Transcript chapter 8 - UniMAP Portal

CHAPTER 8: MICROWAVE
DIODES, QUANTUM EFFECT
& HOT ELECTRON
DEVICES
© S.N. Sabki
QUANTUM EFFECT & HOT
ELECTRON PHENOMENA
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
© S.N. Sabki
COVERAGE OF CHAPTER 8
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 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
© S.N. Sabki
© S.N. Sabki
© S.N. Sabki
© S.N. Sabki
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;
Z0 
L
C
L=inductance (H)
C=capacitance (F)
© S.N. Sabki
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.
© S.N. Sabki
© S.N. Sabki
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)
© S.N. Sabki
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.
© S.N. Sabki
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 

© S.N. Sabki
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.
© S.N. Sabki
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 (continous 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
© S.N. Sabki
IMPATT diode(IMPact ionization
Avalanche Transit Timediode)
∗Areverse biased p-njunction 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.
© S.N. Sabki
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.
© S.N. Sabki
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 similiar to tunnel diodes
They are mainly found in microwave oscillators in the
range from ten to several hundred Gigahertz.
© S.N. Sabki
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.
© S.N. Sabki
TRANSFERRED-ELECTRON DEVICES
(TEDs)
Transferred-electron mechanism to give 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.
© S.N. Sabki
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)
Figure 8.9.
Two cathode contacts for transferredelectron devices (TEDs) (a) Ohmic contact
and (b) two-zone Schottky barrier contact.
• electrons are heated in the high-field
zone  injected to the active region
© S.N. Sabki
• 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
•Time required for domain to travel from cathode to
anode : L/v (L:active device length, v: average
velocity)
Figure 8.10.
Formation of a domain (dipole layer) •Freq. for transit time domain mode: f=v/L
in a medium that has a negative
differential resistivity (NDR).
© S.N. Sabki
QUANTUM-EFFECT DEVICES (QED)
• QED uses quantum mechanical tunneling to
provide carrier transport – active layer thickness
is very small (10nm).
• Give rise to a quantum size effect that can alter
the band structures & enhance device transport
properties
• Resonant Tunneling Diode (RTD) (Basic QED)
• Semicond. double-barrier struc. contains 4
heterojunctions
(GaAs/AlAs/GaAs/AlAs/GaAs), 1 quantum
well in the conduction band
• Important parameter: energy barrier height
E0, energy barrier thickness LB, quantum well
thickness LW
Figure 8.12.
Band diagram of a resonant-tunneling diode.
© S.N. Sabki
• If LW small (10nm) – a set of energy
levels will exist inside the well (E1, E2,
E3, E4) –fig.13 (a)
• If LB small – resonant tunneling will
occur
• incident electron (has energy E =
one of the discrete energy levels in
the well) – it will tunnel thru the
double barrier with a unity (100%)
transmission coeff.
• Transmission coeff. decreases
rapidly as energy E deviates from the
discrete energy levels
• eg.: electron with energy 10meV
higher or lower than the level E1 
105 times reduction in the
transmission coeff. (Tt) – fig.13 (b)
Figure 8.13. (a) Schematic illustration of AlAs/GaAs/AlAs double-barrier
structure with a 2.5 nm barrier and a 7 nm well. (b) Transmission
coefficient versus electron energy for the structure.
© S.N. Sabki
• GaAs/AlAs layers are grown
on an n+ GaAs substrate
• LB = 1.7nm
• LW = 4.5nm
• active regions are defined with
ohmic contacts
Figure 8.15.
A mesa-type resonant tunneling diode
(cross-section)
© S.N. Sabki
• Note: I-V curve is similar to that of a tunnel
diode (fig.4)
• At thermal equilibrium (V=0) energy diagram is
similar to fig.13(a)
• Increase the applied V – electrons in the
occupied energy states near the fermi level to
the left side of the 1st barrier tunnel into the
quantum well
• the electrons tunnel thru the 2nd barrier into
the unoccupied states in the right side.
• Resonance occurs when the energy of the
injected electrons = E1
• V=V1=Vp – the conduction band edge on the
left side is lined up with E1
• When V=V2, the conduction band edge is
above E1 – electrons that can tunnel decreases
– small I
• Iv due mainly to the excess I components:
electrons that tunnel via an upper valley in the
barrier
Figure 8.16.
Measured current-voltage characteristics of the diode in Fig. 8-15.
© S.N. Sabki
To minimize the Iv – must improve the quality of the
heterojunction interfaces & eliminate impurities in the
barrier & well regions
For higher applied V (V>Vv) – Ith due to electrons
injected thru higher discrete energy levels in the well or
thermionically injected over the barrier
Ith increases with inceasing V (similar to tunnel diode)
To reduce Ith: increase the barrier height & design a
diode that operates at relatively low bias voltages
Resonant Tunneling Diodes can be operated at very
high freq. – smaller parasitics
© S.N. Sabki
HOT ELECTRON DEVICES
• Hot electrons: electrons with kinetic
energies substantially above kT (k is
Boltzman’s constant & T is lattie temp.)
• As the dimensions of semicond.
devices shrink & internal fields rise, a
large fraction of carriers in the active
regions of the device during its
operation is in states of high kinetic
energy
• Fig.18: an AlInAs/GaInAs HBT
Figure 8.18. Energy band diagram of a
hot electron heterojunction bipolar
transistor.
•Electrons are injected by thermionic
emission over the emitter-base barrier
at an energy Ec=0.5eV above the
conduction band-edge in the p-GaInAs
base
© S.N. Sabki
Real-Space-Transfer Transistor
• In thermal equilibrium: mobile electrons
reside in the undoped GaAs quantum
wells & spatially seperated from their
parent donors in AlGaAs layers –
fig.19(a)
• Give power input to the structure – the
carriers heat up and undergo partial
transfer into the wide-gap layer –
fig.19(b)
• If the mobility in layer 2 is lower,
negative differential resistance will occur
in the 2-terminal circuit – fig.19(c)
• Transferred-electron effect, based on
the momentum-space intervalley transfer
 named real-space transfer
Figure 8.19.
(a) A heterostructure with alternate GaAs and AlGaAs
layers. (b) Electrons, heated by an applied electric field,
transfer into the wide-gap layers. (c) If the mobility in
layer 2 is lower, the transfer results in a negative
differential conductivity.
© S.N. Sabki
Real-Space-Transfer Transistor
• Source & drain contacts are to
undoped GaInAs channel
• Collector contact is to a doped
GaInAs conducting layer – separated
from the channel by a larger bandgap
material (i.e. AlInAs; Eg=1.45eV)
• At VD=0, electron density is induced
in the source-drain channel by +ve Vc
– but no Ic flows because of the
AlInAs barrier
• VD increases, ID begins to flow & the
channel electrons heat up to some
effective temp. Te
Figure 8.20. Cross section and band
diagram of a real-space-transfer
transistor in a GaInAs/AlInAs material
system.
• The injected electrons are swept
into the collector by the Vc – induced
electric field, giving rise to Ic
Transistor action results from control
of the Te in the source-drain channel
© S.N. Sabki
Exercise (Microwave)
Find the characteristic impedance of a nearly
lossless transmission line (R is very small) that
has a unit-length inductance of 10nH and a unitlength capacitance of 4pF
Solution
Impedance
L
10 10 9
3
Z0 


2
.
5

10
 50
12
C
4 10
© S.N. Sabki
Exercise (microwave)
For a cavity of the dimensions a=5cm (0.05m),
b=2.5cm (0.025m) and d=10cm (0.1m), find the
resonant frequency in the dominant TE101 mode
Solution:
2
2
c m n  p
fr 
     
2  a  b  c 
c

(20) 2  (0)  (10) 2
2
3 108 m/s

 500
2
 3.354GHz
2
© S.N. Sabki
Exercise (Transferred Electron
Devices)
A GaAs TED is 10m long and is operated
in the transit-time domain mode. Find the
minimum electron density n0 required and
the time between current pulses
Solution:
For transit-time domain mode, we require
n0L1012 cm-2
n0 1012/L = 1012/10x10-4 = 1x1015cm-3
The time between current pulses is the time
required for the domain to travel from the
cathode to anode:
t = L/v = 10x10-4/107 = 10-10s = 0.1ns
© S.N. Sabki
INFORMATION!
TEST 2
DATE: 5th October 2007 (FRIDAY)
TIME: 8.30pm – 9.30pm
VENUE: DKG 1 (will confirm later)
TOPICS:
CHAPTER 5,6,8,9
© S.N. Sabki
DON’T FORGET!!
Submit Mini Project Report NEXT
WEEK (19th Sep 2007) during lecture.
Mini Project Presentation on 24th &
25th Sep 2007 during lab session
© S.N. Sabki