Mosfet - Basic Properties, Terminology and Theory

What is a Mosfet?

Mosfet is an acronym for Metal Oxide Semiconductor Field Effect Transistor. There are a number of different types of Mosfets, and the standard for high power, high voltage-switching applications is the n-channel enhancement mode device, shown in the figure 3.2.

The Mosfet is a three terminal voltage controlled device, as opposed to the bipolar transistor that is current controlled. The drain, source, and gate terminal of the Mosfet are analogous to the collector, emitter, and base terminals of the bipolar transistor. The drain current of the Mosfet is controlled by the gate to source voltage. Therefore, the gate voltage of the Mosfet is analogous to base current for the bipolar.
Unlike a bipolar, the current gain of a Mosfet is extremely high. The gate current consists only of the current required to charge the gate input capacitance and supply a small leakage current through the gate oxide. It is essentially meaningless to specify gain for a Mosfet. It is more appropriate to specify transconductance.

The Mosfet chip is made up of a high density of individual cells connected in parallel to achieve a low on-state resistance. The center of the P-well, (deep-P region), is highly doped P+ to achieve low base to emitter resistance of the parasitic bipolar transistor. This low resistance improves the effectiveness of the source metallization short and prevents turn on of the parasitic bipolar during dynamic operation. The source and gate and ultrasonically wire bonded to their respective terminals.

The maximum voltage ratings of a Mosfet should never be exceeded, even on an instantaneous basis.

VDSS is the maximum drain to source voltage that can be blocked without avalanching the Mosfet, usually specified at 25 degrees C, and with the gate shorted to the source. Blocking characteristics should never be measured with the gate open. Due to the high input impedance of a Mosfet, the device can turn on as a result of stray charge accumulation on the gate. The VDSS blocking characteristic, and its strong positive temperature coefficient are illustrated in figure 3.3.

The gate of a power Mosfet is electrically isolated from the rest of the device by a thin layer of silicon dioxide. Exceeding the maximum gate-source voltage rating, usually ± 20 volts, can puncture the gate insulation and destroy the device. Note that the device will support equal positive and negative gate to source voltages. There is no built-in zener protection diode. Prior to installation in equipment, the gate should be protected by proper handling procedures.

The on-resistance, RDS(ON), of a power Mosfet, determines the amount if power dissipated at a given drain current. RDS(ON) is the major figure of merit for a power Mosfet. The value of RDS(ON) is equal to the ratio of on drain-source voltage, VDS(ON), to drain current, ID. The RDS(ON) of a power Mosfet consists of two major components, the channel resistance and the bulk, or epi, resistance. For low voltage devices, the channel resistance dominates and low RDS(ON) devices can be achieved by increasing unit cell densities. For high voltage, (above 100 volts), the epi region resistance dominates. Since the resistivity of the epi region controls the device blocking voltage, the epi region resistance is difficult to reduce.

Power Mosfets can be damaged by electrostatic discharge, (ESD), due to improper handling or installation. Avoid possible damage by static charge when handling, testing or installing in a circuit by using the following precautions:

• Store devices in anti-static bags, conductive foam, metal or conductive tote bins
• Use a grounded wrist strap when handlingHandle device by the package, not the leads
• Work/Test stations should use electrically conductive floor and table mats
• Do not insert devices into circuits or test equipment with power applied
• Use 100 ohm resistor in series with the gate when testing Mosfet on a curve tracer to suppress oscillations
• Reduce power to zero before switching ranges on test equipment
• Use soldering irons with grounded tips