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CHAPTER 8: MICROWAVE DIODES,
QUANTUM EFFECT
& HOT ELECTRON
DEVICES
Part 1
QUANTUM EFFECT & HOT
ELECTRON PHENOMENA
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Quantum effect & hot-electron phenomena  to
enhance circuit performances
Advantage of quantum effect devices (QEDs) & hot
electron devices (HEDs) :
 Higher functionality/speed
 can perform relatively complex circuit functions
with a greatly reduced device count
replacing large numbers of transistors or passive
circuit components
COVERAGE OF CHAPTER 8

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Millimeter-wave devices over those
operated at lower frequencies
The quantum tunneling phenomenon and
its related devices – tunnel diode, resonant
tunneling diode (RTD)
The IMPATT (IMPact ionization Avalanche
Transit-Time) diode – the most powerful
semiconductor source of millimeter wave
power
The Transferred-Electron Device (TED) and
its transit-time domain mode
The Real-Space-Transfer (RST) transistor
The most commonly used transmission lines (stripline
and microstrip line) aren't the only way to transmit a
signal from one place to another.
Figure 8.2. Basic types of planar transmission lines: (a) microstrip,
(b) coplanar waveguide stripline, and (c) suspended-substrate stripline.
A transmission line is a sub-category of waveguides that uses some
physical configuration of metal and/or dielectrics to direct a signal
along the desired path. Most familiar transmission lines (e.g.,
microstrip line) use two conductors; the impedance is:
Z0 
L
C
L=inductance (H)
C=capacitance (F)
RESONANT CAVITY
• metal-walled chamber made of lowresistivity material enclosing a good
dielectric
• Cavity supports: transverse electric
(TE) & transverse magnetic (TM) modes
of propagation
• Electromagnetic wave is confined by
the wall of the cavity
• E in electric field  capacitance C
• E in magnetic field  inductance L
• LC tuned resonant tank circuit present
in the cavity
• resonant mode occur in a cavity – freq
with length d along Z axis (/2) – fig. 3(a)
Figure 8.3. Resonant cavity: (a)
resonator shape, (b) magnetic field
pattern, and (c) electric field pattern.
RESONANT CAVITY
General mode dependent
equation for resonant freq. of
the cavity:
fr 
1
2 
2
2
m n  p
     
 a  b d 
2
2
c m n  p
fr 
     
2  a  b  c 
Mode in cavity Txm,n,p:
2
2
 0 0  c 1
x: E for electric dominant mode, M for
magnetic dominant mode
m: no. of half-wavelength in a
dimension
: permeability
n: no. of half-wavelength in b
dimension
0: permeability for vacuum (=0)
p: no. of half wavelength in the d
dimension
c: speed of light in vacuum
: permittivity
0: permittivity for vacuum (=0)
TUNNEL DIODE
The tunnel diode has a region in its voltage current characteristic where
the current decreases with increased forward voltage, known as its
negative resistance region. This characteristic makes the tunnel diode
useful in oscillators and as a microwave amplifier.
In
the
TUNNEL
DIODE,
the
semiconductor
materials
used
in
forming a junction are doped to the
extent of one-thousand impurity atoms
for ten-million semiconductor atoms.
This heavy doping produces an
extremely narrow depletion zone . Also
because of the heavy doping, a tunnel
diode exhibits an unusual currentvoltage
characteristic
curve
as
compared with that of an ordinary
junction diode.
Figure 8.4.
Static current-voltage characteristics of a
typical tunnel diode. The upper figures
show the band diagrams of the device at
different bias voltages.
• Forward bias:
• electrons tunnel from n-side to pside
• When V=(Vp+Vn)/3 – I reaches Ip
• When V is further increased
(Vp<V<Vv) tunnel I decreased (fewer
available unoccupied states in pside) until I can no longer flow
• With further increased V – normal
thermal I will flow (V>Vv)
• Summary:
•In the forward direction the
tunneling I increases from ‘0’ to a
peak current Ip as the V increases
• Further increase in V, the I
decreases to ‘0’ when V=Vn+Vp,
(V=applied voltage)
• Empirical form for the I-V
characteristics is given by
V
I  Ip
V
 p


 exp 1  V

 V
p



  I 0 exp  qV 

 kT 

Current ratios, I p
Iv
I ratios of:
Ge – 8:1
GaSb – 12:1
GaAs – 12:1
Figure 8.5. Typical current-voltage
characteristics of Ge, GaSb, and GaAs tunnel
diodes at room temperature.
IMPATT DIODE
• IMPATT: IMPact ionization Avalanche Transit-Time
• Employ impact ionization & transit-time properties  to produce a
negative resistance at microwave frequencies
• One of the most powerful solid-state sources of microwave power
• Can generate the highest cw (continuous wave) power output of all
solid-state devices at millimeter-wave frequencies (above 30GHz).
• Extensively used in radar systems & alarm systems
• Noteworthy difficulty in IMPATT applications: the noise is high
because of random fluctuations of the avalanche multiplication
processes
•1st IMPATT diode obtained from silicon p-n junction diode biased
into reverse avalanche breakdown and mounted in a microwave
cavity
IMPATT diode(IMPact ionization
Avalanche Transit Timediode)
∗A reverse biased p-n junction capable of producing
oscillations at up to100GHz
Applied voltage is slowly increased from zero. The electric field at the
junction builds up until it reaches the threshold for avalanche breakdown.
The avalanche produces holes which move quickly to the cathode and
electrons which take much longer to reach the anode. As the electrons
move towards the anode the electric field at the junction drops and so
shuts off the avalanche. When the electrons reach the anode the electric
field can build up again and a new avalanche develops.
The frequency of the oscillation is fixed by the transit time of the electrons
through the n-region.
IMPATT DIODE
• One-sided abrupt p-n junction:
• Most avalanche multiplication
occurs in a narrow region near the
highest field between 0 & xA
(width of avalanche region)
• Hi-lo structure:
• Avalanche region confined within
the N1 region
• Lo-hi-lo structure:
• a “clump” of donor atoms is
located at x=b
•High field region exists from x=0
to x=b, xA=b, max. field can be
Figure 8.6. Doping profiles and electric-field
much lower than hi-lo structure
distributions at avalanche breakdown of three
single-drift IMPATT diodes: (a) one-sided abrupt
p-n junction; (b) hi-lo structure; and (c) lo-hi-lo
structure.
TRANSFERRED-ELECTRON
DEVICES (TEDs)

Transferred Electron Devices (TEDs), widely know as
Gunn diodes, are gallium arsenide (GaAs) or indium
phosphide (InP) devices which are capable of
converting direct current (DC) power into radio
frequency (RF) power when they are coupled to the
appropriate resonator. Typical applications for Gunn
diode oscillators include local oscillators, voltage
controlled oscillators (VCOs), radar and communication
transmitters, Doppler motion detectors, intrusion
alarms, police radar detectors, smart munitions, and
Automotive Forward Looking Radars (AFLRs). Gunn
Diodes
are
two-terminal
negative-impedance
semiconductors which are similar to tunnel diodes They
are mainly found in microwave oscillators in the range
from ten to several hundred Gigahertz.
TRANSFERRED-ELECTRON
DEVICES (TEDs)
Negative Differential Resistance (NDR)
• Electron effect – the transfer of the
conduction electrons from a high-mobility
energy valley to low-mobility higherenergy satellite valley.
• Current density:
J  qn1 E
J  qn 2 E
0  E  Ea
E  Eb
• NDR region – between ET & EV
Figure 8.8. The current versus electricfield characteristic of a two-valley
semiconductor. ET is the threshold field
and EV is the valley field.
TRANSFERRED-ELECTRON
DEVICES (TEDs)


Transferred-electron mechanism is to give a rise to
Negative Differential Resistance (NDR):
 The lattice temp. must be low enough that in the
absence of energy E most of electrons are in lower
valley (conduction band minimum) – separation
between 2 valleys E>kT
 In the lower valley the electrons must have high
mobility & small effective mass, in the upper valley
electrons must have low mobility & large effective
mass
 Energy separation between 2 valleys must be smaller
than the semicond. bandgap (i.e. E<Eg) so that
avalanche breakdown does not begin before the
transfer of electrons into the upper valleys
N-type GaAs & InP – widely studied & used.
Device operation
• Have epitaxial layers on n+ substrates
• Typical donor conc. range: 1014 to
1016cm-3
• typical device lengths L range: few 
to several hundreds 
• Fig (a) & (b): energy band diagram at
thermal equilibrium & electric-field
distribution when V=3VT
• VT: product of threshold field ET &
device length L
• To improve performance: use 2-zone
cathode contact (consists of high-field
zone & n+ zone – fig. (b)(similar to lohi-lo IMPATT diode)
• electrons are heated in the high-field
Figure 8.9.
Two cathode contacts for transferred-electron zone  injected to the active region
devices (TEDs) (a) Ohmic contact and (b) twozone Schottky barrier contact.
• Operational of TED depends on 5 factors:
• Doping concentration
• Doping uniformity
• Length of the active region
• Cathode contact characteristics
• Type of circuit
• Operating bias voltage
• Important mode of operation for TED: transit-time
domain mode (+ve & -ve charges separated by a
small distance – dipole formation/domain)
• Electric field E in dipole > E on either side of it
• Because of NDR, I in low field region > I in high
field region
Figure 8.10.
Formation of a domain (dipole layer)
in a medium that has a negative
differential resistivity (NDR).
•Time required for domain to travel from cathode to
anode : L/v (L:active device length, v: average
velocity)
•Freq. for transit time domain mode: f=v/L
Test 2
will be on Thursday 16th Oct. 2008
@ K. Perlis (DKP1) 8.30pm – 9.30pm