I measured the input capacitance of a MOSFET and found it was greater than an IGBT. Why is this? Is it due to the Miller capacitance seen in a MOSFET?
Miller capacitance is nothing that you can easily measure with a cap-meter. The effect comes from the voltage on one pin falling such that less voltage difference is seen on more of the area, such that more charge can flow on the gate. For an IGBT this effect is not present so much, since the MOSFET in front of the BJT does not see the full voltage swing which a single mosfet would see if it was used instead. Further, since there is more amplification going on in an IGBT, it probably has a much smaller MOSFET inside while the BJT is big.
So it is to be expected that in an IGBT and a MOSFET with roughly the same capabilities, the IGBT has smaller capacity.