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Good morning so today we will start millimetre
wave activity devices Now different applications they need different types of requirements
For example one satellite let us say geo stationary satellite it is 36000 kilometres above earth
surface so obviously it will need very high power to communicate Now mobile phone it needs
maybe milliwatts that should be sufficient but for mobile base stations it is a few watts
so different applications have different types of requirements Now for any millimetre wave
system we need millimetre wave sources and for any amplifier design we also need different
types of amplifiers power amplifiers low noise amplifiers oscillators switches so all involved
active devices So we will see different types of popular active devices how they operate
what are their characteristics so let us start with the basic chart So in general microwave devices they can be
divided into 2 parts solid state devices and microwave tube based devices Again solid state
devices they can be categorised according to their working principles into 4 major categories
transistor field effect transistor transferred electron devices and avalanche transit time
devices So under transistor we have bipolar junction transistor HBT tunnel diode et cetera
Under field effect transistor we have JFET MOSFET MESFET HEMT and different types of
memories and CCDs Under transferred electron devices we have gunned diode LSA diode Indium
Phosphide diode then (diff) different other types of drives and under avalanche transit
time devices we have READ diodes IMPATT diode TRAPATT diode BARITT diode so they are categorised
according to their working principles So among all different types of active devices
popular are HEMT (High Electron Mobility Transistor) Gunn diode and the IMPATT diode So in todays
discussion we are going to discuss about these popular active devices and under now microwave
tubes we have 2 different types of tube linear beam tubes or sometimes we call simply O type
tubes and Crossed field tube or sometimes also called M type tube So we are not going
to discuss about the tubes so mainly we will concentrate on active devices solid state
devices So here is one chart which relates the average
power versus frequency so if we look at this chart where the this comparison uhh we are
doing among different types of tubes and different types of solid state devices are being considered
here So if we look at this chart for any given device if we increase frequency maximum available
power it decreases And not only that the high frequency parts and high power together these
are mainly dominated by tubes but the problem with tubes is that they are expensive and
bulky so we try to avoid them in low cost uhh applications And now looking at the different
solid state devices we have again solid state devices based in gallium arsenide technology
gallium nitride technology and uhh also different examples are HEMT BJT so again for these solid
state devices also power handling (capa) capacity it decreases with increasing frequency And if you look at this plot not only that
so gallium nitride usually it provides high power and gallium arsenide based devices they
are used at much higher frequencies So the stop curve it belongs to Gyrotron but the
problem with Gyrotron is its size and cost For example Gyrotron is shown here it occupies
almost one building so you can compare the size one with 1 human being Now Microwave power requirements so in this
chart we see that different applications they have different types of power requirement
and they correspond to different frequency bands So right now at millimetre wave frequencies
mainly we have defence and space locations so consumer applications is very much limited
but with the incoming 5G Wireless application it might be at millimetre wave frequencies
may be at 60 gigahertz so it is expected that millimetre wave activity devices will be used
for this type of communication Now for solid state devices gallium nitride based device
it can provide more power compared to other solid state devices and this is the theoretical
limit for gallium nitride devices and the 2nd line it shows the theoretical limit for
gallium arsenide devices So we will see which parameter determines high frequency operation
and uhh its maximum power capacity Now here are some more examples of microwave
and millimetre wave power uhh sources So in most of these applications till now we have
many different types of tubes they are being continuously used but the disadvantage of
these tubes they are of large size they are bulky usually they operate at fixed frequency
so frequency tune ability it becomes a problem with them And not only that they are usually
narrowband and it has also spurious spectrum with that so for example our in our today
use we use microwave oven usually it uses a magnetron to generate microwave power at
frequency 2 point 45 gigahertz Now in this plot I am showing you microwave spectrum of
1 magnetron so you can see the pic it is here around 16 point 5 gigahertz but it is associated
with many sidebands So we have many spurious bands so the quality of wave form spectrum
it is very poor and another disadvantage is its high cost So then going back to solid state device so
2 points become very important one is power handling and another one is high frequency
operation Now if we compare different types of solid state devices which are already available
in market so here is the chart So in the 1st figure we are showing the output power versus
frequency so if we increase frequency maximum output power from the device it decreases
and again we can see that gallium nitride based HEMPT it provides maximum power whereas
gallium arsenide based PHEMPT it provides high frequency operation We also have indium
phosphide HEMPT it also goes beyond 100 gigahertz Now another important parameter is power density
considering component miniaturisation so how much power is available per square millimetre
versus frequency Again gallium nitride based devices are the
winner but their frequencies are much limited to let us say 30 to 40 gigahertz So if we
want to use them at higher millimetre wave frequencies then again we have to go for gallium
arsenide based devices but gallium arsenide based devices it has again one problem compared
to silicon based devices that it consumes much power so we will discuss this point later
in detail So some of the popular devices their applications
and frequency limitations so for example IMPATT device they are typically used below 300 gigahertz
so it can cover the whole millimetre wave spectrum starting from 30 to 300 gigahertz
This so this frequency whatever is mentioned here uhh it is it represents the popular applications
so but there are examples where IMPATT diode has been used above 300 gigahertz and physical
substrate used for IMPATT diode fabrication are silicon gallium arsenide indium phosphide
and they are popular for transmitter amplifiers So in high power amplifiers where we need
power amplifier we can use IMPATT diode but they are not much popular as a source millimetre
wave source because they are associated with phase noise we will see later Next is Gunn diode typically used below 180
gigahertz substrate gallium arsenide and indium phosphide It is a very popular millimetre
wave source which we use in laboratory experiments in the universities so it is popular in local
oscillators and also it can be used it transmitter amplifiers Next FET and HEMT typically used
below 140 gigahertz again based on gallium arsenide and indium phosphide and they are
widely used in different types of amplifiers oscillators switches mixers mixers phase shifters
so different types of applications Next is p i n diode typical frequency below
100 gigahertz and the materials used for fabrication silicon gallium arsenide p i n diode is mainly
popular in switching applications and at millimetre wave frequencies sometime they are also used
as variable resistor Next Varacter and it can give variable capacitor so where we need
any tunable component we can use varactor and we can electronically tune the capacitance
of a varactor diode Typical applications multipliers tuning phase shifters and different types
of modulators Now how to choose the material for any given
solid state devices There are several parameters when we go for millimetre wave frequency applications
the frequency is so high that the signal is changing very fast So device it should be
narrow enough so so that carriers from left side to right side it takes minimum time typically
less than the time period of the given signal so we characterise it by transit time So then
the 1st conclusion is the device size should be small whatever carriers we are using here
so carrier velocity should be very high Now carrier velocity inside the substrate material
it depends on many parameter it depends on electric field it depends on mobility electron
mobility and hole mobility so electron mobility and hole utility it is a it depends on the
(sub) type of substrate So depending upon our application requirement we can choose
a proper substrate for fabricating the solid state devices So here are some examples gallium arsenide
substrates they are used because of its high mobility Silicon substrate the fabrication
procedure uhh it is very low cost and also high yield so for consumer market silicon
substrate is very popular Gallium nitride substrate it is mainly used for high power
applications but their high frequency application is limited typically they are used below 30
or 40 gigahertz Now we see the characteristics of some popular semiconductor materials so
we are comparing the band gap energy and mobility at room temperature 300 Kelvin So if we look at the silicon it has a band
gap of 1 point 12 electron volt at room temperature whereas for gallium arsenide it is 1 point
43 so the band gap value is higher than means its power handling capacity will be higher
Now look at the mobility values so if I look at the electron mobility silicon electron
mobility is 1600 centimetres square per Volt second whereas for gallium arsenide 8500 So
obviously for gallium arsenide based devices electron mobility will be much higher under
a given electric field and we can increase (())(15:24) of the device we can go for high
frequency operations with gallium arsenide based devices So some important parameters how we characterise
any solid state device so for example output power or what is the maximum power available
from the device power density what is the maximum frequency of operation then Power
added efficiency so these are the mostly well used parameters So output power P max it is
proportional to V max multiplied by I max where V max it represents the breakdown voltage
and I max it depends on how fast we can remove heat from the device also gate width and length
because the resistance depends on it Next is Power density power density is equal to
V max current density so V max is the breakdown voltage and current density it is limited
by the band gap and thermal conductivity Next high frequency operation F max it is
proportional to V s by L where V s it is the saturated carriers velocity and L is the gate
length for a given let us say gate and P max it is proportional to 1 by F square so if
we increase the frequency it is expected that the power will decrease very fast so what
we seen in the previous plot Next is Power added efficiency so here what we do let us
say any given solid state device we are using in amplifier applications so we will be having
RF good power and then the amplified RF output power And to amplify the signal we have to
apply some energy to the device and this is being done by DC source Now how much power it will absorb from the
DC source and what would be the efficiency of the device we call it the power added efficiency
and it is defined as 100 multiplied by output RF power minus input RF power divided by P
DC total total DC power what it consumes So it depends on many parameters such as wave
shape what is the impedance of the device then what is the leakage current and power
gain of the device Now let us start with the 1st device Bipolar
Junction Transistor so bipolar junction transistor also we used in basic electronics lab It has
very similar principle and only thing is that (in) whenever we use for low frequency applications
let us say at megahertz frequency we do not use different capacitances offered by PN junctions
We have two PN junctions in BJP but for high frequency applications we have to consider
all these capacitances and the device also is associated with some inductance because
of its leads we have to also consider the effect of inductance so high frequency model
it will be little different than what we use at lower frequencies So let us look at one BJT so this is a typical
BJT used at lower millimetre wave frequencies so you can see the base emitter and collector
so the bottom most portion it is called the sub collector and actual collector it sits
over the sub collector and on top of this collector we have a thin layer of base Gain
of the transistor it depends on base width thinner is the base we will have higher gain
so look at this plot then (act) this base lead it is connected to this thin layer and
over it we have emitter and collector connection is coming from right hand side So for high
frequency operation to we have to decrease the transit time and if we need to decrease
the transit time we have to reduce the base width Now looking back to basic operation of BJT
I will skip the very basic things only I am going to discuss the limitations which arise
at higher frequencies so let us consider one NPN type BJT Usually an NPN type BJT is popular
at higher frequencies since it involves mostly electron and electron mobility is higher compared
to whole mobility so emitter it will eject electron and for normal operation we will
forward bias the base emitter PN junction and we will keep the base collector junction
in reverse bias condition So due to the applied electric field then electrons drifts into
base where we have recombination with the holes available in base and then rest of the
part we have 1st diffusion and then drift inside the collector Now for high frequency
operation we have to decrease the transit time so that means we have to decrease the
base width But if we will decrease base width the problem
we will be facing that (base ret) based resistance will become very high so we have to avoid
this problem so how we can avoid them this (resita) high resistance problem One solution
is that we can increase doping inside base but if we increase doping inside base than
the hole which drifts into electron from base that part will increase so the reverse situation
current will increase so then we just cannot keep on increasing doping concentration inside
the base so that is why conventional BJP it is not used at millimetre wave frequencies
We have some other version which can take care of this diverse saturation current and
this modification is called Hetero junction bipolar transistor or HBT that comes just
after this So HBT modelling the 1st model this is uhh
it shows low signal low frequency model you can see here we are just considering r Pi
the current roles g m V be and output resistance R 0 and we are not considering any capacitor
this is the low frequency model Now if I go back to device we have a forward biased PN
junction for this base emitter junction so forward biased PN junction it is associated
with some capacitance we also have reverse biased base collector junction it is again
associated with some capacitor and usually reverse biased capacitance is smaller than
the forward biased capacitor so we have to consider all these capacitors their typical
values of the order of Pico farad at low frequency that is why we avoid this but at millimetre
wave or at microwave frequencies we cannot avoid their effect So then the modified circuit how it looks
here shown here In addition to the previous one we have introduced two more capacitors
C be it represents the base emitter forward biased PN junction capacitor and C bc it represents
the base collector uhh reverse biased PN junction capacitor and this is the internal model of
BJP In addition to this we have packaging effect we have external leads so we have to
add more resisters capacitors and inductors for that will take into account the effect
of packaging effect of the leads Next high frequency limitations so high frequency
limitations it depends on many parameters and the condition sometimes we call it junction
condition so it depends on the following parameters saturated grip velocity So carriers inside
the substrate it has some velocity under given electric field if we keep on increasing electric
field value then this carrier velocity will increase but finally it will reach a saturation
which after which it cannot increase for silicon or germanium or this type of materials semiconductor
materials We call that velocity as the saturated velocity V s but there are another categories
of material usually group 3 group 5 semiconductor conductor materials like gallium arsenide
So for this type of materials after a half highest velocity V s if we keep on increasing
electric field then V s decreases So we will consider the maximum velocity of
carriers V s and then next is the dielectric breakdown dielectric breakdown it depends
on applied electric field and it is the property of that given dielectric so let us called
the break down electric field is E m then maximum current it is also limited by the
base width So considering all that effect if we plot current gain for a BJT it becomes
a function of frequency So here in this graph we are showing the plot of HBT for current
gain or sometimes we call it Beta So typically you see at lower frequencies it is fairly
constant but at higher frequency decreases and at a frequency current gain it becomes
one or unity we call that frequency f T of the device so it mainly depends on the capacitance
value Now let us see some parameters which determine
its maximum frequency of operation so the 1st one is voltage frequency limitation So
here V m is the maximum allowable applied voltage device it is given by E m multiplied
by L minimum so L minimum is distance between emitter and collector and V m multiplied by
f T where f T this is 1 by twice Pi Tau transit time cut off frequency it is equal to E m
V s by twice Pi So E m that is the maximum allowable electric field and V s that is this
saturated trip velocity E m V s by twice Pi then we see that it is related to the maximum
allowable applied voltage multiplied by f T so if we increase frequency then maximum
allowable applied voltage it decreases We can also express this quantity E m V s by
twice Pi in terms of current frequency in terms of power frequency so simply we have
to replace the voltage by the corresponding expression of current and corresponding operation
for power so it is shown in next slide So current frequency limitation here I m into
X c multiplied by f T equal to E m V s by twice Pi where I m is the maximum current
of the device and X c this is the reactive impedance of the device so it depends on f
T and mainly based to collector junction capacitance then the power frequency limitation so square
root of P m into X c multiplied by f T equal to E m V s by twice Pi We also can define
power gain frequency limitation so G m V th V m whole square root multiplied by f T this
is equal to E m V s by twice Pi where G m this is the maximum available power gain and
V th this is the thermal voltage so it depends on room temperature so what we see then gain
of any BJT it becomes a function of frequency At higher frequencies we will aspect lower
gain and not only that it is also a function of temperature So we will take a break and
then we will move to next topic HBT thank you

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