Heterojunction bipolar transistors are bipolar junction transistors, they are made up of at least two dissimilar semiconductors. The energy bandgap as well as all other material properties can be different in the emitter, base and collector. A steady change or ranking of the material is likely within each region. The use of heterojunction provides and additional degree of freedom, which can in vastly improved devices compared to the homojunctions counterparts. GaAs heterojunction bipolar transistors (HBTs) are used for digital and analogy microwave applications with frequencies as high as Ku band. HBTs can provide faster switching speeds than silicon bipolar transistors mainly because of reduced base resistance and collector-to-substrate capacitance[2 ].
Fig 1.0 Cross section of an example HBT.
THE BASIC OPERATION HBT
The barriers for hole junction [∆Vp] and electron injection[∆Vn] in a graded E-B junction different by the band-gap difference [∆ Eg] between the Al GaAs emitter and the GaAs base. The equation is
q[∆ Vp-∆Vn]= Eg 1
∆Eg =Eg(AlGaAs)-Eg(GaAs) 2
Advantages:
single power supply polarity
It used in optical process
Disadvantages :
The heat dissipation can be problem at small emitter size.
The typical reverse isolation is not as high as with PHEMT amplifier leading to poor amplifier directivity.
Collector resistor are required to stabilize amplifier and these cut into power efficiency
Fig 2.0 AlGaAs (1) abrupt E -B junction and(2) graded e-b junction.
The small band-gap difference Eg effects the ratio of In/Ip.
Where In is the electron injection current from the emitter into base
Ip is the hole injection current from the base into the emitter.
The parameter are q= the electronic charge, K= Boltzmann constant, T = Temperature, A= the emitter-base junction, Dn= the electron diffusion into the emitter, Ne= the emitter doping concentration, Dp= the hole diffusion in emitter, W= the base width.
The degradation mechanisms that have been reported in hetrojunction bipolar transistors include following,
Decrease in current gain and increase in base-emitter voltage at high emitter currents.
Increases in contact resistance caused by degradation of the interface between the emitter ohmic contact metallization and the emitter semiconductor
Getting of crystillation defects at the emitter-base heterojunction.
Decrease in current gain and increases in base -emitter voltage for a specified cottector current cased by oxidation of the emitter mesa surface in the region of the emitter base heterojunction.
COMPLEMENTRY METAL- OXIDE SEMICONDUCTOR(CMOS).
The complementary metal oxide semiconductor (CMOS) device technology is the process of choice for high integration and performance at low-power . The simplest example 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 voltage supply, VSS represents the zero volts line. CMOS gate are all based on the fundamental inverter circuit shown below and note that transistors are enhancement-mode MOSFETs; one
Figure3.0 Connections required to build the inverter.
BASIC OF CMOS CELL .the memory that , semiconductor memories have improved both in density and performances, with the production of the 256Mega byte Mb dynamic memories.
Why do we use CMOS?
The logical gates that are represented could very easily be realized by using the bottom half? Of the circuit only.
The power dissipated by the circuit, is reduced.
The minimum supply voltage (VDD) at which a static CMOS circuit can properly operate is given by
(VDD)min≥ pVT
Where VT = KT/q is the thermal voltage ( which K being the Boltzmann constant 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 minimum supply voltage can be as low as 0.1V.
POWER CONSUMPTION IN CMOS CIRCUITS
In other to reduce the power dissipation of a CMOS circuit the various sources must be identified. There are two types of power consumption relevant to circuit design. The peak power and the average power. The power is related to the maximum instantaneous current drawn from the supply which can result in the large drop/bounces on the resistive power/ground rails. The average power dissipation dictates the battery size and weight need to operate the circuit for a given amount of time.[low power].
A typical digital CMOS circuit, there are two main classes of power dissipation, dynamic power P(dynamic) and statics power P(statics). Where the parasitic capacitance are lumped at the output in the capacitor CL, the total energy drawn from the supply E(SUPPLY) and the energy stored in the capacitor E(CAP).
ECAP=CLVDD2 /2=ESUPPLY /2 4
(1)Switching current . (2)short- circuit current.
(3)Dc current. Reverse-bias drain and sub- threshold leakage currents threshold leakage currents.
Figure3.0 CMOS circuit model for power calculation.
The switching power is dissipated during logic transistor as a result of charging parasitic capacitance of gate, diffusion and interconnect. The use of low cost standard CMOS technology for fabrication of this kind of high speed integrated circuit (ICs) will lead to integration of radio frequency (RF).[TWA CMOS].
DSIGN A SIMPLE DISTRIBUTED AMPLIFIER
The distributed amplifier is composed of two coupled lumped transmissions lines. Power is coupled from the gate-line to the drain-line through transistor, where power is coupled in the reverse direction parasitically through the feedback capacitor S21>> S12.the behaviour of the amplifier is dominated by the forward- direction coupling of power from the gate line to the drain line. The number of stages that may be used is limited by the inherent losses on the drain and gate line.
Figure 4.0 A three-stages distributed amplifiers using CMOS.
IMAGE PARAMETER METHOD
The image parameter method may be applied to the distributed amplifier since it consists of a cascade of identical two-port network forming an artificial transmission line. The image impedance Zi and other terminal is also terminated in Zi..
Zi=√Z/Y
Where the S-parameter
S11 S12
S21 S22
S11= S22 by symmetry
The propagation constant for the current and voltage is given by
е-γ = √S11 S22-√S12S21
wave propagation occur if γ has an imaginary component
Zi = √Z1Z2√1+Z1/4Z2
е-γ = 1+ Z1/2Z2+√Z1/Z2+Z12/4Z22
FigurÄ™ 5.0 T -section
LOSSLESS LUMED TRANSMISSION LINE
For a losses cascade of T- section such as that show figure5.o the propagation factor and image impedance are given by [Pozar ,1997].
Zi=
е-γ =1-2
STATE OF THE ART
(MESFET) Metal-Semiconductor Field Effect Transistor.
INTRODUCTION.
The MESFET or GaAs FET has another name which is a high performance of field effect transistor, it is used mainly for high performance microwave applications and semiconductor RF amplifiers. MESFET consists of conducting channel positioned between a source and drain contact region and also share other features with junction FET or JFET.
Figure1.0 Cross section view of MESFET http://www.ee.ui.ac.id/~astha/courses/ts/teksem/mesfet.htm
MESFET HISTORY AND DEVELOPMENT.
The MESFET can be trace back to the early days of semiconductor technology, in 1949 which was the bipolar transistor had a milestone in semiconductor technology . In 1953 was the first practical form of a field effect transistor and as a result made it simpler and cost effective to manufacture the extremely pure forms of semiconductor. In1960 oxide layers was the major work which lead to development of MOSFETs. In 1966 MESFETs were developed and afterwards the unexpected high frequency/RF microwave performance was demonstrated.
MESFET STRUCTURE
The MESFET is a form of semiconductor technology which is very similar to junction FET or JFET. MESFET has a metal contact directly with the silicon, and this form a Schottky barrier diode junction. The material used is GaAs (gallium arsenide). The substrate for the semiconductor device is semi-insulating for low parasitic capacitance, and then the active layer is deposited epitaxaially. The resulting channel is typically less than 0.02 microns thick. To make a low noise device that has good linearity, the doping is usually non uniform in a way that is perpendicular to the gate. Since most of the devices requires high speed operation then it is best to use an n-channel to dope since the electrons move faster than holes (electron mobility) the contact of the gate provides a barier that is high to reduce leakage current can be made from different materials like Aluminum, a Titanium-Platinum-Gold layered structure, Platinum itself, or Tungsten. The gate length to depth ratio is an important as this determines a number of the performance parameters.
There are two main structures that are used for MESFETs:
Non-self aligned source and drain: For this form of MESFET, the gate is placed on a section of the channel. The gate contact does not cover the whole of the length of the channel. This arises because the source and drain contacts are normally formed before the gate.
Non-self aligned MESFET structure
Self aligned source and drain: This form of structure 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 annealing process required after the formation of the source and drain areas by ion implantation, the gate contact must be able to withstand the high temperatures and this results in the use of a limited number of materials being suitable.
Self aligned MESFET structure
MESFET operation
Like other forms of field effect transistor the o MESFET has two forms that can be used:
Depletion mode MESFET: If the depletion region does not extend all the way 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 voltage is applied and is turned "OFF" upon the application of a negative gate-to-source voltage, which increases the width of the depletion region such that it "pinches off" the channel.
Enhancement mode MESFET: In an enhancement-mode MESFET, the depletion region is wide enough to pinch off the channel without applied voltage. Therefore the enhancement-mode MESFET is naturally "OFF". When a positive voltage is applied between the gate and source, the depletion region shrinks, and the channel becomes conductive. Unfortunately, a positive gate-to-source voltage puts the Schottky diode in forward bias, where a large current can flow.
MESFET APPLICATIONS
The MESFET is used in many RF amplifier applications. The MESFET semiconductor technology provides for higher electron mobility, and in addition to this the semi-insulating substrate there are lower levels of stray capacitance. This combination makes the MESFET ideal as an RF amplifier. In this role MESFETs may be used as microwave power amplifiers, high frequency low noise RF amplifiers, oscillators, and within mixers. MESFET semiconductor technology has enabled amplifiers using these devices that can operate up to 50 GHz and more, and some to frequencies of 100 GHz.
The GaAS FET / MESFET has a number of differences and advantages when compared to bipolar transistors. The MESFET has a very much higher input as a result of the non-conducting diode junction. In addition to this it also has a negative temperature co-efficient which inhibits some of the thermal problems experienced with other transistors.
When compared to the more common silicon MOSFET, the GaAs Fet or MESFET does not have the problems associated with oxide traps. Also a MESFET has better channel length control than a JFET. The reason for this is that the JFET requires a diffusion process to create the gate and this process is far from well defined. The more exact geometries of the GaAS FET / MESFET provide a much better and more repeatable product, and this enables very small geometries suited to RF microwave frequencies to catered for.
In many respects GaAs technology is less well developed than silicon. The huge ongoing investment in silicon technology means that silicon technology is much cheaper. However GaAs technology is able to benefit from many of the developments and it is easy to use in integrated circuit fabrication processes.
