Mumbai University > Electronics Engineering > Sem 4 > Discrete Electronic Circuits

Marks: 5M

Year: May 2015

• MOSFET is a power electronic device and is called as Metal Oxide Semiconductor Field Effect Transistor. They have improved current carrying capacity and high OFF state blocking voltage capacity. They are capable of switching at very high switching frequency about 100KHz.
• It is a voltage control device which has low input current. The switching speeds are of the order nano seconds and they not have the problem of second breakdown.

Types of MOSFET's, There are two types of MOSFET:

1. Depletion MOSFET
2. Enhancement MOSFET

Basic structure of power MOSFET:

• A power MOSFET is three terminal device and the three terminals are drain, source and gate.
• It has a vertically oriented four layer structure of alternating P-type and N-type layers.
• The vertically oriented structure increased the cross sectional area of the device. Therefore the ON state resistance of the device is reduced.
• The P-type middle layer is termed as body and is moderately doped. The $n^-$ layer is known as drift layer and is lightly doped. The bottom end top $n^+$ regions are heavily doped and lead to the source and drain terminals.

Principle of operation:

If gate to source voltage is zero $Vgs = 0$ in MOSFET is equivalent to two diodes connected back to back. The diodes are formed between $P$ and $n^+$ layer. The structure of MOSFET is similar to a BJT except for the presence of a MOS capacitor that isolates the gate and the body. **Depletion layer creation:** ![enter image description here][3] 1. Conduction can not take place through a MOSFET from drain to source due to the presence of P-layer. 2. But practically it is possible due to a phenomenon called inversion layer creation. 3. The MOSFET is forward biased by connecting a positive voltage to the drain terminal with respect to source terminal and the gate is made positive with respect to body. 4. The body and source are internally shorted. 5. The P-layer consist of a number of holes and a few electrons. 6. Due to the positive voltage applied the gate and body, the electrons are attracted towards the gate and gathered below the $SiO_2$ layer and produce a depletion region by combining with the holes. 7. If the gate voltage is increased further the number of electrons under the $SiO_2$ layer will increase and the total number of electrons will become more than the holes. 8. The value of gate to source voltage for which the number of electrons are more than number of holes is called the threshold voltage. 9. When the Vgs exceeds Vgs(th) an inversion layer is created which dominates over the P-layer.

Formation of an inversion layer:

1. The process of generation of an inversion layer due to the externally applied gate voltage is called "field effect".
2. The inversion layer is also called as induced channel.
3. In this way an N-type channel is created in the P-type body and conduction can take place through this P-type layer.
4. The MOSFET acts as variable resistor where resistance of the channel depends upon the Gate to Source voltage.

Application of Power MOSFET:

• SMPS
• UPS
• High frequency inverter.
• Motor speed control applications.
• Industrial process control.