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Microwave Devices
By: Amit ShankarChoudharyB.Tech (Electronics &
Communication)PGDM (Finance & Operation)
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Microwave Solid State Devices
Two problems with conventional transistors athigher frequencies
are:
1. Stray capacitance and inductance.- remedy is interdigital
design.
2.Transit time.
- free electrons move quicker thanholes therefore change from
silicon to GalliumArsenide
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Microwave Transistors
Conventional bipolar transistors are notsuitable for microwave
frequencies.
Electrons move faster than holes.
Component leads introduce elevatedreactance.XL increases and XC
decreases therefore
collector feedback becomes worse as
frequency increases.Transit time and mobility of carriers.
Astransit time approaches signal periodphase shifts occur.
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Microwave Transistors
REMEDIES: Interdigital design of emitter and base
minimizes capacitances.
Gallium arsenide. Faster than silicon. N type GaAsFET. Why N
type? Flat component leads.
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Microwave Transistors
REMEDIES contd.: Low noise design considerations:
* Planar and epitaxial methods of construction use diffusion and
surface passivationto protect surfaces from contamination asopposed
to diffusion method of mesa structureimplementing acid etching.
* Shot noise is proportional to the square of current therefore
operate at moderate Ic.
* Thermal noise is reduced at lower powerlevels. With
interdigital base design Rb is lowtherefore lower voltage drop and
less power.
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Gunn Devices
Uses phase shift to minimize transmit time.Transferred-electron
device (TED).N type GaAs electron mobility decreases
as electric field strength increases.Characterized by a negative
resistanceregion.
A domain is developed that sustains
oscillations as a voltage is applied to thesubstrate of GaAS.A
pulse current develops as domain of
charge travels to the positive terminal.
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Other Devices
Pin Diodes - R.B.(R II C) F.B. (variable R)Varactor Diodes R.B.
(variable junction
capacitance)
YIG Yitrium-Iron-Garnet DevicesDielectric ResonatorsMMICs
monolithic microwave integrated
circuits
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HEMT
High Electron Mobility TransistorSimilar to GaAsFET
construction.Difference is that motion of charge carriers is
confined to a thin sheet within a GaAs bufferlayer.
GaAs/AlGaAs heterostructure epitaxy.The thickness of the channel
remains constant
while the number of carriers is modulated bythe gate bias as
opposed to a MESFET thatmodulates the channel thickness.
PHEMT- pseudomorphic HEMT used above 20 GHz(mm wave)
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Microwave Tubes
MagnetronsKlystronsTravelling-Wave Tube
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Microwave Horn Antennas
E-planeH-planePyramidal
ConicalSlot
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