Farzana R. ZakiCSE 177/ EEE 1771 Lecture – 19

Farzana R. ZakiCSE 177/ EEE 1772 MOSFET Construction & operation of Depletion type MOSFET Plotting transfer characteristic curve

Farzana R. ZakiCSE 177/ EEE 1773 MOSFETs Basics MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) have been used in power electronics applications since thee early 80's due to their appreciable current carrying and off- state voltage blocking capability with low on- state voltage drop. They have managed to replace BJTs in many applications due to their simpler gate drive requirements and higher positive temperature coefficient which allows devices to be paralleled for higher current capabilities.

Farzana R. ZakiCSE 177/ EEE 1774 MOSFET Types N-channel enhancement type MOSFETS are the most popular for use in power switching circuits and applications.

Farzana R. ZakiCSE 177/ EEE 1775 Depletion Type MOSFET Depletion type MOSFET has characteristic similar to those of a JFET between cut off and saturation at I DSS but then has the added feature of characteristics that extend into the region opposite polarity for V GS. It is also known as IGFET (Insulated Gate Field Effect Transistor).

Farzana R. ZakiCSE 177/ EEE 1776 Basic Construction: n-channel depletion type MOSFET A slab of p-type material is formed from Si base and is referred to as Substrate. Source (S) and Drain (D) terminals are connected through metallic contacts to n-doped region linked by n- channel. The gate is also connected to a metal contact surface but remains insulated from n-channel by a very thin SiO 2 layer. SiO 2 is a particular type of insulator referred to as a dielectric that sets up opposing electric fields within dielectric when exposed to an externally applied field. It is the insulating layer of SiO 2 in the MOSFET construction that for the very desirable high input impedance of the device.

Farzana R. ZakiCSE 177/ EEE 1777

Farzana R. ZakiCSE 177/ EEE 1778 Basic Operation and Characteristics V GS =0V is done by direct connection from one terminal to other and V DS is applied across the drain-to-source terminal. The result is an attraction of the positive potential at the drain by the free electrons of n-channel and a current I D =I S =I DSS will flow. When V GS =0V

Farzana R. ZakiCSE 177/ EEE 1779 When V GS is negative : When V GS is negative, the negative potential at the gate will tend to pressure electrons toward p-type substrate (like charges repel) and attracts holes from p- type substrate (opposite charges attract). Depending on magnitude of –ve bias established by V GS, a level of recombination between electrons and holes will occur that will reduce the number of free electrons in the n-channel available for conduction. The resulting level of drain current is therefore reduced with increasing negative bias for V GS.

Farzana R. ZakiCSE 177/ EEE Transfer Characteristic curve for n-channel depletion type MOSFET I D mA V GS V GS = +1V

Farzana R. ZakiCSE 177/ EEE When V GS = +ve For positive values of V GS, the positive gate will draw addition electrons (free carriers) from p- type substrate due to reverse leakage current and establish new carriers through the collisions resulting between accelerating particles. As V GS continues to increase in the positive direction, I D will also increase at a rapid rate. Due to the rapid rate, the user must be aware of maximum drain current rating since it could be exceeded with a positive gate voltage.

Farzana R. ZakiCSE 177/ EEE The application of the +ve V GS has “enhanced” the level of free electrons in the channel compared to that encountered with V GS =0V. For this reason, the region of +ve gate voltages on drain or transfer characteristic is called enhancement region, with the region between cut-off and saturation level of I DSS referred to as the depletion region. Shockley’s equation is applicable for depletion type MOSFET.

Farzana R. ZakiCSE 177/ EEE Example: Sketch the transfer characteristics for an n- channel depletion type MOSFET with I DSS =mA and V P =-V

Farzana R. ZakiCSE 177/ EEE P-channel depletion type MOSFET The construction of p-channel depletion type MOSFET is exactly the reverse of that of n-channel. It has n-channel substrate and p-type channel. The terminals remains the same, but all the voltage polarities and the current directions are reversed. Drain characteristic would exactly the same pattern but with V DS having –ve, I D having +ve and V GS having opposite polarities.