JPS63144579A - Field-effect element - Google Patents

Abstract

PURPOSE:To change an electric field, in which negative differential conduction appears by applying an electric field from the outside, by giving structure existing between a quantum level of the lowest order formed in a second valley and the bottom of the second valley to a quantum level of the lowest order shaped in a first valley. CONSTITUTION:An AlSb clad layer 102, an active layer 105 formed by alternately laminating five layers of undoped GaSb 103 and Te-doped n<-> type AlSb 104, and an undoped AlSb clad layer 106 are grown onto a zinc-doped p-type gallium antimonide (GaSb) substrate 101 through a molecular-beam epitaxy method, and a gate electrode 112 is prepared. An ohmic contact 107 to the active layer 105 is prepared through a self-alignment method, and a source electrode 111 and a drain electrode 113 are attached. An impurity is doped selectively to an AlSb barrier layer having electron affinity smaller than GaSb regarding the active layer 105. Accordingly, a two-dimentional electron gas is shaped into a GaSb well, resistance between a source and a drain parallel with a thin-film is kept at a low value, and resistance in the direction vertical to the thin-film is kept at a high value, thus applying sufficiently intense electric field in the film thickness direction to a GaSb well layer.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a novel field effect element that changes the conduction characteristics of a semiconductor through which electrons flow due to a field effect.

(Prior Art) It is known that when a high electric field is applied to n-type gallium arsenide or indium phosphide, the Gunn effect appears above the critical electric field (Eih), and the material exhibits negative differential conductivity (NDC). This situation is shown in FIG. The horizontal axis represents the electric field E, and the vertical axis represents the current density J flowing through the semiconductor. It is known that high frequency oscillation occurs in a semiconductor having such an NDC. The mechanism by which negative differential conductivity occurs due to the Gunn effect is that electrons in the semiconductor are accelerated by the application of an electric field and transition from a low-energy, high-mobility valley to a high-energy, low-mobility valley. Figure 5 shows this situation. The vertical axis is the electron energy ε in the conduction band, and the horizontal axis is the electron wave vector. The black circles in the figure schematically show the electron distribution.

The manner in which the electron distribution changes depending on the intensity of the electric field E applied to the semiconductor is determined based on the magnitude relationship between the threshold electric field Eth and E, where negative differential conduction occurs: (a) E< Etb, (b)
Three cases are shown: E=Eth, (c) E>Eth. To realize such a valley-to-valley transition of electrons, the following is required.

1) When no electric field is applied, most of the electrons are distributed in valleys with low energy. For example, the lattice temperature is sufficiently low compared to the energy difference between two valleys.

2) The effective mass of an electron is small in low valleys and large in high valleys.

3) The energy difference ΔεB between the two valleys is smaller than the forbidden band width Eg of the semiconductor so that an avalanche effect does not occur before the valley transition occurs.

Condition 2) is satisfied in most semiconductors. Therefore, negative differential conduction can be realized by selecting a semiconductor that satisfies the condition 3) and setting the temperature so as to satisfy the condition 1). The energy difference Δε8 between the two valleys is greatly related to the conditions 1) and 3). If Δε8 is small, electrons transition to a high-energy valley in a low electric field,
If ΔεB is large, a high electric field is required for the transition to the high energy valley. Here, the value of ΔεB is a quantity specific to each substance, and therefore the threshold electric field EL at which NDC occurs
h was also a substance-specific quantity. As an example of how to change ΔεB externally, N.R. Haston (A.
Physical Review Letter (Physical Review Letter) by R, 1uston) etc.
rs) Vol. 14, p. 639, methods were limited to applying large pressure to the semiconductor or greatly changing the temperature from room temperature. Therefore, there has been no conventional method for externally changing Etb by electrical means such as applying a voltage to change it.

(Problems to be Solved by the Invention) An object of the present invention is to provide a structure in which the electric field 1ath in which negative differential conduction appears in a semiconductor serving as an electron channel can be changed by applying an electric field from the outside. The object of the present invention is to provide a novel field effect element that can change between two states, a state exhibiting differential conduction and a state not exhibiting differential conduction, using a field effect.

(Means for Solving the Problems) The present invention provides a first semiconductor thin film on a semi-insulating or P-type substrate, which has a larger electron affinity than the substrate and has a thickness equal to or less than the mean free path of electrons. and a second semiconductor layer having a lower electron affinity than the first semiconductor thin film. A field effect element having one electrode, a second electrode for injecting electrons in an in-plane direction of the first semiconductor thin film, and a third electrode for ejecting electrons, the field effect element comprising: A semiconductor thin film has at least two valleys in the conduction band in order of decreasing energy, a first valley and a second valley, and the effective mass of the second valley is larger than the effective mass of the first valley. An electric field effect characterized in that the lowest quantum level formed in the second valley has a structure that exists between the lowest quantum level formed in the second valley and the bottom of the second valley. It is element.

(Function) The first electrode described in the claims will be referred to as a gate, the second electrode as a source, and the third electrode as a drain. The first semiconductor is a thin film with a mean free path of electrons or less,
Since the electron affinity is greater than that of the substrate and the second semiconductor, quantum wells are formed within the first semiconductor, and quantum levels appear corresponding to each valley of the conduction band. The lowest quantum level formed in the first valley will be referred to as quantum level 1, and the lowest quantum level formed in the second valley will be referred to as quantum rank 2.

When the gate voltage V is not applied to the gate electrode, the energy difference Δε between quantum level 1 and quantum level 2 is determined as follows.

Δε=ΔεB+ΔεQ2−ΔεQl...(1
) where Δε is the energy difference between quantum level 2 and quantum level 1, ΔεB is the energy difference between the bottom of the second valley and the bottom of the first valley of the first semiconductor, ΔεQ1(i・1.2)
represents the energy difference from the bottom of the i-th valley to the lowest quantum level occurring in that valley. This situation is shown in Figure 2 (a).
Shown below. In the left diagram of Figure 2(a), the vertical axis represents energy and the horizontal axis represents distance in real space in the film thickness direction.In the right diagram of Figure 2(a), the vertical axis represents energy and the horizontal axis represents electron film thickness. It shows a diagram of the wave number vector in the direction. Also Δε9. (i・1,2
) is most simply expressed as an approximation that assumes that the quantum well in the first semiconductor is infinitely deep. Here, t is (blank constant)/(2π), ml represents the effective mass of electrons in the first valley, and L represents the film thickness of the first semiconductor. Here ms>mi and (
From Equation 21, ΔεQl>ΔεQ2, and therefore from Equation (1), ΔεB. In each valley, electrons can only be distributed in an energy state higher than the lowest quantum level El, so by making the first semiconductor a thin film, the energy difference between the valleys in the conduction band can be made smaller. I can do it. This means that the threshold electric field value Eth at which negative differential conductivity occurs can be controlled by setting the thickness of the thin film. An electric field Eso is applied between the source and drain electrodes, and a threshold electric field E where ESD occurs due to the electron transition mechanism is applied.
Since ΔεB replaces Δε, the conditions for exhibiting negative differential conduction beyond the value of tb are 0<kT<Δε<Ex
(Eg: forbidden band width of semiconductor band 1) (3).

Next, a case will be described in which the gate voltage VQ is applied to the gate electrode. To apply VG, an electric field is applied in the thickness direction of the first semiconductor thin film, which becomes an electron channel. In the quantum well to which an electric field is applied, the quantum level formed in the first semiconductor shifts to the lower energy side, or due to bastard, etc., as reported in Physical Review (P.
Physical Review), volume 828, page 3241.

Letting this deviation amount be δε, δε is approximately expressed by the following equation.

Here, mo is the rest mass of an electron in vacuum, F is the applied electric field strength, is the film thickness, and α is the proportionality constant. The unit of δε is me
If V is taken, F is kV/cm, and L is taken as nm, then
The value of α is approximately 4×10−6. From Equation 441, it can be seen that δε is proportional to the effective mass of the electron.

Generally, the electric field strength at which an avalanche effect occurs in a semiconductor is about 150 kV/cm, so from equation (4), the maximum value of the deviation amount is δεmhx < t x to-1(-,,,,'')・L
4< m eV) +++ (51. If the maximum deviation of the energy level due to the electric field for the first valley is set as 66m1lX + δε!Meng↓ for the second valley, then the maximum electric field is applied. The energy difference ΔεF between quantum level 1 and quantum level 2 in this case is given by the following equation.

Δ ε , =Δ ε 10 δε 25 Todoroki −δ ε2 Meng Todoro ... (6) Here δε! A! (j・1.2
) is set as positive. ml〈From ml, δε2A! ＜δ
Therefore, in equation (6), ΔεF×Δε. From the above, the energy difference between the quantum levels in the two valleys can be changed by applying an electric field. However, since the value of δε2meng↓ does not change by more than 6692, the energy value of quantum level 1 is between the energy value of quantum level 2 and the second valley, as described in the claims of this patent. Only when the energy value is between the bottom energy values, as Δε expressed by equation (1), set Δε1 given by equation (6) to ΔεF<0...('71 while satisfying equation 31) This is possible.The situation at this time is shown in Figure 2(b).When the condition of equation m is satisfied, the electrons move to the second valley in a state of thermal equilibrium without applying the source-drain voltage F'sp. distribution and no longer exhibits negative differential conductivity.

Gallium antimonide (G^sb
>, aluminum antimonide (A
The case where QSb) is used will be explained.

In this case, the first valley is the GaSb valley, and the second valley is the GaSb valley.
Corresponds to L valley in b. By performing quantitative calculations, G
It is estimated that a value of 45λ≦L≦70λ is appropriate for the film thickness of aSb.

When −45λ, Δεa=70 meV; Δε. 1-8
1-8O, Δe o2 = 15 m eV, and δe
+sax S 2m eV, δε proverbial = 8meV, so Δε= 5 meV> 0 Δεp=-1meV<0, and in fact, the energy magnitude relationship between quantum level 1 and quantum level 2 is It is reversed by applying gate voltage VQ.

As described above, the field effect element according to the present invention can change the distribution state of electrons between the first valley and the second valley by the gate voltage VG, so that it is possible to change the distribution state of electrons between the first valley and the second valley. Can be used as a possible gun oscillator.

(Embodiment) FIG. 1 is a cross-sectional view showing an embodiment of the field effect l transistor of the present invention, in which a zinc-doped P-type gallium antimonide (hereinafter abbreviated as GaSb) substrate 101 is formed by molecular beam epitaxy. On top, an aluminum antimonide (hereinafter AQSb) cladding layer 102.50^ thick undoped GaSb 103 with a thickness of about 2000 Å.
and 50 thick tellurium (hereinafter Te) doped type 11 8QSb
An active layer 105 formed by alternately stacking five layers of 104 and a 1000-layer-thick cladding layer 106 are sequentially grown to form a gate electrode 112. An ohmic contact 107 is made to 105, and a source electrode 111 and a drain electrode 113 are attached. Here, a P-type substrate was used for the substrate 101 for isolation. In addition, regarding the active layer 105, by selectively doping the 8Q Sb barrier layer, which has a smaller electron affinity than GaSb, G
By forming a two-dimensional electron gas in the aSb well, keeping the resistance parallel to the thin film low between the source and drain, and keeping the resistance high perpendicular to the thin film, a sufficiently strong electric field in the film thickness direction is created in the GaSb well layer. It was designed to take. In this embodiment, the first valley mentioned in the section of the effect is the r valley, and the second valley is the r valley.
The valley becomes the L valley. Under the above design, vI] is Ov
FIG. 3 shows the current-electric field curve between the source and drain when the voltage is varied from 10V to 10V. With the application of vG, N
The area showing DC decreases and completely disappears at VG/IOV. At the same time, the current value also decreases on the low electric field side. The case where GaSb/A Q Sb is used as the first and second semiconductors has been described above, but the material system includes semiconductors/semiconductors such as gallium arsenide/gallium arsenide, aluminum, indium arsenide/zinc telluride, etc. heterozygous,
Any combination of the first and second materials that satisfies the conditions set forth in the claims can be used, such as semiconductor/insulator heterojunctions such as gallium arsenide/calcium fluoride and strontium.

(Effects of the Invention) According to the present invention, by applying a gate voltage, the distribution of electrons in the first valley and electrons in the second valley in the first semiconductor can be reversed, and the distribution of electrons in the first valley can be reversed. The electrons in the valley of
Due to the difference in effective mass of electrons in the second valley, the conduction characteristics of the current flowing between the source and drain can be changed. The field effect transistor according to the present invention can be applied to a mobility modulated field effect transistor, a voltage modulated Gunn oscillator, or the like.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a field effect transistor according to the present invention;
FIG. 2(a) shows an energy band diagram in real space (left) and an energy band diagram in electron wavenumber space (right) in a field effect transistor according to the present invention, with no gate voltage applied. , Fig. 2(b) shows the energy band diagram in real space with gate voltage applied (left) and the energy band diagram in electron wavenumber space (right), and Fig. 3 shows the energy band diagram according to the present invention. Figure 4 is a diagram showing the dependence of the current between the source and train of a field effect transistor, and the electric field characteristics on the gate voltage. Figure 5 is a conceptual diagram showing the valley-to-trough transition of electrons. In the figure, 101, P-type GaSb substrate 102... undoped QSb cladding layer 103...
- Undoped GaSb potential well layer 104...
n-type AQ Sb potential barrier layer 105...
Active layer 106...Undoped^QSb cladding layer 107...
Source and drain ossic contact layer 111...source electrode 112...gate electrode 113...drain electrode 201...-potential formed by the first valley 202...formed by the second valley Potential 211...Quantum level 1 212...Quantum level 2 Figure 3 Figure 4 Electric field Figure 5 Figure 5

Claims (1)

[Claims] a first semiconductor thin film on a semi-insulating or P-type substrate, which has a larger electron affinity than the substrate and has a film thickness equal to or less than the mean free path of electrons; and a first semiconductor thin film having a smaller electron affinity than the first semiconductor thin film. It has at least one two-layer structure consisting of a pair of second semiconductor layers, and a first electrode for applying an electric field in the stacking direction on the top of the stacked structure, and a surface of the first semiconductor thin film. A field effect element having a second electrode for injecting electrons inward and a third electrode for ejecting electrons, wherein the first semiconductor thin film is arranged in a conduction band in order of decreasing energy. The effective mass of the second valley is larger than the effective mass of the first valley, and the lowest quantum level formed in the first valley is the second valley. A field effect element characterized by having a structure that exists between the lowest quantum level formed between two valleys and the bottom of the second valley.