BJT Capacitances in Integrated Circuits
Vertical NPN and substrate PNP transistors are crucial components in integrated circuits. Learn about their structures, characteristics, and small-signal models. Explore BJT output characteristics and capacitances in forward active regions.
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Presentation Transcript
BJTs in integrated circuits: Vertical NPN buried-L p-base BiCMOS: CMOS + p-base and buried-layer The n-pocket can be an n-well The buried-layer a buried-well of a triple-well CMOS A diode between collector and substrate is present capacitance p-base buried-L Same as (b) but with the Metal 1 removed P. Bruschi Microelectronic System Design 1
The lateral PNP Slower than vertical devices due to large base series resistance (rbb') and base-to substrate capacitance Lower early voltage (VA), due to non-optimal collector doping. Larger than vertical devices for the same current capability P. Bruschi Microelectronic System Design 2
The substrate PNP: compatible with standard CMOS n-well processes Limitation: The collector is committed to the substrate, (forced to VSS) P. Bruschi Microelectronic System Design 3
BJT output characteristics Forward Active Region Saturation 0.2 V CESAT V VBE, VCE: control voltages IC, IB: dependent currents P. Bruschi Microelectronic System Design 4
BJT model in the forward active zone V V BE = + V 1 CB I I e T C S V = V V V A CB CE BE I = C I B F Sometimes this expression is used in order to refer to VBEand VCEas control voltages: V V V BE V + CE 1 I I e T C S A For calculation of ICand IBin all operating zones (saturation, cut-off, forward active, reverse active) the Ebers-Moll model should be used. P. Bruschi Microelectronic System Design 5
BJT: small signal model 1 Small signal dc model I V V I = r = C g r A be F m g 0 T m C Equivalence with the MOSFET parameters 1 I 1 g 1 I = = = = D g r m d V I TE d D D BJT V MOSFET V T TE 1 V A P. Bruschi Microelectronic System Design 6
BJT capacitances in forward active region (vertical npn) C = JC c bc m + 1 jc V CB V JC C = JS c cs m + 1 js V CS V JS Transition frequency C = + = JE c C c g 1 be de m de F m f je V V 1 BE T 2 F JE P. Bruschi Microelectronic System Design 7
BJTs in Integrated Circuit: instance parameters area area minimum size BJT ("elemental" BJT) area=1.5 area may not be an integer (not all PDKs allow this) parameter "area" (dimensionless) area=2 (,3, 4 .) area = integer P. Bruschi Microelectronic System Design 8
BJT sizing: Effect of the area parameter on the electrical parameters Electrical effects of area parameter: elemental BJT with area specified as an instance parameter elemental BJT IS VA CJE .. area IS VA area CJE .. P. Bruschi Microelectronic System Design 9
BJT sizing: Gummel plot and beta plot My BJT has to carry this current (200 mA). The elemental BJT would be damaged Using a BJT with area=100 would be equivalent to make the elemental BJT work with a current 100 times smaller. This corresponds to the operating point given by the blue line This refers to the elemental BJT (area=1) IC VBE Gummel Plot Beta Plot P. Bruschi Microelectronic System Design 10
