Download presentation
Presentation is loading. Please wait.
1
Chap. 5 Field-effect transistors (FET) Importance for LSI/VLSI –Low fabrication cost –Small size –Low power consumption Applications –Microprocessors –Memories –Power Devices Basic Properties –Unipolar device –Very high input impedance –Capable of power gain –3/4 terminal device, G, S, D, B –Two possible device types: enhancement mode; depletion mode –Two possible channel types: n-channel; p- channel 5-1
2
MOSFET Structure Physical structure of a n-channel device: Typically L = 0.35 to 10 m, W = 2 to 500 m, and the thickness of the oxide layer is in the range of 0.02 to 0.1 m. Gate (G) insulated by thin layer of silicon dioxide Source (S) and Drain (D) regions are heavily doped n+ regions in the substrate (B) (also called body) 5-2
3
MOSFETs MOS - metal oxide semiconductor structure (original devices had metal gates, now they are silicon) NMOS - n-channel MOSFET PMOS - p-channel MOSFET CMOS - complementary MOS, both n-channel and p- channel devices are used in conjunction with each other (most popular in IC’s) MESFET - metal semiconductor structure, used in high- speed GaAs devices JFET - junction FET, early type of FET 5-3
4
With V GS = 0 there are two pn junctions between drain and source. Current cannot flow in either direction because one or the other of the junctions would be reverse-biased. However, if V GS > V T (threshold voltage), electrons are attracted to the region below the gate, and an induced, conducting n-channel forms between the drain and source. NOTE: i S = i D and i G = 0 A p-channel enhancement-type MOSFET is similar in construction but has an n-type substrate with p+ regions for the drain and source. 5-4
5
Cross section of a CMOS integrated circuit. Note that the PMOS transistor is formed in a separate n-type region, known as an n well. The two devices are isolated from each other by a thick region of oxide. CMOS 5-5
6
Symbols G D S B G D S B p Channel MOSFET (enhancement-type) -simplified symbol shown below n Channel MOSFET (enhancement-type) -simplified symbol shown below + V DS - + V GS - drain source gate source drain gate iSiS iDiD i G = 0 (Substrate is connected to source) 5-6
7
An n-channel MOSFET with v GS and v DS applied and with the normal directions of current flow indicated. The characteristics for p-channel devices are exactly the same except that voltage polarities and current directions are inverted. (Operates in triode and cutoff regions as a switch.) Output characteristics (n-channel) + V DS - 5-7
8
Input characteristics (n-channel) + V DS - I D = K(V GS -V T ) 2 5-8
9
Summary of MOSFET behavior V GS > V T (threshold voltage) for the device to be on V DS > V GS - V T for device to be in saturation region I D = K(V GS -V T ) 2 Enhancement mode device, V T > 0 (we will be dealing with enhancement mode devices in Chapter 13– MOS Digital Circuits) Depletion mode device, V T < 0 (conducts with V GS = 0) 5-9
10
Comparison of BJT and FET FET voltage controlled V GS > V T for device to be on operates in saturation region (amplifier); V DS > V GS - V T I D = K(V GS -V T ) 2 BJT current controlled V BE 0.7 V for device to be on operates in linear region (amplifier); BE junction forward biased, CB junction reversed biased I C = I B 5-10
11
I D = K(V GS -V T ) 2 K = transconductance parameter K = k' (W/L) where k’ is the process transconductance parameter k' = n C o x, where n is the mobility of electrons, and C ox is the capacitance of the oxide (It’s value is determined by fabrication technology.) W/L is the aspect ratio, W is the width of the gate, L is the length of the gate. I D W/L MOSFET aspect ratio 5-11
Similar presentations
© 2024 SlidePlayer.com Inc.
All rights reserved.