Microwave Engineering

ECE 327

Reference: Microwave Devices and

Circuits

Final: 80
Mid-term, Quiz, Attendance: 45
Total: 125
 Microwave solid-state devices
1- Microwave bipolar junction transistor (BJT), the heterojunction bipolar transistor
(HBT), and the tunnel diodes.
2- Microwave field-effect transistor (FET) such as the junction field-effect transistor
(JFET), metal-semiconductor field-effect transistor (MESFET), high electron mobility
transistor (HEMT), metal-oxide-semiconductor field-effect transistors (MOSFET), the
metal-oxide-semiconductor transistors and memory devices, and the charge-coupled
device (CCD).
3- Devices which is characterized by the bulk effect of the semiconductor, is called the
transferred electron device (TED) including the Gunn diode, limited space-charge-
accumulation diode (LSA diode), indium phosphide diode (InP diode), and cadmium
telluride diode (CdTe diode).
4- Devices which are operated by the avalanche effect of the semiconductor, are referred
to as avalanche diodes: the impact ionization avalanche transit-time diodes (IMPATT
diodes), the trapped plasma avalanche triggered transit-time diodes (TRAPATT
diodes), and the barrier injected transit-time diodes (BARITT diodes).
 Semiconductor diodes

Transferred electron Avalanche Quantum Parametric Schottky

mechanic Step recovery barrier diode
devices (TED) transit time devices
tunneling (SBD)
Gunn IMPATT PIN
Tunnel Varactor

LSA TRAPATT

InP
Read
CdTe
BARITT
Microwave transistor
• Theinvention of the transistor by William Shockley had a revolutionary
impact on electronic technology.
• The microwave transistor is a nonlinear device, and its principle of
operation is similar to that of the low-frequency device, but
requirements for dimensions, process control, heat sinking, and
packaging are much more severe.
• Formicrowave applications, the silicon (Si) bipolar transistors
dominate.
• Thebipolar junction transistor (BJT) is an active three-terminal device
which is commonly used as an amplifier or switch.
Principle of operation of microwave transistor
•A bipolar transistor can be used as an amplifier or a switching
device.
•A bipolar transistor can operate in different modes of operation
depending on the voltage polarities across the two junctions:
1. Active (normal) mode: transistor acts as an amplifier.
2. Saturation mode: transistor acts like a closed switch (short circuit)
for the on state.
3. Cutoff mode: transistor acts like an open switch (open circuit) for
the off state.
 Geometry of microwave transistor
• Allmicrowave transistors are now planar in form and almost all are
of the silicon n-p-n type.
• Thegeometry can be characterized as follows: interdigitated, overlay
and matrix (mesh or emitter grid).

1- Interdigitated 2- Overlay 3- Matrix

 Heterojunction bipolar transistor (HBT)
• Bipolar transistor can be constructed as homojunction or heterojunction.
• When the transistor junction is jointed by two similar materials such as silicon
to silicon or germanium to germanium, it is a homojunction transistor.
• When the transistor junction is formed by two different materials such as Ge to
GaAs, it is called a heterojunction transistor.
• When the lattice constants of two semiconductor materials are matched, they
can be formed together as a heterojunction transistor. This lattice condition is
very important. Currently, Ge and GaAs are the two materials commonly used
because they are matched to within 1%.

Lattice constant = a = 5.646 Å for Ge

Lattice constant = a = 5.653 Å for GaAs
Energy-band diagram for isolated n -Ge and p -GaAs
Energy-band diagram for isolated n -Ge and p -GaAs
• n-Ge is designated as 1, and p-GaAs is referred to as 2.
• The vacuum level is used as reference.
• When an n-Ge and a p-GaAs are isolated, their Fermi energy levels
are not aligned,
•𝜙 : the work function.
•𝑥 : electron affinity in eV
• 𝐸𝑔 : bandgap energy in eV
• The different energy of the conduction-band edge

• The different energy of the valence-band edge

 Microwave tunnel diode
• Many scientists noticed the unusual properties of some p-n junctions, but
irregularities were rejected immediately because they did not follow the "classic"
diode equation.
• Esaki described this phenomenon by applying a quantum tunneling theory.
• The tunneling phenomenon is a majority carrier effect.
• The tunneling time of carriers through the potential energy barrier is not governed by
the classic transit time concept that the transit time is equal to the barrier width
divided by the carrier velocity but rather by the quantum transition probability per
unit time.
• The tunnel diode is a negative-resistance semiconductor p-n junction diode.
Principles of operation of tunnel diode
• The negative resistance is created by the tunnel effect of electrons in the p-n junction.
• The doping of both the p and n regions of the tunnel diode is very high-impurity
concentrations of 1019 to 1020 atoms/ cm3 are used and the depletion layer barrier at
the junction is very thin on the order of 100 Å or 10−6 cm.
• Classically, it is possible for those particles to pass over the barrier if and only if they
have an energy equal to or greater than the height of the potential barrier.
• Quantum mechanically, however, if the barrier is less than 3 Å there is an appreciable
probability that particles will tunnel through the potential barrier even though they do
not have enough kinetic energy.
• In addition to the barrier thinness, there must also be filled energy states on the side
from which particles will tunnel and allowed empty states on the other side into which
particles penetrate through at the same energy level.
Principles of operation of tunnel diode
• In ordinary diodes the fermi level exists in the forbidden band (FB).
• Since the tunnel diode is heavily doped, the fermi level exists in the valence band in
p type and in the conduction band in n type semiconductors.
• Fermi level representing the energy state with 50% probability of being filled if no
forbidden band exists.
• When the tunnel diode is forward biased by a voltage between zero and the value
that would produce peak tunneling current Ip (0 < V < Vp), the electrons tunnel
through the barrier from the n type to the p type, giving rise to a forward tunneling
current from the p type to the n type.
• A maximum number of electrons can tunnel through the barrier giving rise to the
peak current Ip.
Principles of operation of tunnel diode
• If the bias voltage is further increased, the tunneling current decreases.
• Finally, at a very large bias voltage, no electrons can tunnel through the barrier
and the tunneling current drops to zero.
• When the forward-bias voltage V is increased above the valley voltage Vv , the
ordinary injection current I at the p-n junction starts to flow.
• This injection current is increased exponentially with the forward voltage.
• The total current, given by the sum of the tunneling current and the injection
current, results in the volt-ampere characteristic of the tunnel diode.
• the total current reaches a minimum value Iv (or valley current) somewhere in the
region where the tunnel diode characteristic meets the ordinary p-n diode
characteristic.
I-V characteristic of tunnel diode
Equivalent circuit of tunnel diode
• The tunnel diode can be connected either in parallel or in series with a resistive
load as an amplifier.
• When the negative resistance 𝑅𝑛 of the tunnel diode approaches the load
resistance 𝑅𝑙, the gain A approaches infinity, and the system goes into oscillation.
Parallel loading Series loading

𝑹𝒏 𝑹𝒍
𝑨= A=
𝑹𝒏 −𝑹𝒍 𝑹𝒍 −𝑹𝒏