02_Lecutre

02_Lecutre - Lecture 2 EE114 Lecture 2 Long Channel Model...

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Lecture 2 EE114 R. Dutton, B. Murmann 1 Lecture 2 Long Channel Model EE114 R. Dutton, B. Murmann Stanford University R. Dutton, B. Murmann 2 Basic MOS Operation (1) With zero voltage at the gate, device is "off" – Back-to-back reverse biased pn junctions EE114 0V V D (>0V) 0V 0V
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Lecture 2 EE114 R. Dutton, B. Murmann 3 Basic MOS Operation (2) With a positive gate bias applied, electrons are pulled toward the positive gate electrode Given a large enough bias, the electrons start to "invert" the surface (p n); a conductive channel forms – Magic "threshold voltage" V t (more later) EE114 >0 R. Dutton, B. Murmann 4 Basic Operation (3) If we now apply a positive drain voltage, current will flow How can we calculate this current as a function of V GS , V DS ? EE114 >0 V DS >0 I D =?
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Lecture 2 EE114 R. Dutton, B. Murmann 5 Assumptions 1) Current is controlled by the mobile charge in the channel. This is a very good approximation. 2) "Gradual Channel Assumption" - The vertical field sets channel charge, so we can approximate the available mobile charge through the voltage difference between the gate and the channel 3) The last and worst assumption (we will fix it later) is that the carrier velocity is proportional to lateral field ( ν = μ E). This is equivalent to Ohm's law: velocity (current) is proportional to E-field (voltage) EE114 >0 V DS >0 R. Dutton, B. Murmann 6 First Order IV Characteristics (1) What we know: EE114 [ ] t GS ox n V y V V C y Q ! ! = ) ( ) ( W v Q I n D ! ! = E v ! = μ [ ] W E V y V V C I t GS ox D ! ! ! " " = # ) ( ν E μ I D W
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Lecture 2 EE114 R. Dutton, B. Murmann 7 First Order IV Characteristics (2) For V DS /2 << V GS -V t , this looks a lot like a linear resistor: I=1/R
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02_Lecutre - Lecture 2 EE114 Lecture 2 Long Channel Model...

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