DMT121 – ELECTRONIC DEVICES CHAPTER 5 FIELD-EFFECT TRANSISTOR (FET) -MOSFET-
MOSFET • MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) • Different from JFET – no pn junction structure. • Gate of MOSFET is insulated from the channel by silicon dioxide (SiO2) layer. • 2 types – enhancement and depletion.
DEPLETION-TYPE MOSFET • P-type material is formed from silicon substrate. • Source and Drain terminals are connected through metallic contacts to n-doped region linked by n-channel. • Gate connected to metal contact surface but insulated from n-channel by thin SiO2 layer – no direct connection gate and channel of MOSFET. • SiO2 is a dielectric which sets up opposing electric fields within the dielectric when exposed to externally applied field. n-channel depletion-type MOSFET
BASIC OPERATION & CHARACTERISTICS @ VGS=0 V • Gate-to-Source voltage is set to 0 V. • A voltage VDS is applied across the Drain-to-Source terminals. • An attraction for positive potential at Drain by free electron of n-channel – produce current through channel. • At VGS = 0V, ID = IDSS
BASIC OPERATION & CHARACTERISTICS @ VGS<0 V • If VGS is set at a negative voltage: • Negative potential at Gate will pressure electron towards p-type substrate and attract holes from substrate. • Recombination between hole and electron will occur – reduce number of free electron in n-channel for conduction. • More negative the bias, recombination rate is higher. • ID is reduce with increasing negative bias of VGS. • At pinch-off voltage, VP, ID=0A
BASIC OPERATION & CHARACTERISTICS @ VGS>0 V • For positive value of VGS: • Positive Gate will draw additional electron from p-substrate due to reverse leakage current and established new carrier through the collisions between accelerating particles. • ID will increase at rapid rate – user must aware of ID maximum current rating. • Application of positive VGS has enhance the level of free carriers in the channel. • Region of positive gate voltage on drain or transfer curve is called enhancement region while region between saturation and cutoff is called depletion region.
P-CHANNEL DEPLETION-TYPE MOSFET • Construction is reverse of n-channel. • All voltage polarities and current direction are reverse.
ENHANCEMENT-TYPE MOSFET • Primary difference between depletion-type and enhancement-type is the absence of channel between Source and Drain terminals. N-channel enhancement-type MOSFET
BASIC OPERATION & CHARACTERISTICS @ VGS=0 V • VGS is set at 0 V and a voltage applied between Drain and Source. • With VDS at positive voltage, VGS=0 V and terminal substrate (SS) connected to Source – exist two (2) reverse-biased pn-junction between n-doped region and p-substrate. • It is not sufficient to have a large accumulation of carriers (electron) at Drain and Source if a path (channel) is fails to exist between both terminals. • ID = 0 A
BASIC OPERATION & CHARACTERISTICS VGS>0 V • VDS and VGS>0 V: • Positive potential at the Gate will pressure the holes in p-sub along the edge of SiO2 to enter deeper p-sub. • Result in a depletion region near SiO2. • Electron in p-sub (minority carrier) attracted to positive Gate and accumulate in the region near the surface of SiO2 layer. • As VGS increase in magnitude, the concentration of electron increases until eventually induced n-type region to support current flow between Drain and Source. • The level of VGS that results in significant increase in ID is called threshold voltage, VT.
BASIC OPERATION & CHARACTERISTICS VGS>VT • VGS>VT: • The density of free carriers in the induced channel will increase - increased ID. • If increase VDS but VGS constant, ID will saturate. • VDG and Gate will become less and less positive with respect to Drain. • VDG=VDS-VGS • Reduction in Gate-to-Drain voltage will reduce the attractive forces for free carriers (electron) – reduction in channel width. • Channel will reduce to pinch-off and a saturation condition established. • Any further increase in VDS at fixed value of VGS will not affect the saturation level of ID until breakdown conditions are encountered.
BASIC OPERATION & CHARACTERISTICS • Saturation level for VDS is related to applied VGS by: • VDsat = VGS – VT • VGS < VT, ID=0 A • VGS > VT, ID=k(VGS-VT)2
P-CHANNEL ENHANCEMENT-TYPE MOSFET • Construction is reverse of n-channel. • All voltage polarities and current direction are reverse.
D-MOSFET BIASING • Similarities in appearance between transfer curve of JFET and D-MOSFET. • Primary difference: D-MOSFET permit operating points with positive value of VGS and level of ID that exceed IDSS.
D-MOSFET BIASING Self-Biased Configuration:
D-MOSFET BIASING • ID=IDSS(1-VGS/VP)2 • Self-biased configuration results in VGS=-IDRS
D-MOSFET BIASING Voltage-Divider Bias Configuration:
D-MOSFET BIASING • ID=IDSS(1-VGS/VP)2 • Voltage-divider configuration results in: • VGS=VG-IDRS • Where VG=R2xVDD/(R1+R2)
E-MOSFET BIASING • Transfer curve for E-MOSFET is quite different from JFET and D-MOSFET. • ID=0 A if VGS<VT. • VGS>VT, ID=k(VGS-VT)2
E-MOSFET BIASING Voltage-Divider Biasing
E-MOSFET BIASING • Voltage-divider configuration results in: • VGS=VG-IDRS • Where VG=R2xVDD/(R1+R2) • VDS=VDD-ID(RS+RD)
E-MOSFET BIASING Feedback Biasing
E-MOSFET BIASING • IG=0 V • VD=VG • VDS=VGS • VDS=VDD-IDRD • VGS=VDD-IDRD • When ID=0 A: VGS=VDD • When VGS=0 V: ID=VDD/RD