m icrowave fet microwave fet : operates in the microwave frequencies unipolar transistors –...
TRANSCRIPT
MICROWAVE FET
• Microwave FET : operates in the microwave frequencies
• unipolar transistors– current flow is carried out by majority carriers
alone
• It’s a voltage controlled device– voltage at the gate terminal controls the current
flow.
Advantages of FET’s compared to BJT
• It has voltage gain in addition to current gain• Efficiency is higher• Noise figure is low• Input resistance is very high, upto megaohms.• Operating frequency is upto X band/
Physical Structure
• N-channel JFET:– N-type material is sandwiched between 2 highly doped of
p-type material (p+ regions)• If the middle part is a p-type semiconductor, then its p-channel
JFET.• 2 p-type regions in the n channel JFET – Gates• Each end on n-channel is joined by a metallic contact.• Source : Contact which supplies source of the flowing electrons • Drain :Contact which drains electrons out of the material• Id : flows from drain to the device
• For p-channel JFET, polarities of Vg & Vd are interchanged.• Electrons have higher mobility
– n-channel JFET provides higher conductivity.– Higher speed
Operation • Under normal conditions, Vg = zero, Id = zero.
• Channel between gate junctions is entirely open.• When Vd is applied
– n-type semiconductor bar acts as resistor– current Id increases linearly with Vg
• For p-channel JFET, polarities of Vg & Vd are interchanged.• As Vd is further increased
– majority of free electrons get depleted from the channel.– Space chare extends into the channel.– space charge regions expand & join together.– All the free electrons are completely depleted in the
joined region -> PINCH OFF• If Vg is applied : pinch off voltage reduces
I-V CHARACTERISTICS
Pinch off Voltage• It is the gate reverse voltage that removes all
the free charges from the channel.• Poisson’s equation for the voltage in n-
channel
• Integrating once again and applying boundary condition V=0 at y=0 yield
• Integrating the above equation and applying boundary condition ie. E=0 at y=a yield
(a : the height of the channel in metres)
Pinch off voltage under saturation condition is
• The N-channel resistance
Substitution and rearrangement gives
BREAKDOWN REGION
• As Vd increases for a constant Vg, the bias voltage causes avalanche breakdown across the junction.
• Drain current Id increases sharply.• The breakdown voltage is
MOSFETs- Metal Oxide Semiconductor Field Effect Transistors
• 4 terminal – Source, Gate, Drain and Substrate• Simple structure and economic• Types
– NMOS– PMOS– CMOS
• Current is controlled by electric field : o Junction Field Effect Transistors
PHYSICAL STRUCTURES
• N-CHANNEL MOSFET• P-type substrate• 2 highly doped n regions diffused – source &
drain separated by 0.5um• Thin layer of silicon dioxide grown over the
surface.• Metal contact on the insulator – acts as gate.
Electronic Mechanism1. No gate voltage applied
– connection b/w source & drain : 2 back to back pn junctions
– Reverse leakage current b/w Drain and Source2. Gate voltage is +ve w.r.t. Source.
– Positive charge deposition on the gate metal– Negative charges are induced in the p-substrate at the
semiconductor-insulator interface– Formation of channel conduction of Id
3. Threshold Voltage : Minimum gate voltage for channel formation
Modes of Operation
• Enhancement Mode– Normally off mode– Gate voltage = 0 V– Very low Channel conductance – Considered as the OFF state– Positive gate voltage to turn on the device– Channel length is “Enhanced”– Application :
• As Linear Power Amplifiers
• Depletion Mode– Normally ON mode– A channel is present even at zero bias– To turn off the device Negative gate voltage– “Depletion” of charge carriers by the application
of negative gate voltage
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