JP2522358B2 - Heterostructure bipolar transistor using germanium - Google Patents

Description

The present invention relates to a heterostructure bipolar transistor (HBT) having an emitter layer made of a semiconductor having an energy gap wider than that of a semiconductor constituting a base layer. ), Especially for HBTs operating at liquid nitrogen temperatures.

(Prior Art) Documents IEEE Transaction on Electron
Devices, Vol. 34, No. 1, pp. 1 to 3, in a digital circuit with highly integrated miniaturized transistors, the wiring resistance increases as the wiring becomes finer, and the integrated circuit speeds up. The problems that hinder the above, and the problems such as electromigration occur due to the increase in the density of the current flowing in the wiring, and the problems of reliability become apparent. As a method for solving this, it is possible to operate the semiconductor element at the liquid nitrogen temperature. However, a silicon bipolar transistor, which is widely used as a high-speed operation element at room temperature, has been used in the literature when it is operated at liquid nitrogen temperature.
n Devices) Vol. 28, No. 5, p. 494 to p. 500, the base resistance increases due to carrier freezing in the base layer, which impairs high-speed operation, or in a base layer with impurity compensation. Carriers injected from the emitter layer are trapped by impurities, and the current gain decreases.
Alternatively, band-gap narrowing occurs in the heavily doped emitter layer, which causes a problem that the current gain remarkably decreases with operation at low temperature. In addition, in order to suppress the amount of heat generated by the integrated circuit due to the low temperature operation, it is necessary to reduce the power consumption of the transistor, but the power supply voltage of the bipolar transistor is determined by the band gap of the semiconductor material used. As long as it is used, it is impossible to lower the power supply voltage compared to room temperature operation, and lowering the operating current slows down the operating speed of the transistor, so it is not possible to reduce power consumption while maintaining high speed. .

On the other hand, gallium arsenide is used as a base layer and a collector layer, and aluminum gallium arsenide is used as an emitter layer as described in Proceeding of the IEEE Vol. 70, pages 13 to 25. Since the heterostructure bipolar transistor (HBT) has an emitter layer having a wide band gap as shown in the transistor cross-sectional schematic diagram of FIG. 9, the current gain is reduced due to the band gap narrowing of the emitter layer described above. However, since the band gap of gallium arsenide is larger than that of silicon, the power consumption of the transistor is larger and it is not suitable for operation at liquid nitrogen temperature.

(Problems to be Solved by the Invention) As described above, when a bipolar transistor using silicon is used at a low temperature such as liquid nitrogen temperature, the band gap narrowing in the emitter layer and the impurity-compensated base layer There is a problem that the current gain is remarkably lowered due to the trapping of minority carriers due to the impurities in (1), the carrier is frozen in the base layer, the base resistance is increased, and the high speed operation characteristics are deteriorated. Further, the HBT composed of a heterojunction of aluminum gallium arsenide and gallium arsenide has a large power consumption, and therefore has a drawback that it is not suitable for low-temperature operation such as liquid nitrogen temperature where it is necessary to suppress the amount of heat generated from the integrated circuit chip.

The object of the present invention is to eliminate the drawbacks of these conventional bipolar transistors and to provide a new heterostructure bipolar transistor.
To provide a transistor.

(Means for Solving the Problems) According to the present invention, in a heterostructure bipolar transistor in which a collector layer made of germanium, a base layer, and an emitter layer made of a mixed crystal of germanium and silicon are provided on a germanium substrate, the emitter layer is formed. The mixed crystal ratio of the mixed crystal of silicon and germanium is spatially changed from zero to a finite value from the base side toward the inside of the emitter layer. Also, a collector layer made of germanium, a base layer made of a mixed crystal of germanium and silicon, and an emitter layer are provided on a germanium substrate, and the mixed crystal ratio of the mixed crystal of silicon and germanium forming the base layer is changed from the collector side to the emitter side. It is characterized by continuously changing spatially from zero to a finite value.

(Function) The band gap of germanium is 0.66 eV, which is smaller than the band gap of silicon which is 1.12 eV, and N-type dopants of antimony (Sb), arsenic (As) and phosphorus (P) and gallium (Ga) and boron (Ga) and boron ( The impurity binding energy of the P type dopant such as B) is as small as 10 meV, and the carrier does not freeze even at a low temperature such as liquid nitrogen temperature. Further, compared with silicon, germanium has a large electron mobility and hole mobility, and is an advantageous material for high-speed operation.

The lattice spacing of silicon and germanium is 4%, which makes it difficult to grow epitaxially without interfacial defects. However, the document "Modulated Semiconductor Structur"
e) A thin film mixed crystal of germanium and silicon on a silicon substrate or a germanium substrate by a molecular beam epitaxial method as described in Collected Papers pp.717-723 of the 2nd International Conference 1985. It has been reported that epitaxial growth is possible.

As described above, by combining germanium and germanium-silicon mixed crystal on a germanium substrate, germanium HB that can operate at low temperature such as liquid nitrogen temperature
T can be configured.

(Embodiment) FIG. 1 is a schematic sectional view showing an embodiment of the germanium HBT according to the first aspect of the invention. A high-concentration N-type germanium 2 on the P-type germanium substrate 1, an N-type germanium 3 forming a collector layer, a high-concentration P-type germanium 4 forming a base layer, and a mixed crystal ratio of 20% of silicon forming an emitter layer. Mixed crystal of N-type germanium and silicon 5,
High concentration N-type germanium 6 is continuously epitaxially grown. A molecular beam epitaxial growth method can be adopted as the epitaxial growth method. In the figure, 7 is a collector electrode, 8 is a base electrode, and 9 is an emitter electrode. Arsenic is used as the N-type dopant and gallium is used as the P-type dopant. Aluminum is used as the electrode material.

FIG. 2 shows an energy diagram of this transistor in operation. Figure 10 is a conduction band edge portion, 11 is the filled band edge portion, 12 and 13 respectively indicate a discontinuity Delta] E _V respectively generated in the filled band edge portion discontinuous Delta] E _C of the conduction band edge portion generated in the heterojunction portion. The band gap of the silicon-germanium mixed crystal Si _0.2 Ge _0.8 forming the emitter layer is 0.
It becomes 86 eV, which is larger than the band gap of germanium constituting the base layer of 0.66 eV, and the injection efficiency is improved. Therefore, a large current gain can be obtained even if the impurity concentration of the emitter layer is lowered, and a high current gain characteristic can be expected even at a low temperature. Further, even if the donor-type impurity 14 such as arsenic is present in the base, the impurity bond energy is small and the electrons injected from the emitter layer are not trapped.
In addition, the impurity binding energy of gallium, which is the main impurity of the base layer, is small, the carriers do not freeze even at the temperature of liquid nitrogen, and therefore the base resistance can be kept low.

FIG. 3 is a schematic sectional view showing an embodiment of the germanium HBT of the invention described in claim 2. The mixed crystal ratio of the mixed crystal 15 of silicon germanium constituting the emitter layer goes from the base layer 4 side to the N-type high-concentration layer 6 and continuously increases from 0 to 20%, and then monotonically changes from 20% to 0. Has a decreasing structure. Others are the same as the embodiment of FIG. FIG. 4 is an energy diagram of the transistor in operation. Corresponding to the spatially continuous change of the mixed crystal ratio of the emitter layer, the conduction band edge 10 and the filling band edge 11
Are continuously connected from the emitter layer to the base layer. Therefore, the threshold voltage required to turn on this transistor is smaller than that in the embodiment of FIG. 1 by the energy difference ΔE _C of the conduction band edge discontinuity, and the operation at a lower voltage, that is, lower power consumption. Characteristic can be expected. In this example, the threshold value drops by about 0.1V.

FIG. 5 is a schematic sectional view showing an embodiment of the germanium HBT according to the third aspect of the invention. A high-concentration N-type germanium 21, a N-type germanium 22 forming a collector layer, a mixed crystal 24 of P-type germanium and silicon forming a base layer, and a high-concentration N-type germanium 25 are successively formed on a P-type germanium substrate 20. Epitaxially grow. A molecular beam epitaxial growth method can be adopted as the epitaxial growth method. In the figure, reference numerals 26, 27 and 28 denote a collector electrode, a base electrode and an emitter electrode, respectively, which are made of aluminum. Arsenic is used for the N-type dopant and gallium is used for the P-type dopant. The mixed crystal ratio of the mixed crystal 23 of silicon and germanium forming the base layer is from the collector 22 side to the emitter.
It changes spatially continuously from 0 to 10% toward the 24 side. The mixed crystal ratio of the mixed crystal 24 of silicon and germanium constituting the emitter layer continuously increases from 10% to 20% once from the base 23 side toward the high concentration N-type germanium layer 25,
After that, it decreases monotonically from 20% to zero.

FIG. 6 shows the energy diagram of this transistor in operation. In the figure, 29 and 30 respectively indicate a conduction band edge part and a filled band edge part. Silicon that constitutes the emitter layer
The maximum mixed crystal ratio of germanium mixed crystal 24 is 20%, and the maximum mixed crystal ratio of silicon-germanium mixed crystal 23 on the base side is 10%.
%, The injection efficiency is improved. Therefore, a large current gain can be obtained even if the impurity concentration of the emitter layer is lowered, and a high current gain characteristic can be expected even at a low temperature. Further, even if a donor-type impurity 31 such as arsenic is present in the base, the bond energy of the impurity is small, and electrons injected from the emitter layer are not trapped. In addition, the impurity binding energy of gallium, which is the main impurity of the base layer, is small, the carriers do not freeze even at the temperature of liquid nitrogen, and therefore the base resistance can be kept low.

Further, in this embodiment, since the mixed crystal ratio continuously changes in the base, an internal electric field is generated. The band gap difference between the emitter and collector ends of the base layer is 0.1 eV
Since it exists, the internal electric field reaches 10 ⁴ V / cm when the thickness of the base layer is 100 nm. Therefore, the electrons injected into the base are accelerated by the internal electric field and travel at high speed in the base. Therefore, the base running time is shortened,
High-speed operation is expected.

FIG. 7 is a schematic sectional view showing another embodiment of the germanium HBT according to the third aspect of the invention. The mixed crystal ratio of the mixed crystal 24 of silicon and germanium that constitutes the emitter layer is 20% of silicon, which is spatially uniform. Others are the same as the embodiment of FIG.

FIG. 8 shows an energy diagram of this transistor in operation. In the figure, 29 and 30 respectively indicate a conduction band edge part and a filled band edge part. Silicon that constitutes the emitter layer
The silicon mixed crystal ratio of the germanium mixed crystal 32 is 20%, which is larger than the maximum value of 10% of the silicon mixed crystal ratio of the silicon-germanium mixed crystal 23 on the base side, and the injection efficiency is improved. Therefore, a large current gain can be obtained even if the impurity concentration of the emitter layer is lowered, and a high current gain characteristic can be expected even at a low temperature. Further, similar to the embodiment of FIG. 5, trapping of injected electrons by the donor-type impurity 31 in the base that is impurity-compensated does not occur, and holes that are majority carriers in the base do not freeze. Further, as in the embodiment of FIG. 5, an internal electric field of about 10 ⁴ V / cm is generated in the base, and the electrons injected into the base are accelerated by the electric field, resulting in a short base transit time, and thus a high speed operation. realizable.

In the above-described embodiments, all npn type transistors have been described, but it is obvious that pnp type transistors may be used. The operating temperature is not limited to the liquid nitrogen temperature (77k) and may be another temperature (for example, 100k). Although arsenic is used as the n-type impurity and gallium is used as the P-type impurity in the embodiment, antimony, phosphorus, boron or the like can be used. Further, in the embodiments, all are emitter top type, but collector top type may be used.

As described above, according to the present invention, since germanium is used as the semiconductor material, the impurity binding energy is small, the carrier does not freeze even at a low temperature such as liquid nitrogen temperature, and the base resistance is not increased. . Further, even at the temperature of liquid nitrogen, it does not work as a trap for the carriers in which the impurities in the base are injected, so that the gain is not lowered. The band gap energy is also smaller than that of silicon, low voltage operation is possible, and low power consumption characteristics can be expected.

Further, since the mixed crystal of silicon and germanium having a large band gap energy is used as the emitter layer, the injection efficiency is large and the high current gain characteristic can be realized even if the impurity density of the emitter layer is lowered.

Further, as shown in the embodiment of FIGS. 5 and 7,
By using a mixed crystal of silicon and germanium for the base layer, an electric field is generated inside the base layer, electrons injected from the emitter layer into the base layer are accelerated, the base transit time is shortened, and high-speed operation can be realized. . In particular, since the carrier diffusion constant becomes small at low temperatures, if the base layer travels only by diffusion, the base transit time becomes long, so the above effect is important for maintaining high-speed operation at low temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view showing an embodiment of the germanium HBT according to claim 1, FIG. 2 is an energy diagram of the transistor in operation, and FIG. 2 is a schematic sectional view showing an example of the germanium HBT described in 2.
FIG. 4 is an energy diagram during operation of the transistor, FIG. 5 is a schematic sectional view showing an embodiment of the germanium HBT according to claim 3, FIG. 6 is an energy diagram during operation of the transistor, and FIG. 7 is claim 3
FIG. 8 is a schematic cross-sectional view showing another embodiment of the germanium HBT as described in FIG. 8, FIG. 8 is an energy diagram during operation of the transistor, and FIG. 9 is a hetero-bipolar composed of a heterojunction of gallium arsenide and aluminum gallium arsenide. -A schematic cross-sectional view of a transistor. 1,20 ... P-type germanium substrate, 2,21 ... high-concentration N-type germanium, 3,22 ... N-type germanium, 4 ... high-concentration P-type germanium, 5,15,24 ... mixed crystal of N-type germanium and silicon, 6,25 ... High-concentration N-type germanium, 7,26 ... Collector electrode, 8,27 ... Base electrode, 9,28 ... Emitter electrode, 10,29 ...
Conduction band edge part, 11,30 ... Filling band edge part, 12 ... Discontinuity of conduction band edge part, 13 ... Discontinuity of filling band edge part, 14,3
1 ... Donor type impurities, 23 ... Mixed crystal of high concentration P-type silicon and germanium

Claims (2)

(57) [Claims] 1. A heterostructure bipolar transistor comprising a germanium substrate, a collector layer made of germanium, a base layer, and an emitter layer made of a mixed crystal of germanium and silicon.
In the transistor, a heterostructure bipolar characterized in that a mixed crystal ratio of a mixed crystal of silicon and germanium forming the emitter layer is spatially changed from zero to a finite value from the base side toward the inside of the emitter layer. -Transistor. 2. A germanium collector layer, a base layer made of a mixed crystal of germanium and silicon, and an emitter layer are provided on a germanium substrate, and a mixed crystal ratio of a mixed crystal of silicon and germanium constituting the base layer is set from the collector side. Heterostructure bipolar characterized by continuously varying spatially from zero to a finite value toward the emitter side
Transistor.