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MSE 510 Part 4: Heterojunctions - MOS Devices MOS Device Uses: MOSCAP – capacitor: storing charge, charge-coupled device (CCD), etc. MOSFET – transistor: switch, current amplifier, dynamic random access memory (DRAM-volatile), NVM (non-volatile memory), etc. Present day Challenges - Ultrathin SiO2: begin to observe quantum effects + Vgate VSource M VDrain W + poly-crystalline Si O n++ Poly Si Gate Contact or Electrode Source Contact Insulator S Drain Contact Insulator SiO2 - Gate oxide n+source - - - - - - - - - - - - - - n+drain channel p-Si Wafer Crystalline Si tox L Cox o r A Knowlton tox o kox A tox 1 MSE 510 MOSFET Current – Voltage Characteristics IDS (mA) ID (mA) VDS(sat) VGS=10V 10 VDS = 20 V 10 Saturation, ID IDS 8V 5 6V 5V 4V 0 5 Vth = 4 V 0 0 10 VDS (a) 20 30 0 Kasap, Fig. 6.38, p. 538 5 VGS 10 (b) (a) Typical ID vs VDS characteristics of an enhancement MOSFET (Vth = 4 V) for various fixed gate voltages VGS. (b) Dependence of ID on VGS at a given VDS ( >V DS(sat)). I D Cox Knowlton 2 VDS W VGS VT VDS ; (+ = nMOS; - = pMOS) L 2 I Dsat Cox W 2 VGS VT ; (+ = nMOS; - = pMOS) 2L Fig. From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) 2 1 MSE 510 m < sc Ee-M Heterojunctions - MOS O S Evac M O S Ee- Ec Ec sc sc m Ef,m Eg eVFB Ec Eg Ec Eg E f,s/c Ev Φs band bendinginSi Eg Ef,m Ef,s/c Ev p-Si Ev Ev p-Si eVox bandbending SiO2 inoxide SiO2 eVFB eVox Φs Flat band Equilibrium Depletion Regime Knowlton 3 MSE 510 m > sc M Ee- Heterojunctions - MOS O S sio2 M O Ee- Ec sc sc m Eg Eg eVFB Ef,m Evac M S eVox bandbendinginoxide Ec Ef,s/c Ev Φs bandbendinginSi Eg Ec Eg Ef,m p-Si Ef,s/c Ev p-Si Ev SiO2 Ec Ev SiO2 eVFB eVox Φs Flat band Knowlton Equilibrium Accumulation Regime 4 2 Heterojunctions MOSCAP MSE 510 m < sc EeM O Eg qVA Ee- M S Eg O Φs band bendingin Si Ec Ef,s/c Ev Ee- M S Eg Ec Eg qVA eVox band eVox band Inversion Ec Eg p-Si p-Si bendingin SiO2 Φs band bendingin Si A bendingin Si SiO2 Eg S Ef,s/c qV Φs band Ef,m O Ef,s/c Ev p-Si bendingin SiO2 SiO2 Accumulation SiO2 Very strong accumulation Knowlton 5 MSE 510 Heterojunctions - MOS Knowlton Neaman, Semiconductor Devices 6 3 MSE 510 Ee- m < sc Heterojunctions MOSCAP Ee- qVA Depletion Accumulation Knowlton Ee- qVA Very strong accumulation 7 Heterojunctions MOSCAP MSE 510 Ee- Knowlton 8 4 MSE 510 m < sc Heterojunctions: MOSCAP q SiO2 Ee- Ee- q Si qSi qm qF Depletion Depletion Knowlton Heterojunctions: MOSCAP MSE 510 Ee- q SiO2 Ee- At Si/SiO2 Interface: p=n q Si qSi qm So: Ef = Ei qVA =q VG qs [ ] qF qF NOTE: Flat Band: qF s= surface potential Knowlton 9 depletion s = F Ei Ef qF At Flat Band, no Surface Potential, qs, exists. But, when Ef coincides with Ei at the interface, then qF = qs F N kT ln D o p a n t q ni onset of inversion 10 5 Heterojunctions: MOSCAP MSE 510 Assume: m < sc Ee- or S Oxide S/C F Ec F Ei F F F Ef & EA Ev F Knowlton 1Colinge MSE 510 & Colinge, Physics of Semiconductor Devices (Kluwer Academic Publishers, 2002 ) p. 178-179, 182-183 11 Heterojunctions: MOSCAP EeColinge & Colinge, Physics of Semiconductor Devices (Kluwer Academic Publishers, 2002 ) At Si/SiO2 Interface: Ef = Ei qVG q F s = F s = F = ms – Vox + VG Knowlton onset of inversion F kT N ln A q ni VG = s – ms + Vox = F – ms + Vox 12 6 Heterojunctions: MOSCAP MSE 510 [1] [1] One might think that the definition for the threshold of inversion should be the onset of inversion … BUT IT IS NOT. There are varying opinions on this! Onset of strong inversion: S = F Threshold of inversion2,6 , Onset of strong inversion3 or Onset of strong inversion1,2,4,5: S = 2F When: S = 2F, Vg = VT s = 2F = ms – Vox + VT F VG = s – ms + Vox = 2F – ms + Vox kT N ln A q ni 1Colinge & Colinge, Physics of Semiconductor Devices (Kluwer Academic Publishers, 2002 ) p. 178-179, 182-183 2Anderson & Anderson, Fundamentals of Semiconductor Devices, (McGraw Hill, 2005) p. 394 3Muller & Kamins, Device Electronics for Integrated Circuits, 3rd Ed. (Wiley, 2003) p. 390-392 4Streetman & Banerjee, Solid State Electronic Devices, 5th Ed (Prentice Hall, 2000) p.263-275 5Taur & Ning, Fundamentals of Modern VLSI Devices, (Cambridge, ) p..58-74; 6Kasap, Prin. Of Electronic Materials & Devices 3rd Ed (McGraw Hill,, 2006) p. 540 Knowlton 13 Heterojunctions: MOSCAP MSE 510 Assume: m < sc O S Ee- Ec ED Ei EA & Ef Ev Flatband F 2F F Depletion [1] At Si/SiO2 Interface: Ef = Ei n=p onset of inversion S = F At Si/SiO2 Interface: Ef = ED n = NA At Si/SiO2 Interface: Ef > ED threshold of inversion Inversion Or Onset of Strong Inversion strong S = 2F Knowlton 14 7 MSE 510 Heterojunctions: MOSCAP Threshold Voltage: Vt VT,start: modeled by MEDICI, a TCAD program VT,2F: VT at 2F VT,extrapol: extrapolated VT Knowlton MSE 510 D. Schroder, Semiconductor Material & Device Characterization 2rd Ed (Wiley Interscience, 1998) p. 242 15 Heterojunctions: MOSCAP [4] [1] [3] [5] Something is important when: S = 2F Inversion takes over depletion & Ef=ED 1Colinge & Colinge, Physics of Semiconductor Devices (Kluwer Academic Publishers, 2002 ) p. 178-179, 182-183 & Anderson, Fundamentals of Semiconductor Devices, (McGraw Hill, 2005) p. 394 & Kamins, Device Electronics for Integrated Circuits, 3rd Ed. (Wiley, 2003) p. 390-392 & Banerjee, Solid State Electronic Devices, 5th Ed (Prentice Hall, 2000) p.263-275 5Taur & Ning, Fundamentals of Modern VLSI Devices, (Cambridge, ) p..58-74 2Anderson 3Muller Knowlton 4Streetman 16 8 Heterojunctions: MOSCAP MSE 510 Degenerate Ef is in the CB Degenerate Ef is in the VB 0ΦF 2ΦF 1ΦF VT VFB Knowlton 5Taur Eg Something is important when: S = 2F Inversion takes over depletion & Ef=ED & Ning, Fundamentals of Modern VLSI Devices, (Cambridge, ) p..58-74 17 MSE 510 Heterojunctions: MOSCAP Capacitance – Voltage: C Q Q or V [5] [4] Ei VT [1] 1Colinge & Colinge, Physics of Semiconductor Devices (Kluwer Academic Publishers, 2002 ) p. 178-179, 182-183 & Anderson, Fundamentals of Semiconductor Devices, (McGraw Hill, 2005) p. 394 & Kamins, Device Electronics for Integrated Circuits, 3rd Ed. (Wiley, 2003) p. 390-392 & Banerjee, Solid State Electronic Devices, 5th Ed (Prentice Hall, 2000) p.263-275 5Taur & Ning, Fundamentals of Modern VLSI Devices, (Cambridge, ) p..58-74 2Anderson 3Muller Knowlton 4Streetman 18 9 MSE 510 Heterojunctions: MOSCAP Capacitance – Voltage: C Q Q or V 0ΦF 1ΦF 2ΦF [4] Ei Knowlton MSE 510 Ee- 4Streetman VT & Banerjee, Solid State Electronic Devices, 5th Ed (Prentice Hall, 2000) p.263-275 19 Heterojunctions: MOSCAP Ee- Ee- VA = VG VA At Si/SiO2 Interface: Ef = Ei depletion Knowlton onset of inversion At Si/SiO2 Interface: Ef >> Ei strong inversion 20 10 Heterojunctions MOSCAP MSE 510 Ee- Knowlton 21 MSE 510 Heterojunctions Ee- e- Drain (V+) eGate ee- Source (V-) e - MOSFET Knowlton 22 11 MSE 510 MOSFET Current – Voltage Characteristics IDS (mA) ID (mA) VDS(sat) VGS=10V 10 VDS = 20 V 10 Saturation, ID IDS 8V 5 5 6V 5V 4V 0 Vth = 4 V 0 0 10 VDS (a) 20 30 0 Kasap, Fig. 6.38, p. 538 5 VGS 10 (b) (a) Typical ID vs VDS characteristics of an enhancement MOSFET (Vth = 4 V) for various fixed gate voltages VGS. (b) Dependence of ID on VGS at a given VDS ( >V DS(sat)). I D Cox Knowlton MSE 510 2 VDS W VGS VT VDS ; (+ = nMOS; - = pMOS) L 2 I Dsat Cox W 2 VGS VT ; (+ = nMOS; - = pMOS) 2L Fig. From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) 23 Heterojunctions MOSCAP depletion Knowlton Neaman, Semiconductor Devices 24 12
