Transcript I v - UniMAP Portal

CHAPTER 8:
MICROWAVE DIODES,
QUANTUM EFFECT
& HOT ELECTRON
DEVICES
Part 2
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)
• Semiconductor double-barrier structure
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.
• 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 coefficient.
• Transmission coefficient decreases
rapidly as energy E deviates from the
discrete energy levels
• e.g.: electron with energy 10meV
higher or lower than the level E1 
105 times reduction in the
transmission coefficient (Tt) – fig.8.13
(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.
• 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)
• 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.
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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 increasing 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
freqency.
HOT ELECTRON DEVICES
• Hot electrons: electrons with kinetic
energies substantially above kT (k is
Boltzman’s constant & T is lattice
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.8.18: 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
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.8.19(a)
• Give power input to the structure – the
carriers heat up and undergo partial
transfer into the wide-gap layer –
fig.8.19(b)
• If the mobility in layer 2 is lower,
negative differential resistance will occur
in the 2-terminal circuit – fig.8.19(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.
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
• The injected electrons are swept
into the collector by the Vc – induced
electric field, giving rise to Ic
Figure 8.20. Cross section and band
diagram of a real-space-transfer transistor
in a GaInAs/AlInAs material system.
Transistor action results from control
of the Te in the source-drain channel
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
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, where
v=107.
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