Transistor Characteristics EMT 251

Outline Introduction MOS Capacitor nMOS I-V Characteristics (ideal) pMOS I-V Characteristics (ideal)

Introduction So far, we have treated transistors as ideal switches An ON transistor passes a finite amount of current –Depends on terminal voltages –Derive current-voltage (I-V) relationships Transistor gate, source, drain all have capacitance –I = C (  V/  t) ->  t = (C/I)  V –Capacitance and current determine speed

MOS Capacitor Gate and body form MOS capacitor Operating modes –Accumulation –Depletion –Inversion -V V+ V++

Terminal Voltages Mode of operation depends on V g, V d, V s –V gs = V g – V s –V gd = V g – V d –V ds = V d – V s = V gs - V gd Three regions of operation –Cutoff –Linear –Saturation

nMOS Cutoff No channel Vgs < Vt I ds = 0

nMOS Linear Channel forms –Vds = Vgs-Vgd –If Vds=0 (i.e. Vgs=Vgd) No electrical field tending to push current fr. d to s. Small positive potential Vds Current flows from d to s –e - from s to d I ds increases with V ds Similar to linear resistor

nMOS Saturation Channel pinches off Ids independent of Vds We say current saturates Similar to current source

Nmos region of operation “CUTOFF” region: V G < V T 1.Cutoff region: V GS < V T, any value of V DS I D = 0 2.Linear (or Resistive, or Triode) region: V GS > V T, V DS < (V GS – V T ) 3.Saturation region: V GS > V T, V DS > (V GS – V T )

Example We will be using a 0.6  m process for your project –From AMI Semiconductor –t ox = 100 Å –  = 350 cm 2 /V*s –V t = 0.7 V Plot I ds vs. V ds –V gs = 0, 1, 2, 3, 4, 5 –Use W/L = 4/2

pMOS I-V All doping and voltages are inverted for pMOS Mobility mp is determined by holes –Typically 2-3x lower than that of electrons mn –120 cm2/V*s in AMI 0.6 mm process Thus pMOS must be wider to provide same current –In this class, assume mn / mp = 2 –*** plot I-V

Transistor Operation Current depends on region of transistor behavior For what V in and V out are nMOS and pMOS in –Cutoff? –Linear? –Saturation?

Schematic of CMOS inverter

nMOS Operation CutoffLinearSaturated V gsn < V tn V in < V tn V gsn > V tn V in > V tn V dsn < V gsn – V tn V out < V in - V tn V gsn > V tn V in > V tn V dsn > V gsn – V tn V out > V in - V tn V gsn = V in V dsn = V out

pMOS Operation CutoffLinearSaturated V gsp > V tp V in > V DD + V tp V gsp < V tp V in < V DD + V tp V dsp > V gsp – V tp V out > V in - V tp V gsp < V tp V in < V DD + V tp V dsp < V gsp – V tp V out < V in - V tp V gsp = V in - V DD V dsp = V out - V DD V tp < 0

Calculation of V IL When the input voltage is V in = V IL, the slope of the VTC is equal to [-1](dV out /dV in ). At this point B, nMOS transistor operates in saturation while the pMOS transistor operates in the linear region. Using KCL at output node: [I d,n(saturation) = I d,p(linear) ]

At point D, input voltage is equal to V IH, the nMOS transistor operates in the linear region and pMOS transistor operates in saturation. By applying KCL at the output node: [ I d,n(linear) = I d,p(saturation) ] Calculation of V IH

Threshold voltage is defined as V th =V in =V out. For V in =V out, both transistor operate in saturation region, refer point C. By applying KCL at output node: [ I dn(saturation) =I dp(saturation) ] Calculation of V th

Consider a CMOS inverter circuit with the following parameters, calculate the (W/L) ratios of the nMOS and pMOS transistors if the switching voltage is Vth = 1.5 V. V DD = 3.3 V V T,n = 0.6 V V T,p = -0.7V µ n c ox = 80 µA/V 2 µ p c ox = 30 µA/V 2 Exercise 1

Exercise 2 Consider a CMOS inverter circuit with the following parameters, calculate the Switching voltage (Vth) of the circuit. V DD = 3.3 V V T,n = 0.6 V V T,p = -0.7V µ n c ox = 80 µA/V 2 µ p c ox = 30 µA/V 2 (W/L)n = 8 (W/L)p = 12

Voltage transfer characteristic

Inverter with n-Type MOSFET load Depletion-Load nMOS Inverter

When the input voltage V in is smaller than the driver threshold voltage VTO, the driver transistor is turned off and thus not conduct any drain current. I D,load = 0, V OH = V DD  I D,load = 0 Calculation of V OH

Assume V in = V OH = V DD. Driver transistor operate in linear region and load transistor operate in saturation. By using KCL. I d,driver(linear) = I d,load(saturation) Calculation of V OL

When V in = V IL the slope of the VTC is equal to (-1),(dVout/dVin). Driver transistor operate in saturation region and load transistor operate in linear region. KCL I d,driver(saturation) = I d,load(linear) Calculation of V IL

Calculation of VIH Driver transistor operate in linear region and load transistor operate in saturation region. I d,driver(linear) = I d,load(saturation)

RegionnMOSpMOS ACutoffLinear BSaturationLinear CSaturation DLinearSaturation ELinearCutoff Operating Regions (Summary)

Exercise 3 VDD = 3.3 V Threshold voltage nMOS (VT,n) = 0.6 V Threshold voltage pMOS (VT,p) = -0.7V Kn = 200 µA/V 2 Kp = 80 µA/V 2 Based on the inverter circuit and parameter in figure above, what is the value of: i)Switching Voltage, V TH ii)Input Voltage High, VIH

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