654_ch10

654_ch10 - ECE 654 Prof. S. Mohammadi Solid State Devices...

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ECE 654 Solid State Devices II Prof. S. Mohammadi - 149 - Chapter 9 Advanced CMOS Devices In this chapter we study the basics of metal oxide semiconductor field effect transistors (MOSFETs) and their state of the art performance. It is important to realize that they are two distinctive models for MOS transistor. In long channel limit (typically L>1μm) a simple quadratic equation can model device performance. Deviations from the quadratic model are captures as secondary effects. For submicron devices (typically L<0.5μm) which includes almost all current processes, the simple quadratic equation does not hold. Such devices are referred to as short channel devices as short channel effects dominate their performance. There are several approaches to model short channel transistors. Advanced short channel MOS models involve more than 150 parameters, out of which only a handful are physical parameters. Therefore, modeling MOS transistor becomes a tedious task. As the gate length of the transistor shortens by scaling to deep submicron technology, not only one gets the benefit of higher packing density and lower supply voltage (low power application) but also there are benefits in terms of the speed of the transistor. f T and f max increase to above 200GHz for 90nm MOS technology. All of a sudden, MOS transistor seems to be a reasonable choice for RF and microwave application. In this chapter, we will study the implications of MOS transistors as RF and microwave devices. Basic Principles of Long Channel MOSFET N-channel b electrons are the conducting charges b faster P-channel b holes are the conducting charges b slower Source Gate Drain n + n + p - sub n-channel gate oxide
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ECE 654 Solid State Devices II Prof. S. Mohammadi - 150 - Set th GS DS V V V - = in equation (I) Weak inversion b behaves like a resistor (10.1) As you now increase DS V b - - = 2 ) ( 2 DS DS th GS ox n D V V V V L W C I μ (10.2) As you now increase DS V b your channel narrows down close to drain until it pinches off. b current saturates. b 2 ) ( 2 th GS ox n D V V L W C I - = (10.3) I D is now independent of DS V . Condition for pinch off: th GS DS V V V - > . eff V DS th GS ox n D V V V L W C I ) ( - = n + n + n n + n + n G Depleted S D quadratic increase in current 1 mA 4 mA 9 mA D I DS V v V V tn GS 1 = - v V V tn GS 2 = - v V V tn GS 3 = -
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Solid State Devices II Prof. S. Mohammadi - 151 - Secondary Effects 1. Channel length modulation [ ] ( ) ) ( 1 ) ( 2 2 th GS DS th GS ox n D V V V V V L W C I - - + - = λ μ (10.4) ( ) 1 - V b Channel length modulation coefficient b D out D D D out I R I V I R 1 1 = = = (10.5) 2. Body effect Body effect influences both DC and AC performance of the transistor: I DS V
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654_ch10 - ECE 654 Prof. S. Mohammadi Solid State Devices...

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