Semiconductor device having a structure for accelerating carriers

Abstract: A semiconductor device comprises a channel (12) of a semiconductor material for passing carriers, a carrier injecting part (16) for injecting the carriers into the channel and establishing an ohmic contact with the channel at a first location, a carrier collecting part (18) for collecting the carriers from the channel, the carrier collecting part establishing an ohmic contact with the channel at a second, different location, a carrier control part (14) provided on the channel at a third location located between the first and second locations, the carrier control part being applied with a control voltage and controlling the passage of the carriers through the channel from the carrier injecting means to the carrier collecting means in response to the control voltage, and an acceleration part (22, 24) provided between the first and third locations including the third location. The acceleration part is supplied with an acceleration voltage and producing an electric field that accelerates the carriers such that the electric field has a magnitude generally proportional to the acceleration voltage applied thereto.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to semiconductor devices, and in particular to a semiconductor device having a channel of carriers wherein a structure for accelerating the carriers is provided. With the progress in the lithographic patterning technique, field effect transistors having an extremely short gate length are becoming available. In such short gate length devices, a very large electric field is induced in the channel of carriers in correspondence to a region located under the gate electrode and a large acceleration of carriers is achieved in such a region of increased electric field. FIG.1(A) shows the fundamental structure of a conventional MESFET and FIG.1(B) shows the band diagram corresponding to the structure of FIG.1(A). Referring to FIG. 1(A), the MESFET comprises a substrate 10 of semi-insulating GaAs on which an n-type GaAs layer 12 is formed. The channel of carriers is formed in this device layer 12. Thus, the layer 12 will be referred to hereinafter as channel layer. In correspondence to the channel of carriers, a gate electrode 14 is provided on the top surface of the channel layer 12, and source and drain electrodes 16 and 18 are provided on the top surface of the layer 12 at both sides of the gate electrode 14 as usual. Thus, the carriers are injected into the channel layer 12 via the source electrode 16 and transported along the channel in the layer 12 to a drain region formed under the drain electrode 18 under the control of a gate voltage applied to the gate electrode 14, and collected by the drain electrode 18 as a drain current. FIG.1(B) shows the band diagram of the device of FIG.1(A) in the state that a source-drain voltage V sd is applied across the source and drain electrodes 16 and 18 and a source-gate voltage V sg applied across the source and gate electrodes 16 and 14, wherein the conduction band is designated by Ec and the valence band is designated by Ev. As can be seen in FIG.1(B), the conduction band and the valence band are generally flat in the region between the source electrode 16 and the gate electrode 14 because the resistance of the channel between the source and gate electrodes is smaller than that under the gate. On the other hand, the conduction band Ec and the valence band Ev are curved steeply in the region between the gate electrode 14 and the drain electrode 18 due to the larger channel resistance under the gate, and the carriers are accelerated by the large electric field associated with the large potential drop. This acceleration of carriers is particularly conspicuous in the region immediately under the gate electrode 14 and is enhanced further when the gate length is reduced. The reduction of the gate length increases the slope of the band and hence the electric field as will be easily understood. FIG.2 shows the effect of carrier overshoot that occurs when the gate length is reduced to increase the carrier velocity as reported by Ruch (J.G. Ruch, IEEE Trans. Electron Devices ED-19, pp.652 - 654, 1972). As can be seen in this diagram, an overshoot occurs in the carrier velocity under high electric filed when the gate length is reduced below 1 µm. For example, it can be seen that a conspicuous overshoot is observed in the device having the gate length of less than 0.5 µm when the electric field is increased to 10 kV/cm. Thereby, the carrier velocity is increased more than three times than the case where the device has a gate length larger than 1 µm. When the carrier velocity is increased, the operational speed as well as the transconductance and cut-off frequency of the FET is increased. In the conventional device of FIGS.1(A) and 1(B), this feature of carrier overshoot is not exploited completely, as the acceleration of the carriers occurs after the carriers have passed under the gate electrode 14 toward the drain electrode 18. The acceleration of the carriers occurring after passing through the pinch-off point, formed under the gate electrode 14, does not contribute significantly to the improvement of the operational characteristics. Further, it should be noted that, as can be seen in the band diagram of FIG.1(B), the carriers are not accelerated substantially in the region of the channel between the source electrode 16 and the gate electrode 14. The conduction band Ec and the valence band Ev are generally flat in this region and thus, the electric field for accelerating the carriers is not formed. Further, the parasitic resistance of the channel layer 12, shown in FIG.3(A) schematically as a resistor Rs, prevents the high current injection into the channel under the gate. Thereby, the desired improvement of transconductance is not achieved even when there is a desirable overshoot of the carrier velocity in the part of the channel located immediately under the gate electrode 14. In order to circumvent the latter problem, a self-aligned gate structure shown in FIG.3(B) is proposed. In this structure, doped source and drain regions 20a and 20b, doped to the n + -type for example, are formed in the channel layer 12 by the ion implantation of impurities, using the gate electrode 14 as the mask. Thereby, the carrier density is increased in the regions 20a and 20b, leading to the decrease of the parasitic resistance Rs. This conventional structure has a problem in that, because of the region of high carrier density formed adjacent to the gate electrode 14, the breakdown voltage across the gate electrode 14 and the drain electrode 18 tends to be deteriorated. Further, such a structure increases the parasitic capacitance between the gate and source represented schematically as C gs in FIG.3(B). Thereby, the high frequency response of the transistor is inevitably deteriorated. Further, the FETs having short gate lengths generally have the problem of deviation of the threshold voltage from the designed value. The invention is based on a prior art according the preamble of claim 1. This prior art is described in Appl.Phys.Letters 54(1989)9 January, no.2. SUMMARY OF THE INVENTION It is an object of the present invention to provide a field effect transistor having an increased transconductance and an improved high frequency response. Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGS.1(A) and 1(B) are diagrams respectively showing the device structure and band structure of a conventional MESFET; FIG.2 is a diagram showing the effect of carrier overshoot; FIGS.3(A) and 3(B) are diagrams showing the problem pertinent to the conventional MESFET; FIGS.4(A) and 4(B) are diagrams respectively showing the device structure and band structure of a MESFET which is not an inventive embodiment but a useful base for the later explanation of the invention; FIG.5 is a diagram showing the relationship between the carrier velocity and the electric field in various semiconductor materials including GaAs; FIG.6 is a diagram showing the effect of carrier velocity enhancement achieved by the MESFET according to Figs. 4(A) and 4(B); FIGS.7(A) - 7(D) and FIGS.8(A) - 8(C) are diagrams showing the fabrication process of the device according to Figs. 4(A) and 4(B); FIG.9 is a diagram showing a MESFET according to the present invention; FIGS.10(A) - 10(D) are diagrams showing the fabrication process of the MESFET of FIG.9; DETAILED DESCRIPTION FIG.4(A) shows a MESFET which is a not inventive embodiment but a useful example for the later explanation of the invention. In the drawing, those portions that correspond to the portion described already with reference to previous drawings are designated by the same reference numerals and the description thereof will be omitted. In the present example, there is provided a thin vertical doped region 24 of n + -type GaAs in the channel layer 12 between the source electrode 16 and the gate electrode 14 at a location close to the gate electrode 14. The thin vertical doped region 24 may comprise GaAs doped with silicon with an impurity concentration level of 1 X 10 18 cm -3 and has a lateral thickness t that allows the tunneling of carriers therethrough. Typically, the thickness t has a value of several ten nanometers or less. It is desirable to provide the region 24 as close as possible to the gate electrode 14. Typically, the distance between the region 24 and the gate electrode 14, measured along the upper major surface of the channel layer 12 is set to 50 nm. The channel layer 12 comprises an n-type GaAs doped by silicon with an impurity concentration level of 1 X 10 17 cm -3 and may have a thickness of 1000 Å. In correspondence to the n + -type region 24, there is provided an electrode 22 on the upper major surface of the channel layer 12 in ohmic contact with the underlying vertical region 24, and a positive voltage V' sg is applied across the electrode 22 and the source electrode 16. Thereby, the electric potential in the channel layer 12 is pulled down in the band diagram of FIG.4(B) and there appears a large gradient in the conduction band Ec and the valence band Ev in correspondence to the region between the source electrode 16 and the electrode 22. Thus, there appears an electric field E generally proportional to the gradient of the band Ec in the channel of electrons in the region between the layer electrode 22 and the source electrode 16, and the electrons are accelerated not only in the region between the gate electrode 14 and the drain electrode but also between the source electrode and the electrode 22. Because of this reason, the electrode 22 will be referred to hereinafter as acceleration electrode or velocity enhancement electrode. It should be noted that the gradient of the band Ec and thus the accelerating electric field E is generally proportional to the voltage V' sg . FIG.5 shows the effect of electric field on the carrier velocity for various semiconductor materials. In FIG.5, the solid line represents the velocity of the electrons while the broken line represents the velocity of the holes. As can be seen, the electron velocity increases generally linearly with the electric field E in the range between 10 2 V/cm and 10 3 V/cm. For example, the electron velocity is about 8 X 10 5 cm/s in GaAs when the electric field is 100 V/cm, while when the electric field is 1 kV/cm, the electron velocity becomes about 8 X 10 6 cm/s. The maximum velocity is obtained when the electric field is 3 - 4 kV/cm. In this case, the electron speed is about 2 X 10 7 cm/s. Meanwhile, the capability of conducting the current in the channel region of the MESFET is determined by the current density J of the channel region that is defined as J = nev = σE = E/ρ, where n stands for the number of electrons, e stands for the electric charge of an electron, v stands for the velocity of electrons, E stands for the electric field formed in the channel region, σ stands for the conductivity of the channel region and ρ stands for the resistivity of the channel region. Conventionally, efforts have been made to increase the current density J by decreasing the resistivity as shown in the example of FIG.3(B). However, the approach of FIG.3(B) raises various problems as described previously. The current density J can be increased also by increasing the electron velocity v . In this approach, the voltage V' sg is set such that a desirable electric field E is formed between the source electrode 16 and the electrode 22. It should be noted that the magnitude E of the electric field E is determined as V' sg = E/L' sg where L' sg represents the distance between the gate electrode 16 and the acceleration electrode 22. By suitably choosing the value of V' sg , the electron velocity v can be increased by 20 times or more as compared to the conventional electron velocity, which is typically in the order of 100 V/cm. In response to this, the carrier density J representing the capability of conducting the current in the transistor is increased by 20 times or more. By using the present construction, the desirable feature of velocity over-shoot of electrons under the gate electrode 14 is fully exploited and the current that can flow through the channel is significantly increased. In the device of FIG.4(A), the region 24 is applied with the positive voltage V' sg for inducing the electric field E for the acceleration of the carriers. For this purpose, a positive voltage is applied to the electrode 22 as the voltage V' sg . Thereby, the thickness t of the region 24 has to be chosen sufficiently small such that the electrons are not trapped by the region 24 and form the drain current. Because of this reason, the thickness t has to be chosen sufficiently small to allow the tunneling of electrons therethrough, preferably smaller than the mean free path of the electrons in the channel. In the illustrated example, the thickness t is set to 10 nm. FIG.6 shows the result of simulation for the potential distribution and the mean velocity of electrons in the channel of the device of FIG.4(A). For the purpose of comparison, the potential distribution and the mean velocity of the conventional device of FIG.1(A) that lacks the region 24 and the acceleration electrode 22 is presented. In FIG.6, the result for the present example is shown by the continuous line while the result for the conventional device is shown by the broken line. The calculation was made about the device structure having a gate width Wg of 50 µm and the channel carrier density Nd of 5 X 10 17 cm -3 while applying the voltage V' sg of 0.2 volts. The temperature was set to 300 °K. As can be seen, the potential gradient in the channel is generally steeper in the device of the present example than in the conventional device as expected, and in response to this, the mean velocity of the electrons in the device of the present example is generally larger than the conventional device. In FIG.6, the location of the acceleration electrode 22 is shown by the vertical arrow pointed in the downward direction. According to this simulation, it was shown that the drain-source current I DS increases to 18.3 mA as compared with the conventional device wherein the current I DS is 11.7 mA. This result indicates a remarkable improvement of the capability of conducting the current and the transconductance of the transistor. FIGS.7(A) - 7(D) and FIG.8(A) - 8(C) show the process for fabricating the device of the present example. Referring to the drawings, the channel layer 12 of n-type GaAs doped with silicon with the impurity concentration level of 1 X 10 17 cm -3 is formed on the semi-insulating substrate 10 by the molecular beam epitaxy (MBE) process with the thickness of 1000 Å as shown in FIG.7(A). Next, an insulator layer 13 of SiON is deposited on the surface of the layer 12 as shown in FIG.7(B) with the thickness of 500 Å by the plasma-assisted CVD process. Further, a focused ion beam implantation of silicon is made into the channel layer 12 in correspondence to where the region 24 is to be formed, through the insulator layer 13. Thus, the structure of FIG.7(C) is formed. As already described, the region 24 is doped to the high impurity concentration level such as 1 X 10 18 cm -3 and annealed thereafter at 960 °C for 5 seconds. Next, an electron beam resist 15 is deposited on the insulator layer 13 and patterned subsequently to expose a part of the SiON layer 13 located immediately above the doped region 24 and those parts located immediately above the region of the channel layer 12 where the source and drain electrodes 16 and 18 are to be formed. Further, the exposed part of the SiON layer 13 is removed by the dry etching process using a mixture of CF 4 and helium as the etching gas as shown in FIG.7(D). Next, a gold-germanium alloy layer and a gold layer are deposited consecutively on the structure of FIG.7(D) with the entire thickness of 3000 Å and subsequently lifted off together with the electron beam resist 15. Thereby the source electrode 16, the drain electrode 18 and the acceleration electrode 22 are formed as shown in FIG.8(A). This structure is further annealed at 450 °C for one minute to form an alloy region 24a at the top surface of the channel layer 12 where the region 24 makes a contact with the electrode 22 as shown in FIG.8(A). A similar alloy region 12a is formed on the top surface of the channel layer 12 on which the source electrode 16 and the drain electrode 18 are formed. Thereby, the electrodes 16 and 18 as well as the electrode 22 develop an ohmic contact with the underlying channel layer 12 or the doped region 24. Further, the structure of FIG.8(A) is covered by an electron beam resist 19 that is patterned subsequently to expose a part of the top surface of the SiON insulating layer 13 in correspondence to where the gate electrode 14 is to be formed. See FIG.8(B). Further, the exposed part of the layer 13 is removed by the dry etching process described previously such that the top surface of the channel layer 12 is exposed. Next, an aluminum layer is deposited with a thickness of 3000 Å and lifted off subsequently together with the electron beam resist 19. Thereby the structure as shown in FIG.8(C) is obtained. In the structure of FIG.8(C), the top surface of the channel layer 12 that is not covered by any of the electrodes 14, 16, 18 and 22 is protected by the SiON insulating layer 13. In this respect, the device of FIG.8(C) is more practical to the device of FIG.4(A). It should be noted that the device of FIG.4(A) also has the alloy regions 12a and 24a under the electrodes 16, 18 and 22, and these electrodes establish an ohmic contact with the underlying GaAs layer similar to the device of FIG.8(C). In the present example, the doped region 24 is not limited to n + -type GaAs as described, but p + -type GaAs or even a metal layer may be employed as long as the acceleration electric field E is formed in the channel layer 12. Next, the control mode of the MESFET of the present embodiment will be described briefly. Referring to FIG.4(A) again, this diagram shows, in addition to the MESFET device itself, a control unit 101 that is connected to a d.c. voltage source 102 for applying the acceleration voltage V' sg to the acceleration electrode 22. In the simplest example, this control unit 101 may apply a constant acceleration voltage V' sg as already described. Alternatively, this control unit 101 may be controlled by the gate voltage V sg to interrupt the application of the acceleration voltage V' sg to the acceleration electrode 22 when the MESFET is in the turned-off state. Thereby, the formation of unwanted unavailable drain current that is formed as a result of trapping of electrons in the doped region 24, is positively eliminated. It should be noted that such unavailable drain current becomes conspicuous in the turned-off state of the transistor in which the flow of electrons in the channel in the layer 12 from the source electrode 16 to the drain electrode 18 passing through the doped region 24, is absent. By eliminating the unavailable drain current, the power consumption of the device can be reduced. Next, an embodiment of the present invention will be described with reference to FIG.9. In FIG.9, those parts that have been described already with reference to the previous drawings are designated by the same reference numerals and the description thereof will be omitted. The device of the present embodiment is also a MESFET. In the present embodiment, a region of metal such as tungsten is provided within the GaAs substrate 10 and is covered by an insulating region 24A. In the illustrated example, the region 23 is embedded in the substrate 10 with an upper surface substantially flush with the upper major surface of the substrate 10, and the insulating region 24A covers the exposed upper surface of the region 23. Further, the insulating region 24A is embedded within the channel layer 12. Other part of the device is identical with the MESFET of the example. In this device, too, the metal region 23 induces the electric field E that accelerates the electrons similar to the example. The region 23 extends vertically to the plane of the drawing of FIG.9 for the connection to the control unit 101. The location of the region 23 may be anywhere between the source electrode 16 and the gate electrode 14, including the region immediately below the gate electrode. In order to obtain the desired acceleration of the electrons, it is desired to provide the region 23 as close as possible to the gate electrode 14 including the region immediately below the electrode 14. Further, the vertical level of the region 23 is not limited to the position illustrated in FIG.9 but the region 23 may be provided within the substrate 10. Alternatively, the region 23 may be provided within the channel layer 12 as long as the region 23 does not cause obstruction to the flow of carriers. Of course, the region 23 is not limited to metals but doped semiconductors may also be used. FIGS.10(A) - 10(D) show the fabrication process of the device of FIG.9. Referring to FIG.10(A), an electron beam resist 22A is provided on the top surface of the GaAs substrate 10 and is patterned subsequently to expose a part of the top surface in which the region 23 is to be provided. Further, a dry etching process is applied using a mixture of CCl 2 F 2 and helium to form a groove extending vertically to the plane of the drawing as shown in FIG.10(A). The depth of the groove may be 1000 Å, for example. In the step of FIG.10(B), a tungsten layer is deposited with the thickness of 1000 Å on the structure of FIG.10(A), including the groove formed in the previous step. Next, the electron beam resist 22A is removed by lift-off together with the tungsten layer remaining thereon. Thereby the surface of the substrate is exposed. Further, an insulator layer 24 of SiON or silicon oxide that forms the region 24A is deposited on the exposed top surface of the substrate 10 and another electron beam resist is applied on the top surface of the insulator layer 24. The electron beam resist thus deposited is then patterned, leaving a resist strip 22B shown in FIG.10(B) such that the resist strip 22B extends vertically to the plane of FIG.10(B). This resist strip 22B protects the region of the insulator layer 24 corresponding to the region 24A of FIG.9. Next, in the step of FIG.10(C), the insulator layer 24 is patterned by a dry etching process using a mixture of CF 4 and helium to leave the region 24A as shown in FIG.10(C). Further, the channel layer 12 of n-type GaAs is grown on the top surface of the substrate 10 including the region 24A, and an electron beam resist layer is applied on the top surface of the layer 12. This electron beam resist is subsequently patterned to form a resist region 22C that protects the region of the channel layer 12 wherein the channel of electron is to be formed. As a result, the structure shown in FIG.10(C) is obtained. Next, the layers of gold and gold-germanium alloy are deposited on the structure of FIG.10(C) with the thickness of 3000 Å and subsequently lifted off. Thereby, the source and the drain electrodes 16 and 18 are formed. The structure thus obtained is then annealed at 450 °C for one minute to form the alloy region 12a under the source and drain electrodes 16 and 18 similar to the case of the example. Further, another photoresist layer (not shown) is deposited on the structure thus obtained and then patterned to expose the surface of the channel layer 12 where the gate electrode is to be formed. On this structure, an aluminum layer is deposited with a thickness of 3000 Å by sputtering and is lifted off subsequently. Thereby, the gate electrode 14 is formed with a Schottky contact with the underlying channel layer 12. As already noted, the region 23 may be formed by the focused ion beam implantation. In this case, the groove is not formed in the channel layer 12 in the step of FIG.10(A). Instead, an ion implantation is conducted through the window formed in the electron beam resist 22A.

Claims: A semiconductor device, comprising: a channel (12) of a semiconductor material for passing carriers; carrier injecting means (16) for injecting the carriers into the channel, said carrier injecting means establishing an ohmic contact with the channel at a first location; carrier collecting means (18) for collecting the carriers from the channel, said carrier collecting means establishing an ohmic contact with the channel at a second, different location; carrier control means (14) provided on the channel at a third location located between the first and second locations, said carrier control means being applied with a control voltage (V sg ) and controlling the passage of the carriers through the channel from the carrier injecting means to the carrier collecting means in response to the control voltage; and acceleration means (23) between the first and third locations, said acceleration means being supplied with an acceleration voltage (V' sg ) and producing an electric field (E) that accelerates the carriers such that the electric field has a magnitude (E) generally proportional to the acceleration voltage applied thereto, said channel (12) comprises a semiconductor layer (12) having an upper major surface and a lower major surface, said third location being defined on the upper major surface of the semiconductor layer that forms the channel, said third location is located between the first and second locations along the direction of the channel, said carrier control means (14) being provided on the upper major surface of the semiconductor layer forming the channel at said third location, said carrier control means creating a depletion region in the semiconductor layer in response to the control voltage applied thereto, characterized in that said carrier injecting means and said carrier collecting means being provided on the upper major surface of the semiconductor layer at said first and second locations respectively, said acceleration means (23) comprising a conductive region (23) provided in the vicinity of the lower major surface excluding the upper major surface of the semiconductor layer (12) at a fourth location defined between the first and third locations along the direction of the channel including the third location. A semiconductor device as claimed in claim 1 in which said semiconductor device further comprises a substrate (10) having an upper major surface and a lower major surface, said substrate being provided such that the semiconductor layer is provided on the upper major surface of the substrate, said conductive region (23) being embedded in the substrate.

Inventors: AWANO YUJI, AWANO, YUJI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: H01L29
IPC codes: H01L29