Mosfet - what is it? Application and testing of transistors

In the article you will learn about MOSFET transistors , what is it, what kind of connection schemes are there. There is a type of FET, in which the input is electrically isolated from the main current of the carrier channel. And that's why it's called a field-effect transistor with an isolated gate. The most common type of such a field effect transistor, which is used in many types of electronic circuits, is a metal-oxide-semiconductor field-effect transistor based on a junction or a MOS transistor (abbreviated abbreviation for this element).

What is MOSFET transistors?

The MOSFET is a voltage-controlled field-effect transistor that differs from the field one in that it has a "metal oxide" gate electrode that is electrically isolated from the main semiconductor by an n-channel or p-type channel with a very thin layer of insulating material. As a rule, it is silicon dioxide (and, if it is easier, glass).

This ultra-thin insulated metal gate electrode can be considered as one capacitor plate. Isolation of the control input makes the resistance of the MOSFET extremely high, almost infinite.

Like field, MOSFETs have a very high input resistance. It can easily accumulate a large amount of static charge, which leads to damage if the circuit is not carefully protected.

Differences MOSFET from field effect transistors

The main difference from the field is that MOSFETs are produced in two basic forms:

1. Depletion - the transistor requires a gate-source voltage to switch the device to the "Off" position. The MOSFET depletion mode is equivalent to a "normally closed" switch.
2. Saturation - the transistor requires a gate-source voltage to turn on the device. The MOSFET amplifier mode is equivalent to a switch with "normally closed" contacts.

Graphical designations of transistors on circuits

The line between the drain and source connections is a semiconductor channel. If the circuit on which MOSFET transistors are depicted is represented by a solid solid line, then the element operates in a depletion mode. Since the current from the drain can flow with a zero gate potential. If the line of the channel is shown dotted or broken, then the transistor operates in saturation mode, since a current with zero gate potential flows. The direction of the arrow indicates a conductive channel, a p-type or a semiconductor device of the p-type. And domestic transistors are indicated in exactly the same way as foreign analogs.

The basic structure of the MOSFET transistor

The design of the MOSFET (which is described in detail in the article) is very different from the field ones. Both types of transistors use an electric field created by the voltage on the gate. To change the flow of charge carriers, electrons for the p-channel or the p-channel opening, through the semiconductor drain-source channel. The gate electrode is placed on the vertex with a very thin insulating layer, and there is a pair of small p-type regions just under the drain and the source of the electrodes.

Using an isolated gate device for a MOSFET, no restrictions apply. Therefore, you can connect the source of the signal in any polarity (positive or negative) to the gate of the 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 without external interference they usually do not conduct current. And the reason for this is the high input impedance of the shutter. Consequently, very small or insignificant control is necessary for MOSFETs. After all, they are devices controlled externally by voltage.

MOSFET depletion mode

The depletion mode occurs much less frequently than the amplification regimes without applying a bias voltage to the gate. That is, the channel spends at zero voltage on the gate, hence, the device is "normally closed". The circuits use a solid line to denote a normally closed conductive channel.

For the n-channel MOSFET, the negative gate-source voltage is negative, will drain (hence the name) the conducting channel of its free electrons of the transistor. Similarly, for a p-channel MOSFET, depletion of the positive gate-source voltage will deplete the channel of its free holes, transferring the device to a non-conductive state. But the continuity of the transistor does not depend on what mode of operation.

In other words, for the depletion mode of the n-channel MOSFET:

1. A positive voltage on the drain means more electrons and current.
2. Negative voltage means less electrons and current.

Reverse statements are also true for the p-channel transistors. Then the MOSFET depletion mode is equivalent to a "normally open" switch.

N-channel MOSFET in depletion mode

The depletion mode of the MOSFET is constructed in the same way as for FETs. And the drain-source channel is a conducting layer with electrons and holes, which is present in the p-type or p-type channels. This channel doping creates a conductive path of low resistance between the drain and a source with zero voltage. Using a transistor tester, you can measure the currents and voltages at its output and input.

MOSFET amplifier mode

More common in MOSFET transistors is the gain mode, it is the reverse for the depletion mode. Here the conducting channel is weakly doped or even unalloyed, which makes it non-conductive. This leads to the fact that the device does not conduct a current in the resting mode (when the gate bias voltage is zero). In circuits for designating MOSFETs of this type, a broken line is used to denote a normally open current-carrying 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 MOSFET p-type (which it is, operation modes, switching circuits, described in the article), it attracts more electrons towards the oxide layer around the gate, thereby increasing the gain (hence the name) of the channel thickness, allowing more free flow Current.

Features of the gain mode

An increase in the positive gate voltage will cause resistance in the channel. This will not show a transistor tester, it can only verify the integrity of the transitions. To reduce further growth, you need to increase the drain current. In other words, for the amplification mode of the n-channel MOSFET:

1. The positive signal transistor translates into a conducting mode.
2. The absence of a signal or its negative value transforms the transistor into a non-conducting mode. Therefore, in the amplification mode, the MOSFET is equivalent to a "normally open" switch.

The reverse statements are valid for the regimes of amplification of p-channel MOS transistors. At zero voltage, the device is in the OFF mode and the channel is open. Applying a voltage of negative value to the p-type gate in MOSFET increases the conductivity of the channels, translating its "on" mode. You can check using a tester (digital or switch). Then for the gain mode of the p-channel MOSFET:

1. A positive signal translates the transistor "Off".
2. Negative switches the transistor on.

Mode of amplification of N-channel MOSFET

In the amplification mode, MOSFETs have a low input resistance in conducting mode and extremely high in non-conductive mode. Also their infinitely high input resistance due to their isolated shutter. The transistor amplification mode is used in integrated circuits to obtain the type of CMOS logic gates and the switching of power circuits in the form, like PMOS (P-channel) and NMOS (N-channel) inputs. CMOS is a complementary MOS in the sense that this logical device has both PMOS and NMOS in its design.

Amplifier on MOSFET

Just like the field ones, MOSFET transistors can be used to produce Class A amplifiers. Amplifier circuits with an N-channel MOS transistor of the common initial gain mode are the most popular. On MOSFETs, the depletion-type amplifiers are very similar to those using field devices, 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 resistive circuit formed by resistors R1 and R2. In addition, the output signal for the common amplifier source on the MOSFET transistors in the gain mode is inverted, because when the input voltage is low, the transistor transition is open. This can be verified by having only a tester (digital or even switch) in the arsenal. With a high input voltage transistor in the on mode, the output voltage is extremely low.