金属氧化物半导体场效应晶体管

金属氧化物半导体场效应晶体管,简称金氧半场效应晶体管是一种可以广泛使用在模拟电路与数字电路的场效应晶体管。是一种可以广泛使用在模拟电路与数字电路的场效晶体管。金属氧化物半导体场效应管依照其沟道极性的不同,可分为电子占多数的N沟道型与空穴占多数的P沟道型,通常被称为N型金氧半场效晶体管(NMOSFET)与P型金氧半场效晶体管(PMOSFET)
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1. Lecture #24 ANNOUNCEMENT • Special session (led by the TA’s): – MEDICI demonstration (for Part II of Design Project) – 3-4 PM, Friday April 18, 521 Cory (Hogan Room) OUTLINE The MOSFET (cont.) • Theory of Operation • Long-channel I-V (Square-Law Theory) (Reading: Textbook Chapter 17.2) Spring 2003 EE130 Lecture 24, Slide 1 Bias-Temperature Stress Measurement Used to determine mobile charge density in MOS dielectric (units: C/cm2) Na+ located at lower SiO2 interface ∆VFB Æ reduces VFB Na+ located at upper SiO2 interface Æ no effect on VFB Positive oxide charge shifts the flatband voltage in the negative direction: tox QF 1 QIT (ψ s ) VFB = φ MS − − Cox ε SiO2 ∫ xρ 0 ox ( x )dx − Cox QM = −Cox ∆VFB Spring 2003 EE130 Lecture 24, Slide 2 1

2. Clarification: Effect of Interface Traps “Donor-like” traps are charge-neutral when (a) (c) filled, positively charged (b) when empty Positive oxide charge causes C-V curve to (a) shift toward left (b) (more shift as VG decreases) Traps cause “sloppy” C-V and also (c) greatly degrade mobility in channel Q (ψ ) ∆VG = − IT s Cox Spring 2003 EE130 Lecture 24, Slide 3 NMOSFET Operation (Qualitative) VGS < VT: depletion layer VGS > VT : VDS ≈ 0 I DS = WQinv v = WQ inv µeff E VDS > 0 V  = WQ inv µeff  DS   L  Spring 2003 EE130 Lecture 24, Slide 4 2

3.Pinch-Off & Channel-Length Modulation VGS > VT : VDS = VGS-VT VDS > VGS-VT Spring 2003 EE130 Lecture 24, Slide 5 Ideal MOSFET I-V Characteristics (Enhancement Mode Transistor) Saturation Linear Spring 2003 EE130 Lecture 24, Slide 6 3

4. Effective Mobility V  I DS = WQinv v = WQ inv µeff E = WQ inv µeff  DS   L  = (W / L) µeff Coxe (VG − VT )VDS • Note that the MOSFET can be modelled as a resistor, for low VDS VDS L RDS = = I DS Wµeff Coxe (VG − VT ) • µeff is the effective electron mobility Spring 2003 EE130 Lecture 24, Slide 7 (Vgs + V t + 0.2)/6Toxe (MV/cm) Scattering mechanisms: (NFET) • coulombic scattering • phonon scattering • surface roughness scattering Empirically, we see: µ0 (PFET) µeff = 1 + θ (VG − VT ) –(Vgs + 1.5V t – 0.25)/6Tox e (MV/cm) Spring 2003 EE130 Lecture 24, Slide 8 4

5. NMOSFET I-V Characteristics • VD > VS • Current in the channel flows by drift • Channel voltage VC(y) varies continuously between the source and the drain 2qN Aε Si ( 2ψ B + VCB ( y )) VT = VFB + VC ( y ) + 2ψ B + Cox • Channel inversion charge  Q ( y)  Qinv ( y ) = −Coxe VG − VFB − VC ( y ) − 2ψ B − dep  Coxe  W Spring 2003 EE130 Lecture 24, Slide 9 1st-Order Approximation • Neglect variation of Qdep with y Qdep = 2qN Aε Si (2ψ B + VSB ) ⇒ Qinv = −Coxe [VG − VT + VS − VC ] where VT = threshold voltage at source end Spring 2003 EE130 Lecture 24, Slide 10 5

6. NMOSFET Current (1st-order approx.) • Consider an incremental length dy in the channel. The voltage drop across this region is dy dy I dy dVC = I DS dR = I DS = I DS = − DS σWTinv qµeff nWTinv Qinv µeff W L VD ∫I 0 DS dy = − ∫ µeff WQinv (VC )dVC VS W VD I DS = − µeff ∫ Qinv (VC )dVC L VS W  V  I DS = µeff Coxe VGS − VT − DS VDS L  2  Spring 2003 EE130 Lecture 24, Slide 11 Saturation Current • saturation region: VD ≥ VDsat = VGS − VT W I Dsat = Coxe µeff (VGS − VT ) 2 2L Spring 2003 EE130 Lecture 24, Slide 12 6