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MOSFET ID-VGS Characteristic and Circuit Models

This outline discusses the ID-VGS characteristic of MOSFETs, including the resistive switch model and small-signal model. It also covers the different regions of operation and the measurement of VT.

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MOSFET ID-VGS Characteristic and Circuit Models

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  1. Week 9a OUTLINE • MOSFET IDvs.VGS characteristic • Circuit models for the MOSFET • resistive switch model • small-signal model Reading • Rabaey et al.: Chapter 3.3.2 • Hambley: Chapter 12 (through 12.5); Section 10.8 (Linear Small-Signal Equivalent Circuits)

  2. IDvs.VDS Characteristics • The MOSFET ID-VDS curve consists of two regions: • 1) Resistive or “Triode” Region: 0 < VDS < VGS VT • 2) Saturation Region: • VDS > VGS VT process transconductance parameter “CUTOFF” region: VG < VT

  3. Overview of NMOSFET Regions • Cutoff region: • Conditions: VGS < VT, any value of VDS • ID = 0 • Linear (or Resistive, or Triode) region: • VGS > VT, (VGS – VT) > VDS • ID = (f1 x f2 x f3 )VDS where • f1 = mCox (depends on the fabrication process) • f2 = W/L (chosen by the design engineer) • f3 = f3(VGS, VT, VDS) = [VGS – VT – VDS/2] • ~ (VGS – VT) if (VGS – VT) >> VDS /2 • Saturation region: • VDS > (VGS – VT) = VDSaturation2= (VGS – VT)2 • ID = (1/2) f1 x f2 x (VGS – VT)2

  4. MOSFET IDvs.VGS Characteristic • Typically, VDS is fixed when ID is plotted as a function of VGS Long-channel MOSFET VDS = 2.5 V > VDSAT Short-channel MOSFET VDS = 2.5 V > VDSAT

  5. MOSFET VT Measurement • VT can be determined by plotting IDvs.VGS, using a low value of VDS : ID (A) VGS (V) 0 VT

  6. Subthreshold Conduction (Leakage Current) • The transition from the ON state to the OFF state is gradual. This can be seen more clearly when ID is plotted on a logarithmic scale: • In the subthreshold (VGS < VT) region, This is essentially the channel- source pn junction current. (n, the emission factor, is between 1 and 2) (Some electrons diffuse from the source into the channel, if this pn junction is forward biased.) VDS > 0

  7. Qualitative Explanation for Subthreshold Leakage • The channel Vc (at the Si surface) is capacitively coupled to the gate voltage VG: Using the capacitive voltage divider formula: DEVICE CIRCUIT MODEL VG VG VD n+ poly-Si Cox + Vc – n+ n+ Cdep The forward bias on the channel-source pn junction increases with VG scaled by the factor Cox / (Cox+Cdep) Wdep depletion region p-type Si

  8. Slope Factor (or Subthreshold Swing) S • S is defined to be the inverse slope of the log (ID) vs.VGS characteristic in the subthreshold region: VDS > 0 Units: Volts per decade Note that S≥ 60 mV/dec at room temperature: 1/Sis the slope

  9. VT Design Trade-Off(Important consideration for digital-circuit applications) • Low VT is desirable for high ON current IDSAT (VDD - VT) 1 <  < 2 where VDD is the power-supply voltage …but high VT is needed for low OFF current log IDS Low VT High VT IOFF,low VT IOFF,high VT VGS 0

  10. The MOSFET as a Resistive Switch • For digital circuit applications, the MOSFET is either OFF (VGS < VT) or ON (VGS = VDD). Thus, we only need to consider two ID vs. VDS curves: • the curve for VGS < VT • the curve for VGS = VDD ID VGS = VDD (closed switch) Req VDS VGS< VT(open switch)

  11. Equivalent Resistance Req • In a digital circuit, an n-channel MOSFET in the ON state is typically used to discharge a capacitor connected to its drain terminal: • gate voltage VG = VDD • source voltage VS = 0 V • drain voltage VD initially at VDD, discharging toward 0 V The valueof Req should be set to the value which gives the correct propagation delay (time required for output to fall to ½VDD): Cload

  12. V + dd V + dd = = Figure 0.1 CMOS circuits and their schematic symbols

  13. Typical MOSFET Parameter Values • For a given MOSFET fabrication process technology, the following parameters are known: • VT (~0.5 V) • Coxand k (<0.001 A/V2) • VDSAT ( 1 V) • l ( 0.1 V-1) Example Req values for 0.25 mm technology (W = L): How can Req be decreased?

  14. P-Channel MOSFET Example • In a digital circuit, a p-channel MOSFET in the ON state is typically used to charge a capacitor connected to its drain terminal: • gate voltage VG = 0 V • source voltage VS = VDD (power-supply voltage) • drain voltage VD initially at 0 V, charging toward VDD VDD 0 V iD Cload

  15. Common-Source (CS) Amplifier • The input voltage vs causes vGS to vary with time, which in turn causes iD to vary. • The changing voltage drop across RD causes an amplified (and inverted) version of the input signal to appear at the drain terminal. VDD RD iD vs + vOUT = vDS   + + vIN = vGS  + – VBIAS

  16. Notation • Subscript convention: • VDSVD– VS , VGS VG– VS , etc. • Double-subscripts denote DC sources: • VDD , VCC , ISS , etc. • To distinguish between DC and incremental components of an electrical quantity, the following convention is used: • DC quantity: upper-case letter with upper-case subscript • ID , VDS , etc. • Incremental quantity: lower-case letter with lower-case subscript • id , vds , etc. • Total (DC + incremental) quantity: lower-case letter with upper-case subscript • iD , vDS , etc.

  17. Load-Line Analysis of CS Amplifier • The operating point of the circuit can be determined by finding the intersection of the appropriate MOSFETiDvs.vDS characteristic and the load line: iD (mA) load-line equation: vGS (V) vDS (V)

  18. Voltage Transfer Function (1): transistor biased in cutoff region (2): vIN > VT; transistor biased in saturation region (3): transistor biased in saturation region (4): transistor biased in “resistive” or “triode” region vOUT Goal: Operate the amplifier in the high-gain region, so that small changes in vIN result in large changes in vOUT vIN

  19. Quiescent Operating Point • The operating point of the amplifier for zero input signal (vs = 0) is often referred to as the quiescent operating point. (Another word: bias.) • The bias point should be chosen so that the output voltage is approximately centered between VDD and 0 V. • vs varies the input voltage around the input bias point. Note: The relationship between vOUT and vIN is not linear; this can result in a distorted output voltage signal. If the input signal amplitude is very small, however, we can have amplification with negligible distortion.

  20. Bias Circuit Example VDD RD R1 R2

  21. Rules for Small-Signal Analysis • A DC supply voltage source acts as a short circuit • Even if AC current flows through the DC voltage source, the AC voltage across it is zero. • A DC supply current source acts as an open circuit • Even if AC voltage is applied across the current source, the AC current through it is zero.

  22. NMOSFET Small-Signal Model D G id + vgs  + vds  gmvgs ro S S transconductance output conductance

  23. Channel-Length Modulation • If L is small, the effect of DL to reduce the inversion-layer “resistor” length is significant • ID increases noticeably with DL (i.e. with VDS) ID ID= ID(1 + lVDS) l is the slope ID is the intercept VDS

  24. Small-Signal Equivalent Circuit G D + vout  + vin  + vgs  gmvgs R1 R2 RD ro S S voltage gain

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