0 I >0 V < 0 V > 0 Minority carrier profiles: emitter base collector p n n p pn pno n po p no x -W -X -X 0 W W +X W +X +W
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-7 ollector injects electrons into base, emitter collects electrons from base: I = I S exp qv kt ase injects holes into collector, recombine at collector contact and buried layer: ollector current: I = I S β R (exp qv kt 1) I = I I = I S exp qv kt I S β R (exp qv kt 1) Typically, β R 0.1 5 β F. I A I A
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-8 Forward-active Gummel plot (V =3V ): Reverse Gummel (V =3V ):
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-9 ut-off: V < 0, V < 0 n-mitter p-ase n-ollector I >0 I >0 I <0 V < 0 V < 0 Minority carrier profiles: emitter base collector p n n p pn pno n po p no x -W -X -X 0 W W +X W +X +W
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-10 ase extracts holes from emitter: I 1 = I S β F = I ase extracts holes from collector: I 2 = I S β R = I These are tiny leakage currents ( 10 12 A).
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-11 Saturation: V > 0, V > 0 n-mitter p-ase n-ollector I I I <0 V > 0 V > 0 Minority carrier profiles: emitter base collector p n n p pn pno n po p no x -W -X -X 0 W W +X W +X +W
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-12 Saturation is superposition of forward active + reverse: I = I S (exp qv kt exp qv kt ) I S β R (exp qv kt 1) I = I S (exp qv β F kt 1) + I S β R (exp qv kt 1) I = I S (exp qv β F kt 1) I S(exp qv kt exp qv kt ) I and I can have either sign, depending on relative magnitude of V and V, and β F and β R. In saturation, collector and base flooded with excess minority carriers takes lots of time to get transistor out of saturation. electrons holes
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-13 2. Large-signal equivalent circuit model System of equations that describes JT operation: I = I S (exp qv kt exp qv kt ) I S β R (exp qv kt 1) I = I S (exp qv β F kt 1) + I S β R (exp qv kt 1) I = I S (exp qv β F kt 1) I S(exp qv kt exp qv kt ) quivalent-circuit model representation: Non-Linear Hybrid-π Model I S qv (exp β R kt -1) qv I S (exp qv - exp ) kt kt I S β F qv (exp kt -1) Three parameters in this model: I S, β F, and β R. Model equivalent to bers-moll model in text.
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-14 Simplifications of equivalent-circuit model: Forward-active regime: V > 0, V < 0 I S exp qv kt I β F I I S β F qv (exp kt -1) V,on For today s technology: V,on 0.7 V. I depends on outside circuit. Reverse: V < 0, V > 0 I S qv (exp β R kt -1) V,on I S exp qv kt β R I I For today s technology: V,on 0.5 V. I also depends on outside circuit.
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-15 I vs. V for V =3V : I vs. V for V =3V :
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-16 Saturation: V > 0, V > 0 + V,on V,on V,sat V,on V,on - Today s technology: V,sat = V,on V,on 0.2 V. I and I depend on outside circuit. ut-off: V < 0, V < 0 Only negligible leakage currents.
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-17 3. Output characteristics First, I vs. V with I as parameter: I I I =0 0 0 V,on V Next, common-emitter output characteristics (I vs. V with I as parameter): I I I =0 0 0 V =V +V V,sat
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-18 I vs. V for 0 I 100 µa: I vs. V for 0 I 100 µa:
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-19 I vs. V for 0 I 100 µa:
6.012 - Microelectronic Devices and ircuits - Fall 2005 Lecture 18-20 Key conclusions Forward-active regime: most useful, device has gain and isolation. For bias calculations: I V,on β F I Saturation: device flooded with minority carriers. Not useful. For bias calculations: + V,on V,sat V,on - ut-off: device open. Useful. For bias calculations: