Mosfet is voltage controlled field effect transistor has a metal oxide gate electrode which is electrically insulated from the main semiconductor n-channel or p-channel by a very thin layer of insulating material i.e. silicon dioxide known as glass. In our previous articles, we explain in detail about Fet operation and JFET Transistor operation. In this article, we are going to explain Mosfet types, operation, structure etc.
MOSFETs generally work by electronically varying the width of a channel through which charge carriers i.e. electrons or holes travels because the device conduction is directly proportional to the channel width. The wider the channel, the better the device conducts.
(i) Mosfet terminals:
Like Jfet, Mosfet is a three terminal device i.e. source, drain and gate. The charge carriers either holes or electrons enters through the source terminal and exits through the drain terminal. The channel width is controlled by the voltage applied to the electrode i.e gate present between the source and Drain terminal which is insulated from the channel by a thin layer of metal oxide.
Note: Just like Jfet, Mosfet is also considered as a voltage controlled resistor because the current flowing the main channel between the source and drain is proportional to the input voltage.
(ii) Mosfet symbols:
Generally, a variety of symbols is available for Mosfet’s. It operates in two modes; enhancement mode and depletion mode. As shown in Figure three line segments are used for enhancement mode and a solid line is for depletion mode.
Mosfet symbol used above indicates that the device has a bulk substrate- indicated by the arrow on the central area of the substrate. The Substrate is the fourth terminal which is not normally used as either an input or output connection but instead used for grounding the substrate while in discrete type Mosfet substrate lead is connected internally to the source terminal.To the body or metal tab of the Mosfet, the substrate connects to the main semiconductive channel through a diode junction.
The Semiconductive channel is the channel between the source and drain through which majority carriers travel. As shown in the figure, if this is a solid unbroken line then it represents a Depletion (normally ON) type Mosfet while If the channel line is shown dotted then it represents an Enhancement (normally OFF) type Mosfet. The direction of the arrow indicates whether the conductive channel is a p-type or an n-type semiconductor device.
Metal oxide semiconductor field effect transistor structure is obtained by growing a layer of silicon dioxide (Sio2) on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon. When a voltage is applied across the MOS structure, the majority or minority carriers start moving inside the semiconductor depends upon the polarity i.e. p-type or n-type.
Let us take the example of p-type semiconductor. When we apply the positive voltage across the gate to body (Vgb) terminal then the positive charge holes start moving away from the gate/insulator semiconductor interface, leaving behind the negatively charged acceptor ions which results in the formation of the depletion layer. If we further increase the voltage across the gate to body (Vgb) terminal then the high concentration of negative charge carriers starts forming in an inversion layer. Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is same as the volume density of holes in the body is called the Threshold voltage. When the voltage between the transistor gate and source exceeds the threshold voltage then it is known as overdrive voltage.
Today’s Mosfet is used in many applications such as in electronic switches or to make logic gates because with no bias they are normally non-conducting. Both the p-channel and n-channel MOSFETs are available in two basic forms; one is Depletion mode and another is Enhancement mode.
(i) Depletion mode Mosfet:
Being a depletion mode device, normally on Mosfet type acts as normally closed switch and requires no gate current to function. In depletion mode MOSFET, we apply more negative voltage than the threshold voltage which depletes or shutting off the majority current carriers in the preformed channel beneath the gate. As a result, the size of depletion region got changed which further increase the channel resistance and reduce the current flow.
In p-channel depletion MOS transistor a positive gate-source voltage +Vgs will turn off the transistor by depleting the channel of its free holes while in n-channel depletion MOS transistor a negative gate-source voltage -Vgs will turn off the transistor by depleting the channel of its free electrons.
In n-channel depletion mode:
- +Vgs means more electrons and more current.
- -Vgs or zero voltage less electrons and less current.
In p-channel depletion mode:
- +Vgs means less electrons and less current.
- -Vgs or zero voltage more electrons and more current.
(ii) Enhancement mode Mosfet:
Enhancement mode Mosfet is the reverse of the depletion mode type into which the conducting channel is lightly doped which remain the device normally off when the gate bias voltage Vgs is equal to zero.
In n-channel enhancement Mos transistor, when a gate voltage is applied to the gate terminal greater than the threshold voltage level then the drain current will start flow which making it a transconductance device.
In n-channel enhancement mode:
- +Vgs turns the transistor ON
- -Vgs or zero voltage turns the transistor OFF
In p-channel enhancement mode:
- +Vgs turns the transistor OFF
- -Vgs or zero voltage turns the transistor ON
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