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Explore the fundamentals of Metal Oxide Silicon Field-Effect Transistors (MOSFETs), versatile devices crucial for amplification, memory, logic, and more, with detailed insights into their structure, operation, and applications in integrated circuits.
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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
