I have worked with quite a few MOSFET packages over time but I always wondered that why is the drain terminal of the MOSFET physically bigger compared to the source terminal?
Example the Toll, TO-252-5, TDSON, HDSOP-16-2 (this guy has a top cooled which is internally connected to drain).
Why is this so? Is there any MOSFET that has a bigger source terminal compared to its drain?
why is the drain terminal of the MOSFET physically bigger compared to the source terminal
It has to be (irrespective of MOSFET construction type) for most devices.
This is because the drain region usually gets hotter than the source region and, it needs more heatsinking because of this. It's especially so when the MOSFET is operating in its saturated region and, less so when operating in its triode region. The drain tab has therefore more surface area to conduct heat away. It's down to the channel pinch-off occurring primarily much closer to the drain than the source: -
Basic image from What is Channel-Length Modulation in MOSFETs? with my addition in red/blue. As you can see the channel gets naturally more restricted (pinched) towards the drain hence, for a given drain current (source current) the volt-drop of the channel per fractional-distance is much greater at the drain end of the channel.
This is why MOSFETs are built that way.
Is there any MOSFET that has a bigger source terminal compared to its drain ?
I'm not aware of one but, there's sometimes the odd exception to the rule.
There are plenty of ways to make FETs. One way to distinguish them is lateral vs vertical:
Lateral MOSFET: current travels parallel to the plane of the semiconductor chip.
Vertical MOSFET: current travels through the flat semiconductor chip, from one side to the other side.
Take any wire, the thicker it is, the less resistance it'll have: resistance is inverse proportional to conductor cross section. Same thing happens with MOSFETs: in a vertical MOSFET, current travels from one side of the chip to the other, so the available cross section is close to the area of the chip.
In a lateral MOSFET, current travels across the chip, so the available cross section is limited by the thickness of the MOSFET, which is tiny. In other words, vertical MOSFETs allow lower RdsON, so they are popular in power switching applications.
Here's a cutaway view to illustrate (source):
There are many different types of discrete Vertical silicon MOSFETs, but they all have the drain on the bottom side of the chip. That's also the bottom side of the wafer. It is flat and there is only one contact there, whereas on the top side there's both source and gate. So it is practical to solder the chip on a metal tab, which then becomes the drain:
Soldering the chip on the copper tab gives very low thermal and electrical resistance, so usually the drain tab is also used for cooling.
To add to the other answers: there are MOSFETs with bigger source terminals by Infineon. They call them "source down MOSFETs" because they effectively flip the chip so that the source is facing downwards:
It is supposed to be helpful in situations where the drain is a noisy node that you want to keep small. At the same time you need to get rid of heat. With source down MOSFETs, the main heatsink is the source terminal.
You can find more about it in Infineon's marketing article where the image came from.
Links to datasheets from the QR code on the above here, for example, the datasheet for this 850µΩ 30V one.
