History Of The Heterojunction Bipolar Transistor Engineering Essay

Heterojunction bipolar transistors are bipolar junction transistors, they are made up of at least two dissimilar semiconducting materials. The energy bandgap every bit good as all other stuff belongingss can be different in the emitter, base and aggregator. A steady alteration or ranking of the stuff is likely within each part. The usage of heterojunction provides and extra grade of freedom, which can in immensely improved devices compared to the homojunctions opposite numbers. GaAs heterojunction bipolar transistors ( HBTs ) are used for digital and analogy microwave applications with frequences every bit high as Ku set. HBTs can supply faster exchanging velocities than Si bipolar transistors chiefly because of decreased base opposition and collector-to-substrate electrical capacity [ 2 ] .

Fig 1.0 Cross subdivision of an illustration HBT.

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The barriers for hole junction [ a?†Vp ] and electron injection [ a?†Vn ] in a ranked E-B junction different by the band-gap difference [ a?† Eg ] between the Al GaAs emitter and the GaAs base. The equation is

Q [ a?† Vp-a?†Vn ] = Eg 1

a?†Eg =Eg ( AlGaAs ) -Eg ( GaAs ) 2


individual power supply mutual opposition

It used in optical procedure


The heat dissipation can be job at little emitter size.

The typical contrary isolation is non every bit high as with PHEMT amplifier taking to hapless amplifier directionality.

Collector resistance are required to stabilise amplifier and these cut into power efficiency

Fig 2.0 AlGaAs ( 1 ) abrupt E -B junction and ( 2 ) graded e-b junction.

The little band-gap difference Eg effects the ratio of In/Ip.

Where In is the negatron injection current from the emitter into base

Ip is the hole injection current from the base into the emitter.

The parametric quantity are q= the electronic charge, K= Boltzmann invariable, T = Temperature, A= the emitter-base junction, Dn= the negatron diffusion into the emitter, Ne= the emitter doping concentration, Dp= the hole diffusion in emitter, W= the base breadth.

The debasement mechanisms that have been reported in hetrojunction bipolar transistors include following,

Decrease in current addition and increase in base-emitter electromotive force at high emitter currents.

Additions in contact opposition caused by debasement of the interface between the emitter ohmic contact metallization and the emitter semiconducting material

Geting of crystillation defects at the emitter-base heterojunction.

Decrease in current addition and additions in base -emitter electromotive force for a specified cottector current cased by oxidization of the emitter mesa surface in the part of the emitter base heterojunction.


The complementary metal oxide semiconducting material ( CMOS ) device engineering is the procedure of pick for high integrating and public presentation at low-power. The simplest illustration of a CMOS is that of a logical inverter. Here, the input is connected to the gate of both the PMOS and NMOS transistor, VDD represent the positive electromotive force supply, VSS represents the nothing Vs line. CMOS gate are all based on the cardinal inverter circuit shown below and observe that transistors are enhancement-mode MOSFETs ; one

Figure3.0 Connections required to construct the inverter.

BASIC OF CMOS CELL.the memory that, semiconducting material memories have improved both in denseness and public presentations, with the production of the 256Mega byte Mb dynamic memories.

Why do we utilize CMOS?

The logical Gatess that are represented could really easy be realized by utilizing the bottom half? Of the circuit merely.

The power dissipated by the circuit, is reduced.

The minimal supply electromotive force ( VDD ) at which a inactive CMOS circuit can properly operate is given by

( VDD ) mina‰? pVT

Where VT = KT/q is the thermic electromotive force ( which K being the Boltzmann invariable and Q is the electron charge ) , and P is factor between 2 to 4.at room temperature ( T=300k ) , VT is about 26mV and so the minimal supply electromotive force can be every bit low as 0.1V.


In other to cut down the power dissipation of a CMOS circuit the assorted beginnings must be identified. There are two types of power ingestion relevant to circuit design. The peak power and the mean power. The power is related to the maximal instantaneous current drawn from the supply which can ensue in the big drop/bounces on the resistive power/ground tracks. The mean power dissipation dictates the battery size and weight demand to run the circuit for a given sum of clip. [ low power ] .

A typical digital CMOS circuit, there are two chief categories of power dissipation, dynamic power P ( dynamic ) and statics power P ( statics ) . Where the parasitic electrical capacity are lumped at the end product in the capacitance CL, the entire energy drawn from the supply E ( SUPPLY ) and the energy stored in the capacitance E ( CAP ) .


( 1 ) Switch overing current. ( 2 ) short- circuit current.

( 3 ) Dc current. Reverse-bias drain and sub- threshold escape currents threshold escape currents.

Figure3.0 CMOS circuit theoretical account for power computation.

The exchanging power is dissipated during logic transistor as a consequence of bear downing parasitic electrical capacity of gate, diffusion and interconnect. The usage of low cost criterion CMOS engineering for fiction of this sort of high velocity integrated circuit ( ICs ) will take to integrating of wireless frequence ( RF ) . [ TWA CMOS ] .


The distributed amplifier is composed of two coupled lumped transmittals lines. Power is coupled from the gate-line to the drain-line through transistor, where power is coupled in the rearward way parasitically through the feedback capacitance S21 & gt ; & gt ; S12.the behavior of the amplifier is dominated by the forward- way yoke of power from the gate line to the drain line. The figure of phases that may be used is limited by the built-in losingss on the drain and gate line.

Figure 4.0 A three-stages distributed amplifiers utilizing CMOS.


The image parametric quantity method may be applied to the distributed amplifier since it consists of a cascade of indistinguishable two-port web organizing an unreal transmittal line. The image electric resistance Zi and other terminus is besides terminated in Zi..


Where the S-parameter

S11 S12

S21 S22

S11= S22 by symmetricalness

The extension invariable for the current and electromotive force is given by

?µ-I? = a?sS11 S22-a?sS12S21

moving ridge extension occur if I? has an fanciful constituent

Zi = a?sZ1Z2a?s1+Z1/4Z2

?µ-I? = 1+ Z1/2Z2+a?sZ1/Z2+Z12/4Z22

FigurA™ 5.0 T -section


For a losingss cascade of T- subdivision such as that show figure5.o the extension factor and image electric resistance are given by [ Pozar,1997 ] .


?µ-I? =1-2


( MESFET ) Metal-Semiconductor Field Effect Transistor.


The MESFET or GaAs FET has another name which is a high public presentation of field consequence transistor, it is used chiefly for high public presentation microwave applications and semiconducting material RF amplifiers. MESFET consists of carry oning channel positioned between a beginning and drain contact part and besides portion other characteristics with junction FET or JFET.

Figure1.0 Cross subdivision position of MESFET hypertext transfer protocol: //www.ee.ui.ac.id/~astha/courses/ts/teksem/mesfet.htm


The MESFET can be trace back to the early yearss of semiconducting material engineering, in 1949 which was the bipolar transistor had a milepost in semiconducting material engineering. In 1953 was the first practical signifier of a field consequence transistor and as a consequence made it simpler and be effectual to fabricate the highly pure signifiers of semiconducting material. In1960 oxide beds was the major work which lead to development of MOSFETs. In 1966 MESFETs were developed and afterwards the unexpected high frequency/RF microwave public presentation was demonstrated.


The MESFET is a signifier of semiconducting material engineering which is really similar to junction FET or JFET. MESFET has a metal contact straight with the Si, and this signifier a Schottky barrier rectifying tube junction. The stuff used is GaAs ( gallium arsenide ) . The substrate for the semiconducting material device is semi-insulating for low parasitic electrical capacity, and so the active bed is deposited epitaxaially. The resulting channel is typically less than 0.02 micrometers thick. To do a low noise device that has good one-dimensionality, the doping is normally non uniform in a manner that is perpendicular to the gate. Since most of the devices requires high velocity operation so it is best to utilize an n-channel to dope since the negatrons move faster than holes ( electron mobility ) the contact of the gate provides a barier that is high to cut down escape current can be made from different stuffs like Aluminum, a Titanium-Platinum-Gold superimposed construction, Platinum itself, or Tungsten. The gate length to depth ratio is an of import as this determines a figure of the public presentation parametric quantities.

There are two chief constructions that are used for MESFETs:

Non-self aligned beginning and drain: A For this signifier of MESFET, the gate is placed on a subdivision of the channel. The gate contact does non cover the whole of the length of the channel. This arises because the beginning and drain contacts are usually formed before the gate.

Non-self aligned MESFET construction

Self aligned beginning and drain: A This signifier of construction reduces the length of the channel and the gate contact covers the whole length. This can be done because the gate is formed first, but in order that the tempering procedure required after the formation of the beginning and drain countries by ion nidation, the gate contact must be able to defy the high temperatures and this consequences in the usage of a limited figure of stuffs being suited.

Self aligned MESFET construction

MESFET operation

Like other signifiers of field consequence transistor the O MESFET has two signifiers that can be used:

Depletion manner MESFET: A If the depletion part does non widen all the manner to the p-type substrate, the MESFET is a depletion-mode MESFET. A depletion-mode MESFET is conductive or “ ON ” when no gate-to-source electromotive force is applied and is turned “ OFF ” upon the application of a negative gate-to-source electromotive force, which increases the breadth of the depletion part such that it “ pinches off ” the channel.

Enhancement manner MESFET: A In an enhancement-mode MESFET, the depletion part is broad plenty to squeeze off the channel without applied electromotive force. Therefore the enhancement-mode MESFET is of course “ OFF ” . When a positive electromotive force is applied between the gate and beginning, the depletion part psychiatrists, and the channel becomes conductive. Unfortunately, a positive gate-to-source electromotive force puts the Schottky rectifying tube in forward prejudice, where a big current can flux.


The MESFET is used in many RF amplifier applications. The MESFET semiconducting material engineering provides for higher negatron mobility, and in add-on to this the semi-insulating substrate there are lower degrees of isolated electrical capacity. This combination makes the MESFET ideal as an RF amplifier. In this function MESFETs may be used as microwave power amplifiers, high frequence low noise RF amplifiers, oscillators, and within sociables. MESFET semiconducting material engineering has enabled amplifiers utilizing these devices that can run up to 50 GHz and more, and some to frequences of 100 GHz.

The GaAS FET / MESFET has a figure of differences and advantages when compared to bipolar transistors. The MESFET has a really much higher input as a consequence of the nonconductive rectifying tube junction. In add-on to this it besides has a negative temperature co-efficient which inhibits some of the thermic jobs experienced with other transistors.

When compared to the more common Si MOSFET, the GaAs Fet or MESFET does non hold the jobs associated with oxide traps. Besides a MESFET has better channel length control than a JFET. The ground for this is that the JFET requires a diffusion procedure to make the gate and this procedure is far from good defined. The more exact geometries of the GaAS FET / MESFET supply a much better and more quotable merchandise, and this enables really little geometries suited to RF micro-cook frequences to provide for.

In many respects GaAs engineering is less good developed than Si. The immense ongoing investing in Si engineering means that Si engineering is much cheaper. However GaAs engineering is able to profit from many of the developments and it is easy to utilize in incorporate circuit fiction processes.



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