lab2_fets_notes - ECE 327 Electronic Devices and Circuits...

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Unformatted text preview: ECE 327: Electronic Devices and Circuits Laboratory I Notes for Lab 2 (Field Effect Transistor Lab) 1. Trans-istor: trans conductance (variable) resistor • Bipolar (“accident”) and unipolar (FET, “valve”, like “tube”) varieties – New technologies share features of both (e.g., insulated gate bipolar transistors (IGBTs)) • Voltage-controlled current sources (VCCS) – Vary channel resistance in order to make current constant – Think of “transistor man” from Horowitz and Hill (1989) • For a bipolar transistor, base–emitter diode is forward biased, so it carries current too. So i B = I S parenleftbigg exp parenleftbigg v BE kT/q parenrightbigg − 1 parenrightbigg where kT q ≈ 25 . 3 mV @ room temperature and i C = βI S parenleftbigg exp parenleftbigg v BE kT/q parenrightbigg − 1 parenrightbigg = βi B where I S ≈ 0 and β ≫ 1 are determined by material properties. Fortunately, i C = βi B . This relationship and the (steep) exponential i – v curves help make the bipolar transistor so handy. • For a unipolar transistor (FET), no current enters the gate. So i G = 0 and i D = K ( v GS − V T ) 2 where 0 . 5 V < V T < 10 V and (small) K are determined by material properties and temperature. • Graphically: 1 2 3 4 1 2 3 4 5 i B , i C , i D (mA) v BE , v GS (V) i D i C i B V BE V T FET threshold voltage BJT: v BE ≈ V BE ≈ . 7Vforall i C FET: v GS variesgreatlyover i D (and V T canbeunpredictable) – Notice vertical scale. Consider v GS required for a significant drain current – Additionally, threshold V T can have a 5 V manufacturing spread (!!) – NOTE: In tiny region below V T , unipolar has steep exponential curve similar to bipolar • BJTs: high transconductance (+), very non-linear curve ( − ), predictable base–emitter drop (+?) Copyright © 2007–2009 by Theodore P. Pavlic Creative Commons Attribution-Noncommercial 3.0 License Page 1 of 6 ECE 327 — Lab 2: Field Effect Transistor Lab Notes 2. Analogous operation modes (gate ∼ base, source ∼ emitter, drain ∼ collector): Condition = ⇒ Unipolar (FET) ∼ Bipolar (BJT) Low gate–source voltage = ⇒ “Subthreshold” ∼ “Cutoff” High gate–source voltage = ⇒ “Pinch-off” (or “Saturation”) ∼ “Active” Low source–drain voltage = ⇒ “Linear” (or “Triode” or “Ohmic”) ∼ “Saturation” • Bipolar and unipolar “saturation” are different (BJT: out of active mode, FET: into active mode) • BJT “Saturation” versus FET “Ohmic” — only FETs can really be switches – Bipolar saturation is not practically usable (slow to exit, and can be impossible to use on IC) – In “Ohmic” mode, FET is like programmable resistor (e.g., electronic volume control, switches) 3. Diffusion reminder (demonstration: cold H 2 O, hot H 2 O, shaken H 2 O, tilted H 2 O (like diode)) • Diffusion is a statistical certainty (entropy increases) – Thermal agitation causes movement, but eventually “now” and “later”...
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This note was uploaded on 08/05/2011 for the course ECE 209 taught by Professor Staff during the Fall '08 term at Ohio State.

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lab2_fets_notes - ECE 327 Electronic Devices and Circuits...

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