Field-effect transistor

The field-effect transistor (FET) is a type of transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with the dual-carrier-type operation of bipolar (junction) transistors (BJT). The concept of the FET predates the BJT, though it was not physically implemented until after BJTs due to the limitations of semiconductor materials and relative ease of manufacturing BJTs compared to FETs at the time.

History

Field-effect transistors were invented by Julius Edgar Lilienfeld in 1925 and by Oskar Heil in 1934, but practical devices were not made until much later.

Terminals

All FETs have a gate, drain, and source terminal that are roughly similar to the base, collector, and emitter of BJTs. Aside from the JFET, all FETs also have a fourth terminal called the body, base, bulk, or substrate. This fourth terminal serves the technical purpose of biasing the transistor into operation; it is rare to make non-trivial use of the body terminal in circuit designs, but its presence is important when setting up the physical layout of an integrated circuit.


Cross section of an n-type MOSFET

The names of the terminals refer to their functions. The gate terminal may be thought of as controlling the opening and closing of a physical gate. This gate permits electrons to flow through or blocks their passage by creating or eliminating a channel between the source and drain. Electrons flow from the source terminal towards the drain terminal if influenced by an applied voltage. The body simply refers to the bulk of the semiconductor in which the gate, source and drain lie. Usually the body terminal is connected to the highest or lowest voltage within the circuit, depending on type. The body terminal and the source terminal are sometimes connected together since the source is also sometimes connected to the highest or lowest voltage within the circuit, however there are several uses of FETs which do not have such a configuration, such as transmission gates and cascode circuits.


Composition

The FET can be constructed from a number of semiconductors, silicon being by far the most common. Most FETs are made with conventional bulk semiconductor processing techniques, using the single crystal semiconductor wafer as the active region, or channel.

Among the more unusual body materials are amorphous silicon, polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field effect transistors that are based on organic semiconductors and often apply organic gate insulators and electrTypes of field-effect transistors

The channel of a FET (explained below) is doped to produce either an N-type semiconductor or a P-type semiconductor. The drain and source may be doped of opposite type to the channel, in the case of enhancement mode FETs, or doped of similar type to the channel as in depletion mode FETs. Field-effect transistors are also distinguished by the method of insulation between channel and gate. Types of FETs are:

  • The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) utilizes an insulator (typically SiO2) between the gate and the body .
  • The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction to separate the gate from the body.
  • The MESFET (Metal–Semiconductor Field-Effect Transistor) substitutes the p-n junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.
  • Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High Electron Mobility Transistor), also called an HFET (heterostructure FET). The fully depleted wide-band-gap material forms the isolation between gate and body.
  • The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region.
  • The IGBT (Insulated-Gate Bipolar Transistor) is a device for power control. It has a structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These are commonly used for the 200-3000 V drain-to-source voltage range of operation. Power MOSFETs are still the device of choice for drain-to-source voltages of 1 to 200 V.
  • The FREDFET (Fast Reverse or Fast Recovery Epitaxial Diode FET) is a specialized FET designed to provide a very fast recovery (turn-off) of the body diode.
  • The ISFET is an Ion-Sensitive Field Effect Transistor used to measure ion concentrations in a solution; when the ion concentration (such as pH) changes, the current through the transistor will change accordingly.
  • The DNAFET is a specialized FET that acts as a biosensor, by using a gate made of single-strand DNA molecules to detect matching DNA strands.

FET operation

The FET controls the flow of electrons (or electron holes) from the source to drain by affecting the size and shape of a "conductive channel" created and influenced by voltage (or lack of voltage) applied across the gate and source terminals. (For ease of discussion, this assumes body and source are connected). This conductive channel is the "stream" through which electrons flow from source to drain.

Consider an n-channel "depletion-mode" device. A negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel. If the depletion region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch. Likewise a positive gate-to-source voltage increases the channel size and allows electrons to flow easily.

Now consider an n-channel "enhancement-mode" device. A positive gate-to-source voltage is necessary to create a conductive channel, since one does not exist naturally within the transistor. The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET; this forms a region free of mobile carriers called a depletion region, and the phenomenon is referred to as the threshold voltage of the FET. Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain; this process is called inversion.

For either enhancement- or depletion-mode devices, at drain-to-source voltages much less than gate-to-source voltages, changing the gate voltage will alter the channel resistance, and drain current will be proportional to drain voltage (referenced to source voltage). In this mode the FET operates like a variable resistor and the FET is said to be operating in a linear mode or ohmic mode.[1][2]

If drain-to-source voltage is increased, this creates a significant asymmetrical change in the shape of the channel due to a gradient of voltage potential from source to drain. The shape of the inversion region becomes "pinched-off" near the drain end of the channel. If drain-to-source voltage is increased further, the pinch-off point of the channel begins to move away from the drain towards the source. The FET is said to be in saturation mode;[3] some authors refer to it as active mode, for a better analogy with bipolar transistor operating regions.[4][5] The saturation mode, or the region between ohmic and saturation, is used when amplification is needed. The in-between region is sometimes considered to be part of the ohmic or linear region, even where drain current is not approximately linear with drain voltage.

Even though the conductive channel formed by gate-to-source voltage no longer connects source to drain during saturation mode, carriers are not blocked from flowing. Considering again an n-channel device, a depletion region exists in the p-type body, surrounding the conductive channel and drain and source regions. The electrons which comprise the channel are free to move out of the channel through the depletion region if attracted to the drain by drain-to-source voltage. The depletion region is free of carriers and has a resistance similar to silicon. Any increase of the drain-to-source voltage will increase the distance from drain to the pinch-off point, increasing resistance due to the depletion region proportionally to the applied drain-to-source voltage. This proportional change causes the drain-to-source current to remain relatively fixed independent of changes to the drain-to-source voltage and quite unlike the linear mode operation. Thus in saturation mode, the FET behaves as a constant-current source rather than as a resistor and can be used most effectively as a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current through the channel.


Induktor tersimulasi


Fungsi utama dari girator adalah untuk mensimulasi unsur induktif pada sirkuit elektronik kecil atau sirkuit terintegrasi. Sebelum penemuan transistor, lilitan kawat dengan induktansi tinggi digunakan untuk membuat tapis elektronik. Induktor yang sebenarnya dapat digantikan dengan rangkaian yang lebih kecil yang terdiri dari kondensator, penguat dan resistor. Hal ini sangat penting pada teknologi sirkuit terintegrasi karena induktor biasanya secara relatif sangat besar. Selain itu, pada kenyataannya, kondensator yang ada biasanya lebih dekat kepada keadaan ideal daripada induktor. Karena itu, sebuah induktor sintetik terbuat dari girator mungkin jauh lebih dekat pada induktor ideal daripada yang bisa dilakukan dengan induktor sebenarnya. Selain itu, penggunaan girator mungkin menambah kualitas jaringan tapis daripada jika menggunakan induktor. Faktor Q dari sebuah induktor sintetis juga dapat ditentukan dengan mudah. Walaupun begitu, girator tidak dapat menggantikan induktor yang digunakan untuk menimbulkan efek 'flyback', seperti pembuatan gaya elektromotif lawan yang besar saat terjadi perubahan arus.

Cara kerja sirkuit

Sirkuit ini bekerja dengan cara membalik efek kapasitansi kondensator. Efek yang diinginkan pada girator adalah induktansi "L" dengan resistansi deret RL:

Z = R_\mathrm{L} + j \omega L \,\!

Impedansi masukan sirkuit op-amp adalah:

Z_\mathrm{in} = \left(   R_\mathrm{L} + j \omega R_\mathrm{L} R C \right) \| \left( R + {1 \over {j \omega C}} \right)

Dengan RLRC = L, dapat terlihat bahwa impedansi dari induktor tersimulasi adalah impedansi yang diinginkan yang berjajar dengan impedansi dari C dan R. Pada desain yang umum, R dibuat cukup besar sehingga persamaan menjadi:

Z_\mathrm{in} = R_\mathrm{L} + j \omega R_\mathrm{L} R C \,\!

Ini sama dengan resistansi RL berderet dengan induktansi L = RLRC.

Pada penggunaan biasa, induktansi dan resistansi dari girator jauh lebih besar daripada induktor sesungguhnya. Girator dapat digunakan untuk membuat induktor dalam jangkah mikrohenry hingga megahenry. Induktor asli biasanya terbatas hanya hingga puluhan henry dengan resistansi deret diantara beberapa mikroohm hingga beberapa ratus ohm. Resistansi deret dari girator bergantung pada topologi sirkuit yang digunakan, tetapi biasanya diantara puluhan ohm hingga ratusan kiloohm. Untuk frekuensi yang sama, sebuah girator mempunyai induktansi yang jauh lebih besar, kapasitansi yang jauh lebih rendah, tetapi resistansioya lebih tinggi. Girator pada umumnya memiliki tingkat ketepatan yang lebih tinggi daripada induktor, dikarenakan kondensator presisi jauh lebih murah daripada induktor presisi.

Penggunaan

Penggunaan utama dari girator adalah untuk menggantikan induktor yang terlalu besar, berat dan mahal. Sebagai contoh, tapis lulus jalur dapat dibangun dengan menggunakan kondensator, resistor dan penguat, tanpa induktor.

Sirkuit girator sering digunakan pada peranti telefon yang terhubung ke sistem POTS. Memungkinkan telepon untuk menjadi lebih kecil. Girator juga digunakan untuk equalizer grafik hi-fi, equalizer parametrik, filter lulus-jalur, filter stop-jalur, filter desah, dan filter sinyal pembantu FM.

Ada beberapa penggunaan dimana penggunaan girator sebagai pengganti induktor tidak memungkinkan.

  • Tegangan tinggi, dimana tegangan jauh melebihi tegangan kerja penguat.
  • Sistem frekuensi radio, dimana induktor untuk frekuensi radio relatif kecil dan mudah dibuat.
  • Pengubah daya, dimana lilitan digunakan sebagai penyimpan daya sementara.