 Hello. Myself Sunil Kalshatti, Assistant Professor, Department of Electronics Engineering, Valchin Institute of Technology, Solapur. Today, I am going to explain the Depletion Type MOSFET, Learning.com. At the end of this session, students can describe construction, working and characteristics of Depletion Type MOSFET. Up till now, we studied the BJT, Bipolar Junction Transistor. These are the current control device. Field effect transistors are the another type of transistors. Field effect transistors are available in two types. Junction Field Effect Transistor, JFET, and Metal Oxide Semiconductor Field Effect Transistor, MOSFET. It is also called as IZFET, Insulated Gate Field Effect Transistor. It is the voltage controlled device. Voltage power is controlled by the input gate voltage. That's why name is the voltage control device. These are the unipolar device. Here, current conduction takes place because of majority charge carriers, either electrons or holes. That's why MOSFETs are called as a unipolar device. Operating frequency above 100 kHz, switching time 50 to 100 nanosecond, switching speed faster than BJT, available for high voltage and high current rating. No possibility of second breakdown because MOSFETs are positive temperature coefficient. As the temperature increases, resistance increases and current decreases. So there is no possibility of second breakdown. But in the BJT, there is a possibility of second breakdown because BJTs are the NTC. High input impedance, other trade names for this device, HexFET, International Rectifiers, CMOS, manufactured by Siemens, TIMOS, manufactured by Motorola. MOSFETs are available in two types, Depletion type MOSFET and Enhancement type MOSFET. Further, Depletion type MOSFETs are divided in two types, N-channel D-MOSFET and P-channel D-MOSFET and Enhancement type MOSFET, N-channel E-MOSFET and P-channel E-MOSFET. These are the schematic symbols of the MOSFETs, source, gate, drain, these are the three terminals and fourth one subset. The arrow represents the direction of majority charge carriers from source to drain, whereas in P-channel MOSFET, the arrow is outward the direction of majority charge carriers from drain to source. This is the construction diagram of En-channel D-MOSFET. It is the lightly doped P-type semiconductor material in which two heavily doped N-regions are diffused and that two heavily doped N-regions are represented as a source and drain. In between two heavily doped N-regions, there is a connectivity and that connectivity is made up by the N-type material, that is why name is the N-channel Depletion type MOSFET. And the third terminal gate, gate is isolated from the channel. In between gate and channel, there is the thin layer of SiO2 layer, that is why input impedance is high. Why the name is MOSFET, Metal Oxide Semiconductor Field Effect Transistor. M metal, source, gate and drain are connected through the metallic contact to the body or for oxide layer. The gate is isolated from the channel. In between gate and channel, there is the thin layer of SiO2 layer. And by having the gate potential, the conductivity of the channel changes and as the conductivity changes according to that, the electric field is developed across the channel. And that developed electric field controls the output power, that is why name is the Metal Oxide Semiconductor Field Effect Transistor. This is the basic structure of P-channel D-type MOSFET. The construction is like this. It is the light ledopter and type semiconductor material in which two heavy ledopter P-regions are diffused and that two heavy ledopter P-regions are represented as a drain and source. In between two P-regions, there is the connectivity and that connectivity is made up of by using the P-type material, that is why name is the P-channel D-type MOSFET. Why input impedance of MOSFET is very high. In the MOSFET, gate is isolated from the channel. In between gate and channel, there is the thin layer of oxide layer. There is no direct electrical connection in between the gate and channel, that is why input impedance is very high. Generally, the input impedance is in the range of 1 megaohm, working of N-channel D-type MOSFET. Operation with VGS is equal to 0. In this mode, VGS is equal to 0 volt and ID is the maximum current IDSS. Short the gate to the source and apply the finite voltage in between the drain and source. So, because of 0 volt VGS, the electrons are starts flowing from the source to drain. So, effect of this, the current starts flowing from the drain to source. In this mode, current is maximum. Operation with negative VGS. In this mode, VGS is negative, that is, VGS is less than 0 and ID is less than maximum current. Because of negative gate potential, the electrons from the channel are repelled away from the oxide layer. And whenever the electrons are repelled, they leave behind the positive immobile ion effect of this, depletion layer is created below the oxide layer. At the same time, the holes from the p-region try to attract towards the gate potential, but they cannot reach up to the gate. They recombine with electrons. So, effect of this, the number of electrons are reduces and the rate of change of current is reduces and the current is reduces. At the specific value of VDS, the width of channel becomes minimum and through which high density charge carriers flows through the device. The voltage at that point is called as a pinch-off voltage. And after that, the MOSFET enters in the pinch-off region or saturation region. Effect of positive gate to source voltage. In this mode, VGS is greater than 0 and ID is greater than maximum current. If VGS is positive, increases the number of free electrons passing through the channel. The greater the gate voltage, greater is the number of free electrons passing through the channel. Thus, the level of free electrons has been enhanced, that is, enhanced the conductivity of the channel and operating mode is called as an enhancement mode. When VGS is positive, the MOSFET operates in the enhancement mode. Here MOSFET operates in three regions, cut-off region, ohmic region and saturation region. In the cut-off region, VGS is sufficiently negative, so effect of this, the current remains 0 and MOSFET remains in the offset. If you want to use the MOSFET as an open switch, it must operate in the cut-off region, ohmic region. In this region, if there is small change in the VDS, there is large change in drain current. The MOSFET offers the constant resistance region and it obeys the Ohm's law, that's why name is the ohmic region. If you want to use the MOSFET as an closed switch, it must operate in the ohmic region. And once the input crosses the pinch-off voltage, the MOSFET operates in the pinch-off region or saturation region. In this region, the current remains constant. If you want to use the MOSFET for the amplification purpose, it must operate in the saturation region. These are the different operating regions of the MOSFET. Transfer characteristics of N-channel D MOSFET, when it is the relation between the VGS and drain current, when VGS is 0, the drain current is maximum. As the VGS increases positively, the MOSFET operates in the enhancement mode and the current crosses the maximum value. And when VGS is negative, the width of conducting channel is reduces, so effect of this, the drain current is reduces. The formula used to plot the transfer curve is, Id is equal to Ids s into bracket 1 minus VGS upon Vp bracket square. This formula is used to plot the transfer curve. Why D MOSFET is called as a normally on-device? When VGS is 0, the output current is maximum, that is why the D MOSFET is called as a normally on-device. Why N-channel MOSFETs are popular than P-channel? In the N-channel MOSFET, majority charge carriers are electrons, whereas in the P-channel, majority charge carriers are holes. The mobility of electron is 1300 centimeter square per volt second, whereas mobility of hole is 500 centimeter square per volt second. In the N-channel MOSFET, the current is twice than P-channel and the packing density of N-channel MOSFET is very high, so the N-channel MOSFETs are more compact, that is why practically N-channel MOSFETs are more popular and practically these are used in the commercial application. These are references. Thank you.