transistor (HBT) with a current gain as high as 100,000 has been fabricated. This HBT utilizes GaN for the emitter and. Sic for the base and collector. The devices ...
High-Temperature GaNlSiC Heterojunction Bipolar Transistor with High Gain J . Pankove1,2, S.S. Chang2, H.C. Lee2, R . J . Molnar3, T.D. Moustakas3, and B. Van Zeghbroeck*
lAstralux Inc., 2386 Vassar Drive, Boulder, CO 80303 2Department of Electrical and Computer Engineering, University of Colorado, Campus Box 425, Boulder, CO 80309-0425 3Boston University, Boston. MA 02215 Abstract A new high temperature heterojunction bipolar transistor (HBT) with a current gain as high as 100,000 has been fabricated. This HBT utilizes GaN for the emitter and S i c for the base and collector. The devices exhibit near ideal current -voltage characteristics, as demonstrated by their high current gain along with the absence of any observable Early effect, with the exception of high leakage currents at voltages above 10 V. High temperature operation has been demonstrated up to 26OOC with minimal degradation in output, except for an increase in leakage currents. Introduction Silicon carbide exhibits many attractive characteristics, such as a large bandgap, 2.9 eV, and high thermal conductivity, 5 W/cm"C [l], which make it a suitable material for high temperature semiconductor device operation. Recently, 6-H S i c bipolar junction transistors have been fabricated with demonstrated high temperature operation of up to 400OC. Until now, these devices have achieved a maximum current gain of 10.4, an emitter efficiency of 0.94 and a base transport factor of 0.96 at 300K [ 2 ] . Silicon carbide bipolar junction transistors with a wider bandgap emitter would have a larger current gain due to a larger emitter efficiency. The wider bandgap emitter restricts the diffusion of holes from the base to the emitter, resulting in a high electron injection efficiency into the base. Additionally, the wider bandgap emitter allows the base to he heavily doped, thereby decreasing the base resistance and the sensitivity to the Early effect without sacrificing the emitter efficiency. Also, silicon carbide has a longer lifetime, compared to a direct bandgap material such as GaN, since it is an indirect bandgap material. The high lifetime yields a long diffusion length, thus a high base transport factor. Furthermore, these devices have a short
base width which further enhances the transport factor, thereby increasing the current gain. Gallium nitride is a natural choice for a high bandgap emitter. Gallium nitride not only has a higher bandgap, 3.4 eV, than S i c , it also has a high thermal conductivity, 1.3 W/cm°C. With a lattice constant of 3.18 A for GaN and 3.08 A for S i c both materials are closely lattice matched ~ 1 [31. . In this paper, we will discuss what we believe to be the first heterojunction bipolar transistor successfully fabricated in the GaN/SiC material system [4]. Due to high leakage currents, the HBTs were operated in common-base mode yielding a common emitter, differential current gain of at least 100,000 while exhibiting virtually no Early effect. We shall describe our fabrication steps, present our experimental results and discuss how this result may be further improved by decreasing its leakage current. Structure and Fabrication The cross section of the GaN/SiC HBT is shown in Figure 1. The device consists of a 0.57 pm thick n-type l0lg c m 3 unintentionally doped GaN emitter grown by MBE with an electron cyclotron resonance (ECR) nitrogen plasma source; a 0.2 pm p-type 9 x 1OI8 c m 3 6-H S i c base grown by LPE on top of an n-type 1.8 x lo1* ~ m S -i c ~ substrate. Before growth of the GaN emitter, the S i c substrate was dipped in HF to remove surface contaminants. The substrate was first degassed at 1000 O C while the GaN was grown at 800 OC. The base pressure in the growth system was less than Torr before the introduction of nitrogen gas. The gallium source temperature was 840 OC. The nitrogen flow rate for this growth was at 6.0 sccm which corresponded to a process pressure of 1.2 x lo4 Torr. The GaN was grown for 3 hours at a growth rate of 0.19 pm/hr. Reference [5] describes this process in more detail.
This work was supported in part by a DNA SBIR phase I contract.
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A1 Emitter Contact 15
GaN Emitter, 0.57 pm Al/Cr Base Contact
SIC Base, 0.2pm
p=9x10
S i c Collector/Substrate
n
=
18
1 8x10
cm 18
-3
cm
-3
0 0
-5
Al/Cr Collector Contact
5
10
Collector to Base Voltage Figure 1. Cross section of the GaNISiC transistor The first device processing step is the deposition and etching of an 0.28 pm thick layer of aluminum which forms the ohmic contact to the emitter [ 6 ] , 171. A 2 pm thick layer of positive photoresist was used to mask the aluminum during etching. The same resist layer also serves as a mask during the reactive ion etching of the GaN, thereby providing a self-aligned emitter contact with respect to the GaN emitter region. The GaN layer was etched in a CC12F2 plasma. The etch rate of the GaN at a flow of 10 sccm, pressure of 100 mTorr, and power density of 0.86 W/cm2 was 23 nm/min, while the etch rate for S i c under the same conditions was 96 n m h i n . The large difference in etch rate caused substantial etching into the base layer. A total of 0.676 pm was etched away, so that 0.106 pm or about half of the base layer was also etched away. The base contact was then formed by lifting off 200 nm of A1 with a 50 nm layer of Cr on top. The collector contact was created by first depositing 280 nm of A1 followed by an 80 nm layer of Cr to the back of the wafer. Thirty-five devices were fabricated on a 0.25 cm2 wafer.
15
[v
Figure 2. Common base I-V characteristics at room temperature. The curves correspond to an emitter current of 0 mA, for the bottom curve, 1 mA,2mA,3mA,4mA,5mA,6mA,7mA,8 mA, and 9 mA for the top curve. The emitter area is 75 pm by 75 pm. The current gain ( = dIc/dIB) measured at constant V,, for the device described above is shown in figure 3 . The gain varies significantly with emitter current and collectorbase voltage, reaching a maximum gain of 108,701 at VC,=2V and I, = 100 mA.
Experimental Results Current-voltage characteristics measured at room temperature are shown in Figure 2. The device measured has an emitter contact area of 75 pm by 75 pm. The transistor, as measured in common base mode, exhibits no current variation when changing the collector to base voltage for voltages less than 10 V. Soft breakdown of the base-to-collector junction was observed around 1OV. The absence of the Early effect as well as the low breakdown voltage can be explained by the high doping concentration in the base as well as in the collector regions. The large area (0.25 cm2) of the base-to-collector junction also contributes to the leakage current at higher voltages.
0.1
10.0
100.0
Emitter Current [mAJ Figure 3. Differential current gain, dIc/dIB, as a function of the emitter current for different values of the collector to base voltage, V,,. High temperature operation up to 26OOC has been achieved for the above device, as shown in figure 4. Higher temperature operation was prevented due to limitations of
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the hot chuck. However, due to the large energy bandgap and the high thermal conductivity of S i c and GaN, 5 W/(cm°C) and 1.3 W/(cm°C) respectively, operating temperatures over 4OOOC is expected. At 26OOC close to ideal behavior is observed for voltages less than 5 V. Above 5 V, the leakage current starts to dominate. At 260°C the differential gain, dIc/dIB, for this device is over 1,600, measured at VcB = 0.6 and I, = 6 mA.
where xh is the width of the base region and L, is the diffusion length of electrons. Using a mobility of 110 cm2/Vs, and a lifetime of 5 ps, the diffusion length is calculated to be 37.7 Fm and the base transport factor is calculated to be 0.999987. The current gain, which is expressed as
p=-
a.1Y
l-uTY is calculated to be 80,409 which is in reasonable agreement with the experimental results. As the numbers indicate, the gain of this device is limited by the loss of electrons in the base region due to recombination and not by the emitter efficiency.
1
-5
0 -1
-2
I
-5
5
0
IO
2 -3 L
15
wE 4
Collectorto Baw Voltage M
-5
Figure 4. Common base I-V characteristics at 26OOC. The curves corresponds to an emitter current of 0 mA at the bottom increasing in 1 mA increments to 9 mA at the top.
-6 -7
-0.2
As discussed in the Introduction, an HBT will have high current gain due to the band offsets in a forward biased pn heterojunction. The energy band diagram illustrating this concept for the GaN/SiC transistor under active bias is shown in figure 5. The emitter efficiency is improved through the band offsets in the heterojunction and may be calculated by the ratio of the electron current crossing the emitter base junction to the total emitter current or LE In,
+
0.2
0.4
Depth [microns]
Theoretical Analysis
7=
0
I,,
In our transistor, the emitter efficiency is found to be greater than 0.999999. The base transport factor is dependent on the ratio between the width of the quasi-neutral base region and the diffusion length of electrons in the base and can be written as
Figure 5. Energy band diagram of the HBT under active bias with V,, = 2.5 V and V,, = 3.5 V. The thickness of the emitter and collector layers have been reduced to show the depletion regions more clearly. The current gain is sensitive to many parameters. For example, if the base width were to be decreased to 0.1 pm, the gain would increase by more than a factor of 4 to 366,694. If the lifetime were 1 ps, instead of 5 ps, the gain would decrease by a factor of 5 to 16,081. Therefore, variations in base width or quality of material can drastically affect the gain of an HBT.
Discussion The I-V characteristics at room temperature indicate high leakage currents above 10 V. This effect may be explained by examining the base-collector breakdown voltage. Figure 6 shows the I-V characteristics at room temperature for the base-collector junction. The breakdown voltage was determined by finding the voltage intercept for
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a line tangent to the I-V curve at -15 V as indicated on the figure. For this device, the breakdown voltage occurs at -11 V. The breakdown field at this voltage is 1.0 x lo6 V/cm, assuming -11 V appears across the depletion region. This number is somewhat lower than the 2 - 3.7 x IO6 V/cm which has been reported previously [PI. We attribute the soft breakdown and low breakdown field to the quality of the S i c p-n junction. Furthermore, blue luminescence was observed at collector-base voltages greater than 9 V, which is consistent with the low breakdown voltage.
2
7
-E1
v
U
E L L
5
O
-1
-15
-10
-5
0
5
10
Voltage [VI Figure 6. I-V characteristics of the base-collector junction (p-n) at room temperature. A higher breakdown voltage may be obtained by lowering the doping concentration in the collector junction. The breakdown voltage in lightly doped S i c is 1000 V [8]. However, as the doping increases, the breakdown voltage decreases, and at a doping concentration of 1.6 x 1017 cm9 the breakdown voltage drops to 200 V [9]. Therefore, if the doping concentration in the collector were lowered, the breakdown voltage in the device would improve. Furthermore, as processed, all devices share a common base and collector. If the devices were isolated by etching a trench through the base layer around each device, the leakage current will be reduced. All of the devices were operated in the common base mode. The peak gain of each devices occurred when the base current was less than 1 PA. Therefore, due to the high leakage current present in each device, common emitter
References
H. Morkoc. S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, and M. Bums, "Large-band-gap S i c , 111-V nitride, and 11-VI ZnSe-based semiconductor device technologies." J . Appl. Phys. vol 76, pg. 1363, 1994. J.W. Palmour, J.A. Edmond, and C.H. Carter, Jr., "Demonstrating the Potential of 6H-Silicon Carbide for Power Devices," Technical Digest, 1993, 51st Annual Device Research Conference. paper VAI. S. Strite and H. Morkoc, "GaN, AIN, and InN: A Review," I . Vac. Sci. Technol. B vol. lO(4). pg. 1237, 1992. J . Pankove, U.S. Patent 4,985,742. "High Temperature Semiconductor Devices having at least on Gallium Nitride Layer," January 15, 1991. R. J . Molnar and T.D. Moustakas, "Growth of gallium nitride by electron-cyclotron-resonance plasma-assisted molecular-beam epitaxy: The role of charge species," J . Appl. Phys. Vol 76(8), 1994. J.S. Foresi and T.D. Moustakas, "Metal Contacts to gallium nitride." Appl. Phys. Lett. Vol. 62. pg. 2859, 1993. M.E. Lin. Z.Ma, F.Y. Huang, Z.F. Fan, L.H. Allen, and H. Morkoc, "Low resistance ohmic contacts on wide band-gap GaN," Appl. Phys. Lett. Vol. 64, pg. 1003. 1994. L.G. Matus. J.A. Powell, and C.S. Salupo, "High-voltage 6H-Sic pnjunctiondiodes." Appl. Phys. Lett. Vol. 59, pg. 1770, 1991. Mohit Bhatnagar and B. Jayant Baliga, "Comparison of 6H-SiC, 3CSIC, and SI for Power Devices," IEEE Trans. Electron Devices, Vol. 40. pg. 645, 1993.
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Summary
The first widegap heterostructure bipolar transistor using a GaN emitter and a S i c base and collector has been presented. The transistors were operated in the common base mode with high leakage currents preventing operation in the common emitter mode. A high differential common emitter current gain of over 100,000 has been obtained. High temperature operation up to 26OOC has been demonstrated. The properties of this device make it very attractive for high power and high temperature applications.
/
=-11
operation was not possible. When the devices were operated in the common emitter mode, the high gain amplified the base and leakage currents causing the leakage current to dominate the I-V characteristics. For these devices to function in the common emitter mode, either the leakage current or the current gain must be decreased. The current gain may be reduced by increasing the base width to lower the base transport factor, while the leakage current can be reduced by lowering the collector doping concentration.
