microwave solid state device
TRANSCRIPT
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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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