El Mosfet

Power MOSFET Basics
By Vrej Barkhordarian, International Rectifier, El Segundo, Ca.

Breakdown Voltage......................................... 5 On-resistance.................................................. 6 Transconductance............................................ 6 Threshold Voltage........................................... 7 Diode Forward Voltage.................................. 7Power Dissipation........................................... 7 Dynamic Characteristics................................ 8 Gate Charge.................................................... 10 dV/dt Capability............................................... 11

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Power MOSFET Basics
Vrej Barkhordarian, International Rectifier, El Segundo, Ca. Discrete power MOSFETs employ semiconductorprocessing techniques that are similar to those of today's VLSI circuits, although the device geometry, voltage and current levels are significantly different from the design used in VLSI devices. The metal oxide semiconductor field effect transistor (MOSFET) is based on the original field-effect transistor introduced in the 70s. Figure 1 shows the device schematic, transfer characteristics anddevice symbol for a MOSFET. The invention of the power MOSFET was partly driven by the limitations of bipolar power junction transistors (BJTs) which, until recently, was the device of choice in power electronics applications. Although it is not possible to define absolutely the operating boundaries of a power device, we will loosely refer to the power device as any device that can switch at least1A. The bipolar power transistor is a current controlled device. A large base drive current as high as one-fifth of the collector current is required to keep the device in the ON state.

Source Contact

Field Oxide

Gate Gate Oxide Metallization

Drain Contact

n* Source

n* Drain

tox

p-Substrate Channel l

(a)
ID

0 0

VT

VGS

(b)
ID
D SB (Channel or Substrate) G S(c)

Figure 1. Power MOSFET (a) Schematic, (b) Transfer Characteristics, (c) Also, higher reverse base drive Device Symbol. currents are required to obtain fast turn-off. Despite the very advanced state of manufacturability and lower costs of BJTs, these limitations have made the base drive circuit design more complicated and hence more expensive than the power MOSFET.

Another BJTlimitation is that both electrons and holes 2000 contribute to conduction. Presence of holes with their higher carrier lifetime causes the switching speed to be several orders of 1500 magnitude slower than for a power MOSFET of similar size and Bipolar Transistors voltage rating. Also, BJTs suffer from thermal runaway. Their 1000 forward voltage drop decreases with increasing temperature causingdiversion of current to a single device when several MOS 500 devices are paralleled. Power MOSFETs, on the other hand, are majority carrier devices with no minority carrier injection. They 0 are superior to the BJTs in high frequency applications where 1 10 100 1000 switching power losses are important. Plus, they can withstand Maximum Current (A) simultaneous application of high current and voltagewithout Figure 2. Current-Voltage undergoing destructive failure due to second breakdown. Power Limitations of MOSFETs and BJTs. MOSFETs can also be paralleled easily because the forward voltage drop increases with increasing temperature, ensuring an even distribution of current among all components. However, at high breakdown voltages (>200V) the on-state voltage drop of the power MOSFET becomeshigher than that of a similar size bipolar device with similar voltage rating. This makes it more attractive to use the bipolar power transistor at the expense of worse high frequency performance. Figure 2 shows the present current-voltage limitations of power MOSFETs and BJTs. Over time, new materials, structures and processing techniques are expected to raise these limits.
Source Gate Oxide...