ECE249 MOSFET Notes PDF

ECE249: BASIC ELECTRICAL AND ELECTRONICS ENGINEERING Fundamental of Semiconductor Devices UNIT-2 Lecture Prepared by Dr KRISHAN KUMAR  Topics Unit III Fundamental of semiconductor devices : PN junction diode and its applications, Bipolar junction transistor (PNP...

ECE249: BASIC ELECTRICAL AND ELECTRONICS ENGINEERING Fundamental of Semiconductor Devices UNIT-2 Lecture Prepared by Dr KRISHAN KUMAR  Topics Unit III Fundamental of semiconductor devices : PN junction diode and its applications, Bipolar junction transistor (PNP and NPN), MOSFET (working and applications), Op-amp (features and virtual ground concept), Op-amp (inverting and non-inverting)  MOSFET Metal Oxide Silicon Field Effect Transistors commonly known as MOSFETs are electronic devices used to switch or amplify voltages in circuits. It is a voltage controlled device and is constructed by three terminals.  MOSFET MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. Figure shows how a practical MOSFET looks like.  Construction of a MOSFET In the construction of a MOSFET an oxide layer is deposited on the substrate to which the gate terminal is connected. This oxide layer acts as an insulator (sio2 insulates from the substrate), and hence the MOSFET has another name as IGFET. In the construction of MOSFET, Depending upon the substrate used, they are called as P-type and N-type MOSFETs.  The voltage at gate controls the operation of the MOSFET. In this case, both positive and negative voltages can be applied on the gate as it is insulated from the channel. With negative gate bias voltage, it acts as depletion MOSFET while with positive gate bias voltage it acts as an Enhancement MOSFET.  Working of N - Channel depletion mode MOSFET In this construction there is no PN junction present between gate and channel, insulating dielectric SiO2 and the aluminum metal layer of the gate together form a parallel plate capacitor. If the NMOS has to be worked in depletion mode, the gate terminal should be at negative potential while drain is at positive potential, as shown in the following figure. When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some negative voltage is applied at VGG. Then the minority carriers i.e. holes, get attracted and settle near SiO2 layer. But the majority carriers, i.e., electrons get repelled. With some amount of negative potential at VGG a certain amount of drain current ID flows through source to drain. When this negative potential is further increased, the electrons get depleted and the current ID decreases. Hence the more negative the applied VGG, the lesser the value of drain current ID will be. The channel nearer to drain gets more depleted than at source likeinFET and the current flow decreases due to this effect. Hence it is called as depletion mode MOSFET.  Drain Characteristics The drain characteristics of a MOSFET are drawn between the drain current ID and the drain source voltage VDS. The characteristic curve is as shown below for different values of inputs. Actually when VDS is increased, the drain current ID should increase, but due to the applied VGS, the drain current is controlled at certain level. Hence the gate current controls the output drain current.  Transfer Characteristics Transfer characteristics define the change in the value of VDS with the change in ID and VGS in both depletion and enhancement modes. The below transfer characteristic curve is drawn for drain current versus gate to source voltage. Advantages It generates enhanced efficiency even when functioning at minimal voltage levels. These devices can function at minimal power levels and uses minimal current Disadvantages When these devices are functioned at overload voltage levels, it creates instability of the device. As because the devices have a thin oxide layer, this may create damage to the device when stimulated by the electrostatic charges Applications of MOSFET 1. Mosfet is used for some switching applications an in electronics device. 2. It is used in some amplifying circuits. 4. MOSFET can be used as a high-frequency amplifier. 5. It can be used in voltage regulator circuits. 6. It is used as an inverter in some of the electronics circuits. 7. It can be used as a passive element e.g. resistor, inductor used in a circuit. 8. It is used in SMPS ( switch mode power supply ).  MOSFETs are used in smartphones to control the flow of electrical current. They are typically integrated into the circuitry of the device. THANKS