nanoscale devices mnt-204 unit-5

7/31/2019 Nanoscale Devices MNT-204 UNIT-5

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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.

nanoscale devices mnt-204 unit-5

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