Some disadvantages of field effect transistors are high drain resistance, moderate input impedance, and slow operation. To overcome these shortcomings, the MOSFET was invented, which is an advanced FET.
Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor, abbreviated as MOSFET. It is also called IGFET Insulated Gate Field Effect Transistor. The FET is operated in both depletion and enhancement mode.
MOSFET has three terminals source, drain, and gate. The flow of current from the source to the drain is controlled by the voltage applied at the gate terminal. The presence of an insulating layer of metal oxide results in high input impedance to the device.
Construction of a MOSFET
The construction of a MOSFET is very similar to that of a FET. An oxide layer is applied to the substrate to which the gate terminal is attached. This oxide layer acts as an insulator. SiO2 is insulated from the substrate. Hence, another name for MOSFET is IGFET. In manufacturing a MOSFET, a lightly doped substrate is combined with a heavily doped region. Depending on the substrate used, they are called p-type and n-type MOSFETs. One of the basic applications of field-effect transistors is the use of MOSFETs as switches.
The operation of the MOSFET is controlled by the gate voltage. Both positive and negative voltages are applied to the gate as it is insulated from the channel. With a negative gate bias voltage, it acts as a depletion MOSFET while with a positive voltage gate bias it acts as an enhancement MOSFET.
Classification of MOSFET
MOSFETs are classified into two parts based on the type of material used during manufacture and the type of operation: enhancement mode and depletion mode.
- Symbol of N-Channel MOSFET: N-Channel MOSFETs are commonly called NMOS.
- Symbol of P-Channel MOSFET: P-channel MOSFETs are commonly called PMOS.
Construction of N-Channel MOSFET
In an n-channel MOSFET, a lightly doped N-type substrate is taken in which two heavily doped n-type regions are diffused, which act as source and drain. Between these two N+ regions, diffusion occurs to form an N-channel, which interconnects the drain and source.
A thin layer of silicon dioxide (SiO2) is formed over the entire surface and holes are made to draw ohmic contacts to the drain and source terminals. A conducting layer of aluminum is placed over the entire channel. The SiO2 layer forms the gate from the source to the drain. The SiO2 substrate is connected to the common or ground terminals.
Due to its construction, a MOSFET has a much smaller chip area than a BJT, which is 5% of the occupancy compared to a bipolar junction transistor. This device is operated in modes which are Enhancement and Depletion mode.
Working of N-Channel MOSFET in Depletion Mode
MOSFET is the opposite of FET which has no PN junction between gate and channel. The diffuse channel N between the two N+ regions, the insulating dielectric SiO2, and the aluminum metal layer of the gate are aligned to form parallel plate capacitors.
If the NMOS is to operate in depletion mode, the gate terminal must be at a negative potential, while the drain is at a positive potential.
When no voltage is applied between the gate and the source, some current will flow due to the voltage between the drain and the source. Some negative voltage is applied across VGG. The minority carrier holes then get attracted and settle near the SiO2 layer. But most of the carriers, electrons get repelled.
In VGG a certain amount of drain current ID flows from the source to the drain with some amount of negative potential. When this negative potential is increased further, electrons are depleted and the current ID decreases. Therefore, the more negative VGG is applied, the lower will be the value of the drain current ID.
The channel near the drain is depleted more than the source like in a FET and due to this effect, the current flow is reduced. That’s why it is called depletion mode MOSFET.
Working of N-Channel MOSFET in Enhancement Mode
The corresponding MOSFET can be operated in enhancement mode if the voltages can change the polarity of VGG, then the gate-source voltage to the MOSFET is positive with VGG.
When no voltage is applied between the gate and the source, some current will flow due to the voltage between the drain and the source. Some positive voltage is applied across the VGG. Then the minority carrier holes are repelled and the majority of carrier electrons are attracted to the SiO2 layer.
A certain amount of drain current ID flows from source to drain with some positive potential in VGG. When this positive potential is further increased, the current ID increases due to the flow of electrons from the source, and further increases due to the voltage applied to the VGG. So the more positive the applied VGG, the higher the value of the drain current ID. An increase in current flow leads to a better increase in electron flow than a depletion mode. Hence this mode is called Enhanced Mode MOSFET.
Construction of P-Channel MOSFET
The construction and function of PMOS are similar to that of NMOS. A lightly doped N-substrate is taken in which two heavily doped P+ regions are diffused. These two P+ regions act as source and drain. A thin layer of SiO2 is laid on the surface. Holes are cut through this layer to make contact with the P+ regions.
Working of P-Channel MOSFET
When the gate terminal is provided with a negative potential at VGG compared to the drain-source voltage VDD, the hole current through the diffused P channel is amplified due to the presence in the P+ regions and the PMOS operates in enhancement mode.
When the gate terminal is provided with a positive potential at Vgg as compared to the drain-source voltage Vdd, there is a depletion due to repulsion thereby reducing the flow of current. Thus, PMOS operates in depletion mode. Although the construction is different, the working in both types of MOSFETs is the same. Therefore both types of voltages can be used in both modes with polarity changes.
Drain Characteristics of MOSFET
The drain characteristics of the MOSFET are drawn between the drain current ID and the drain-source voltage VDS. the characteristic curve for different values of the input,
Actually, when VDS is increased, the drain current ID increases but due to VGS being applied the drain current gets controlled to a certain level. Hence the gate current controls the output drain current.
Transfer Characteristics of MOSFET
The transfer characteristics define a change in the value of VDS, with a change in ID and VGS in both Depletion and Enhancement modes. Transfer characteristic curve for drain current vs gate to source voltage,
Comparison between BJT, FET, and MOSFET
Related Tutorial: Various types of Transistors.