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MICROWAVE ENGINEERING
EKT 345
CHAPTER 4: MICROWAVE
SEMICONDUCTOR DEVICES
Outline:
4.1Microwave Diodes
- Shottky Diodes, Pin Diodes, Varactor Diodes, Tunnel Diodes, Gunn Diodes, IMPATT, TRAPATT
4.2Microwave Transistors
- Bipolar, Unipolar FET, HEMT
- Limitation of Transistor at High Frequencies
4.3 Parametric Amplifier
- Manley Rowe Relationship
4.1 Microwave Diodes
Two elements device which will do much more
than rectify or clamp a dc voltage.
General functions : able to switch, attenuate, mix
frequencies, detect, amplify, oscillate and
modulate signals in microwave and millimeter
bands frequencies.
Definition: two terminal electronic device that will conduct electricity much more easily in one direction than in the other.
Cont’d…
Schottky Diodes – for mixing and detection
Pin diodes – for attenuation, modulation,
switching, phase shifting and limiting.
Varactor diodes – for frequency multiplications,
parametric amplification and tuning
Tunnel and Gunn Diodes – for oscillation
Impact Ionization Avalanche Transit Time
(IMPATT) and Trapped Plasma Avalanche
Transit Time (TRAPATT) – for amplification and
oscillation
Cont’d…
a. Schottky Diodes
Rj = resistance of metallic junction
Cj = barrier capacitance
Rs = bulk resistance of heavily doped Si substrate (4-6 ohm)
Ls= inductance of gold whisker wire (0.4 – 0.9nH)
Cc = Case Capacitance
Cont’d…
Metal-semiconductor barrier diodes
Exhibits a square–law characteristic and have
higher burnout rating
Better reliability compared to point contact
diodes
Constructed on a thin silicon (n+ type) substrate
by growing epitaxially on n-type active layer of
about 2 microns thickness.
Metal semiconductor is formed by depositing
metal over SiO2.
Cont’d…
b. Pin Diodes
Rj = junction resistance (variable)
Cj = junction capacitance (~0.2pf)
Rs = bulk semiconductor (p+ and n+) layer and contact resistance.
Lp, Cp = package inductance,
capacitance
Cont’d…
Consists of a high-resistivity intrinsic semiconductor layer between highly doped p+
and n+Si layers.
Acts as an electrically variable resistor related to i layer thickness.
Intrinsic layer has a very large resistance in reverse bias and it decreases in forward bias.
When mobile carriers from p and n regions are injected into the i layer, carriers take time -diode ceases to act as a rectifier at microwave frequency and appear as a linear resistance:
variable attenuator
Cont’d…
Pin device is designed under reverse or zero
biasing, Rj is extremely large and Cj plays the
dominant role, where Cp, Lp, Rs are small.
For reverse bias, high capacitance impedance
is presented to the microwave signal.
For forward bias, the diodes represents a very
low resistance .
Changing the bias, it acts as a switch
Single switch
Double switch – use two diodes called single-pole
double-throw (SPDT)
Cont’d…
Cont’d…
Cont’d…
c. Varactor Diodes
Cont’d…
Operated under reverse bias where the
junction resistance is 10 MΩ or more.
Equivalent circuit reverse-bias at
microwave frequencies : a simple of
capacitance and resistance in series.
Equivalent circuit of forward-bias at
microwave frequencies more complicated
must include the diffusion of the injected
carriers.
Cont’d…
Due to skin effect, Rs increases with increase
in frequency
So, frequency used normally below 0.2fc
Cont’d…
d. Tunnel Diodes
Rj = junction
resistance (100 ohms)
Cj = junction
capacitance (pf)
Ls = lead inductance
(nH)
Rs = bulk resistance
(few ohms)
-Rj
Equivalent circuit of tunnel diode
Cont’d…
Heavily doped p-n junction diodes with a
negative resistance over a portion of its I-V
characteristic (refer page 416)
Used as microwave amplifiers or oscillators.
Heavy doping – depletion width becomes very
thin and overlap occurs between the
conduction-band level (n-side) and the
valence-band level (p-side).
Cont’d… Tunnel-diode oscillator
Consists of a tank circuit coupled with the diode by means of a capacitive divider
When the power switched on, a surge current produces oscillationin the tank circuit
The R-C values make the dc bias at the centre of the negative resistance characteristic of the diode
Sustained oscillation occurs if magnitude of the negative resistance of the diode is equal or greater than the resistance of tank circuit.
The oscillator circuit can generate microwave signals up to frequencies: 100GHz
Cont’d…
Cont’d…
Cont’d…e. Gunn Diodes
Cont’d…
Negative resistance devices which normally
used as low-power oscillator at the microwave
frequencies in transmitter and as local oscillator
in receiver front ends.
Two main principles of operation:
Gunn Mode or Transit-Time (TT) mode
Limited-Space Charge Accumulation (LSA) mode
Other modes:
Quenched domain mode
Delayed mode
Cont’d…
TT mode
Low efficiency of power generation
Uncontrollable frequency by external circuit
LSA mode
Produce several watts of power with minimum
efficiency 20%.
Power output decreases with frequency:
example:
1 Watt at 10 GHz
1 mW at 100 GHz
Cont’d…
Cont’d…f. IMPATT
IMPact Avalanche and Transit Time
diodes
An efficiency of 3% CW power and 60%
pulsed power.
Operated from 500 MHz to 100GHz.
Power output between 1W (CW) and over
400W (pulsed).
Many forms:
n+ pip + or p + nin + Read device
p + nn + abrupt junction
p + in + diode
Manufactured from Ge, Si, GaAs or InP,
The highest efficiency, highest operating
frequency and least noise figure but
complicated fabrication process: GaAs
Cont’d…
Cont’d…
Reverse-biased condition n+ pip +
When reverse bias voltage exceeds the breakdown voltage VB, a maximum electric field of very high value (MV/m) appears at the n+p junction.
The holes moving in the high field region acquire sufficient energy to excite valence electrons of the atom into the conduction band resulting in avalanche multiplication of electron hole-hole pairs
To prevent burning diode: a constant diode current bias source is used to maintained the average current at safe limit I0.
The diode current is contributed by the conduction electrons which move to n+
region and associated holes which drift through i space charge region to the p+
region, under the influence of a lower but steady electric field.
Cont’d…
Cont’d…
IMPATT disadvantages:
Since dc power is drawn due to induced electron
current in the external circuit, IMPATT diodes
have low efficiency.
Tend to be noisy due primarily to the avalanche
process and to the high level of operating
current.
Typical noise figure is 30dB which is worse than
Gunn Diodes.
Cont’d…
g. TRAPATT
TRApped Plasma Avalanche Triggered
Transit Diode
Suitable for low frequency (1-3GHz) applications
with pulse output of several hundreds watts
Efficiency 20-60%
Manufactured from Si and have p+n n+ or n+ pp+
configurations
p-n junction reverse-biased beyond the
breakdown junction – current density is higher
Cont’d…
TRAPATT diode is mounted inside a coaxial resonator at position of maximum RF voltage swing.
When avalanche occurs at the dc reverse-bias plus RF swing beyond the threshold of breakdown, a plasma of holes and electrons are generated
The plasma density results in a high potential difference across the junction in opposition to the dc reverse bias voltage
At this low voltage, the plasma gets trapped
Since the external circuit current flow, the voltage rise and the trapped plasma is released producing current pulse across the drift space.
Total transit time is the sum of the delay time in releasing the trapped plasma and the drift time.
Since the transit time is longer due to low voltage, the operating frequency is limited below 10 GHz – current pulse is associated with low voltage, the power dissipation is low and efficiency is higher
Cont’d…
TRAPATT disadvantages:
High noise figure ~60dB limits it use as an
amplifier
Generates strong harmonics due to the short
duration current pulse
Cont’d…
COMPARISON BETWEEN GUNN, IMPATT, TRAPATT AND BARITT
4.2 Microwave Transistors
Ordinary transistors are pnp or npn junction transistors :
bipolar and unipolar FET
High frequency operations of these devices are limited by:
Diffusion capacitance at base-emitter junction
Space charge capacitances at base-emitter and base-collector
junction
Base spreading resistance
Bulk resistance of the base
Microwave transistors (MT) are miniaturized design to
reduce device and package parasitic capacitances and
inductances and to overcome the finite transit time of
charge carriers in the semiconductor materials.
Cont’d…
Three categories of MT:
Low-noise transistor which is employed in first stage
Low-level transistor which is used to drive power stage
Power transistor which amplifies final power output.
MT constructions:
Hybrid Microwave Integrated Circuits (HMIC)
The transmission lines and matching networks are
realised by microstrip circuit elements and the discrete
components.
Monolithic Microwave Integrated Circuits (MMIC)
All active devices and passive circuit elements are
fabricated on a single semiconductor crystal
Cont’d…
a. Bipolar Transistor
Planar in form, Si n-p-n type operating up to
5GHz
GaAs used for performance improvements in
the operating frequency in high temperature
and radiation field
Devices Geometry:
Inter-digitated
Overlay
Matrix form with wide emitter area
Cont’d…
Cont’d…
Cont’d…
Epitaxial n layer is formed by condensing a
single crystal film of semiconductor material
upon a low resistivity Si wafer of substrate
n+.
A p-type diffused base and n+-type diffused
emitter are formed
Typical value:
emitter width is 1 micron
base thickness is 2 microns
emitter length is 25 microns
Cont’d…
High frequency limitations:
At high frequencies, the reactance due to junction
capacitances limit the gain. The values of these
capacitance depend on the depletion layer’s width or
the bias voltage and provides feedback paths. The
feedback effect is reduced in CB mode of operation. The
junction capacitance must be reduce as much as
possible.
Signal loss in the lead inductance can be minimized by
reducing the length in packaging
The transit time taken by the majority of charge
carriers to cross from the emitter to collector limits the
high frequency
Cont’d…b. Unipolar Field Effect Transistor (FET)
Manufactured from GaAs for higher frequency of operation which
is achieved due to higher electron mobility compared to Si.
Frequency up to 40GHz, Noise figure 3dB.
Operates with only one type of carrier: electrons
Earliest FET
Junction FET (JFET)
Metal oxide semiconductor FET (MOSFET)
Insulated gate FET (IGFET)
Metal semiconductor FET (MESFET) – Competitor to bipolar
Different design from bipolar
Bipolar : 10V – 12V on the collector, biased the base and attached
emitter to ground
Unipolar: 10V – 12V on the drain, biased the gate and attached source
to ground.
Cont’d…
MESFET Construction MESFET equivalent circuit
Cont’d…
c. High Electron M0bility Transistor (HEMT)
Latest addition to the line up of
solid state devices design.
Looks similar to GaAs FET
Single stage gain 15dB at 8 GHz,
noise figure 0.4dB
4.3 Parametric Amplifier
Parametric amplifier Named because its operation due to the periodic variation of the device’s parameter such as capacitance of varactor diode under the influence of a suitable pump signal.
If a small input signal at a frequency fg and the ac power source operated as a pumping signal at a frequency fp are applied together to the varactor diode, linear amplification of a small signal results, due to time varying capacitance of the diode.
Pump signals provides the power required for amplification and the power output is either at the input frequency fg or at the idler frequency fi = fp-fg
Cont’d…
Manley-Rowe Relations Derived a set of general energy relations regarding power
flowing into and out of an ideal nonlinear reactance.
These relations are useful in predicting whether power gain
is possible in a parametric amplifier.
Sometimes called Power-Conservation Relations (PCR)
Cont’d… Advantages and limitations of Parametric Amplifiers
Noise figure
Less thermal noise due to minimum resistive elements
Range 1-2 dB
Frequency
Upper frequency limit (40-200GHz), set by difficulty of obtaining source power at the pump frequency and by the frequency at which the varactor capacitance can be pumped.
Lower frequency is set by the cut-off frequency of the microwave components used in the circuit
Bandwidth
Small due to the presence of tuned circuit
Gain
Limited to (20-80dB) by the stabilities of the pump source and the time varying capacitance
Recommended