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MNT-204
UNIT-5
Heterojunction Bipolar Transistors (HBTs)
Modulation-doped field effect transistor MODFETs
Single electron transistor
Resonant tunneling diodes
Temperature effects
Fundamentals of carrier transport in quantum structures
Topic
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Double Barrier Tunneling
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Resonant Tunneling Diode
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Figure 1 shows the band diagram of a RTD. It has a semiconductor double-barrier structure
containing four heterojunctions, a GaAs/AlAs/GaAs/AlAs/GaAs structure, and one quantum
well in the conduction band.
There are three important device parameters for a
RTD:
1. The energy barrier height E0, which is the
conduction band discontinuity,
2. The energy barrier thicknessLB,
3. The quantum well thicknessLw.
Figure 1 Conduction band of a RTD:
If the well thickness LW, is sufficiently small (on the order of 10 nm or less), a set of
discrete energy levels will exist inside the well (such as E1, E2, E3, andE4, in Fig 2a). If
the barrier thicknessLB, is also very small, resonant tunneling will occur.
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Figure 2
When an incident electron has an energy E that exactly equals one of the discrete
energy levels inside the well, it will tunnel through the double barrier with a unity (100%)
transmission coefficient.
The transmission
coefficient decreases
rapidly as the energy E
deviates from the discrete
energy levels. For
example, an electron with
an energy 10 meV higher
or lower than the level E,
will result in 105 times
reduction in the
transmission coefficients,
as depicted in Fig.2b
Relation Between LW, LB and En
The energy levels, En, at which the
transmission coefficient exhibits its first and
second resonant peaks in GaAs/AlAs RTD are
shown in Fig as a function of barrier thickness
LB, with the well thicknessLW, as a parameter.
It is apparent that En is essentially
independent ofLB, but is dependent onLW.
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Construction of RTD:
The cross section of a RTD is shown in Fig. The alternating GaAsIAIAs layers are
grown sequentially by molecular beam epitaxy (MBE) on ann+GaAs substrate. The
barrier thicknesses are1.7nm and the well thickness is 4.5 nm. The active regions
are defined with ohmic contacts.
The top contact is used as a mask to isolate the region under the contact by etching
mesas.
V-I characteristic:
The I-V curve is similar to that of a tunnal diode.
At thermal equilibrium, V =0, the energy diagram is similar to that in Fig a (here only the
lowest energy level E1 is shown).
As we increase the applied voltage, the electrons in the occupied energy states near the
Fermi level to the left side of the first barrier tunnel into the quantum well.
The electrons subsequently tunnel through the second barrier into the unoccupied states in
the right side.
Resonance occurs when the energy of the injected electrons becomes approximately equal
to the energy level E1, where the transmission probability is maximum.
This is illustrated by the energy diagram for V =V1 = V,, where the conduction band edge
on the left side is lined up with El. The magnitude of the peak voltage must be at least 2El/q
but is usually larger because of additional voltage drops in the accumulation and depletion
regions:
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When the voltage is further increased, that
is, at V =V2, the conduction band edge isabove E1 and the number of electrons that
can tunnel decreases, resulting in a smallcurrent. The valley currentIv, is due mainlyto the excess current components, such
as electrons that tunnel via an upper valley
in the barrier
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Metalsemiconductor junction
A metalsemiconductor (MS) junction is a type of junction in which a metal comes inclose contact with a semiconductor material.
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MODFET(Modulation Doped Field Effect Transistor)
The modulation-doped field-effect transistor (MODFET) is a heterostmcture field-effect
device, is a type of FET.
Other names commonly applied to the device include high electron mobility transistor
(HEMT), two-dimensional electron gas field-effect transistor (TEGFET), and selectively
doped heterostructure transistor (SDHT), heterojunction field-effect transistor (HFET).
MODFETs are used in integrated circuits as digital on-off switches. MODFETs can also be
used as amplifiers for large amounts of current using a small voltage as a control signal.
A perspective view of a conventional MODFET. The special features of a MODFET are its
heterojunction structure under the gate, and the modulation doped layers.
MODFET is a transistor built on themodulation doping principle, which is the doping of a
heterostructure (e.g. AlGaAs-GaAs) implemented in such way that the resulting free
electrons are separated from the positive donor ions, due to the separation, electrons
remain free and mobile even at the very low temperatures.
Basic Structure:
The most-common heterojunctions for the MODFETs are the AlGaAs/GaAs,
AlGaAdInGaAs, and InAlAs/InGaAs heterointerfaces. A basic MODFET structure based on
the AlGaAs/GaAs system is shown (next).
It is seen here that the barrier layer AlGaAs under the gate is doped, while the channel layer
GaAs is undoped. (it is the principle of modulation doping).
such that carriers from the doped barrier layer are transferred to reside at the
heterointerface and are away from the doped region to avoid impurity scattering.
The doped barrier layer is typically around 30-nm thick.
The top layer of n +-GaAs is for better source and drain ohmic contacts.
The top layer of n+-GaAs is for
better source and drain ohmic contacts. These contacts are made from alloys containing
Ge, such as AuGe.
The sourceldrain deeper n+-regions are formed either by ion implantation.
Most MODFETs reported are n-channel devices for higher electron mobility.
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For the device in Fig, AlGaAs is the wide bandgap semiconductor, whereas GaAs is the
narrow bandgap semiconductor.
The two semiconductors are modulation doped, i.e., the AlGaAs is doped (d1), except for a
narrow region (do), which is undoped, whereas the GaAs is undoped.
Electrons in the AIGaAs will diffuse to the undoped GaAs, where aconduction channel can
be formed at the surface of theGaAs.
The band diagram of a MODFET in thermal
equilibrium conditionFig a.
Similar to a standard Schottky barrier, q Bn, is the
barrier height of the metal on the wide-bandgap
semicondutor.Ec is the conduction band
discontinuity for the heterojunction structure, and the
pinch-off voltage (VP) given by
Operation:
A key parameter for the operation of a MODFET is
the threshold voltage VT, , which is the gate bias at
which the channel starts to form between the source
and drain. WithFig b.
VT, corresponds to the situation when the bottom of
the conduction band at the GaAs surface coincides
with the Fermi level.
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Other MODFET structures
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Advantages of MODFET structure
The major development effort for MODFETs has
been on a channel material that can furtherimprove the electron mobility.
Instead of GaAs, InxGa1-x,as has been pursueddue to its smaller effective mass.
These advantages are found to be directlyrelated to the indium contents: the higher thepercentage, the higher the performance.
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Single Electron Transistor
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Heterojunctions
A heterojunction is defined as a junction formed between two dissimilar semiconductors.
The two semiconductors are assumed to have different energy bandgapsEg, different
dielectric permittivities s, different work function q s, and different electron affinities (The
work function is defined as the energy required to remove an electron from Fermi levelEF
to a position just outside the material.
The electron affinity is the energy required to remove an electron from the bottom of the
conduction bandE, to the vacuum level.
Heterojunction Bipolar Transistor (HBTs)
A heterojunction bipolar transistor (HBT) is a transistor in which one or bothp-njunctions
are formed between dissimilar semiconductors.
The primary advantage of an HBT is its high emitter efficiency ( ).
The circuit applications of the HBT are essentially the same as those of bipolar transistors.
The HBT has higher-speed and higher frequency capability in circuit operation.
The HBT has gained popularity in photonic, microwave, and digital applications.
Drawbacks of BJT
To achieve a fast base transit time, and hence a high value of cut-off frequency, the
basewidth needs to be very small, as shown in equation.
Where is associated with the excess minority carrier charge in the, base depletion region.
The mechanism that limits the extent that the base width can be reduced is punch-through ofthe base, which occurs when the emitter/base depletion region intersects the collector/basedepletion region in the base.
Thinner depletion regions can be achieved by increasing the base doping concentration,
so that narrower base widths could be achieved without encountering punch-through.
But increasing the base doping, degrades the gain, as can be seen from equation
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Design HBT:
SiGe has a lower bandgap than Si.
If a bipolar transistor could be created with SiGe in the base and Si in the emitter much
higher values of gain would be achieved.
A SiGe HBT is produced by sandwiching a SiGe base between a Si collector and a Si
emitter.
The band diagram of the SiGe HBT is indicated
by the solid line and that for the Si BJT by the
dashed line.
In the valence band, the bandgap difference is
seen as discontinuities at the emitter/base and
collector/base heterojunctions, while in the
conduction band it is seen as spikes.
A comparison of the band diagrams in Figure shows that the barrier height to electron flowfrom emitter to baseEb(conduction band barrier) is much smaller in the SiGe HBT thanthe Si BJT. This means that the collector current at a given base/emitter voltage will bebigger in a SiGe HBT than in a Si BJT.
Collector current
where it has been assumed thatNaeffis the same in SiGe and Si. the base currentof a SiGe HBT is the same as that for a Si bipolar transistor, and hence the gainEnhancement. obtainable from a SiGe HBT can be obtained by taking the ratio ofcollector currents:The barrier height to hole flow from the base to the emitter (valence band barrier) isapproximately the same in the SiGe HBT and the Si BJT, which means that the basecurrents of the two types of device will be approximately the same.
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It can be seen that the gain of the HBT is much higher than that of the BJT and that
this increased gain is due to an increased collector current.
The increased collector current of a SiGe HBT can be thought of in another way.