EE143 F05 Lecture 22 Electrical Characteristics of MOS Devices The MOS Capacitor Voltage components Accumulation Depletion Inversion Modes Effect of channel bias and substrate bias Effect of gate oxide charges Threshold voltage adjustment by implantation Capacitance vs voltage characteristics MOS Field Effect Transistor I V characteristics Parameter extraction Professor N Cheung U C Berkeley 1 EE143 F05 Lecture 22 1 Revisit EE143 Week 2 Reading Assignment Introduction to IC Devices www icknowledge com Streetman Chap 3 Energy Band and Charge carriers in Semiconductors 2 Visit the Device Visualization Website http jas eng buffalo edu and run the visualization experiments of 1 Charge carriers and Fermi level 2 pn junctions 3 MOS capacitors 4 MOSFETs Professor N Cheung U C Berkeley 2 EE143 F05 Lecture 22 Metal Oxide Semiconductor Transistor n channel VG Vthreshold Negligible electron concentration underneath Gate region Source Drain is electrically open Professor N Cheung U C Berkeley VG Vthreshold High electron concentration underneath Gate region Source Drain is electrically connected 3 EE143 F05 Lecture 22 Work Function of Materials METAL Work function q SEMICONDUCTOR Eo Ef Vacuum energy level q Ef Eo EC EV q M is determined q S is determined by the metal material by the semiconductor material the dopant type and doping concentration Professor N Cheung U C Berkeley 4 EE143 F05 Lecture 22 Work Function q M of MOS Gate Materials Eo vacuum energy level Ef Fermi level EC bottom of conduction band EV top of conduction band q 4 15eV electron affinity Eo Eo Eo q M q M q 4 15eV EC EC Ef 0 56eV Ef Al 4 1 eV TiSi2 4 6 eV q 4 15eV Ei Ei 0 56eV EV n poly Si Professor N Cheung U C Berkeley q M 0 56eV Ef 0 56eV EV p poly Si 5 EE143 F05 Lecture 22 Work Function of doped Si substrate Depends on substrate concentration NB Eo Eo q s Ef F kT N B ln q ni Ei q 4 15eV EC 0 56eV q F EV n type Si s volts 4 15 0 56 F Professor N Cheung U C Berkeley q s Ef 0 56eV q 4 15eV EC 0 56eV Ei 0 56eV q F EV p type Si s volts 4 15 0 56 F 6 EE143 F05 Lecture 22 The MOS Capacitor VG VFB xox metal oxide Vox VSi semiconductor V G V FB V ox V Si Cox ox xox F cm2 Oxide capacitance unit area Professor N Cheung U C Berkeley 7 EE143 F05 Lecture 22 Flat Band Voltage VFB is the built in voltage of the MOS VFB M S Gate work function M Al 4 1 V n poly Si 4 15 V p poly Si 5 27 V Semiconductor work function S s volts 4 15 0 56 F for n Si s volts 4 15 0 56 F for p Si Vox voltage drop across oxide depends on VG VSi voltage drop in the silicon depends on VG Professor N Cheung U C Berkeley 8 EE143 F05 Lecture 22 MOS Operation Modes A Accumulation VG VFB for p type substrate M O Si p Si holes Thickness of accumulation layer 0 VSi 0 so Vox VG VFB QSi charge unit area in Si Cox VG VFB Professor N Cheung U C Berkeley 9 EE143 F05 Lecture 22 MOS Operation Modes B Flatband VG VFB No charge in Si and hence no charge in metal gate VSi Vox 0 M Professor N Cheung U C Berkeley O S p Si 10 EE143 F05 Lecture 22 MOS Operation Modes cont C Depletion VG VFB xd M 2 Si VSi qN B O S p Si xd qNB V G V FB qN B x d qN B x d C ox 2 s Vox Professor N Cheung U C Berkeley 2 Depletion layer can solve for xd VSi 11 EE143 F05 Lecture 22 Depletion Mode Charge and Electric Field Distributions by Superposition Principle of Electrostatics x Q Metal x Q Oxide Semiconductor x xo x d x Metal x Q Oxide x 0 E x Metal Oxide Semiconductor Metal E x Oxide x xo Professor N Cheung U C Berkeley x 0 x xo Semiconductor Metal x x x x o d x xo x d x 0 Q x xo x Semiconductor x xo x d x Oxide x x 0 Metal Semiconductor x 0 x x o E x Oxide x xo Semiconductor x x x o x x 0 x x d o 12 EE143 F05 Lecture 22 MOS Operation Modes cont D Threshold of Inversion VG VT This is a definition for onset of nsurface NB for p type substrate VSi 2 F strong inversion M O S p Si xdmax qNB Q n V G V T V FB Professor N Cheung U C Berkeley 2 s 2 F qN C ox B 2 F 13 EE143 F05 Lecture 22 MOS Operation Modes cont E Strong Inversion VG VT xdmax is approximately unchanged x d max 4 Si F qN B when VG VT M O S p Si xdmax V ox qN B x d max Q n C ox Q n C ox V G V T Professor N Cheung U C Berkeley qNa Q n electrons 14 EE143 F05 Professor N Cheung U C Berkeley Lecture 22 15 EE143 F05 Professor N Cheung U C Berkeley Lecture 22 16 EE143 F05 Lecture 22 p Si Professor N Cheung U C Berkeley 17 EE143 F05 Lecture 22 Suggested Exercise Most derivations for MOS shown in lecture notes are done with p type substrate NMOS as example Repeat the derivations yourself for n type substrate PMOS to test your understanding of MOS Professor N Cheung U C Berkeley 18 EE143 F05 Lecture 22 p Si substrate NMOS Accumulation holes strong inversion electrons depletion VFB VT VG more positive n Si substrate PMOS VG more negative Strong inversion holes Professor N Cheung U C Berkeley VT Accumulation electrons depletion VFB 19 EE143 F05 Lecture 22 Voltage drop area under E field curve Accumulation Vox Qa Cox VSi 0 Vox qNaxd Cox Depletion VSi qNaxd2 2 s Vox qNaxdmax Qn Cox Inversion VSi qNaxdmax2 2 s 2 F For simplicity dielectric constants assumed to be same for oxide and Si in E field sketches Professor N Cheung U C Berkeley 20 EE143 F05 Lecture 22 Appendix Electron Energy Band Fermi Level Electrostatics of device charges Professor N Cheung U C Berkeley 21 EE143 F05 Electron Potential Energy Lecture 22 Conduction Band and Valence Band Available states at discreet energy levels Isolated atoms Professor N Cheung U C Berkeley Available states as continuous energy levels inside energy bands Atoms in a solid 22 EE143 F05 Lecture 22 The Simplified Electron Energy Band Diagram Professor N Cheung U C Berkeley 23 EE143 F05 Lecture 22 Energy Band Diagram with E field Electron Electron Energy Energy E field 2 1 Electric potential 2 1 EC 2 EV x E field 1 EC EV Electric potential 2 1 x Electron concentration n q 2 kT n 2 e q 2 1 kT q 1 kT e n …
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