In this lecture, we will discuss the Bipolar Junction Transistor or BJT, and its modern successor, the Insulated Gate Bipolar Transistor, or IGBT. Here's a sketch of the Bipolar Junction
Transistor. it, it's an npn device. And we have lightly doped n region or intrinsic region in the collector, so that
this base collector junction, pn junction really has
the same structure as the classic power diode that we've
already discussed. So, under reverse biased conditions, where the collector voltage is
positive, the base is more negative then the collector and so is
the emitter. And in fact for, to turn this device off,
the np or pn junction between a base and emitter is
reverse biased and turned off. So, under these conditions the device is
off. And the electric field is in the n minus
region that is able to block the, the collector voltage. this is a vertical device, and again this
is a cross-sectional area of the transistor or the chip, like the
cross-sectional areas we've been drawing previously for
the MOSFET. So, to turn this device on, what we do is
we have a a driver circuit, connected to the base and emitter that will forward
bias the base emitter junction. So, apply a positive voltage at the base with respect to the
emitter to forward bias this junction and when we
do that, electrons from the emitter are able to
diffuse across this junction into the base, where they
become minority carriers. because the collector is, is more positive
voltage these electrons can easily continue to, to flow and they'll actually
flow into the collector region. So by forward biasing the base emitter
junction, we get the minority carriers into the base and into the n minus region that are needed to conduct the collector to emitter
current. In the process, you can see that these minority carrier
electrons can go into the n minus region and cause conductivity modulation that
effectively reduces its on-resistance, and so Like in the diode we can this
connectivity modulation gives us the best of both worlds as far as
high voltage breakdown plus low an, on-resistance. But the price we pay is that we have a lot
of stored minority charge in the n minus region, that has to be removed
if we want to turn the transistor off. Okay, so to turn the transistor off, when we're in this situation, we have two
choices one is we can simply reverse bias the pn base emitter junction, with zero base
current. And wait for these charges to recombine
and eventually they will, and at once they do then the transistor
will turn off. But the other thing we could do is
actively remove the, the electrons through the base
terminal. Actually we put, we would pull, the electrons backwards with a negative base
emitter current, that will remove the stored
charge in the base and in n minus region. So, [COUGH]. So, here's a sketch of what, the typical
waveforms look like.
Here we have a, a base drive circuit, that I'm representing with some thevenin
equivalent voltage source and resistance. here is the voltage of that base driver
voltage source, it starts out negative, which reverse biases the base emitter junction and turns the transistor
off. at this point our driver voltage goes
high, to turn on the base emitter junction and you
find the base emitter voltage then of the base
emitter diode goes forward and goes to plus 0.7
volts. then the transistor turns on, so it has
low collector to emitter voltage and a high
collector current. And, and it's in the on state.
To turn the transistor off actively, what we do is we make our driver actually
go to a negative voltage, and with that negative voltage, we actively remove,
the store charge out of the base. Okay, and so the base current that we get,
base current waveform will go negative. And actually look a lot like the reverse
recovery current of a diode. It's very similar in shape, so you get a large negative current to
actively remove the stored charge. During this time, the base emitter voltage
is high. for at least part of the reverse recovery
time eventually the voltage will go low and at that point we'll see
the transistor fully turn off, [COUGH]. so in order to switch the transistor off
quickly, we need this large negative current and so an ideal base
current waveform then looks like this. We have a large positive current initially
to supply the charge to the base and turn it on,
then we have a small maintenance current to
supply recombination in the base of the minority carriers,
maintain the minority carriers in the base, and the collector n minus
region. And then to turn the transistor off, we
have a large negative current coming out of the base to remove
the stored charge quickly. Finally, once the transistor is off, we
have no base current. Okay, one thing you might ask is, well,
why can't we just build big, big base current driver
that can pull the all this stored charge out very
quickly and turn the transistor off just as fast as a
MOSFET. Well first of all it takes a lot more
current than a MOSFET, but second there are limits to how large
the base current can be. Very large negative base currents during the turn-off switching time lead to a
failure mechanism that is traditionally called a second breakdown mechanism that
comes from current crowding. So, here's a, here's an example or a
sketch of what happens. So, we have a lot of stored charge in the
n minus region and in the base p region and we're going to
pull current out of the base, [INAUDIBLE] and pull that stored charge out. So, you can see the way the base and
emitter contacts are built. A big negative current requires that we have lateral current flowing through the
base region. And, really, through the n minus region,
also. this lateral current causes a voltage drop
in the resistance of the silicon. So, the silicon material has some
resistance. And so, from the direction of the current
you can see that the voltage in the base would more positive in the
center, than it will be at the edge. And what that does is it makes the base
emitter junction be more positive in the middle than it is at
the edge and so during this time, the collector current
will tend to want to flow there, in the center. So, we get what's called current crowding. And that current crowding heats up the
center part of the base emitter junction more than it
heats up the edges and we get this phenomenon where the
center part of the, the base emitter diode junction,
will hog the current. It can even go into thermal runaway, as we discussed previously with, the case of
trying to parallel diodes. Effectively this, you can think of this is
like a distributed diode. And we want the current to, to flow
uniformly across the whole region but in fact here we're making it
crowd in the center. And so this part will run hotter than the
rest. And, it will tend to hog all of the current, and this can make the
transistor fail. So, this is called current crowding. If you have a transistor that fails you
can actually cut it open and look at it under a
microscope. And see that it failed at the, in the
middle of the emitter and you know then the engineer get's told that there,
there base, negative base current at turn-off is
too large. You can do a similar thing like turn on,
where the current is going in the other direction and then in that case it can make the current all crowd at the
edges of the emitter. And you can see that with a microscope
also. So, this is called a second breakdown
mechanism that causes BJTs to fail. And the effect is that it limits, how fast
we can switch the practical BJT. Another different characteristic of the
BJT relative to the MOSFET, is that the, at high current, the BJT runs
out of gain and we're not going to go into the various reasons
for this, but they're well understood. And so this is a plot of base current
versus collector current. For an actual BJT, power BJT that is rated
uh,10 amps and 600 volts. And so, if the BJT operates with a constant current gain beta, then you would
expect the base current versus collector current, to
be a straight line with a slope equal to that current
gain. But you can see that at high values of
collector current, the slope becomes low and the transistor
runs out of current gain. And this is classic for any BJT.
For this particular device, it's rated 10 amps, and you can see that
you can barely get 10 amps out of this
transistor at all. That's really the limit. And so the current rating is determined,
really, by the this gain characteristic of the
transistor. with a MOSFET this doesn't happen a, a 10
amp-rated MOSFET will have curves like this that extend up to 20 and
30 and much higher currents. And the MOSFET then is actually thermally limited. So, you can run a MOSFE, a 10 amp MOSFET
at 20 amps for a very short time, as long as the average is low
enough and you don't over heat the device. so this loss of current gain provides
another mechanism but that limits the power that we can get
through the BJT. the Darlington-connected BJT is one of the
classic kind of early approaches to improve the gain of high
current or high voltage BJTs. And at high voltage it's difficult to get
high current gain at the same time, so a 1000 volt BJT might have a
current gain of only a few. and a Darlington-connected BJT gives us two transistors worth of gain, and the,
the total gain from the input base current of
the first transistor to the collector current
of the combination, now is the product of the two
betas. So, this, this is one of the traditional
ways to build a high current or high voltage BJT, the one problem with driving this Darlington-connection
comes at the turn-off. So, if we have a, say another base driver,
we connect here. And we do the same thing with it that we
did with the single BJT, a couple of slides ago, where
to turn this device off, we pull a negative current out of the
base. in the, if we just had Q1 and Q2 with no diode, what happens is that our base
driver will turn Q1 off quickly and then once Q1 is
off, there is nothing to actively remove the
stored charge in Q2. So, we have to wait for the, the minority
charge in Q2 to to recombine, and its switching time can be long. [COUGH] so we have a lot of switching loss and we
have slow switching times. So, this diode, D1, is added to, to fix
that, so that when our gate, our base driver turns off
Q1 quickly, we still have a path to remove the stored charge from Q2
through the, through D1 and our base driver is able to turn-off
Q2 quickly as well. 'Kay, so the BJT is one of the classic
power transistors. back when I was a graduate student the BJT was the workhorse of the power
electronics business. [COUGH], and then it was replaced by the MOSFET,
which became commercially significant in the 1980s and so in low voltage
applications, say 600 volts and below the MOSFET, really
is the device of choice today. Then at higher voltage applications, the
BJT, it's not just is being, it has been replaced by the IGBT
for voltages above 600 volts. And so we're going to talk about the IGBT
next. compared to the MOSFET, the BJT ha, is slower switching because it's a minority
carrier device, but its on-resistance for the same chip size
is a lot lower than that of a comparable
voltage MOSFET. 'Kay here is the IGBT really this is a combination MOSFET and BGT and here's a
sketch or a cross sectional drawing of it and
this actually looks exactly like the drawing
for the MOSFET. There's only one small difference that
this region right here is p instead of n. so there is in fact an extra pn junction
in series with what we called the drain of the MOSFET that now is traditionally
called the collector. so the insulated gate bipolar transistor
has a gate just like a MOSFET, the source of the MOSFET is called the emitter in the
IGBT and the drain now is called a collector, and we have this extra pn
junction. Okay, what the pn junction does, is under normal
conditions where the device is turned on, we have current flowing this way
across the pn junction. And we have holes from the p material, that diffuse across the pn junction and go into the n minus region and cause
conductivity modulation. And so, these holes, when the device is
turned on, give us minority carriers that reduce the
on-resistance of the device. So, we, we get a much lower on-resistance
in the n minus region than we had in the comparable
MOSFET, although we do have an extra pn junction in series that
has its forward drop. But what this let's us do in practice is
to, design IGBTs with very high voltages and have much lower on-resistances than
the MOSFET would, would have. So above 600 volts, the on-resistance of
the MOSFET goes up very quickly, but by doing
this trick to get minority carrie inj, carrier injection, we can control that mod, that
on-resistance. And so we have 1200 volt and 1700 volt
devices today that are very good and, and very
significant commercially. And we have much larger voltage devices
also for, for high-powered applications with, with breakdown voltages
of in the 2,000 and 3,000 and even higher range. 'Kay, so effectively, the IGBT looks like
this. We have a MOSFET, channel in the usual
place, the same place as in the MOSFET, and then we have this pn
junction, and to first order you might say well, we have a
diode in series with our MOSFET, but what actually happens is a
little more complicated than that. If you look at the, the structure, the
substrate p material that is shorted to the what we're calling
the emitter now, actually gives us a pnp transistor. and so a more accurate equivalent circuit
for the IGBT is this. It has a MOSFET and then the MOSFET is
connected to a pnp transistor and we can actually get two
different components of current flow. They're called i1 and i2 here.
So we can have current flow this way through the channel of the MOSFET and out,
that's called i1. Or we can have current flow this way, just
through the pnp transistor and bypassing the MOSFET
channel and that's called i2. And effectively the MOSFET drain current
supplies the base current to the pnp transistor. Okay, so, the effect of this on a switching affect the turn-off switching
performance, is this our [INAUDIBLE], our gate driver can turn the MOSFET off
quickly which makes i1 go to 0. But we still have minority charge in the n
minus region that will allow the pnp to continue
to conduct. And so, i2 continues to conduct.
And, in fact, there's no way to actively remove that stored charge from the n minus
region, so all we can do is sit around and wait for the minority charge to recombine. And once it does, then finally the pnp
will turn off. So, the IGBT has this fairly slow turn-off
mechanism as a result, and that turn-off mechanism is traditionally
called current tailing in the IGBT. So, the turn-off current waveform looks
like this. The originally when the device is on, the total current
flowing this way is some combination of i1 and i2. And when we turn the device off, we
turn-off the MOSFET quickly, and so i1 goes away quickly, and
you see that, the current decrease by the switching off of the
MOSFET to this point. But after that, i2 continues to flow and
the fraction of the collector current that, that was i2, slowly decays while the
minority carriers decay. And eventually the device turns off. And so, this long time it takes the i2 component to decay is called the
current tail. Okay, so the turn-off switching transition
of the IGBT can be significant. It depends on the size and the, the rating
of the IGBT, but it may be several microseconds or in a good IGBT it may be,
say, several hundreds of nanoseconds. So, if we multiply the collector to
emitter voltage by the collector current, we get the power loss in the transistor
during this turn-off transition. And it has some area, that is the, the
energy loss during this turn-off transition and we get switch and loss then
equal to the, the energy loss at, during the turn-off tail times the
switching frequency. And in fact there is some energy loss
during the turn-on transition too, so we really ought to add the
turn-on, energy loss also. [COUGH] on data sheets, they will actually usually
specify values of the these turn-on and turn-off energy
losses. There are some things we can do to control
this current tail. One thing we could do is in fact, in fact
by designing the circuit we can actually
control the current gain of this bipolar transistor and we can, can
control how much of this current flows in the i2 path versus
the i1 path. So, this is a design variable that the
device designer has to play with. So if we make most of the current flow
through the MOSFET and very little flow through the BJT, then
what that does is it makes this the i1 portion be large and the i2 portion
be small. And the current tail is smaller, so we'll get less energy loss during the turn-off
transition. And so the designers can make a faster
IGBT that way, but the price they pay is that the forward voltage drop is larger because
more of the current is flowing in the MOSFET
direction and less in the collector direction. So, another thing that, that is often
done, is to make the opposite choice. Which gives us more minority charge,
stored charge. It makes the device switch off slower but
it gives us a smaller forward drop. And often many manufactures even offer,
two versions of their devices. One that is optimized for forward drop, and the other that is optimized for
fast switching times. And, so you have to decide which one will give you more efficiency in in your
application. Okay, so the IGBT really is the modern device
of, of choice and really at voltage's above 600 volts and we have very large IGBT modules.
these modules are composed of multiple individual chips, where the individual
chip might be rated 50 amps or 100 amps. multiple chips like that are packaged
together on a common thermal substrate to make them, run at the same temperature
and share the current. and so then we have a module of many of
these chips that, that module might be rated at hundreds of amps or even
1,000 amps, and at high voltages. You know, possibly several thousand volts. So, these are very good high powered
devices. They can be controlled simply by switching
their gate voltage from zero to 15 volts and
back. And so you can control a lot of power with a simple MOSFET like gate. the forward voltage drop for this devices is a diode in series with an
on-resistance. So, it's the on-resistance of the n minus region, in series with the base emitter,
or not base emitter, but the pn junction
voltage drop that is in the collector of the
device. the temperature coefficient of these
devices is a combination of MOSFET and BJT behavior at
low current the forward voltage has a negative
temperature coefficient, but at high current the
MOSFET on-resistance dominates. And we have a positive temperature coefficient, which means that the devices
tend to share as you get close to the rated
current of the device. the devices are slower then a MOSFET, but
they can be faster then a Darlington or an SCR or gate
turn-off thyristor. you see it's switching frequencies for
the, the low forward drop devices might be a kilohertz or several
kilohertz and for fast devices Sometimes they're run at 10s of kilohertz
and there are some, you know, lower voltage very fast devices that are,
can even be run at a 100 kilohertz or more.