Small-Signal MOSFETs
Low-Voltage and Signal Applications
Operation Modes and Channel Types
- Enhancement Mode (Normally Off)
- Depletion Mode (Normally On)
- N-Channel MOSFETs
- P-Channel MOSFETs
MOSFETs use a vertical DMOS structure in power switches combined with a well-proven, rugged gate oxide process. There are two types of enhancement-mode MOSFETs: P channel and N channel. Characteristic of all MOS structures, these devices are free from thermal runaway and thermally induced secondary breakdown. You can use lateral MOSFETs with no intrinsic body diode and often-ballasted source areas for linear mode operation amplifiers, the most prevalent of which are RF power amplifiers (e.g., LDMOS).
These transistors also use lateral DMOS technology. The key difference is that the depletion-mode MOSFET is normally on and commanded off when Vgs increases.
N-channel MOSFETs are enhanced when the gate is pulled positive with respect to the source. N-channel MOSFETs tend to have lower RDS(on) per unit area compared to P-channel MOSFETs. In a half-bridge stage, the high-side N-channel MOSFET must have a means of holding the gate voltage positive as the device turns on. This requires a voltage above the rail, which you can derive by using bootstrapped drivers that allow a flying capacitor bank tied to the source of the high-side switch to charge when the low-side switch turns on and then flying up with the source to hold it on or with galvanically isolated drivers.
P-channel MOSFETs are enhanced when the gate is pulled negative with respect to the source. This is often advantageous in a high-side switch application because the circuit does not require an isolated or bootstrapped driver but rather a simple clamp to protect the gate oxide and a pull-down circuit or driver. P-channel MOSFETs have been popular in lower-power high-side switches, hot swap applications on the positive rail, electronic relays, electronic circuit breakers and general DC switching.