RU2029415C1 - Field-effect transistor - Google Patents

Abstract

FIELD: cryotronics. SUBSTANCE: field-effect transistor is formed on dielectric substrate. The latter bears buffer layer with conducting channel and gate made of insulating material. Metal electrode is provided for gate. Channel is made of superconducting material. Second buffer layer may be provided between channel and gate. EFFECT: improved design. 5 cl, 2 dwg

Description

 The invention relates to cryoelectronics and can be used to create the element base of superconducting microelectronics and, in particular, fully superconducting integrated circuits.

 An insulated gate field effect transistor containing an induced conductive channel and an adjacent gate, consisting of a dielectric material with a metal contact to the latter, is the basis of modern microelectronics and is described in all monographs in this direction [1].

Currently, a significant part of the work on high-temperature superconductivity is aimed at creating semiconductor integrated circuits in which field-effect transistors should be active elements, and microstrip interconnects are made of a high-temperature superconductor. However, in such integrated circuits, the advantage of superconducting interconnects associated with a decrease in the delay time cannot be realized due to the sharp mismatch of the wave impedance of the superconducting microstrip line, which is units of Ohm, and the channel resistance of the open transistor (in microelectronic design), which is 10 ³ -10 ⁴ ohms. Due to such a mismatch, the signal propagation in the line has not a wave but a diffusion character, in which the delay is determined not by the phase velocity of the signal propagation, but by the linear capacity of the line, independent of its resistance.

 The closest analogue of the invention is one of the varieties of an insulated gate field effect transistor - a hidden channel transistor [2]. Its construction contains a conductive channel made of doped n-silicon, to which a buffer silicon layer adheres on one side, and on the other hand there is a gate made of a dielectric material (silicon dioxide) with a metal contact to it. The purpose of the buffer layer is to reduce the effect of substrate defects on the channel material.

The disadvantage of the closest analogue is the high (10 ³ -10 ⁴ Ohm) channel resistance, which makes it impossible to match the transistor with a superconducting microstrip line.

The aim of the invention is to provide an insulated gate field effect transistor having a channel resistance of less than 10 ¹ Ohm, i.e. consistent with wave impedance of superconducting microstrip lines.

а буферный слой выполнен из материала с нормальным характером проводимости и кристаллической решеткой, одинаковой с кристаллической решеткой материала канала, где ε - диэлектрическая постоянная материала канала, Ф/см; ε_o - диэлектрическая постоянная вакуума, Ф/см; Е_пр - напряженность поля в канале при пробое, В/см; q - заряд электрона, Kул; А - размер одной ячейки монокристалла материала канала, см; Р_о - концентрация свободных носителей, при которой разрушается сверхпроводящее состояние, см^-3, проводящий канал может быть выполнен из сверхпроводника состава YBa₂Cu₃O₇. Между проводящим каналом и диэлектрическим материалом затвора может быть введен второй буферный слой из материала с нормальным характером проводимости и кристаллической решеткой, одинаковой с кристаллической решеткой сверхпроводящего материала канала, толщиной d₁, удовлетворяющей соотношению:
A₁< d₁<

, см, где А₁ - размер одной ячейки монокристалла второго буферного слоя, см; Р₁ - концентрация дырок во втором буферном слое, см^-3.The goal is achieved in a field-effect transistor containing a conductive channel to which the buffer layer adheres on one side, and on the other hand there is a gate made of a dielectric material with a metal contact to the latter, the channel being made of a superconducting material with a concentration of free carriers P _o ≅ P ≅ 10 ²² cm ^-3 and a thickness d, cm corresponding to the ratio
A <d <

and the buffer layer is made of a material with a normal character of conductivity and a crystal lattice identical with the crystal lattice of the channel material, where ε is the dielectric constant of the channel material, f / cm; ε _o - dielectric constant of the vacuum, f / cm; E _CR - field strength in the channel during breakdown, V / cm; q is the electron charge, Kool; A is the size of one cell of a single crystal of the channel material, cm; P _about - the concentration of free carriers at which the superconducting state is destroyed, cm ^-3 , the conductive channel can be made of a superconductor of the composition YBa ₂ Cu ₃ O ₇ . Between the conductive channel and the dielectric material of the gate can be introduced a second buffer layer of material with a normal nature of conductivity and a crystal lattice identical to the crystal lattice of the superconducting channel material, thickness d ₁ , satisfying the ratio:
A ₁ <d ₁ <

, cm, where A ₁ - the size of one cell of a single crystal of the second buffer layer, cm; P ₁ is the concentration of holes in the second buffer layer, cm ^-3 .

Both buffer layers can be made of PrBa ₂ Cu ₃ O ₇ , and the gate can be made of SrTiO ₃ .

 The essence of the invention is illustrated by drawings (Fig. 1 and 2), where the following notation is adopted: conductive channel 1, first buffer layer 2, dielectric material of the gate 3, metal contact 4 to the dielectric material, substrate 5, contacts 6 of the superconducting material to the conductive channel 1 , metal contacts 7 of the source and drain, the second buffer layer 8.

The essence of the proposed design is that the conductive channel is made of a superconductor with a relatively low concentration of free carriers P _o ≅ P ≅ 10 ²² cm ^-3 and has a very small thickness, i.e. the total number of superconducting pairs per unit area of the channel is relatively small. This makes it possible to obtain a very low resistance (in the limit - zero) of the conducting channel in the absence of bias at the gate, and when applying a working bias to the gate, to ensure the removal of free carriers by the electric field into the buffer layer and the destruction of superconductivity, and the electric field in the channel material at this does not exceed the critical (breakdown) value at which the channel breakdown occurs.

dx и равна d =

. При этом поле Е_м должно быть меньше критического поля пробоя материала канала Е_пр, откуда следует выражение для предельно допустимой толщины канала A < d <

. Если выполнить проводящий канал из любого металлического сверхпроводника с Р

10²³ см^-3, что d<1 x 10^-9 см, т. е. тоньше, чем один атомный монослой, что нереально. Открытые недавно высокотемпературные сверхпроводящие керамики типа YBa₂Cu₃O₇ имеют Р ≅ 5 х 10²¹ см^-3, что дает при ε ≃ 100 Р=10²¹ см^-3 и Е_пр=10⁷ В/см d=6 x 10^-8 см. Поскольку заметное изменение концентрации свободных носителей должно происходить на расстоянии порядка 2-3 d, то канал может иметь толщину порядка одной-двух ячеек кристалла YBa₂Cu₃O₇. Эксперименты показали, что слой такой толщины, выращенный на буферном слое состава PrBa₂Cu₃O₇, обладает переходом в сверхпроводимость при температуре ≈50 К.If the shutter creates a field E _m at the boundary with the channel, then the screening depth, i.e. the thickness of the layer from which free carriers will be removed is determined from the solution of the Poisson equation
dE =

dx and is equal to d =

. In this case, the field E _m should be less than the critical breakdown field of the channel material E _pr , whence the expression for the maximum allowable channel thickness A <d <

. If you make a conductive channel of any metal superconductor with P

10 ²³ cm ^-3 , which d <1 x 10 ^-9 cm, i.e., thinner than one atomic monolayer, which is unrealistic. Recently discovered high-temperature superconducting ceramics like YBa ₂ Cu ₃ O ₇ have P ≅ 5 x 10 ²¹ cm ^-3 , which gives ε при 100 P = 10 ²¹ cm ^-3 and E _pr = 10 ⁷ V / cm d = 6 x 10 ^-8 cm. Since a noticeable change in the concentration of free carriers should occur at a distance of the order of 2-3 d, the channel can have a thickness of the order of one or two cells of a YBa ₂ Cu ₃ O ₇ crystal. The experiments showed that a layer of this thickness grown on a buffer layer of the composition PrBa ₂ Cu ₃ O ₇ has a transition to superconductivity at a temperature of ≈50 K.

The first buffer layer is made of a material with a normal character of conductivity at an operating temperature; this type of conductivity makes it possible to obtain a large difference in resistances during the destruction of superconductivity in the channel. This material should have the same crystal lattice with the channel to preserve the channel properties. The thickness of the buffer layer should be large enough so that during its growth healing of growth defects caused by defects of the substrate and diffusion of impurities from it occurs. Since not all dielectric materials can be grown directly on a thin conducting channel without violating its superconducting properties, it is then possible to introduce a second buffer layer between the channel and the dielectric layer. This second buffer layer should have a normal character of conductivity and have the same crystal lattice with the conducting channel, all this contributes to the preservation of the superconducting properties of the channel. If the channel is made of superconducting ceramics YBa ₂ Cu ₃ O ₇ , then both buffer layers should be made of ceramics of the composition PrBa ₂ Cu ₃ O ₇ , which has a normal conductivity and a crystal lattice that matches the lattice YBa ₂ Cu ₃ O ₇ . In the second buffer layer, the charge of the carriers present in this layer shields the gate field from the channel. Therefore, to reduce the value of the shielding charge, the thickness of the second buffer layer should be chosen so that the specific charge in it per unit area is at least an order of magnitude less than the specific charge in the channel, i.e., the relation
Pd ≥ 10p ₁ d ₁ .

On the other hand, the thickness of the second buffer layer should be greater than the size of one single crystal cell of the buffer layer material. The channel in the proposed transistor can be made both from YBa ₂ Cu ₃ O ₇ , and from BiSrCaO, TlBaCaCuO. For these materials, a channel of corresponding materials for buffer layers has not yet been found. The dielectric layer is best made of SrTiO ₃ , BaTiO ₃ , TiO ₂ , ZrO ₂ , 4GaO ₃ xx NdGaO ₃ ˙ LaAlO ₃ can also be used. The drain and source must also be made of the same superconducting material as the channel, so as not to violate the superconductivity of the channel.

 The proposed transistor operates as follows.

When the transistor is cooled to a temperature lower than the temperature of the transition to the superconducting state of the channel material, the conductivity in it is ensured by paired holes and the channel resistance is zero. When an external voltage is applied to terminals 7, the source-drain (I-C, + at the source), current limited by the external circuit flows through channel 1. When a control signal is applied to the gate 3 circuit - drain 7 (3-C, + on the gate), the paired holes are forced out by the field into the lower buffer layer 2 and are steamed. As a result of this, the channel conductivity decreases from the value determined by the resistance to movement of normal holes in the lower buffer layer 2. The difference in resistance in this case is 10 ¹⁰ -10 ¹² , which ensures good key properties of the device. When the control signal is removed from the shutter 3, superconductivity in the channel is restored.

 The transistors were manufactured according to the following flow chart.

с осью С, направленной вертикально. Затем на этой пленке был выращен проводящий канал 1 толщиной 20

и контакты 6 толщиной 500

из YBa₂Cu₃O₇ с Р=5 х 10²¹см^-3. После этого был выращен второй буферный слой из PrBa₂Cu₃O₇толщиной 200

, диэлектрический слой из SrTiO₃ толщиной 500

и методом термического распыления в вакууме нанесены алюминиевые контакты 7 толщиной 0,5 мкм и обкладка затвора 4 толщиной 0,5 мкм. Ширина проводящего канала была 100 мкм, длина - 400 мкм. Без управляющего сигнала на затворе сопротивление сток-исток составляло 2 х 10^-5 Ом, что определялось сопротивлением контактов. При подаче на затвор управляющего сигнала с напряжением U₃=30 В сопротивление возрастало до 2 х 10³ Ом, что примерно равно сопротивлению буферного слоя 2.On a substrate 5 of SrTiO ₃ (100), a film 1000 of PrBa ₂ Cu ₃ O ₇ 1000 was grown by laser spraying.

with the C axis directed vertically. Then, a conductive channel 1 with a thickness of 20 was grown on this film

and contacts 6 with a thickness of 500

from YBa ₂ Cu ₃ O ₇ with P = 5 x 10 ²¹ cm ^-3 . After that, the second buffer layer was grown from PrBa ₂ Cu ₃ O ₇ with a thickness of 200

500 SrTiO ₃ dielectric layer

and by thermal spraying in a vacuum, aluminum contacts 7 with a thickness of 0.5 μm and a gate lining 4 with a thickness of 0.5 μm are applied. The width of the conductive channel was 100 μm, the length was 400 μm. Without a control signal at the gate, the drain-source resistance was 2 x 10 ^-5 Ohms, which was determined by the contact resistance. When applying to the gate of the control signal with a voltage of U ₃ = 30 V, the resistance increased to 2 x 10 ³ Ohms, which is approximately equal to the resistance of the buffer layer 2.

 Thus, the invention allowed to sharply reduce the resistance of the conductive channel of the field-effect transistor and at the same time to obtain a difference in resistance when switching in eight orders of magnitude. This opens up a fundamentally new possibility of creating completely superconducting integrated circuits, where the proposed transistor will be used as the active element, and the interconnects will be made in the form of microstrip superconducting lines.

Claims (5)

1. FIELD TRANSISTOR containing a buffer layer, a conducting channel located on it and a gate made of a dielectric material with a metal contact to the latter, characterized in that the conducting channel is made of a superconducting material with a concentration of free carriers P _{0 0} P ≅ 10 ^{2 2} cm ^{- 3} and thickness d, cm, satisfying the relation

and the buffer layer is made of a material with a crystal lattice identical to the crystal lattice of the channel material, where ε is the dielectric constant of the channel material, F / cm; e _o is the dielectric constant of the vacuum, f / cm; E _{p p} - field strength in the channel material during breakdown, V / cm; q is the electron charge; A is the single crystal cell size of the channel material, cm; P ₀ is the concentration of free carriers at which the superconducting state is destroyed, cm ^{- 3} . 2. The transistor according to claim 1, characterized in that the conductive channel is made of a superconductor of the composition JBa ₂ Cu ₃ O ₇ . 3. The transistor in paragraphs. 1 and 2, characterized in that in addition between the channel and the layer of gate dielectric material, a second buffer layer of material with a normal conductivity and a crystal lattice identical with the crystal lattice of the channel superconducting material, thickness d ₁ , satisfying the ratio

where P ₁ is the concentration of holes in the second buffer layer, cm ^{- 3} ;
A ₁ - cell size of the material of the second buffer layer, see 4. The transistor according to claim 3, characterized in that both buffer layers are made of PrBa ₂ Cu ₃ O ₇ . 5. The transistor according to claims 1 to 4, characterized in that SrTiO ₃ is used as the gate dielectric material.

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