Abstract
A HBT on a GaAs substrate is presented, wherein its base comprises a first base layer comprising In.sub.iGa.sub.1-iAs with an Indium content i with a slope s1 and a second base layer on the emitter side comprising In.sub.iGa.sub.1-jAs with an Indium content j with a slope s2, and an average of s1 is half of the average of s2 or smaller; or the base comprises a first base layer comprising In.sub.mGa.sub.1-mAs with an Indium content m and a second base layer on the emitter side comprising In.sub.nGa.sub.1-nAs with an Indium content n, and an average of n is larger than the m at a second base layer side; or the base comprises a first base layer pseudomorphic to GaAs with a bulk lattice constant larger than GaAs, and the emitter comprises a first emitter layer pseudomorphic to GaAs with a bulk lattice constant smaller than GaAs.
Claims
1. A heterojunction bipolar transistor, comprising a plurality of semiconductor layers epitaxially grown on a GaAs substrate and forming a collector above the GaAs substrate, a base on the collector, and an emitter on the base, wherein the base comprises: a first base layer comprising In.sub.iGa.sub.1-iAs with an Indium content i being in the range 0<i<1, the Indium content i is uniformly distributed or varies from an emitter side to a collector side with a first slope s1 being defined as positive when the Indium content i increases from the emitter side to the collector side, and a second base layer inserted between the first base layer and the emitter and comprising In.sub.jGa.sub.1-jAs with an Indium content j being in the range 0<j<1, the Indium content j varies from the emitter side to the collector side with a second slope s2 being defined as positive when the Indium content j increases from the emitter side to the collector side, wherein an average of the Indium content i is larger than an average of the Indium content j, wherein an average of the slope s2 is positive, and wherein the average of the first slope s1 is half of the average of the second slope s2 or smaller.
2. (canceled)
3. The heterojunction bipolar transistor according to claim 1, wherein the Indium content i in the first base layer is between 0.03 and 0.2.
4. A heterojunction bipolar transistor, comprising a plurality of semiconductor layers epitaxially grown on a GaAs substrate and forming a collector over the GaAs substrate, a base on the collector, and an emitter on the base, wherein the base comprises: a first base layer comprising In.sub.mGa.sub.1-mAs with an Indium content m being in the range 0<m<1, and a second base layer inserted between the first base layer and the emitter and comprising In.sub.nGa.sub.1-nAs with an Indium content n being in the range 0<n<1, and wherein an average Indium content n of the second base layer is larger than the Indium content m of the first base layer at a second base layer side inside the first base layer.
5. The heterojunction bipolar transistor according to claim 4, wherein the Indium content n of the second base layer increases from a first base layer side to an emitter side.
6. The heterojunction bipolar transistor according to claim 4, wherein the Indium content m in the first base layer is between 0.03 and 0.2.
7. The heterojunction bipolar transistor according to claim 4, wherein the emitter comprises a first emitter layer lattice-matched to GaAs, and the emitter further comprises a second emitter layer inserted between the second base layer and the first emitter layer, wherein the second emitter layer comprises In.sub.vGa.sub.1-vP with an Indium content v being between 0.53 and 0.8.
8. A heterojunction bipolar transistor, comprising a plurality of semiconductor layers epitaxially grown on a GaAs substrate and forming a collector over the GaAs substrate, a base on the collector, and an emitter on the base, wherein the base comprises a first base layer which is pseudomorphic to GaAs with a first bulk lattice constant being at least 0.15% larger than the bulk lattice constant of GaAs, and the emitter comprises a first emitter layer which is pseudomorphic to GaAs with a second bulk lattice constant being at least 0.15% smaller than the bulk lattice constant of GaAs.
9. The heterojunction bipolar transistor according to claim 8, wherein the first emitter layer consists of In.sub.xGa.sub.1-xAs.sub.1-yP.sub.y with an Indium content x and a Phosphorus content y being between 0 and 1 and satisfying the condition that the bulk lattice constant of the first emitter layer is at least 0.15% smaller than the bulk lattice constant of GaAs.
10. The heterojunction bipolar transistor according to claim 8, wherein the first emitter layer consists of In.sub.xGa.sub.1-xP with an Indium content x being between 0.1 and 0.44.
11. The heterojunction bipolar transistor according to claim 10, wherein the emitter further comprises a second emitter layer inserted between the first base layer and the first emitter layer, wherein the second emitter layer consists of In.sub.zGa.sub.1-zP with an Indium content z being between 0.53 and 0.8.
12. The heterojunction bipolar transistor according to claim 8, wherein the first emitter layer consists of GaAs.sub.1-yP.sub.y with a Phosphorus content y being between 0.03 and 0.5.
13. The heterojunction bipolar transistor according to claim 8, wherein the first base layer consists of In.sub.oGa.sub.1-oAs.sub.1-pSb.sub.p with an Indium content o and an Antimony content p being between 0 and 1 with the combination of o and p satisfying that the bulk lattice constant of the first base layer is at least 0.15% larger than the bulk lattice constant of GaAs.
14. The heterojunction bipolar transistor according to claim 8, wherein the first base layer consists of In.sub.oGa.sub.1-oAs with an Indium content o being between 0.03 and 0.2.
15. The heterojunction bipolar transistor according to claim 14, wherein the base further comprises a second base layer inserted between the first base layer and the first emitter layer, and the second base layer consists of In.sub.qGa.sub.1-qAs with an average Indium content q being larger than Indium content o of the first base layer at a second base layer side inside first base layer.
16. The heterojunction bipolar transistor according to claim 15, wherein the Indium content q increases from a first base layer side to an emitter side.
17. The heterojunction bipolar transistor according to claim 8, wherein first base layer consists of GaAs.sub.1-pSb.sub.p with an Antimony content p being between 0.03 and 0.2.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0027] FIG. 1 is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention.
[0028] FIGS. 2A and 2B are diagrams showing the distribution of Indium content of the base layers along the stacking direction of the semiconductor layers in two embodiments of the GaAs HBT shown in FIG. 1.
[0029] FIG. 3A is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention. FIG. 3B is a diagram showing the distribution of Indium content of the base layers along the stacking direction of the semiconductor layers in an embodiment of the HBT shown in FIG. 3A. FIG. 3C is a diagram showing the two-dimensional device simulation results of the C.sub.bc plotted as a function of the thickness of the second base layer t.sub.grad of the GaAs HBT shown in FIG. 3A.
[0030] FIG. 4A is a diagram showing a GaAs HBT according to an embodiment of the present invention. FIG. 4B is a diagram showing measured C.sub.bc plotted as a function of I.sub.c with the GaAs HBT structure shown in FIG. 4A (circle) and a GaAs HBT with a GaAs base (triangle). FIG. 4C is a diagram showing measured power-added efficiency (PAE) plotted as a function of output power (Pout) of the GaAs HBT shown in FIG. 4A (solid line) and the GaAs HBT with the GaAs base (dashed line). FIG. 4D is a diagram showing measured Error Vector Magnitude (EVM) for a WiFi IEEE 802.11 modulation for the GaAs HBT shown in FIG. 4A (solid line) and the GaAs HBT with the GaAs base (dashed line).
[0031] FIG. 5A is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention.
[0032] FIGS. 5B, 5C, and 5D are diagrams showing the distribution of Indium content of the base layers along the stacking direction of the semiconductor layers in three embodiments of the GaAs HBT shown in FIG. 5A.
[0033] FIG. 6A is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention. FIG. 6B is a diagram showing the distribution of Indium content of the first and second base layers along the stacking direction of the semiconductor layers in an embodiment of the GaAs HBT shown in FIG. 6A. FIG. 6C is a diagram showing the distribution of Indium content inside the first and second emitter layers along the stacking direction of the semiconductor layers in an embodiment of the GaAs HBT shown in FIG. 6A. FIG. 6D is a diagram showing Vceoff calculated by two-dimensional device simulation for five conventional GaAs HBTs and two embodiments of the present invention.
[0034] FIG. 7A is a diagram showing a GaAs HBT according to an embodiment of the present invention. FIG. 7B is a diagram showing measured collector current Ic plotted as a function of collector voltage Vce for the GaAs HBT shown in FIG. 7A (solid line) and a GaAs HBT with a GaAs base (dashed line). FIG. 7C is a diagram showing measured curves of C.sub.bc plotted as a function of I.sub.c for the GaAs HBT shown in FIG. 7A (circle) and the GaAs HBT with a GaAs base (triangle). FIG. 7D is a diagram showing measured power-added efficiency (PAE) plotted as a function of output power (Pout) for the GaAs HBT shown in FIG. 7A (circle) and the GaAs HBT with a GaAs base (triangle).
[0035] FIG. 8 is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention.
[0036] FIG. 9A is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention. FIG. 9B is a diagram showing the distribution of Indium content inside the first emitter layer along the stacking direction of the semiconductor layers in an embodiment of the GaAs HBT shown in FIG. 9A and the resulting profile of the conduction band edge E.sub.CBM.
[0037] FIG. 10A is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention. FIG. 10B is a diagram showing the distribution of Indium content inside the first emitter layer along the stacking direction of the semiconductor layers in an embodiment of the GaAs HBT shown in FIG. 10A and the resulting profile of the conduction band edge E.sub.CBM.
[0038] FIG. 11A is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention. FIG. 11B is a diagram showing the distribution of Indium content inside the first emitter layer along the stacking direction of the semiconductor layers in an embodiment of the GaAs HBT shown in FIG. 11A and the resulting profile of the conduction band edge E.sub.CBM.
[0039] FIG. 12A is a cross-sectional view of a GaAs HBT according to an embodiment of the present invention. FIG. 12B is a diagram showing the distribution of Indium content inside the first and second emitter layers along the stacking direction of the semiconductor layers in an embodiment of the GaAs HBT shown in FIG. 12A.
[0040] FIG. 13A is a diagram showing a conventional InGaAs base GaAs HBT. FIGS. 13B and 13C are diagrams showing GaAs HBTs according to embodiments of the present invention. FIGS. 13D, 13E, and 13F are diagrams showing the results of the high-temperature operation lifetime (HTOL) test with the GaAs HBTs shown in FIGS. 13A, 13B, and 13C, respectively.
[0041] FIG. 14A is a diagram showing a GaAs HBT according to an embodiment of the present invention. FIG. 14B is a diagram showing a conventional InGaAs base pseudomorphic GaAs HBT. FIG. 14C is a diagram showing the results of the HTOL test for the GaAs HBT according to an embodiment of the present invention shown in FIG. 14A. FIG. 14D is a diagram showing the results of the HTOL test with the conventional InGaAs base GaAs HBT shown in FIG. 14B. FIG. 14E is a diagram showing measured C.sub.bc plotted as a function of I.sub.c for the GaAs HBT shown in FIG. 14A (circle) and a GaAs HBT with a GaAs base (triangle).
[0042] FIG. 15 illustrates the collector current (Ic) and collector voltage (Vce) characteristics showing the Vce offset, Vceoff. Load lines for the device operating as a RF power amplifier are also illustrated.
[0043] FIG. 16 illustrates the conduction band edge (ECBM) profile for the conventional GaAs HBT with a GaAs base and the conventional pseudomorphic GaAs HBT with an InGaAs base. Arrows illustrate the flow of forward junction electron current.
DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENTS
[0044] The embodiments according to the present invention will hereinafter be described with reference to the drawings.
[0045] FIG. 1 shows an embodiment of a GaAs heterojunction bipolar transistor (HBT) 100 provided by the present invention. The GaAs HBT 100 comprises a GaAs substrate 110 and plural semiconductor layers epitaxially grown on the GaAs substrate 110. The plural semiconductor layers sequentially form a sub-collector 120, a collector 130, a base 140, and an emitter 150, in which the base 140 comprises a first base layer 141 and a second base layer 142. The first base layer 141 is formed on the collector 130 and the second base layer 142 is formed between the first base layer 141 and the emitter 150.
[0046] The first base layer 141 comprises In.sub.iGa.sub.1-iAs with an Indium content i being in the range 0<i<1, and the second base layer 142 comprises In.sub.iGa.sub.1-jAs with an Indium content j being in the range 0<j<1. The Indium content i of the first base layer 141 is uniformly distributed or varies from the emitter side to the collector side with a first slope s1, and the Indium content j of the second base layer 142 varies from the emitter side to the collector side with a second slope s2. Here, the sign of the slope s1 and s2 is defined as positive when the Indium content increases from the emitter side to the collector side. The second base layer 142 is designed to have a positive slope s2 on average. On the other hand, the first base layer 141 is designed to have a much larger average Indium content and a much less positive composition gradient than the second base layer 142 in the present invention. In a preferable embodiment of the present invention, the average of the first slope s1 is half of the average of the second slope s2 or smaller. In an extreme case, the Indium content i is a constant. In another extreme case, the first slope s1 may be even slightly negative. In some embodiments, the Indium content i in the first base layer 141 is between 0.03 and 0.2. As shown in FIG. 1, the y-axis is directed from the emitter side to the collector side along the stacking direction of the plural semiconductor layers.
[0047] FIGS. 2A and 2B show two embodiments of the distribution of Indium contents i and j along the y-axis. In the embodiment shown in FIG. 2A, the average of the second slope s is positive, and the average of the first slope s1 is smaller than half of the average of the second slope s2. FIG. 2B shows an extreme embodiment, in which the Indium content gradient only exists in the second base layer 142 and the Indium content in the first base layer 141 is uniform, indicating that the average of the first slope s1 is zero. The total amount of Indium in the base 140 provided by the present invention is reduced compared with the case that the whole base consists of a single InGaAs layer with the same average Indium content as that of the first base layer. As a result, the misfit dislocations are reduced or eliminated. Since the total base thickness is unchanged, the base sheet resistance is also unchanged. Moreover, the built-in drift electric field imposed by the second slope s2 in the second base layer 142 prevents the electron-hole recombination at the emitter/base junction and thus contributes to the increase of the current gain. On the other hand, the first base layer 141 with the large Indium content reduces the base-collector capacitance C.sub.bc. It is because the diffusion capacitance is reduced, as is suggested with the InGaP/GaAs/AlGaAs DHBT, and/or the depletion capacitance is reduced owing to the reduced base onset voltage V.sub.be. Thus, the RF performance is improved.
[0048] FIG. 3A shows another embodiment of the GaAs HBT 100 provided by the present invention, in which the emitter 150 consists of an InGaAs contact layer 151, a GaAs layer 152, and a lattice-matched InGaP layer 153 with an Indium content being 0.48. Either 0.48 or 0.49 as regarded as the Indium content that gives the lattice constant matched to GaAs. In the present invention, 0.48 is regarded as the Indium content for the lattice-matched InGaP. The thickness and the doping concentration of the InGaP layer 151 are 40 nm and 210.sup.17 cm.sup.3, respectively. The thickness of the second base layer 142 t.sub.grad is varied while the total base thickness is fixed at 70 nm. As shown in FIG. 3B, the Indium content is linearly graded from 0 to 0.1 in the second base layer 142, and is fixed at 0.1 in the first base layer 141. At the base-collector junction, there is a thin InGaAs collector layer 131 with the Indium content graded from 0.1 at the base side to 0 at the collector side. This layer is introduced to smoothly connect the conduction band edge between the base and the collector. The thickness of the InGaAs collector layer 131 is typically between 5 nm and 30 nm. The collector layer 132 is composed of GaAs with the thickness being 1 m.
[0049] The base-collector capacitance C.sub.bc in the knee region (low collector voltage/high collector current region) with the GaAs HBT structure shown in FIG. 3A is simulated. FIG. 3C shows the results of a two-dimensional device simulation of the C.sub.bc plotted as a function of the t.sub.grad thickness of the second base layer 142 when the emitter size is 3 m40 m3 fingers, V.sub.ce=0.5V, and I.sub.c=100 mA. Comparing with the extreme case where the whole base layer is linearly graded (i.e. t.sub.grad=70 nm), C.sub.bc is reduced if the thickness of the base layer with uniform composition (the first base layer) is increased. The C.sub.bc is reduced and the misfit dislocation can also be reduced or eliminated simultaneously while t.sub.ad is greater than 0 and smaller than 70 nm.
[0050] FIG. 4A shows an embodiment of the GaAs HBT 100 shown in FIG. 3A, in which the semiconductor layers are fabricated using an epitaxial wafer grown on a GaAs substrate by metal-organic chemical vapor deposition. The Indium content of the second base layer 142 is linearly graded from 0.07 at the first base layer side to 0 at the emitter side. The Indium content in the first base layer 141 is fixed at 0.07. The collector layer 131 has the Indium content which is graded from 0 at the collector side to 0.07 at the first base layer side. In FIG. 4A and also in other tables showing the device structures, the arrow between two numbers, 0.07.fwdarw.0, for example, means that the content of the element is continuously graded from 0.07 at the bottom to 0 at the top of the layer. After optimizing the thickness of the first and the second base layers, a DC current gain equal to 140 is achieved when the first base layer and the second base layer are 50 nm and 20 nm, respectively. FIG. 4B shows a measured C.sub.bc plotted as a function of I.sub.c of the GaAs HBT shown in FIG. 4A (circle) and a conventional GaAs HBT in which GaAs is the base material (triangle) for the emitter size of 3 m40 m3 fingers and Vce=0.5 V. It is shown that the C.sub.bc is smaller for the GaAs HBT of the present invention than for the conventional one, particularly at a large I.sub.c corresponding to the knee region.
[0051] The RF performance of the GaAs HBT shown in FIG. 4A is evaluated. A load-pull measurement of the device is done with the frequency being 0.9 GHz, the emitter size being 3 m40 m3 fingers, V.sub.c being 3.6V, and I.sub.c being 10 mA. FIG. 4C shows the power-added efficiency (PAE) plotted as a function of the output power (Pout) of the present invention (solid line) and of a GaAs HBT in which GaAs is the base material (dashed line). The maximum PAE in the high power region is improved for the GaAs HBT according to the present invention. This is due to the reduced C.sub.bc at the knee region, which reduces the gain suppression at the high power.
[0052] FIG. 4D shows the Error Vector Magnitude (EVM) of power amplifiers operating with a WiFi IEEE 802.11a modulation. The EVM is evaluated for the power amplifiers fabricated using the GaAs HBT shown in FIG. 4A (solid line) and a GaAs HBT in which GaAs is the base material (dashed line) with the frequency being 5.8 GHz, the emitter size being 3 m40 m3 fingers, V.sub.c being 5V, and I.sub.c being 23 mA. The output power (Pout) of the power amplifier fabricated using the GaAs HBT of the present invention at a given EVM is improved by about 0.8 dB with the EVM being between 1.8% and 3% which is required in the IEEE802.11 standards.
[0053] FIG. 5A shows an embodiment of a GaAs HBT 200 provided by the present invention. The GaAs HBT 200 comprises a GaAs substrate 210 and plural semiconductor layers epitaxially grown on the GaAs substrate 210. The plural semiconductor layers sequentially form a sub-collector 220, a collector 230, a base 240, and an emitter 250, in which the base 240 further comprises a first base layer 241 and a second base layer 242. The first base layer 241 is formed on the collector 230 and the second base layer 242 is formed between the first base layer 241 and the emitter 250.
[0054] The first base layer 241 consists of In.sub.mGa.sub.1-mAs with an Indium content m being in the range 0<m<1, and the second base layer 242 consists of In.sub.nGa.sub.1-nAs with an Indium content n being in the range 0<n<1. The average Indium content n of second base layer 242 is larger than the Indium content m at the second base layer side inside first base layer 241. As shown in FIG. 5A, the y-axis is directed from the emitter side to the collector side along the stacking direction of the plural semiconductor layers. In some embodiments, the Indium content n of second base layer 242 continuously increases from the first base layer side to the emitter side as shown in FIGS. 5B, 5C, and 5D. On the other hand, the Indium content m of the first base layer may be uniform or may increase or decrease from the second base side to the collector side. Some preferred Indium content profiles are shown in FIGS. 5B, 5C, and 5D. In FIG. 5B, the Indium content m of the first base layer is uniform. In FIG. 5C, the Indium content m increases from the second base layer side to the collector side. In FIG. 5D, the Indium content m increases from the second base layer side, then becomes uniform near the collector side. The Indium content profile in the first base layer shown in FIG. 5D corresponds to the first and the second base layers in the other embodiment of the present invention (GaAs HBT 100), whose composition profile is shown in FIG. 2B. In the present invention, the Indium content of the InGaAs base is designed to make the base rich in Indium near the emitter-base junction compared with the inside the base. In some embodiments, the Indium content m of the first base layer 241 is between 0.03 and 0.2.
[0055] FIG. 6A shows another embodiment of the GaAs HBT 200 provided by the present invention, in which the emitter 250 comprises an InGaAs contact layer 251, a GaAs layer 252, and a first emitter layer 253. The first emitter layer 253 is lattice-matched to GaAs with an Indium content of 0.48. The thickness and the doping concentration of the first emitter layer 253 are 40 nm and 210.sup.17 cm.sup.3, respectively. The profile of the Indium content may be modified to form a second emitter layer 254 composed of Indium-rich In.sub.vGa.sub.1-vP with the profile of Indium content v shown in FIG. 6C. On the other hand, as shown in FIG. 6B, the Indium content n in the second base layer 242 comprising In.sub.nGa.sub.1-nAs is linearly graded from 0.12 at the emitter side to 0.1 at the first base layer side, and the Indium content m is fixed at 0.1 in the first base layer 241 comprising In.sub.mGa.sub.1-mAs. At the base-collector junction, there is a thin InGaAs collector layer 231 with the Indium content graded from 0.10 at the base side to 0 at the collector side. This layer is introduced to smoothly connect the conduction band edge between the base and the collector. The thickness of collector 231 is typically between 5 nm and 30 nn. The collector layer 232 comprises GaAs with the thickness being 1 m.
[0056] FIG. 6D is the two-dimensional device simulation result of Vceoff with different types of devices. The base and emitter layers in FIG. 6D refer to the HBT structure shown in FIG. 6A. Tin FIG. 6D is the thickness of each layer. The device in which T of the second base layer is equal to zero corresponds to a conventional HBT. In the conventional GaAs base GaAs HBT where the base consists of 44 nm GaAs and the emitter consists of 40 nm lattice-matched InGaP, the simulated Vceoff is 197 mV. If the base is replaced by InGaAs with the Indium content being 0.1, Vceoff is increased to 223 mV. The Vceoff is even larger (228 mV) with the Indium content being 0.12. On the other hand, in Embodiment 1 shown in FIG. 6D, the second base layer is introduced with the total base thickness being unchanged; the Vceoff is reduced to 214 mV. In Embodiment 2, the Indium-rich InGaP second emitter layer is also introduced; the Vceoff is reduced to 181 mV. It is noted that the introduction of the second emitter layer alone results in Vceoff of 190 mV and 196 mV with the Indium content of the second base layer being 0.1 and 0.12, respectively. The reduction of Vceoff is most pronounced when the device comprises both the second base layer and the second emitter layer.
[0057] FIG. 7A shows an embodiment of the GaAs HBT 200 shown in FIG. 6A, in which the GaAs HBT is fabricated using an epitaxial wafer grown on a GaAs substrate by metal-organic chemical vapor deposition. The Indium content n of second base layer 242 is linearly graded from 0.1 at the first base layer side to 0.12 at the emitter side. The Indium content m of first base layer 241 is fixed at 0.10. FIG. 7B shows measured I.sub.c plotted as a function of V.sub.ce of the GaAs HBT in FIG. 7A whose emitter size being 3 m40 m3 fingers (solid line) and of a conventional GaAs HBT in which GaAs is the base material (dashed line). The Vceoff is lower for the present invention than for the conventional device. FIG. 7C shows a measured C.sub.bc plotted as a function of I.sub.c of the GaAs HBT shown in FIG. 7A (circle) and a conventional GaAs HBT in which GaAs is the base material (triangle) for the emitter size of 3 m40 m3 fingers and Vce=0.5 V. The C.sub.bc is smaller for the GaAs HBT of the present invention in which InGaAs is the base material than for the conventional device, particularly at a large I.sub.c corresponding to the knee region.
[0058] The RF performance of the GaAs HBT shown in FIG. 7A is evaluated. A load-pull measurement is done with the device whose emitter size is 3 m40 m3 fingers, the frequency being 1.95 GHz, V.sub.c being 3.4V, and I.sub.c being 6.4 mA. FIG. 7D shows the power-added efficiency (PAE) plotted as a function of output power (Pout) of the present invention (circle) and of the GaAs HBT in which GaAs is the base material (triangle). The maximum PAE is increased with the GaAs HBT according to the present invention. This is due to the reduced C.sub.bc at the knee region, which reduces the gain suppression at the high power operation.
[0059] FIG. 8 shows an embodiment of a GaAs HBT 300 provided by the present invention. The GaAs HBT 300 comprises a GaAs substrate 310 and plural semiconductor layers epitaxially grown on the GaAs substrate 310. The plural semiconductor layers sequentially form a sub-collector 320, a collector 330, a base 340, and an emitter 350. The base 340 comprises a first base layer 341, which is pseudomorphic to GaAs with a first bulk lattice constant being at least 0.15% larger than the bulk lattice constant of GaAs. The emitter 350 comprises a first emitter layer 351, which is pseudomorphic to GaAs with a second bulk lattice constant being at least 0.15% smaller than the bulk lattice constant of GaAs. The pseudomorphic emitter layer generates a tensile strain that compensates the compressive strain in the pseudomorphic base layer. The mechanical instability is thus reduced, and the formation and the multiplication of misfit dislocations during the device operation can be avoidable.
[0060] In an embodiment, the first emitter layer 351 may consist of In.sub.xGa.sub.1-xAs.sub.1-yP.sub.y with the Indium content x and the Phosphorus content y being between 0 and 1 and satisfying the condition that the bulk lattice constant of the first emitter layer 351 is at least 0.15% smaller than the bulk lattice constant of GaAs.
[0061] In an embodiment, the first emitter layer 351 may consist of In.sub.xGa.sub.1-xP with the Indium content x being between 0.1 and 0.44. The In.sub.xGa.sub.1-xP with the Indium content x being 0.1 and 0.44 has the bulk lattice constant smaller than that of GaAs by 2.8% and 0.3%, respectively.
[0062] In an embodiment, the first emitter layer 351 may consist of GaAs.sub.1-yP.sub.y with the Phosphorus content y being between 0.03 and 0.5. GaAs.sub.1-yP.sub.y has a direct band gap for the Phosphorus content y lower than 0.5. Therefore, the high resistance to the conduction electron passing through the GaAs.sub.1-yP.sub.y layer can be avoidable.
[0063] In an embodiment, the first base layer 341 may consist of In.sub.oGa.sub.1-oAs.sub.1-pSb.sub.p with Indium content o and Antimony content p being between 0 and 1 and satisfying the condition that the bulk lattice constant of the first base layer 341 is at least 0.15% larger than the bulk lattice constant of GaAs.
[0064] In an embodiment, the first base layer 341 consists of In.sub.oGa.sub.1-oAs with the Indium content o being between 0.03 and 0.2. In.sub.oGa.sub.1-oAs with the Indium content being 0.03 and 0.2 has the bulk lattice constant larger than bulk GaAs by 0.2% and 1.4%, respectively.
[0065] In an embodiment, first base layer 341 consists of GaAs.sub.1-pSb.sub.p with the Antimony content p being between 0.03 and 0.2.
[0066] In any combination of the abovementioned emitter and base materials, the compressive strain in the base layer is compensated by the tensile strain in the emitter layer.
[0067] FIG. 9A shows an embodiment of the GaAs HBT shown in FIG. 8. In this embodiment, the first emitter layer 351 comprises Gallium-rich In.sub.xGa.sub.1-xP. The Gallium-rich In.sub.xGa.sub.1-xP has a tensile strain that compensates the compressive strain in first base layer 341. The Indium content x is varied across the first emitter layer 351. For example, the Indium content x may be graded with the minimum in the middle of the In.sub.xGa.sub.1-xP layer. Defining the y-axis directed from the emitter side to the collector side along the stacking direction of the plural semiconductor layers, FIG. 9B shows a graded Indium content x along the y-axis with the minimum being near the center of the first emitter layer and the resulting profile of the conduction band edge E.sub.CBM. In this embodiment, the Indium content x is 0.3 or greater as shown in FIG. 9B. It is because InGaP has an indirect band gap if the Indium content is below 0.3. The high resistance to the conduction electron passing through the InGaP layer can be thus avoidable.
[0068] To smoothen the profile of the conduction band edge, a high concentration n-type doping may be introduced in the high Gallium-content region. FIG. 10A is an embodiment of the GaAs HBT 300 shown in FIG. 8, in which a high concentration n-type doping 360 is further introduced at the minimum of the Indium content profile, i.e. the maximum of the Gallium content profile. FIG. 10B shows the resulting profile of the conduction band edge E.sub.CBM with the Indium content distribution being same as that shown in FIG. 9B and a planar n-type doping 360 being located at the minimum of the Indium content profile. Comparing with the result shown in FIG. 9B, the conduction electrons can flow more smoothly from the emitter to the base, and therefore a better high-frequency operation can be obtained.
[0069] FIG. 11A is another embodiment of the GaAs HBT 300 shown in FIG. 8, in which the first emitter layer 351 comprises a thick Gallium-rich In.sub.xGa.sub.1-xP layer with the Indium content x being 0.3. The first emitter layer 351 further comprises two planar n-type doping 360 at both sides of the layer with the Indium content x being 0.3, in order to smoothen the conduction band edge profile, as shown in FIG. 11B. The thick Gallium-rich InGaP layer strongly compensates the compressive strain generated in the pseudomorphic base layer. The formation and the multiplication of misfit dislocations during the device operation are more effectively prevented.
[0070] FIG. 12A shows another embodiment of the GaAs HBT 300 shown in FIG. 8. In FIG. 12A, a second emitter layer 352 comprising In.sub.zGa.sub.1-zP is introduced between the first emitter layer 351 and the base 340. In the second emitter layer 352, the Indium content z at the base side is made larger than that of the layers with lattice being matched to GaAs, as shown in FIG. 12B, in order to reduce Vceoff. In this case, the second emitter layer 352 introduces additional compressive strain. However, the mechanical instability can be reduced by the tensile strain in the Gallium-rich region in the first emitter 351. The strong tensile strain generated by thick Gallium-rich InGaP compensates the compressive strain in both first base 341 and second emitter 352, and thus ensures the mechanical stability. Two planar n-type doping region 360 are introduced to smoothen the conduction band edge profile from the emitter to the base.
[0071] FIGS. 13A, 13B, and 13C show the semiconductor layer structures of GaAs HBTs fabricated to test the effect of strain compensation by the tensile-strained emitter layer of the present invention. FIGS. 13B and 13C show GaAs HBTs according to embodiments of the present invention. The semiconductor layers are fabricated using an epitaxial layers growth by metal-organic chemical vapor deposition. In the GaAs HBT shown FIG. 13A, the emitter comprises a lattice-matched InGaP layer with the Indium content being 0.48 and an additional Indium-rich In.sub.zGa.sub.1-zP layer between the lattice-matched InGaP emitter and the base. The Indium content z of the Indium-rich emitter layer is graded from 0.65 at the base side to 0.48 at the emitter side. In the GaAs HBT shown in FIG. 13B, the Gallium-rich In.sub.xGa.sub.1-xP emitter layers, whose Indium content profile is similar to that shown in FIG. 9B, is introduced to compensate the compressive strain generated in the base and the Indium-rich In.sub.zGa.sub.1-zP emitter. These two layers of the Gallium-rich In.sub.xGa.sub.1-xP correspond to the first emitter layer 351 in FIG. 9A, and the Indium-rich In.sub.zGa.sub.1-zP layer corresponds to the second emitter 352 in FIG. 12A. The base of this embodiment comprising In.sub.oGa.sub.1-oAs corresponds to the first base layer. In the GaAs HBT shown in FIG. 13C, the first emitter layer comprises a thick Gallium-rich In.sub.xGa.sub.1-xP layer to compensate the strain more strongly. The high-temperature operation lifetime (HTOL) test at an operation condition which is typical to handset power amplifiers is made. The bias condition in the HTOL test is V.sub.ce=3 V and Ic=20 mA. The emitter size of the test device is 2 m20 m2 fingers. The junction temperature during test is approximately 210 degree Centigrade. At each type of devices, 45 samples are tested. FIGS. 13D, 13E, and 13F are diagrams showing the result of the HTOL test for the GaAs HBTs shown in FIGS. 13A, 13B, and 13C, respectively. The DC current gain normalized to the initial value is plotted as a function of stress time. Without the strain compensation, a large number of samples show failure after 160 hours test, as shown in FIG. 13D. However, with the strain compensation, the number of failure is reduced in the order of the strength of compensation, as shown in FIGS. 13E and 13F.
[0072] FIG. 14A shows an embodiment of the GaAs HBT 300 shown in FIG. 12A, in which the semiconductor layers are fabricated using an epitaxial growth by metal-organic chemical vapor deposition. The first emitter layer comprises a thick Gallium-rich In.sub.xGa.sub.1-xP layer with the Indium content x being 0.3 and two planar n-type doping regions. The GaAs HBT further comprises the second emitter layer comprising In.sub.zGa.sub.1-zP whose Indium content z is graded from 0.65 to 0.48. The base comprises an In.sub.oGa.sub.1-oAs layer with uniform Indium content being 0.1 (first base layer) and an In.sub.qGa.sub.1-qAs layer with Indium content q graded from 0.1 to 0.12 (second base layer) which is similar to the second base layer shown in FIG. 7A. The purpose of inserting the second base layer is to reduce Vceoff. The use of the first base layer composed of InGaAs is to reduce the C.sub.bc, which is similar to the InGaAs base layers of GaAs HBT 100 and GaAs HBT 200. In this embodiment, the large compressive strain generated by the first and second base layers and the second emitter layer is effectively compensated by the first emitter layer comprising a thick Gallium-rich In.sub.xGa.sub.1-xP layer. The HTOL test at an operation condition typical to handset power amplifiers is made with the GaAs HBT shown in FIG. 14A. The HTOL test is also made with the GaAs HBT shown in FIG. 14B, which is a pseudomorphic InGaAs base GaAs HBT based on the prior art. In FIG. 14B, the base layer is composed of InGaAs with the Indium content being fixed at 0.10, and the emitter comprises a lattice-matched InGaP. The result of HTOL test of the present invention is shown in FIG. 14C and that of the prior art is shown FIG. 14D. Several samples of the prior art fail after 200 hours, while the GaAs HBT according to an embodiment of the present invention does not show any failure even after 1000 hours. It should be noted that the total amount of Indium in the base layer of the GaAs HBT shown in FIG. 14A is larger than that of the GaAs HBT shown in FIG. 14B, even while the total base thicknesses is the same. Furthermore, the Indium content in the InGaP emitter near the emitter-base junction is larger with the GaAs HBT shown in FIG. 14A. Therefore, the total compressive strain is much larger with the GaAs HBT shown in FIG. 14A than with the GaAs HBT shown in FIG. 14B. Nevertheless, the GaAs HBT shown in FIG. 14A passes the test.
[0073] FIG. 14E shows the measured C.sub.bc plotted as a function of I.sub.c with the GaAs HBT shown in FIG. 14A (circle) and a conventional HBT in which GaAs is the base material (triangle). The emitter size is 3 m40 m3 fingers and Vce is 0.5V. As shown in FIG. 14E, the C.sub.bc of the GaAs HBT shown in FIG. 14A, whose base is composed of InGaAs, is much lower than that of the conventional HBT whose base is composed of GaAs, particularly at a large I.sub.c corresponding to the knee region.
[0074] In summary, the present invention has the following advantages:
1. In the GaAs HBT comprising pseudomorphic InGaAs base layers with Indium content being increased from the emitter side to the collector side with varied slope, the total amount of Indium is lowered compared with the single base layer with an uniform or linearly graded Indium content. As a result, the misfit dislocations due to the lattice mismatch are reduced or eliminated. The first base layer on the collector side with a large Indium content reduces C.sub.bc. Thus, the RF performance is improved. On the other hand, the drift electric field across the second base layer on the emitter side with a larger slope of Indium content contributes to the increase of the DC current gain.
2. In the GaAs HBT comprising pseudomorphic InGaAs base layers having a region rich in Indium near the emitter-base junction, the turn-on voltage offset Vceoff in the Ic-Vce characteristics is reduced while not in the conventional GaAs HBT with a pseudomorphic InGaAs base. Thus, the RF performance of power amplifier is improved. The InGaAs base also reduces C.sub.bc and improves the RF performance.
3. In the HBT comprising a base layer which is pseudomorphic to GaAs with a bulk lattice constant larger than the bulk lattice constant of GaAs and an emitter layer pseudomorphic to GaAs with a bulk lattice constant smaller than the bulk lattice constant of GaAs, the pseudomorphic emitter layer generates a tensile strain that compensates the compressive strain in the pseudomorphic base layer. The mechanical instability is thus reduced, and the formation and the multiplication of misfit dislocations during the long-term device operation are prevented.
[0075] To sum up, the HBT provided by the present invention can indeed get its anticipated objects with a GaAs HBT which has an improved DC and RF performance, an enhanced mechanical stability during a long-term operation, and a better high-frequency operation.
[0076] The description referred to the drawings stated above is only for the preferred embodiments of the present invention. Many equivalent local variations and modifications can still be made by those skilled in the art and do not depart from the spirit of the present invention, so they should be regarded to fall into the scope defined by the appended claims.
