Power MOSFET Construction and Working
Metal-oxide semiconductor field-effect transistors (MOSFETs) are a rather new addition to the unipolar transistor family. A distinguishing feature of this device is its gate construction. The gate is, for example, completely insulated from the channel. Voltage applied to the gate will cause it to develop an electrostatic charge. No current is permitted to flow in the gate area of the device. The gate is simply an area of the device coated with metal. Silicon dioxide is used as an insulating material between the gate and the channel.
Power MOSFET Construction and Working
Depletiontype MOSFETs have an interconnecting channel built on a common substrate. Direct connection is made between the source and drain of this device. Channel construction is very similar to that of the JFET. A second type of MOSFET is called an enhancement type of device. E-MOSFETs do not have an interconnecting channel.
Current carriers pulled from the substrate form an induced channel. Gate voltage controls the size of the induced channel. Channel development is a direct function of the gate voltage. E and D-type MOSFETs both have unique operating characteristics.
Enhancement-type MOSFET construction, element names, and schematic symbols are shown in following Figure. Note in the crystal structure that no channel appears between the source and drain.
The gate is also designed to cover the entire span between the source and drain. The gate is actually a thin layer of metal oxide. Conductivity of this device is controlled by the voltage polarity of the gate. A P-N junction is not formed by the gate, source, and drain.
An E-MOSFET is considered to be a normally off device. This means that without an applied gate voltage, there is no conduction of ID.
When the gate of an N-channel device is made positive, electrons are pulled from the substrate into the source-drain region. This action causes the induced channel to be developed.
With the channel complete, conduction occurs between the source and drain. In effect, drain current is aided or enhanced by the gate voltage. Without VG, the channel is not properly developed and no ID will flow.
A family of characteristic curves for an E-type MOSFET is shown in above Figure. This particular set of curves is representative of an N-channel device. A P-channel device would have the polarity of the gate voltage reversed. Note that an increase of gate voltage causes a corresponding increase in ID.
The input impedance of this device is extremely high regardless of the crystal material used or the polarity of the gate voltage.
Depletion-type MOSFET construction, element names, and schematic symbols are shown in following Figure.
This N-channel device has a thin channel of N material formed on a P substrate. Source and gate connections are made directly to the channel. A thin layer of silicon dioxide (SiO2) insulation covers the channel. The gate is a metal-plated area formed on the silicon dioxide layer. The entire unit is then built on a substrate of P material.
The arrow of the schematic symbol refers to the material of the substrate. When it “Points iN” this shows that the substrate is P material and the channel is N.
The construction of P- and N-channel devices is essentially the same. The crystal material of the channel and substrate is the only difference. Current carriers are holes in the P-channel device, while electrons are carriers in the N-channel unit.
The schematic symbol differs only in the direction of the substrate arrow. It does “Not Point” toward the substrate in a P-channel device. This means that the substrate is N material and the channel is P material.
The operation of a D-MOSFET is very similar to that of a JFET. With voltage applied to the source and drain, current carriers pass through the channel. The gate does not need to be energized in order to produce conduction. In this regard, a D-MOSFET is considered to be a normally on device.
Control of ID is, however, determined by the polarity of gate voltage. When the polarity of VG is the same as the channel, current carrier depletion occurs. A reverse in polarity causes ID to increase due to the enhancement of the current carriers.
In a sense, the D-MOSFET should be classified as a depletion-enhancement device. Its conduction normally responds to both conditions of operation.
A family of characteristic curves for an N-channel D-MOSFET is shown in following Figure. The horizontal part of the graph shows the source-drain voltage as VDS. The vertical axis shows the drain current in milli-amperes. Individual curves of the graph show different values of gate voltage.
Note that zero VG is near the center of the curve. This means that the gate can swing above or below zero. For an N-channel device, negative gate voltage reduces ID. This voltage, in effect, pulls holes from the substrate into the initial channel area. Electrons normally in the channel move in to fill the holes. This action depletes the number of electrons in the initial channel. An increase in negative gate voltage causes a corresponding decrease in ID.
When VG swings positive, it causes the number of current carriers in the initial channel to be increased. A positive gate voltage attracts electrons from the P substrate. This action increases the width of the initial channel. As a result, more flows. Making VG more positive increases ID. This means that the channel is aided or enhanced by a positive gate voltage.
A D-MOSFET therefore responds to both the depletion and enhancement of its current carriers. Selection of an operating point is easy to accomplish when this device is used as an amplifier.
In a P-channel D-MOSFET, operation is very similar to that of the N-channel device. In the P-channel device, holes are the current carriers. Current conduction is normally on when VG is zero. A positive gate voltage will reduce ID. Electrons are drawn out of the substrate to fill holes in the channel. This causes a reduction in channel current.
When the gate voltage swings negative the channel current carriers are enhanced. Holes pulled into the initial channel cause an increase in current carriers. Current carriers are enhanced or depleted according to the polarity of VG.