FET Basics - Google

FET Basics - Google

Introduction to FETs ELEC 121 Current Controlled vs Voltage Controlled Devices January 2004 ELEC 121 2 Types of FETs JFET Junction Field Effect Transistor MOSFET Metal Oxide Semiconductor Field Effect

Transistor D-MOSFET - Depletion Mode MOSFET E- MOSFET - Enhancement Mode MOSFET January 2004 ELEC 121 3 Transfer Characteristics The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT In a BJT, (hhFE) defined the relationship between IB (hinput current) and IC

(houtput current). In a JFET, the relationship (hShockleys Equation) between V GS (hinput voltage) and ID (houtput current) is used to define the transfer characteristics, and a little more complicated (hand not linear): VGS ID = IDSS 1 V P 2

As a result, FETs are often referred to a square law devices January 2004 ELEC 121 4 JFET Construction There are two types of JFETs: n-channel and p-channel. The n-channel is more widely used. There are three terminals: Drain (hD) and Source (hS) are connected to n-channel

ELEC 121 Gate (hG) is connected to the p-type material January 2004 5 JFET Operating Characteristics There are three basic operating conditions for a JFET: JFETs operate in the depletion mode only A. VGS = 0, VDS is a minimum value depending on IDSS and the drain and source resistance B. VGS < 0, VDS at some positive value and C. Device is operating as a Voltage-Controlled Resistor

For an n channel JFET, VGS may never be positive For an p channel JFET, VGS may never be negative January 2004 ELEC 121 6 Specification Sheet (JFETs) January 2004 ELEC 121

7 N-Channel JFET Operation The nonconductive depletion region becomes thicker with increased reverse bias. (hNote: The two gate regions of each FET are connected to each other.) January 2004 ELEC 121 8 N-Channel JFET Symbol

January 2004 ELEC 121 9 Saturation At the pinch-off point: any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. ID is at saturation or maximum. It is referred to as I DSS. The ohmic value of the channel

is at121maximum. January2004 ELEC 10 ID IDSS As VGS becomes more negative: the JFET will pinch-off at a lower voltage (hVp). ID decreases (hID < IDSS) even though VDS is increased. Eventually ID will reach 0A. VGS at this point is called Vp or VGS(hoff). Also note that at high levels of VDS the JFET reaches a breakdown situation. ID will increases uncontrollably if VDS > VDSmax.

January 2004 ELEC 121 11 FET as a Voltage-Controlled Resistor The region to the left of the pinch-off point is called the ohmic region. The JFET can be used as a variable resistor, where VGS controls the drain-source resistance (hrd). As VGS becomes more negative, the resistance (hrd) increases. January 2004

ELEC 121 rd = ro VGS 1 VP

2 12 Transfer (Transconductance) Curve From this graph it is easy to determine the value of ID for a given value of VGS It is also possible to determine IDSS and VP by looking at the knee where VGS is 0 January 2004 ELEC 121 13

Plotting the Transconductance Curve Using IDSS and VP (hor VGS(hoff)) values found in a specification sheet, the Family of Curves can be plotted by making a table of data using the following 3 steps: Step 1: Solve VGS ID = IDSS 1 VP 2

for VGS = 0V Step 2 Solve Step 3: Solve VGS ID = IDSS 1 VP

2 VGS ID = IDSS 1 VP January 2004 for VGS = VP (h aka VGS(hoff) ) 2

for 0V VGS VP in 1V increments for VGS ELEC 121 14 Case Construction and Terminal Identification This information is found on the specification sheet January 2004 ELEC 121 15

p-Channel JFETs p-Channel JFET operates in a similar manner as the n-channel JFET except the voltage polarities and current directions are reversed January 2004 ELEC 121 16 P-Channel JFET Characteristics As VGS increases more positively

the depletion zone increases ID decreases (hID < IDSS) eventually ID = 0A Also note that at high levels of VDS the JFET reaches a breakdown situation. ID increases uncontrollably if VDS > VDSmax. January 2004 ELEC 121 17 MOSFETs MOSFETs

MOSFETs have characteristics similar to JFETs and additional characteristics that make then very useful There are 2 types of MOSFETs: Depletion mode MOSFET (hD-MOSFET) Operates in Depletion mode the same way as a JFET when V GS 0 Operates in Enhancement mode like E-MOSFET when V GS > 0 Enhancement Mode MOSFET (hE-MOSFET) Operates in Enhancement mode IDSS = 0 until VGS > VT (hthreshold voltage) January 2004 ELEC 121

19 MOSFET Handling MOSFETs are very static sensitive. Because of the very thin SiO 2 layer between the external terminals and the layers of the device, any small electrical discharge can stablish an unwanted conduction. Protection: Always transport in a static sensitive bag Always wear a static strap when handling MOSFETS Apply voltage limiting devices between the Gate and Source, such as back-toback Zeners to limit any transient voltage January 2004

ELEC 121 20 D-MOSFET Symbols January 2004 ELEC 121 21 Specification Sheet

January 2004 ELEC 121 22 Depletion Mode MOSFET Construction The Drain (hD) and Source (hS) leads connect to the to n-doped regions These N-doped regions are connected via an n-channel This n-channel is connected to the Gate (hG) via a thin insulating layer of SiO 2 The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS January 2004

ELEC 121 23 Basic Operation A D-MOSFET may be biased to operate in two modes: the Depletion mode or the Enhancement mode January 2004 ELEC 121 24

D-MOSFET Depletion Mode Operation The transfer characteristics are similar to the JFET In Depletion Mode operation: When VGS = 0V, ID = IDSS When VGS < 0V, ID < IDSS 2 When VGS > 0V, ID > IDSS VGS ID = IDSS 1 The formula used to plot the Transfer Curve, is: VP

January 2004 ELEC 121 25 D-MOSFET Enhancement Mode Operation Enhancement Mode operation In this mode, the transistor operates with VGS > 0V, and ID increases above IDSS Shockleys equation, the formula used to plot the Transfer Curve, still applies but V GS is 2 positive:

VGS ID = IDSS 1 VP January 2004 ELEC 121 26 p-Channel Depletion Mode MOSFET The p-channel Depletion mode MOSFET is similar to the n-channel except that the

voltage polarities and current directions are reversed January 2004 ELEC 121 27 Enhancement Mode MOSFETs n-Channel E-MOSFET showing channel length L and channel width W January 2004

ELEC 121 29 Enhancement Mode MOSFET Construction The Drain (hD) and Source (hS) connect to the to n-doped regions These n-doped regions are not connected via an n-channel without an external voltage The Gate (hG) connects to the p-doped substrate via a thin insulating layer of SiO 2 The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS January 2004

ELEC 121 30 Specification Sheet January 2004 ELEC 121 31 E-MOSFET Symbols

January 2004 ELEC 121 32 Basic Operation The Enhancement mode MOSFET only operates in the enhancement mode. VGS is always positive IDSS = 0 when VGS < VT As VGS increases above VT, ID increases If VGS is kept constant and VDS is increased, then ID saturates (hIDSS) The saturation level, VDSsat is reached.

January 2004 ELEC 121 33 Transfer Curve ID(on) 2 (VGS(ON) - VT)2 To determine ID given VGS: ID = k (VGS - VT) where VT = threshold voltage or voltage at which the MOSFET turns on. k = constant found in the specification sheet

The PSpice determination of k is based on the geometry of the device: k= W KP k = L 2 January 2004 where KP = C NCOX ELEC 121 34

p-Channel Enhancement Mode MOSFETs The p-channel Enhancement mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed. January 2004 ELEC 121 35 Summary Table JFET

January 2004 D-MOSFET ELEC 121 E-MOSFET 36

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