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Chapter 5. Metal Oxide Silicon Field-Effect Transistors (MOSFETs). 1. Transistor. Three terminal device Voltage between two terminals to control current flow in third terminal Versatile for many applications Amplification Memory Logic voltage controlled current source switch
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Chapter 5. Metal Oxide Silicon Field-Effect Transistors (MOSFETs) 1
Transistor • Three terminal device • Voltage between two terminals to control current flow in third terminal • Versatile for many applications • Amplification • Memory • Logic • voltage controlled current source • switch • Two popular types: • Bipolar Junction Transistor (BJT): used in power amplifier • MOSFET: used in integrated circuits
pn junction • pn junction • Enhancement-type NMOS transistor • p-type material as substrate (i.e., body) • n-type material chemically bonded on body at source and drain • → source and drain are electrically indistinguishable • Equivalent to having two diodes back to back • → current cannot flow between source and drain • Typical dimensions: • L = 0.1 to 3 μm, • W = 0.2 to 100 μm • tox = 2 to 50 nm
Four terminals shown: Source (S), Gate (G), Drain (D) and Body (B). • Body is typically grounded (along with one of the other three terminals) and does not play any role. • Gate is electrically insulated from the body by Silicon Oxide (SiO2) • With no external voltages applied, normally there is no current between S and D. • When certain voltage is applied at G, current flows from D to S. → The gate voltage controls the flow of current.
Channel region • With S and D grounded, apply positive voltage to G (vGS > 0). • Because G is electrically insulated to the body, in the channel, the gate voltage attracts electrons from the body. • A thin layer of “induced n-type channel is formed between S and D. • Across the induced n-type channel, there is no pn junction between S and D. • The thickness of the induced n-type channel is proportional to vGS. • Now, between S and D there is continuous n-type material. • In the n-type region, there are excess electrons floating around (i.e., drift current flowing in random directions).
With vGS > 0, now apply smallvDS > 0. • Then, (diffusion) current starts flowing from D to S. There is no current flowing into G, because of SiO2 insulator. • To form an induced n-type channel sufficient to support current flow, vGS > Vt. Vtis called the threshold voltage.
For smallvDS, iDis a linear function of vDS. • The slope is the inverse of the resistance between D and S. • When vGS< Vt,the resistance is infinite
Increase vDSwithfixed vGS > Vt • Voltage between G and S = vGS • Voltage between G and D = vGS - vDS • n-channel is thickest at S, and thinnest at D. • As vDS increases, the resistance across the channel increases.
When vGS - vDS = Vt, the channel depth at D is ≈ 0. • Channel is then, “pinched off.” • Increasing vDS beyond the point vDS = vGS - Vthas no effect on iD. • This region is called the saturation. vDSsat = vGS - Vt • Device in saturation: region vDS ≥ vDSsat • Device in triode region: vDS < vDSsat
The PMOS transistor works similarly, but with n-type body and p-type S and D. • To establish a p-type channel between S and D for the PMOS transistorvGS< Vtwhere Vt < 0. • The current flows from S to D when vDS < 0. • PMOS is not used by itself very often.
Complementary MOS (CMOS) Transistor • Combines NMOS and PMOS on single substrate • Most popular transistor for integrated circuit • Very dense structure, consumes low power. • Very powerful and versatile
Circuit Symbols for NMOS Most popular one to use
Triode Region • vGS≥ Vt:channel is induces between S and D. • vDS ≤ vGS - Vt:channel is continuous (i.e., no pinch off).
Saturation Region • vGS≥ Vt:channel is induces between S and D. • vDS ≥ vGS - Vt:channel is pinched off. • At the boundary of triode and saturation:vDS = vGS - Vt
More Accurate Model • In saturation, slope in iD – vDS curve is not entirely flat. • There is internal resistance, ro.
Circuit Symbols for PMOS Most popular one to use
Nominal Current Directions and Voltage Polarity • Triode Region • vGS≤ Vt:channel is induces between S and D where Vt< 0. • vDS ≥ vGS - Vt:channel is continuous (i.e., no pinch off) where vDS< 0.
Voltage Characteristics for PMOS Transistor Skip All Sections for PMOS.
MOSFET as Amplifier • Utilize saturation mode. • iD as function of vGS. • Transconductance amplifier
Common source amplifier Often, we want amplifiers to be linear. But iD is a quadratic function of vGS. Use DC biasing technique. Shift small signal around a point that mimics linearity.
Boundary with triode region Quiescent point is determined by VGS and RD. Q3 • Q1 is too close to cut-off and Q2 is too close to triode boundary. We want the quiescent point to be in the middle of saturation region. For example, Q3 may be a reasonable point.
Small Signal (or AC) Equivalent Models λ = 0 λ ≠ 0 • gm and ro are determined for each Q point. • For analyzing small signal circuits, DC sources must be eliminated. • Voltage source: short circuit • Current source: open circuit