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 murmann@stanford.edu 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.
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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)
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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]
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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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