Lecture 04-Subthreshold and Short Channel Effects

# Lecture 04-Subthreshold and Short Channel Effects - Lecture...

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EE 214 Lecture 4 (HO#6) B. Murmann 1 Lecture 4 EE214 Technology: g m /I D Subthreshold Operation Short Channel Effects Boris Murmann Stanford University [email protected] Copyright © 2004 by Boris Murmann

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EE 214 Lecture 4 (HO#6) B. Murmann 2 Overview Reading 1.8 (Weak Inversion) 1.7 (Short Channel Effects) Introduction – Last lecture, we found that one important figure of merit for transistors is the transconductor efficiency, g m /I D . Today we'll look at g m /I D in our EE214 0.35 µ m technology. We will find that additional modeling is needed to explain the behavior of this parameter below and around V t , and also the deviation from the long channel model in strong inversion. This leads us to an analysis of subthreshold current and an introduction to short channel effects.
EE 214 Lecture 4 (HO#6) B. Murmann 3 g m /I D Simulation \$ gm/id vs. gate overdrive \$ Boris Murmann, September 2004 .param gs=1 vgs g 0 dc 'gs' mn1 g g 0 0 nch214 L=0.35um W=10um .op .dc gs 0.4V 1.2V 10mV .probe ov = par('gs-vth(mn1)') .probe gm_id = par('gmo(mn1)/i(mn1)') .options post brief .lib './ee214_hspice.txt' nominal .end 10/0.35

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EE 214 Lecture 4 (HO#6) B. Murmann 4 Result -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0 5 10 15 20 25 30 35 40 V OV [V] g m /I D [1/V] EE214 technology 2/V OV BJT (q/kT)
EE 214 Lecture 4 (HO#6) B. Murmann 5 Observations Our long channel predication is fairly close for V OV > 150mV Unfortunately g m /I D does not approach infinity for V OV 0 It also seems that we cannot do better than a BJT, even though the long channel equation would predict that for 0 < V OV < 2kT/q 52mV at room temperature For further analysis, it helps to identify three distinct operating regions – Strong inversion: V OV > 150mV • Deviations due to short channel effects – Weak Inversion: V OV < 0 • Behavior similar to a BJT, g m /I D nearly constant – Moderate Inversion: 0 < V OV < 150mV • Transition region, an interesting mix of the above

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EE 214 Lecture 4 (HO#6) B. Murmann 6 Weak Inversion Questions: – What determines the current when V OV < 0, i.e. V GS < V t ? – What is the definition of V t ? A closer look at the device current in our previous simulation: -0.5 0 0.5 1 0 0.2 0.4 0.6 0.8 1 V OV [V] I D [mA] -0.5 0 0.5 1 10 -5 -4 -3 -2 -1 0 V [V]
EE 214 Lecture 4 (HO#6) B. Murmann 7 Definition of V t •V t is defined as the V GS at which the number of electrons pulled to the surface equals the number of doping atoms Seems somewhat arbitrary, but makes sense in terms of surface charge control

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Lecture 04-Subthreshold and Short Channel Effects - Lecture...

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