MIT 6 720J - The Long Metal-Oxide-Semiconductor Field-Effect Transistor

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6.7205/3.435 - Integrated Microelectronic Devices - Spring 2007 Lecture 26-1 Lecture 26 - The "Long" Metal-Oxide-Semiconductor Field-Effect Transistor (cont.) April 11, 2007 Contents: I. Current-voltage characteristics of ideal MOSFET (cont.) 2. Charge-voltage characteristics of ideal MOSFET Reading assignment: del Alamo, Ch. 9, 559.4 (9.4.3-9.4.5): 9.5 Cite as: leslis del Alamo, course materials for 6.7201 Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month W]6.7205/3.435 -Integrated Microelectronic Devices -Spring 2007 Lecture 26-2 Key questions Why does the drain current in a MOSFET saturate at high VDs? What charges should we keep track of as we construct a model for the charge-voltage characteristics of the MOSFET? Cite as: leslis del Alamo, course materials for 6.7201 Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month W]6.7205/3.435 -Integrated Microelectronic Devices -Spring 2007 Lecture 26-3 1. I-V characteristics of ideal MOSFET (cont.) Problems with MOSFET current model for linear regime as VDs approaches VGS-VT. Problems centered around g = L: Local gate overdrive goes to zero + lQil4 0.How can current be supported? Gradual-channel approximation becomes invalid. Sheet-charge approximation becomes invalid. Lateral field so large that linearity between field and velocity invalid. Model that can handle VDs values all the way up to VGS-VT is rather complicated; but ... actually, don't need new model! Reason: when VDs approaches VGS-VT, IDchanges very little due to prominent debiasing on the drain side of the channel. Cite as: leslis del Alamo, course materials for 6.7201 Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month W]6.7205/3.435 -Integrated Microelectronic Devices -Spring 2007 Lecture 26-4 Ask a different question: how close can VDs get to VGS-VT before simple model fails? Answer: up to about 80% of VGS-VT which means up to about 96% of ID,,,. Hence, simple model is pretty good up to VDs = VGS-VT. Cite as: leslis del Alamo, course materials for 6.7201 Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month W]6.720J/3.43J -Integrated Microelectronic Devices -Spring 2007 Lecture 26-5 What happens if VDs reaches or exceeds VGS-VT? Electron concentration at y = L drops to very small con-centrations +depletion region appears at y = L: pinch-08. Depletion region is no barrier to electron flow: field "pulls" electrons into drain. inversion III layer /T--7-- -........, ----.....--depletion region P'.---... pinch-off point "--------... drain pinch-off region a) linear regime VDS<VDS,,~ b) pinch-off VDS=VDS,,~ c) saturation regime VDS>VDS,,~ As VDs exceeds VGS-VT, depletion region widens into channel underneath gate; all extra voltage consumed in depletion region; electrostatics of channel, to first order, unperturbed; channel current unchanged + MOSFET in satura-tion. C~teas: Jeslis del Alamo, course materials for 6.7201 Integrated M~croelectron~cDev~ces,Spr~ng2007. MIT OpenCourseWare (http://ocw.m~t.edu/), Massachusetts Inst~tuteof Technology. Downloaded on [DD Month W].6.720J/3.43J -Integrated Microelectronic Devices -Spring 2007 Lecture 26-6 Lateral electrostatics in saturation: local gate overdrive C~teas: leslis del Alarno, course rnater~als for 6.7201 Integrated M~croelectron~c Dev~ces, Spr~ng 2007. MIT OpenCourseWare (http://ocw.rn~t.edu/), Massachusetts Inst~tute of Technology. Downloaded on [DD Month W]6.7205/3.435 -Integrated Microelectronic Devices -Spring 2007 Lecture 26-7 Current model in saturation: IDdoes not increase passed VDs = VGS -VT. Hence, VDs at which transistor saturates is denoted as VDssat: Current-voltage characteristics: Cite as: leslis del Alamo, course materials for 6.7201 Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month W]6.7205/3.435 - htegrated Microelectronic Devices - Spring 2007 Lecture 26-8 Why square dependence? VGS T+ VDSsat T+ higher lateral field in channel at saturation. Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].6.7205/3.435 - h tegrated Microelectronic Devices - Spring 200 7 Lecture 26-9 LI Energy band diagrams (VGS > VT): channel S D Ec EF ------------------------------------------ Efe ----- ------_=, ----------_ .. .. .... ...------ - --. ----__ ________ ---- t ~VDS VDS=O O<VDs<VDssat a) linear b) linear VDS=~DSS~~ VDs>VDssat c) pinch-off d) saturation Pinch-off point: region of "free fall" of electrons. Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].6 720J/3 43J- Integrated Microelectronic Devices - Spring 2007 Lecture 26-10 2. Charge-voltage characteristics of ideal MOSFET Two types of stored charge in a MOSFET: depletion charge in: - source-body pn junction - drain-body pn junction - MOS structure inversion charge vBS + - - - - - - - - -- ,I ;;ision depletion I region ' .--------- Clte as: leslis del Alarno, course rnaterlals for 6.7203 Integrated M~croelectron~c Devlces, Sprlng 2007. MIT OpenCourseWare (http://ocw.mlt.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month W]6 720J/3 43J- Integrated M~croelectronic Devlces - Spring 2007 Source-body junction depletion charge (assume strongly asymmetric junction) : with Clte as: les6s del Alamo, course materials for 6.7201 Integrated M~croelectron~c Devlces, Spring 2007. MIT OpenCourseWare (http://ocw,mlt.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month W].6.7205/3.435 - Integrated IlIicroelectronic Devices - Spring 2007 Lecture 26-12 Drain-body j~~nction depletion charge (assume strongly asymmetric junction) : with Cite as: leslis del Alamo, course materials for 6.7201 Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare


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