With some amount of negative potential at V GG a certain amount of drain current I D flows through source to drain.
But the majority carriers, i.e., electrons get repelled. holes, get attracted and settle near SiO 2 layer. Let some negative voltage is applied at V GG. When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. 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. We can also observe that, the diffused channel N (between two N+ regions), the insulating dielectric SiO 2 and the aluminum metal layer of the gate together form a parallel plate capacitor. Working of N - Channel (depletion mode) MOSFETįor now, we have an idea that there is no PN junction present between gate and channel in this, unlike a FET. They are depletion and enhancement modes. The SiO 2 substrate is connected to the common or ground terminals.īecause of its construction, the MOSFET has a very less chip area than BJT, which is 5% of the occupancy when compared to bipolar junction transistor. A conducting layer of aluminum is laid over the entire channel, upon this SiO 2 layer from source to drain which constitutes the gate. Between these two N+ regions, there occurs diffusion to form an Nchannel, connecting drain and source.Ī thin layer of Silicon dioxide (SiO 2) is grown over the entire surface and holes are made to draw ohmic contacts for drain and source terminals. A lightly doped P-type substrate is taken into which two heavily doped N-type regions are diffused, which act as source and drain. Let us consider an N-channel MOSFET to understand its working. Also, there is no need to mention that the study of one type explains the other too. Usually an NChannel MOSFET is considered for explanation as this one is mostly used. Now, let us go through the constructional details of an N-channel MOSFET. The symbols for P-channel MOSFET are as given below. The P-channel MOSFETs are simply called as PMOS. The symbols for N-channel MOSFET are as given below. The N-channel MOSFETs are simply called as NMOS. Classification of MOSFETsĭepending upon the type of materials used in the construction, and the type of operation, the MOSFETs are classified as in the following figure.Īfter the classification, let us go through the symbols of MOSFET. With negative gate bias voltage, it acts as depletion MOSFET while with positive gate bias voltage it acts as an Enhancement MOSFET. In this case, both positive and negative voltages can be applied on the gate as it is insulated from the channel.
The voltage at gate controls the operation of the MOSFET. The following figure shows the construction of a MOSFET.
Depending upon the substrate used, they are called as P-type and N-type MOSFETs. In the construction of MOSFET, a lightly doped substrate, is diffused with a heavily doped region. This oxide layer acts as an insulator (sio 2 insulates from the substrate), and hence the MOSFET has another name as IGFET. An oxide layer is deposited on the substrate to which the gate terminal is connected. The construction of a MOSFET is a bit similar to the FET. The following figure shows how a practical MOSFET looks like. The FET is operated in both depletion and enhancement modes of operation. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. To overcome these disadvantages, the MOSFET which is an advanced FET is invented. FETs have a few disadvantages like high drain resistance, moderate input impedance and slower operation.
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