In the article, you will learn about MOSFET transistors, what it is, what switching circuits are. There is a type of field effect transistor in which the input is electrically isolated from the main current of the carrier channel. And therefore it is called an insulated gate field effect transistor. The most common type of field-effect transistor that is used in many types of electronic circuits is called a metal-oxide-semiconductor field effect transistor or MOS transistor (abbreviation for this element).
What are mosfet transistors?
The MOS transistor is a voltage-controlled field effect transistor that differs from the field effect in that it has a metal oxide gate electrode that is electrically isolated from the main semiconductor by a p-channel or a p-type channel with a very thin layer of insulating material. As a rule, this is silicon dioxide (or, if simpler, then glass).
This ultrathin insulated metal gate electrode can be considered as one capacitor plate. Isolation of the control input makes the resistance of the MOS transistor extremely high, almost endless.
Like MOSFETs, MOSFETs have a very high input impedance. It can easily accumulate a large amount of static charge, which leads to damage if the circuits are not carefully protected.
Differences of MOSFET from field-effect transistors
The main difference from field ones is that MOS transistors are available in two main forms:
- Depletion - The transistor requires a gate-source voltage to switch the device to the off position. The depletion mode of the MOS transistor is equivalent to a “normally closed” switch.
- Saturation - A transistor requires a gate-source voltage to turn on the device. The amplification mode of the MOS transistor is equivalent to a switch with “normally closed” contacts.
Graphic designations of transistors on circuits
The line between the drain and source connections is a semiconductor channel. If the diagram on which MOSFET transistors are depicted is represented by a solid solid line, then the element operates in the depletion mode. Since the current from the drain can flow with a zero gate potential. If the channel line is shown in dotted or broken lines, then the transistor operates in saturation mode, as current flows with a zero gate potential. The direction of the arrow indicates a conductive channel, p-type, or p-type semiconductor device . Moreover, domestic transistors are denoted in exactly the same way as foreign analogues.
The basic structure of a MOSFET transistor
The design of the MOSFET (which is described in detail in the article) is very different from the field. Both types of transistors use an electric field created by the voltage across the gate. To change the flow of charge carriers, electrons for the p-channel or holes for the p-channel, through the semiconducting channel, the drain-source. The gate electrode is placed on top of a very thin insulating layer, and there are a couple of small p-type areas just below the drain and source of the electrodes.
With an isolated gate device for the MOS transistor, no restrictions apply. Therefore, it is possible to connect a signal source in any polarity (positive or negative) to the gate of a MOSFET. It should be noted that imported transistors are more common than their domestic counterparts.
This makes MOSFET devices particularly valuable as electronic switches or logic devices because, as a rule, they do not conduct current without external influences. And the reason for this is the high input impedance of the shutter. Therefore, very little or little control is needed for MOS transistors. After all, they are devices controlled from the outside by voltage.
MOSFET depletion mode
Depletion mode is much less common than amplification modes without applying bias voltage to the gate. That is, the channel conducts at zero voltage at the gate, therefore, the device is "normally closed". In the diagrams, a solid line is used to indicate a normally closed conducting channel.
For a p-channel MOSFET depletion, a negative gate-source voltage is negative, it will deplete (hence the name) the conducting channel of its free electrons of the transistor. Similarly, for a p-channel MOS transistor, depletion of a positive gate-source voltage will deplete the channel of its free holes, placing the device in a non-conducting state. But the transistor dialer does not depend on what mode of operation.
In other words, for the depletion mode of a p-channel MOS transistor:
- Positive drain voltage means more electrons and current.
- Negative voltage means less electrons and less current.
The converse is also true for p-channel transistors. Then the depletion mode of the MOS transistor is equivalent to a "normally open" switch.
N-channel depletion MOSFET
The depletion mode of the MOS transistor is constructed in the same way as that of field-effect transistors. Moreover, the drain-source channel is a conducting layer with electrons and holes, which is present in p-type or p-type channels. Such channel doping creates a conductive path of low resistance between the drain and the source with zero voltage. Using a transistor tester, it is possible to measure currents and voltages at its output and input.
MOSFET gain mode
More common with MOSFETs is the gain mode; it is the reverse of the depletion mode. Here, the conductive channel is lightly doped or even undoped, which makes it non-conductive. This leads to the fact that the device in standby mode does not conduct current (when the gate bias voltage is zero). In the diagrams, a broken line is used to designate this type of MOS transistor to indicate a normally open current-insulating channel.
To increase the N-channel MOSFET, the drain current will flow only when the gate voltage is applied to the gate more than the threshold voltage. When a positive voltage is applied to the gate to the n-type, the MOSFET (what it is, operating modes, switching circuits, described in the article) attracts a larger number of electrons in the direction of the oxide layer around the gate, thereby increasing the gain (hence the name) of the channel thickness, allowing free flow current.
Amplification Features
An increase in the positive gate voltage will cause resistance in the channel. This will not be shown by the transistor tester, it can only check the integrity of the junctions. To reduce further growth, it is necessary to increase the drain current. In other words, for the gain mode of a p-channel MOS transistor:
- A positive signal transistor translates into conductive mode.
- The absence of a signal or its negative value translates the transistor into non-conductive mode. Therefore, in gain mode, the MOS transistor is equivalent to a “normally open” switch.
The converse is true for amplification modes of p-channel MOSFETs. At zero voltage, the device is in the “Off” mode and the channel is open. Applying a negative voltage to the p-type gate of the MOSFET increases the conductivity of the channels, translating it to On. You can check using a tester (digital or arrow). Then for the gain mode of the p-channel MOS transistor:
- A positive signal translates the transistor "Off".
- Negative turns the transistor on.
Amplification Mode of N-Channel MOSFET
In gain mode, MOSFETs have a low input impedance in a conducting mode and are extremely high in non-conducting. Also, their infinitely high input impedance due to their insulated shutter. The gain mode of transistors is used in integrated circuits to obtain CMOS type logic gates and switch power circuits in the form of both PMOS (P-channel) and NMOS (N-channel) inputs. CMOS is a complementary MOS in the sense that this logic device has both PMOS and NMOS in its design.
Amplifier on MOSFET
Just like field-effect transistors, MOSFETs can be used to make Class A amplifiers. Amplifier circuits with an N-channel MOSFET common source amplification mode, is the most popular. On MOSFETs, depletion amplifiers are very similar to field device circuits, except that the MOSFET (what it is and what types are discussed above) has a higher input impedance.
This impedance is controlled at the input by a bias resistor circuit formed by resistors R1 and R2. In addition, the output signal for the common source of the amplifier on MOSFET transistors in the amplification mode is inverted, because when the input voltage is low, the transition of the transistor is open. This can be verified by having only a tester in the arsenal (digital or even arrow). With a high input voltage, the transistor is on, the output voltage is extremely low.