EE 5340 Semiconductor Device Theory Lecture 22 - Fall 2010 Professor Ronald L. Carter ronc@uta.edu http://www.uta.edu/ronc

Non-ideal effects in BJTs Base-width modulation (FA: xB changes with changes in VBC) Current crowding in 2-dim base High-level injection (minority carriers g.t. dopant - especially in the base). Emitter Bandgap narrowing (NE ~ density of states at cond. band. edge) Junction breakdown at BC junction L22 08Nov2010

Base-width modulation (Early Effect, cont.) Fig 9.16* L22 08Nov2010

Emitter current crowding in base Fig 9.21* L22 08Nov2010

Interdigitated base fixes emitter crowding Fig 9.23* L22 08Nov2010

Base region high- level injection (npn) L22 08Nov2010

Effect of HLI in npn base region Fig 9.17* L22 08Nov2010

Effect of HLI in npn base region (cont) L22 08Nov2010

Effect of HLI in npn base region (cont) L22 08Nov2010

Emitter region high- level injection (npn) L22 08Nov2010

Effect of HLI in npn emitter region L22 08Nov2010

Effect of HLI in npn base region Figs 9.18 and 9.19* L22 08Nov2010

Bandgap narrowing effects Fig 9.20* Replaces ni2 throughout L22 08Nov2010

Junction breakdown at BC junction Reach-through or punch-through when WCB and/or WEB become large enough to reduce xB to zero Avalanche breakdown when Emax at EB junction or CB junction reaches Ecrit. L22 08Nov2010

Hybrid-pi circuit model Adapted from linking current version of E-M model with parasitic Rs and CSubstr C-E branch is linking current B-E branch is the reduced B-E diode with diffusion (for and rev) resistance and capacitance and junction cap. B-C branch is the reduced B-C diode with diffusion (for and rev) resistance and capacitance and junction cap. L22 08Nov2010

Hybrid-pi Circuit model Fig 9.33* L22 08Nov2010

Gummel-Poon Static npn Circuit Model B RBB ILC IBR ICC - IEC = IS(exp(vBE/NFVt - exp(vBC/NRVt)/QB B’ ILE IBF RE E L22 08Nov2010

BJT Characterization Forward Gummel iC RC iB RE RB vBEx vBC vBE + - vBCx= 0 = vBC + iBRB - iCRC vBEx = vBE +iBRB +(iB+iC)RE iB = IBF + ILE = ISexpf(vBE/NFVt)/BF + ISEexpf(vBE/NEVt) iC = bFIBF/QB = ISexpf(vBE/NFVt)/QB L22 08Nov2010

Ideal F-G Data iC and iB (A) vs. vBE (V) N = 1  1/slope = 59.5 mV/dec L22 08Nov2010

References * Semiconductor Physics and Devices, 2nd ed., by Neamen, Irwin, Boston, 1997. **Device Electronics for Integrated Circuits, 2nd ed., by Muller and Kamins, John Wiley, New York, 1986. L22 08Nov2010