For power MOSFETs, there is a good rule of thumb indicating that the newer the part, the better it is optimized for switching applications. Originally, MOSFETs were used as pass elements in linear voltage regulators (no base current degrading the no-load losses or overall efficiency) or class AB audio amplifiers. Today, the driving force for the development of new MOSFET generations is, of course, the ubiquity of switch-mode power supplies and the continuing thrive towards motor control with frequency converters. Whatever has been achieved in this regard is nothing less than spectacular.
Some of the characteristics that were improved with every new generation of switching MOSFETs:
- Lower RDS,on - Because minimizing conduction losses means maximizing the overall efficiency.
- Less parasitc capacitance - Because less charge around the gate helps with reducing driving losses and increases switching speed; less time spent in the switching transitions means less switching losses.
- Less reverse recovery time of the internal diode; linked with a higher dV/dt rating - This also helps towards fewer switching losses, and it also means you can't destroy the MOSFET as easily when you force it to switch off really, really fast.
- Avalanche ruggedness - In switching applications, there's always an inductor involved. Cutting off the current to an inductor means creating large voltage spikes. If poorly snubbed or entirely unclamped, the spikes will be higher than the MOSFET's maximum voltage rating. A good avalanche rating means you get some extra bonus before catastrophic failure will occur.
However, there is one not-so-well-known gotcha for linear applications of MOSFETs that has become more pronounced with their newer generations:
- FBSOA (forward biased safe operating area), i.e. power handling capability in linear mode of operation.
Admittedly, this is an issue with any type of MOSFET, old and new, but the older processes were a bit more forgiving. This is the graph that has most of the relevant information:
Source: APEC, IRF
For a high gate-to-source voltage, an increase in temperature will lead to an increase in on-resistance, and a decrease in drain current. For switching applications, this is just perfect: MOSFETs are driven into good saturation with a high VGS. Think about paralleled MOSFETs, and keep in mind that a single MOSFET has many tiny, paralleled MOSFETs on its chip. When one of these MOSFETs gets hot, it will have an increased resistance, and more current will be "taken" by its neighbors, leading to a good overall distribution with no hotspots. Awesome.
For a VGS lower than the value where the two lines cross, called the zero temperature crossover (cf. IRF's App'note 1155), however, an increased temperature will lead to a decreased RDS,on, and in increased drain current. This is where thermal runaway will knock on your door, contrary to the popular belief that this is a BJT-only phenomenon. Hot spots will occur, and your MOSFET may self-destruct in a spectacular way, taking with it some of the beautiful circuitry in its neighborhood.
Rumor has it that older, lateral MOSFET devices had better-matching transfer characteristics across their internal, paralleled, on-chip MOSFETs compared to the newer trench devices optimized towards the above-mentioned characteristics important for switching applications. This is further backed up by the paper I have already linked to, showing how newer devices have an even increasing VGS for the point of zero temperature crossover.
Long story short: There are power MOSFETs that are better suited for linear applications or switching applications. Since linear applications became something like a niche application, e.g. for voltage-controlled current sinks, extra caution towards the graph for the forward-biased safe-operating area (FB-SOA) is needed. If it doesn't contain a line for DC operation, this is an important hint that the device will likely not work well in linear applications.
Here is one more link to a paper by IRF with a good summarization of most things I have mentioned here.