What is MOSFET?

In addition to the junction area effective transistor (JFET), there is another type of field-effective transistor, called the effective transistor, whose gate entrance is electrically isolated from the mainstream transport channel and therefore receives an insulated door.

The most common insulated door FET type used in many different electronic circuits is called metal oxide semiconductor field effective transistor or MOSFET for short.

Unlike a jfet, IGFET or MOSFET is an area-effect transistor with voltage control. Because it has a "Metal oxide" gate electrode electrically isolated from the main semiconductor n-channel or p-channel, with a very thin layer of insulation material, often known as glass.

This ultra-thinly insulated metal door electrode can be considered a plate of a capacitor. The insulation of the control gate makes Mosfet's input resistance extremely high in the Mega-ohm (MΩ) zone, making it almost infinite.

Since the gate terminal is electrically isolated from the mainstream transport channel between drain and question, "Current does not flow into the Gate" and just like JFET, MOSFET also acts as a voltage-controlled resistance in which the current flowing through the main channel between drain and source is proportional to the input voltage. In addition, Mosfets with very high input resistance, such as JFET, can easily accumulate a large amount of static load, which causes mosfet to be easily damaged, unless carefully used or protected.

As in the previous JFET tutorial, Mosfets are three terminal devices with a gate, drain and source. Both P-channel (PMOS) and N-channel (NMOS) Mosfets are available. The main difference this time is that mosfets are available in two basic forms:

  • Depletion type– The transistor requires gate source voltage (VGS) to "turn off" the device. Depletion mode MOSFET is equivalent to a "normally off" switch.
  • Development type– The transistor requires a gate source voltage (VGS) to "turn on" the device. Development mode MOSFET is equivalent to a "normally open" switch.

The symbols and basic structure for both MOSFET configurations are shown below.

MOSFET
symbols and basic structure

The four MOSFET symbols above indicate an additional terminal called a substrate and are not normally used as an input or output connection. Instead, it is used to ground the substrate. It is connected to the main semiconductor channel, the body of the mosfet or the metal fingernail through a diode junction.

Usually in discrete type Mosfets, this substrate cable is connected to the source terminal, INCLUDING. In this case, as with the development types, it is removed from the symbol for the description.

The line in the MOSFET symbol between the Drain (D) and source (s) connections represents the semiconductor channel of transistors. If this channel line is a solid continuous line, it represents a "depletion" (normally on) type MOSFET, as the discharge current can flow with zero gate bias potential.

If the channel line is shown as a dotted or broken line, a "enhancement" (normally off) type represents MOSFET when the zero drain current flows with zero gate potential. The direction of the arrow pointing to this channel line indicates whether the conductive channel is a p-type or an N-type semiconductor device.

Basic MOSFET Structure and Symbol

MOSFET
Basic MOSFET Structure and Symbol

The structure of metal oxide semiconductor FET is very different from that of the connection FET. Mosfets of both depletion and development type use an electric field produced by a gate voltage to change the flow of load carriers, electrons for the n-channel or holes for the P-channel along the semiconductor drain source channel. The gate electrode is placed on a very thin layer of insulation. Just below the drain and source electrodes is a pair of small n-type regions.

In the previous tutorial, we found that the door of the effective transistor junction receiving JFET should be biased in such a way as to reverse the pn junction. Such a limitation is not applied with an isolated door MOSFET device, so it is possible to bias the gate of an MOSFET as polarity, positive (+and) or negative (-and).

This makes the MOSFET device especially valuable as electronic keys or to make logic gates. Because they are not normally conductive without bias, and this high gate input resistance means that little or no control current is needed because mosfets are voltage-controlled devices. Both p-channel and n-channel Mosfets are available in two basic forms: development type and depletion type.

Depletion Mode MOSFET

Less common than development mode types, exhaustion mode MOSFET normally opens as "on" (conductor) without applying a gate bias voltage. That is, the channel makes it a "normally closed" device when it is VGS = 0. The circuit symbol shown above for a depletion MOS transistor uses a solid channel line to indicate a normally closed conductive channel.

N-channel depletion, which has a negative gate source voltage, for the MOS transistor-vgs consumes the conductive channel of its free electrons, which changes the transistor to "off". Similarly, a positive gate source voltage for a p-channel depletion MOS transistor will make the channel of +VGS free holes "closed". In other words, MOSFET for an n-channel depletion mode: +VGS means more electrons and more current.

Depletion Mode N-Channel MOSFET and Circuit Symbols

MOSFET
Depletion Mode N-Channel MOSFET and Circuit Symbols

Depletion mode MOSFET is built similarly to its JFET transistor counterparts, naturally conductive with electrons and holes already present in the drain source channel, n-type or p-type channel. This doping of the channel produces a low-resistance conductive pathway with zero gate bias between drain and source.

MOSFET Amplifier

The more common development mode Amplifier MOSFET is the inverse of the exhaustion mode type. Here, the conductive channel is slightly additive or even unopened and is not conductive. This causes the device to normally be "off" (non-conductive) when the gate bias voltage is equal to vgs zero. The circuit symbol shown above for an enhancement MOS transistor uses a broken channel line to indicate a channel that is not normally open conductive.

A drain current for the N-channel development MOS transistor will flow only when a gate voltage (VGS) is applied to a gate terminal larger than the threshold voltage (VTH) where conductivity occurs, which will make it a transconductance device.

Applying a positive (+and) gate voltage to an N-type mosfete draws more electrons towards the oxide layer around the gate. Thus, it increases the thickness of the channel, which allows more currents to flow. Therefore, such a transistor is called a development mode device, since the application of a gate voltage increases the channel.

Increasing this positive gate voltage causes the channel resistance to decrease further and causes an increase in drain current throughout the channel. In other words, MOSFET for an n-channel development mode: +vgs makes the transistor "on" and the transistor "off" if zero or-vgs. Therefore, the development mode MOSFET is equivalent to a "normally open" switch.

Conversely, p-channel development applies to the MOS transistor. When Vgs = 0, the device is "off" and the channel is on. Applying a negative (- and) gate voltage to P-type emosfet makes the conductivity of the channels "on". Then mosfet for p-channel development mode: +VGS transistor "off", and vgs makes the transistor "on".

Development Mode N-Channel MOSFET and Circuit Symbols

MOSFET
Development Mode N-Channel MOSFET and Circuit Symbols

The DC bias of this common source (CS) MOSFET amplifier circuit is almost identical to the JFET amplifier. The MOSFET circuit is biased in class A mode by the voltage divider network created by R1 and R2 resistors. AC input resistance is given as rn = rg = 1MΩ.

Metal oxide semiconductor field effective Transistors are three terminal active devices made of different semiconductor materials that can act as an insulator or conductor by applying a small signal voltage.