Field-effect transistor

A field-effect transistor is an electrical semiconductor device whose output current is controlled by a field, therefore, by a voltage of the same sign. The forming signal is fed to the gate, it regulates the conduction of the channel of n or p-type. Unlike bipolar transistors, where the signal is of alternating polarity. The second sign is the formation of the current exclusively by the main carriers( of the same sign).

Classification of field-effect transistors

Let's start the classification. Varieties of field-effect transistors are numerous, each works according to the algorithm:

1. Type of conduction channel: n or p. The factor determines the polarity of the control voltage.
2. By structure. With pn-transition fused, diffusion, MDP( MOP), with a Schottky barrier, thin-film.
3. The number of electrodes is 3 or 4. In the latter case, the substrate is considered a separate subject, allowing you to control the flow of current through the channel( besides the gate).
4. Conductor Material. Silicon, germanium, gallium arsenide are common today. The semiconductor material is marked with the symbol letters( K, D, A) or( in products of the military industry) numbers( 1, 2, 3).
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5. The application class is not included in the marking, indicated by reference books providing information that the field-effect transistor is often included in the composition of amplifiers, radio receivers. In world practice, there is a division in applicability into the following 5 groups: high, low frequency, direct current amplifiers, modulators, key ones.
   

   Semiconductor transistor

6. The range of electrical parameters determines the set of values ​​in which the field-effect transistor remains operable. Voltage, current, frequency.
7. By design features distinguish unitrons, alkathrons, technetrons, grid resistors. Each device is endowed with key features. Alkatron electrodes are made with concentric rings, increasing the amount of current flowing.
8. By the number of structural elements enclosed by one substrate emit double, complementary.

In addition to the general classification, a specialized, defining operating principle has been invented. Distinguish:

1. Field effect transistors with pn-junction control.
2. Schottky Field Effect Transistors.
3. Insulated-Field Effect Transistors:

• With built-in channel.
• With an induced channel.

In the literature, structures are additionally ordered as follows: it is impractical to use the MOP designation, structures on oxides are considered a special case of MIS( metal, dielectric, semiconductor).The Schottky barrier( MeP) should be separately identified, since it is a different structure. Reminds properties p-n-transition. We add that structurally the dielectric( silicon nitride) and oxide( tetravalent silicon) are capable of entering the transistor at the same time, as happened with KP305.Such technical solutions are used by people looking for methods to obtain the unique properties of the product, reduce the cost.

Among foreign abbreviations for field-effect transistors, the combination FET is reserved, sometimes it denotes the type of control with a pn-junction. In the latter case, along with this we meet JFET.Synonyms. Abroad, it is customary to separate oxide( MOSFET, MOS, MOST - synonyms) and nitride( MNS, MNSFET) field effect transistors. The presence of a Schottky barrier is marked with SBGT.Apparently, the material value, the domestic literature the significance of the fact is silent.

The electrodes of field-effect transistors in the diagrams are denoted: D( drain) - drain, S( source) - source, G( gate) - gate. Substrate is called substrate.

Field-effect transistor device

The control electrode of a field-effect transistor is called a gate. The channel is formed by a semiconductor of arbitrary conductivity type. The polarity of the control voltage is positive or negative. The field of the corresponding sign displaces free carriers until the isthmus under the gate electrode becomes empty at all. Achieved by applying a field to either the pn-junction or the homogeneous semiconductor. The current becomes zero. This is how a field effect transistor works.

The current flows from the source to the drain, beginners traditionally are tormented by the question of distinguishing the two indicated electrodes. There is no difference in which direction the charges move. Field effect transistor is reversible. The unipolarity of the charge carriers explains the low noise level. Therefore, in the field of technology transistors occupy a dominant position.

A key feature of the devices is a large input resistance, especially alternating current. The obvious fact arises from the control of the reverse-biased pn-junction( the Schottky transition) or the capacitance of the technological capacitor in the region of the insulated gate.

Substrates are often protruded unalloyed semiconductors. For field-effect transistors with a Schottky gate - gallium arsenide. In its pure form, it is a good insulator to which the product includes the following requirements:

1. No negative phenomena at the junction with the channel, source, drain: photosensitivity, parasitic control over the substrate, hysteresis of parameters.
2. Thermal stability during the technological cycles of product manufacturing: resistance to annealing, epitaxy. The lack of diffusion of impurities in the active layers caused by this degradation.
3. Minimum impurities. The requirement is closely related to the previous one.
4. High-quality crystal lattice, minimum defects.

It is difficult to create a layer of considerable thickness that meets the list of conditions. Therefore, the fifth requirement is added, which consists in the possibility of gradual growth of the substrate to the desired size.

Field effect transistors with control pn-junction and MeP

In this case, the type of gate material conductivity differs from that used by the channel. In practice, there are various improvements. The shutter is composed of five areas, recessed in the channel. Lower voltage can control the flow of current. Mean increase in gain.

The reverse bias of the pn-junction is used in the circuits; the stronger, the narrower the channel for current flow. At a certain voltage value, the transistor is locked. Forward bias is dangerous due to the fact that a powerful controlled circuit can affect the gate circuit. If the junction is open, a large current will flow, or a high voltage will be applied. The normal mode is provided by the correct selection of polarity and other characteristics of the power source, the choice of the operating point of the transistor.

However, in some cases, direct gate currents are intentionally used. It is noteworthy that those MOSFETs can use this mode, where the substrate forms a p – n junction with the channel. The moving charge of the source is divided between the gate and the drain. You can find the area where a significant current gain is obtained. Controlled by shutter mode. With an increase in current iz( up to 100 μA), the parameters of the circuit deteriorate sharply.

A similar inclusion is used by the so-called gate frequency detector circuit. The design exploits the rectifying properties of the pn-junction between the gate and the channel. Forward displacement is small or even zero. The device is still controlled by the gate current. In the drain circuit, a significant signal gain is obtained. Rectified voltage for the gate is blocking, varies according to the input law. Simultaneously with detection, signal amplification is achieved. The drain circuit voltage contains components:

• Constant component. Not used.
• Signal with carrier frequency. Plant on the ground by using filter tanks.
• Signal with baseband frequency. Processed to extract the pledged information.

The disadvantage of the gate frequency detector is considered to be a large non-linear distortion factor. Moreover, the results are equally bad for weak( quadratic dependence of the working characteristic) and strong( exit to the cut-off mode) signals. A somewhat better demonstrates the phase detector on a double gate transistor. A reference signal is fed to one control electrode, an information component is formed on the drain, amplified by a field effect transistor.

Despite large linear distortions, the effect is used. For example, in selective power amplifiers metered by transmitting a narrow frequency spectrum. Harmonics are filtered, do not have a big impact on the final quality of the circuit.

Schottky barrier metal-semiconductor( MeP) transistors are almost identical to those with a pn junction. At least when it comes to work principles. But thanks to the special qualities of the metal-semiconductor transition, the products are capable of operating at an increased frequency( tens of GHz, the boundary frequencies in the region of 100 GHz).At the same time, the MeP structure is simpler to implement when it comes to manufacturing and technological processes. The frequency characteristics are determined by the gate charge time and carrier mobility( for GaAs over 10,000 sq. Cm / V s).

MOSFET

In MOS structures, the gate is reliably isolated from the channel, control is entirely due to the effect of the field. Insulation is carried out by silicon oxide or nitride. It is these coatings easier to apply on the surface of the crystal. It is noteworthy that in this case there are also metal-semiconductor transitions in the area of ​​the source and drain, as in any polar transistor. This fact is forgotten by many authors, or is mentioned in passing through the use of the mysterious phrase “ohmic contacts”.

In the topic about the Schottky diode this question was raised. Not always at the junction of metal and semiconductor barrier. In some cases, ohmic contact. It depends for the most part on the features of technological processing and geometrical dimensions. Technical characteristics of real devices are strongly dependent on various defects of the oxide( nitride) layer. Here are some:

1. The imperfection of the crystal lattice in the surface region is due to broken bonds at the boundary of the change of materials. The influence is exerted as free atoms of a semiconductor, there and impurities like oxygen, which is in any case. For example, when using methods of epitaxy. As a result, energy levels appear that lie in the depth of the forbidden zone.
2. At the boundary of the oxide and semiconductor( 3 nm thick), an excess charge is formed, the nature of which has not yet been explained. Presumably, the role is played by positive empty spaces( holes) of defective atoms of the semiconductor itself and oxygen.
3. The drift of ionized atoms of sodium, potassium and other alkali metals occurs at low voltages on the electrode. This increases the charge accumulated at the boundary of the layers. To block this effect in silicon oxide, phosphorus oxide( anhydride) is used.

Volumetric positive charge in oxide affects the threshold voltage at which the channel is unlocked. The parameter determines the switching speed and determines the leakage current( below the threshold).In addition, the response is influenced by the gate material, the thickness of the oxide layer, and the concentration of impurities. Thus, the result again comes down to technology. To get the specified mode, select materials, geometrical dimensions, manufacturing process with low temperatures. Separate techniques will also reduce the number of defects, which favorably affects the reduction of the parasitic charge.