How do Power MOSFETs (HEXFETs) work?

Question

I understand how regular N/P-channel MOSFETs work and what it takes to turn each of them on and off. However, I am having a hard time understanding how that all works in a HEXFET such as the internal ones in the IRF7343, which is the part I am dealing with. I am not sure what it takes at the gate to turn on and off the drain/source current specially because of the included diode.

As an example for the P-channel: I have 12 V going into the source and a 12 V/0 V controlled by a switch going to the gate. What voltage at the gate would turn on or off the drain/source current?

Answer 1

HEXFET is just a marketing name coined by "International Rectifier".

Please see RF-Wirelessworld Article and Wikipedia article mentioning HEXFET

They are available as P-/N- types and are - at least for the enduser - the same as every other x-FET available.

The body-diode shown in the schematic is present in these devices, as they are in every FET available to the market - the diode is just a PN-Junction between substrate and the "channel-zone".

Answer 2

HEXFET is just Infineon's brand name for their power MOSFET construction, so the drive characteristics are the same as for any other enhancement mode device: for N-channel, bring Vg positive with respect to the source to allow majority carriers to flow from source to drain (current flows into the drain terminal and out of the source); for P-channel, bring Vg negative with respect to the source to allow majority carriers to flow from source to drain (current flows into the source terminal and out the drain terminal).

The threshold voltage Vgs(th) is the minimum gate-to-source voltage at which current between the drain and source can just barely be detected. 250 microamps Id is what Infineon is using to say that the inversion layer is just barely formed and that happens at a Vgs of 1 V for the N-channel device and -1 V for the P-channel. Vgs of +/- 12 V will very comfortably turn on both devices and is well within the maximum Vgs of 20 V. The specific response characteristics are shown in tables 1 and 12 for the N- and P- devices, respectively.

Answer 3

The diode in the symbol is redundant; just emphasizing that, yes, there is in fact a diode here. It's often drawn as a zener to indicate avalanche robustness.

The structure of an N-channel MOSFET consists of two N-doped regions embedded in a P-doped substrate. Thus there is an intrinsic diode (P-N) from substrate to each terminal. Three-terminal MOSFETs always tie substrate to source. (The most common exception is probably when the substrate doesn't exist at all, as in power GaN MOSFETs: the GaN is a thin active layer on top of an insulating layer. 4-terminal MOSFETs are rare but do exist.)

The depiction has three vertical line segments in a row, indicating the drain, channel and source respectively; the arrow/triangle indicates the substrate P-N junction. The lines are broken, indicating the channel is normally in depletion i.e. current doesn't flow at V_gs = 0. Positive gate voltage enhances (hence "enhancement mode") or induces charges to the surface, making it conductive (the surface of the P-substrate becomes N-type, connecting drain and source resistively). A depletion-mode MOSFET should be drawn with a solid line segment, indicating the channel is normally conductive at V_gs = 0 (and is turned off by negative gate voltage).

As far as understanding operation, it's just a 4-terminal MOSFET with source and substrate tied together. It sounds like you already understand the 4-terminal MOSFET, so this should be adequate.