OK, with the next two lectures, we're going to cover the power MOSFET. In this lecture, I'm going to introduce
what the power MOSFET is, how it's constructed, and some
of its basic characteristics. And then in the next lecture we'll talk about MOSFET gate drive circuits and
some practical considerations. Here's a cross-sectional view of a power
MOSFET, so if you take a silicon wafer and cut it, and look at it
from the side what, what it looks like. generally you see something like this, so
we have vertical current flow through the
MOSFET and really through just about every power
device, so for example the drain here is connects to
the bottom of the silicone wafer, and the
source and the gate are put on the top side. What I've shown here the cross hatched
regions represent metal contacts. So we have metal contacts to the drain and
to the source. This shaded area here around the gate is
the oxide layer, silicon dioxide or glass that insulates
the gate from the rest of the MOSFET. The clear area inside that is the gate. That is usually a polysilicon material,
but it's a conductor that it forms the gate. And then here we have the dope silicon.
this is an in-channel MOSFET, the source is
connected right here, the drain is, comes from the bottom at
this in region where there's lightly doped in
material and the drain actually, is right there, and so we have a
MOSFET that connects to the drain and source like
that. The P material is called the substrate,
and in the practical power MOSFET, the substrate is
shorted to the source. This particular structure shown here is
called the Dima structure. In which the gate and the source are built
laterally on the top surface of the MOSFET.
We also have other variants grooved structures such as Vmoss or Umoss or
sometimes known by a trade name such as trench moss are devices where grooves are
cut into the top of the side of the silicon, and the gate is actually built vertically along the
side of the groove. We also have the super moss structure,
which is a, relatively recent improvement that reduces the on
resistance of high voltage devices. And so we have some very good 600 volt MOSFETs built with, the super moss
structure. Okay, the, the power MOSFET is a highly
interdigitated device, meaning that we replicate the MOSFET many times on
the top side of, of a chip and connect these all, all
of these little MOSFETS in parallel, to get one
large high current MOSFET. So here you can see, there's a channel
right here of a MOSFET. There's another one there.
There's another one over here, another one over here, and so on, and so we pack these
MOSFETs as tight as we can together to fill the surface of a silicon wafer and
build a high-current device. Okay, so, then, current will, when a
MOSFET is turned on, current will flow in the drain, and then through the channel
and out the source. That's for positive, positive current
flow. Actually, a better way to say it, perhaps,
is we have electrons coming in from the source
to the in-material. They form a, when the gate is forward
biased, they form an inversion layer or a channel here
that effectively is more in-region that connects the source
to the drain, and then the electrons flow this way and
out the drain. In the off state, where the MOSFET is, the
gate is turned off, positive voltage is put on the
drain with respect to the source, then, we have a depletion
region formed that blocks the voltage, and really, this is the same as
in the pn diode. And in fact, if you look at the substrate,
p material, and this region right here, this actually
looks like a p n diode, as we talked about in the previous
lectures for the diode. We have lightly doped n region in the
drain that gives the device the breakdown
voltage that it requires. And so in the reverse state, we have an
electric field really across here that has the voltage drop from
drain to source. Okay? To obtain a high voltage breakdown rating,
as in any semiconductive device, we require lightly doped, high-resistivity
material. In the on state, then again, current flows
this way as I've mentioned before. A key distinguishing feature here, though,
is that the current has to flow through this in minus region and much of the on
resistance of the device appears across that region. And since it is lightly doped, it has a
high resistance. And this is the on resistance, primarily,
of the MOSFET. We do have to con, you know, put that in series with the resistance of the channel
and the resistance of the contacts and the leads and the
package, but in general, this n minus region dominates
the on resistance. And in the MOSFET, we have a majority
carrier device where there are no minority carriers to cause conductivity
modulation the way there is in, in the, diode. So this accounts for why MOSFETs tend to have a relatively high on resistance,
especially at high voltage levels, and so a high voltage
rated device will have to have an especially
high resistance, n minus region, and the on-resistance goes
up quickly with voltage. So we have very good MOSFETs at low
voltage, but at high voltage, we start to prefer minority carrier
devices that can give lower forward voltage drops. Okay. The MOSFET has this body diode which I
mentioned, last week. The body diode actually is, comes from
this PN junction between the substrate P material
and the drain. You can see there is a PN junction there,
and it's a real diode. So effectively, we have our MOSFET
represented by the channel, and it's in parallel with the
body diode. Okay.
So this is a real diode. It has reverse recovery it can conduct
current if it's forward biased. And that happens if we put plus on the
source and minus on the drain. Then this diode can become forward biased,
and current can flow this way. the diode can conduct the full rated
current of the MOSFET. After all, it is the same device. however the, the caveat is that, the diode
is not necessarily optimized for fast switching speed, and it
depends on the diode. Some MOSFETs are designed to have on
purpose a fast recovery diode, and others are not. Here are the static IV characteristics of
the MOSFET. This is drawn in a little different way
than you might be used to seeing, and it's drawn to
emphasize how to control the MOSFET. So the vertical axis is the drain current. The horizontal axis here is the gate to
source voltage. And we have characteristics or lines of
constant drain voltage. So basically, if the gate voltage is below
a voltage known as the threshold voltage, then the MOSFET is
off. And the gate driver has to bring the gate
voltage, or gate to source voltage, higher than the
threshold to turn the MOSFET on. so here as you can see, as we raise the voltage above the threshold, for the first
few volts, the MOSFET starts to turn on, and when it up here at
maybe ten or 15 volts, the MOSFET is all the way
on. And then we have, you know, for a given
drain current, we have a low drain to source voltage. So, we generally will operate the MOSFET
turned on over here where we have a low voltage and in this
case, the drain to source voltage is approximately proportional to the drain
current, and we approximate this as being, saying the drain voltage is the
drain current multiplied by the on-resistance of the MOSFET. 'Kay? in minor in majority carrier devices like
this we typically have a positive temperature
coefficient of the on resistance. Which means, as the device gets hot, the
on resistance goes up. So, first of all, this makes it easier to
parallel MOSFETs. They don't have this mechanism, this
positive feedback mechanism that will make one device hog all the
current. On the other hand, the on resistance is
highest at high temperature. And when we choose an on resistance, we
have to be cognizant of that, and think about what in the worst case, what
is the hottest the MOSFET will run? And what is the on resistance under those
conditions? Here's an equivalent circuit of the MOSFET
to first order, showing the capacitances. So a MOSFET has capacitance from each
terminal to each other terminal. And we'll talk about what each of these
is, one at a time. So first of all, the gate to source capacitance is this ideal capacitor of the
MOSFET between the gate terminal and the channel,
and it's draw here with respect to the source. So this is the capacitance across here
from gate to channel. And, we charge up this gate, the source
capacitance, to put charge in the channel and turn the
MOSFET on. You know, in addition to that, you can see that there's overlap between the gate
contact and the drain. And this forms what's called the gate to
drain capacitance. Okay, we do what we can to make that capacitance small, but in fact,
charging it up does help put additional charge in the drain region and somewhat reduce the on
resistance. Typically the gate to drain capacitance is small compared to the gate to source
capacitance. So it's a, it's a, a lot fewer picofarads
of capacitance. However, it has a lot of voltage across
it. The gate to source capacitance might get
charged up to ten or 15 volts, whereas the gate to drain capacitance,
gets charged up to basically the full drain
voltage. And so although the capacitance is small,
the voltage across it is large and the amount of charge in
this capacitance can be large. In fact, in some devices, it's larger than the charge in the gate to source
capacitance. This has some impact on and considerations
on how we design the gate driver circuit. And we're going to talk about that in the
next lecture. The third capacitance is the drain to
source capacitance across here, and this is basically the capacitance of the, the body diode.
It's really capacitance from here to here. And this is the same capacitance that the,
this PN junction diode would have. and it behaves a lot like a diode
capacitance. It's, it's nonlinear. We didn't talk about this with respect to
the PN junction diode, but it's actually a
similar kind of function. Here's a classic formula for a depletion
capacitance and it's a non-linear device that where the capacitance varies roughly
as the inverse square root of the voltage. So as the voltage goes up the capacitance, goes down.
The incremental capacitance, and we get this inverse square root formula. ' Kay, these output capacitances of
devices, such as the drain to source capacitance of the MOSFET, and the junction capacitance of
the diode are another mechanism that can lead to
switching loss. Basically, these capacitances get charged up with energy when there's
voltage on it, across them. So, for example, when the MOSFET is off,
we have vg across here. There's no voltage across the, the diode
junction capacitance. But as far as AC signals are going, go. The junction capacitance is effectively in
parallel with CDS. And the two add together. When we turn the MOSFET on, we short out
these capacitances. And any energy stored in them gets dumped
into the channel of the MOSFET during the turn
on transition. So if this is a linear capacitance, we can
say that. These capacitances store energy equal to
one half cv squared, where heat, here C is the effective capacitance
if these were linear capacitances. Of the, the output capacitances of the devices. And vg is the, the voltage at the input to
the converter. So we lose this much energy, everytime we turn the MOSFET on and short the
capacitances out. Okay, so this would cause a switching lost that is equal to this energy stored in this capacitances, multiplied by the
number of times per second that we short out the capacitances, or multiply it by the
switching frequency. So this is another mechanism for switching
loss. If we want to account for the fact that the capacitances are nonlinear, we
can integrate the power that charges up the capacitances
to see how much energy the nonlinear
capacitors store. So we can integrate the power which is
voltage times the current, and plug in the expression
for I is CDBDT for this capacitor, and when we
integrate the square root function, we find that the energy
that is lost is, is equal to this. And it has a 2 3rds factor instead of a
one half factor. And we evaluate the capacitance at the off
state condition of the MOSFET. So, that makes us actually, lose a little
more ca, energy. Instead of one half C V square, we
actually have 4 3rds times as much or 2 3rds CV squared. 'Kay.
Conclusion. So a MOSFET is a majority-carrier device. That's good news because it has very fast
switching speed. We don't have all the time that it takes
to, to remove all of this stored charge that was causing
conductivity modulation in the diode. And so MOSFETs can switch much faster, and
in a MOSFET that is, you know, it's typical to get, you know,
switching times of a few tens of nanoseconds and in
low-voltage devices, even faster. so typical switching frequencies, tens and
hundreds of kilohertz are easy, and we have, You know, low
voltage, computer type power supplies that run in the
megahertz of switching frequency range because of the high speed
of these devices. The bad news though is that on-resistance
increases rapidly with the rated voltage, because these are majority
carrier devices, and don't have conductivity
modulation. So we have good, super junction MOSFETs at
600 volts, but we can't go much higher than that, before the, the
on-resistance has increased too much to where we- it's just not economical to use the device.
They're very easy to drive. We simply switch the, the gate to source
voltage between zero and ten or 15 volts. and very easy to control, then, when
they're on and off. we also have logic-level MOSFETs that are
designed to operate with a five volt gate voltage instead of
ten or 15. Okay, one other thing, these devices
generally are loss-limited. We, when we design a power converter we
generally don't select the MOSFET based on the
current rating. Which is a peak current rating of the
device. Instead, usually we select them on the basis of on-resistance and conduction
loss. So often we will choose a device with a peak current rating much higher
than we intend to use. And instead, we choose the MOSFET size to
get a given low on-resistance as needed in our
application. So, as a result these are very rugged
devices, as well, that have quite a bit of margin, when it
comes to current.