SEMICONDUCTOR DEVICE

Abstract: A device (termed a "permeable base tansistor") has a base layer (3), sandwiched between an emitter layer (7) and a collector layer (4), and an electrode layer (2) having an array of gaps therein which is provided at a heterojunction between the emitter layer and the base layer. The emitter layer (7) has an energy bandgap which is greater in width than that of the base layer. Electrons injected into the base layer through the emitter layer therefore gain kinetic energy, thereby affording a high switching speed.

Description: The present invention relates to a semiconductor device. A device described as a permeable base transistor (PBT) and comprising an emitter layer, a comb-shaped or lattice-shaped base electrode, a base layer, a collector layer, an emitter electrode, and a collector electrode has been proposed in WO-A-8l/00489. In such a PBT, the value of the voltage applied to a comb-shaped base electrode can be used to change the coverage of the depletion layer extending from the base electrode in the horizontal direction. This effect can be used to control the electrons passing between the comb-shaped base electrode from the emitter layer to the collector layer. The thickness of the base electrode is about 0.02 µm, corresponding to a field effect transistor (FET) which also has a short gate length of about 0.02 µm. Therefore, the time taken for electrons to pass through the base electrode in the PBT is very short, enabling a high speed of operation. However, the switching speed of such a PBT is about 15 ps at best. Thus, a higher switching speed PBT is required. According to one aspect of the present invention there is provided a semiconductor device wherein a base layer is sandwiched between an emitter layer and a collector layer, and an electrode layer having an array of gaps therein is provided at a heterojunction between the emitter layer and the base layer, characterised in that the said emitter layer has an energy band gap which is greater in width than that of the said base layer. Reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 shows a cross-sectional view of a previously-proposed PBT; Fig. 2 shows a cross-sectional view of a PBT embodying the present invention; Figs. 3 and 5 show respective partial cross-sectional views of the embodiment of Fig. 2; Figs. 4 and 6 are energy band diagrams for Figs. 3 and 5, respectively; Fig. 7 is an energy band diagram; Fig. 8 is a circuit diagram using a PBT embodying the present invention; Figs. 9A to 9D show cross-sectional views explaining the process of producing a structure according to an embodiment of the present invention; Fig. 10 shows a cross-sectional view of an IC device according to an embodiment of the present invention; Fig. 11A shows a plan view of a comb-shaped base electrode 2; and Fig. 11B shows a plan view of a lattice-shaped electrode 2. Figure 1 shows a cross-sectional view of a PBT proposed previously. As shown in Fig. 1, on a n + type GaAs emitter layer 1, a comb-shaped or lattice-shaped base electrode 2 composed of a high melting point metal such as tungsten (W) or a metal silicide such as tungsten silicide (WSi) is formed. Over the entire surface of the base electrode 2 and the emitter layer 1, an n⁻ type GaAs base layer 3 is formed. On the base layer 3, an n + type GaAs collector layer 4 is formed. Reference numerals 5 and 6 denote an emitter electrode and collector electrode, respectively. In the above-mentioned structure of a PBT, as explained above, the passing time of electrons from the emitter layer 1 to the collector layer 4 through the base electrode 2 is such that the switching speed limit is about 15 ps. Figure 2 is a cross-sectional view of a PBT embodying the present invention. As shown in Fig. 2, an n type AlGaAs emitter layer 7 is additionally formed on the n + type GaAs emitter layer, which is denoted as 1a in Fig. 2. Other structures, for example the base electrode 2, are the same as in Fig. 1. The n type AlGaAs emitter layer 7 has a wider energy bandgap than the n⁻ type GaAs base layer 3 and n⁺ type GaAs collector layer 4. Further, the comb-shaped or lattice-shaped base electrode 2 is formed on a heterojunction surface formed by the n type AlGaAs emitter layer 7 and the n⁻ type GaAs base layer 3. A Schottky junction or insulation layer 10 is interposed at respective interfaces between the electrode 2 and the emitter and base layers (7, 3). Thus the base electrode 2 is not in ohmic contact with the base layer 3 or the emitter layer 7. The operation of the PBT described above will now be explained. Figures 3 and 5 show partial cross-sectional views of the embodiment, explaining depletion layers when V BE > O and V CE > O, and when V BE = O and V CE > O, respectively. Figures 4 and 6 are energy band diagrams for Figs. 3 and 5, respectively. In Figs. 3 to 6, the areas 8 around the comb-shaped or lattice-shaped base electrode 2 denote depletion layers, and e₁ (o) and e₂ (o) denote electrons and hot electrons, respectively. In Fig. 3, V CE (a voltage between the collector layer 4 and the emitter contact layer 1a) > O, and V BE (a voltage between the base electrode 2 and the emitter layer) > O, so the PBT is in an ON state. That is, not all the base layer 3 in the gaps of the base electrode 2 is converted to a depletion layer, so the potential barrier ⊘ p is low as shown in Fig. 4. Consequently, electrons e₁ in an emitter contact layer 1a are injected into the base layer 3 through the emitter layer 7. When the electrons e₁ are injected into the base layer 3, since there is an energy barrier ø B according to the wider band gap emitter layer 7, the injected electrons e₁ obtain a high kinetic energy so that the electrons e₂ become hot electrons which have a high initial speed. Therefore, such hot electrons e₂ can pass the base layer 3 with the high speed so as to reach the collector layer 4, whereby a collector current flows. In the case of Fig. 3, the voltage between the base and emitter (V BE ) is, for example, 0.3 V, and the voltage between the collector and emitter (V CE ) is, for example, 0.6 V. On the other hand, in Fig. 5, V CE > O and V BE = O, so the PBT is in an OFF state. That is, all of the base layer 3 in the gaps of the base electrode 2 is converted to a depletion layer because of the built-in potential between the base electrode 2 and the base layer 3, so that the potential barrier ø p is high, as shown in Fig. 6. Consequently, the electrons e₁ in the emitter contact layer 1a are not injected into the base layer 3, so that no collector current flows. In the case of Fig. 5, V CE is given as 0.2 V. In Fig. 6, the broken line illustrates the conduction band in Fig. 4. Therefore, the PBT turns ON or OFF depending upon whether V BE > O or V BE = O. As shown in Figs. 4 and 6, the barrier height of the emitter layer 7 gradually increases from the emitter contact layer 1a to the base layer 3, so as to enable the electrons e₁ in the emitter contact layer 1a to inject into the base layer 3 through the wide band gap layer 7. The electrons e₁ injected from the emitter layer 7 into the base layer 3 in Fig. 3 lose potential energy therebetween. The electrons e₁ gain kinetic energy in place of the loss of the potential energy so that the electrons e₁ change to hot electrons e₂. Thus, the speed of the electrons e₂ in the base layer 3 is increased, for example to 1 X 10⁸ cm/sec., to provide a significantly higher switching speed as compared with the PBT of Fig. 1. Figure 7 shows an energy band diagram when V CE = O and V BE = O, i.e. in the normally off state. Figure 8 schematically shows an example of a circuit diagram using a PBT embodying the present invention. This is an inverter circuit. Figures 9A to 9D are cross-sectional views explaining the process of producing a structure according to an embodiment of the present invention. As shown in Fig. 9A, on an n + type GaAs emitter contact layer 1a, an n type Al x Ga 1⁻x As ______ emitter layer 7 having a thickness of about 1000 Å (0.1µm) is formed by molecular beam epitaxy (MBE). The x value of the emitter layer 7 is steadily increased from O as the layer is built up, so that at the 1000 Å thickness the x value is 0.3. Accordingly an energy barrier ø B will exist between the emitter layer 7 and the base layer 3. Furthermore, the graded x value of the emitter layer 7 generates a graded energy level between the emitter contact layer 1a and the base layer 3. To form the emitter layer 7, a chemical vapor deposition (CVD) process may alternatively be used. As shown in Fig. 9B, a tungsten layer having a thickness of about 200 Å (0.02 µm) is formed on the n type Al x Ga 1-x As emitter layer 7. The tungsten layer is patterned with an array of gaps, by a conventional photolithography technique, to form a comb-shaped base electrode 2. As shown in Fig. 9C, on the obtained structure, an n⁻ type GaAs base layer 3 having a thickness of about 4000 Å (0.4 µm) with a carrier concentration of 5 x 10¹⁶ cm⁻³ is formed by the MBE process. In place of the MBE process, a vapor phase epitaxy (VPE) process may be used. As shown in Fig. 9D, an n⁺ type GaAs collector layer 4 having a thickness of about 2000 Å (0.2 µm) is formed on the n⁻ type GaAs base layer 3 by using MBE or VPE continuously. Finally, as shown in Fig. 2, respective AuGe/An layers each having a thickness of about 4000 Å (0.4 µm) are formed on the collector layer 4 and the emitter contact layer 1a to form a collector electrode 6 and an emitter electrode 5 respectively. It should be noted that the base electrode 2 connects to the emitter layer 7 and the base layer 3 through a Schottky junction or insulating layer 10. In an embodiment of the present invention, in place of the GaAs, In y Ga 1-x As may be used. In place of the Al x Ga 1-x As, InP and Al z In 1-z As may be used. Further, in an embodiment of the present invention the order of the position of the emitter layer, the base layer, and the collector layer may be reversed. In the above-mentioned embodiment, a single semiconductor device was explained. However, the single semiconductor device can be easily changed to an IC device. For example, as shown in Fig. 10, on a semi-insulating GaAs substrate 1a each semiconductor layer such as an n⁺ type GaAs emitter layer 1 is formed. After that by a selective mesa etching process a part of the surface of the n⁺ type GaAs emitter layer 1 is exposed whereby an emitter electrode 5 may be taken out therefrom. Figure 11A is a plan view of a comb-shaped base electrode 2. In Fig. 11A reference numeral 2a denotes a contact portion. Figure 11B is a plan view of a lattice-shaped base electrode 2.

Claims: A semiconductor device wherein a base layer (3) is sandwiched between an emitter layer (7) and a collector layer (4), and an electrode layer (2) having an array of gaps therein is provided at a heterojunction between the emitter layer (7) and the base layer (3), characterised in that the said emitter layer (7) has an energy band gap which is greater in width than that of the said base layer (3). A device as claimed in claim 1, wherein the said emitter layer (7) is sandwiched between the base layer (3) and an emitter contact layer (1a) such that electrons passing through the emitter layer (7) in the direction from the emitter contact layer (1a) to the base layer (3) encounter an opposing potential barrier which increases with distance, in the said direction, through the said emitter layer (7). A device as claimed in claim 1 or 2, wherein the said electrode layer (2) is surrounded by either a Schottky junction (10) or an insulating layer such that the layer (2) is not in ohmic contact with the emitter layer (7) or the base layer (3).

Inventors: YOKOYAMA, NAOKI
Applicants: FUJITSU LIMITED
Application year: 1985
Main classification: H01L29
IPC codes: H01L29, H01L21