KR20040052276A - Heterojunction bipolar transistor power transistor - Google Patents

Abstract

PURPOSE: A hetero-junction bipolar transistor power transistor is provided to prevent thermal runaway by using stabilizing resistors, coupling capacitors and new-layout transistor cells. CONSTITUTION: A base area(202) is located on a collector area(201) with a base mesa formation. An emitter area(203) is included in the base mesa. Two rectangular collector contacts(204) are located on the collector area(201). A rectangular base contact(205) is located on the base mesa. A rectangular emitter contact(206) is located on the emitter area(203). The rectangular emitter contact(206) is electrically isolated from the base area(202) and includes a long edge region which faces one of long edge regions of the rectangular base contact(205). The base contact is connected to a base stabilizing resistor.

Description

Heterojunction bipolar transistor power transistor {HETEROJUNCTION BIPOLAR TRANSISTOR POWER TRANSISTOR}

The present invention relates to the optimized design of heterojunction bipolar transistor (HBT) power transistors with improved ruggedness, in particular the use of HBT power transistors by combining the use of stabilized resistors with a new layout of transistor cells. Related to the optimized design, thermal runaway can be avoided while maintaining the performance of the HBT power transistor.

Power amplifiers are the most important elements in transmitters of cellular telephones or other wireless communication systems. Usually a power amplifier consists of many device transistors, where each device transistor typically uses a junction bipolar transistor (HBT) as a heter. In HBT power transistors, a problem of so-called "self heating" exists, resulting in thermal runaway and destruction of device operation. The self heating effect originates from a positive-feedback between the collector current and the temperature of the transistor, and the transistor becomes thermally unstable. As expected from the current-temperature relationship of the pn junction, the high temperature creates an increased collector current, which in turn raises the temperature of the transistor and thus further increases the collector current. This unstable state suddenly destroys the operation of the power transistor, or even permanently damages the device.

In order to avoid the problem of thermal runaway and to improve device ruggedness, stability resistors are usually included in the emitter of the HBT. As the collector current increases, the emitter-stable resistor (EBR) will increase the emitter voltage and reduce the voltage drop between the base and the emitter, thus reducing the collector current. In other words, EBR effectively avoids the self-heating problem by forming a negative-feedback loop between the input and the output. However, the use of EBR has some drawbacks. For example, if the resistance of the EBR is too large, the power gain of the HBT device will be undesirably reduced under RF operation and thus reduce the efficiency of the power amplifier.

Another method for stabilizing HBT devices is the use of base-stable resistors (BBR) instead of EBR. The BBR can be connected in series with the input signal and prevent premature current collapse, as well as stabilize the RF signal at the expense of reduced power gain and thereby PAE. This penalty can be eliminated by creating a shunt with an RF coupling capacitor on the signal path and BBR on the DC path. Therefore, voltage transistor ruggedness can be protected without sacrificing gain and efficiency.

In addition, the use of ballast resistors can optimize the cell design of HBT power transistors to avoid thermal runaway at the transistor level rather than the circuit level. Conventional HBT power transistors usually include a number of emitter fingers. However, if such an arrangement of emitter fingers and base contacts includes several "sharp corners", the current flowing to these emitter fingers will not be the same. This unequal flowing current partially heats the higher current density region, which in turn further increases the current density in this region, eventually destroying the operation of the power transistor. Therefore, in order to avoid unequal temperature distributions, the arrangement of these emitter fingers and base contacts in the base mesa is also an important problem.

It is an object of the present invention to develop an optimized design of HBT power transistors with improved ruggedness, so that the problem of thermal runaway can be avoided while maintaining device performance. The layout employed in the present invention to improve the ruggedness of the HBT device is included at the transistor level by locating a new layout of the HBT power transistor cell as well as optimizing the transistor cell design at the circuit level by using a stabilizer.

The characteristic layout of the HBT power transistor cell is the arrangement of emitter fingers and base contacts on the base mesa, where the possibility of a sharp-corner effect is excluded, so that the current flowing through the emitter finger will be evenly distributed and not at the same temperature. The effect of the distribution may be greatly reduced.

Another feature of the present invention is the use of coupling capacitors shunted to the base ballast resistor, which can prevent premature current breakdown and improve the device performance of the HBT power transistor.

1 is a layout side view of a conventional HBT power transistor.

2 is a layout side view of an HBT power transistor formed in accordance with a first preferred embodiment of the present invention.

3 is a layout side view of an HBT power transistor including a base ballast resistor, emitter ballast resistor, and coupling capacitor formed in accordance with a second preferred embodiment of the present invention.

4 is a layout side view of a multi-finger HBT power transistor including a base ballast resistor, an emitter ballast resistor, and a coupling capacitor formed in accordance with a third preferred embodiment of the present invention.

5 is a layout side view of an HBT power transistor including a base ballast resistor, emitter ballast resistor, and coupling capacitor formed in accordance with a fourth preferred embodiment of the present invention.

6A shows the device performance of design A. FIG.

6B shows the device performance of design B. FIG.

7A shows the collector current-voltage characteristic (Ic-Vc) of design C. FIG.

7B shows the collector current-voltage characteristic (Ic-Vc) of design D.

1 shows a layout of a prior art HBT cell design. In general, it includes three regions, the collector region 101, the base region 102, and the emitter region 103. Collector region 101 includes two rectangular collector contacts 104 and a rectangular base region 102 (also referred to as base mesa). Base mesa 102 includes base contact fingers 105 and two rectangular emitter regions 103 independent of base contact fingers 105. Emitter contact fingers 106 are positioned over emitter regions 103 and are electrically separated from base region 102. It is noteworthy that such an arrangement of emitter contact fingers 106 and base contact fingers 105 includes some "sharp corner" as indicated by circle 107 in FIG. 1. The emitter current drawn from the collector contacts 104 around the sharp corner area will not be the same. This unequal flowing emitter current partially heats the region of higher current density, which in turn further increases the current density in this region, eventually destroying the operation of the power transistor.

Figure 2 shows a more compact layout of the HBT cell without the sharp corner effect which is the first preferred embodiment of the present invention. In general, it includes three regions: the collector region 201, the base region 202, and the emitter region 203. Collector region 201 includes a rectangular base mesa forming two collector contacts 204 and base region 202. Base mesa 202 includes a rectangular base contact 205 and a rectangular emitter region 203 in contact with the base contact 205. Emitter contact 206 is located above emitter region 203 and is electrically separated from base region 202. As shown in this figure, this arrangement avoids the possibility of sharp corner effects, and device rudness can be improved.

As shown in FIG. 3, a second preferred embodiment of the present invention is that a HBT cell with the compact layout shown in FIG. 2 additionally shunts to the base stable resistor, emitter stabilizer and / or base stable resistor. It includes a coupling capacitor connected to. As in the first preferred embodiment, the HBT cell comprises three regions, the collector region 301, the base region 302 and the emitter region 303. Collector region 301 includes two collector contacts 304 and a rectangular base mesa 302. Base mesa 302 includes a rectangular base contact 305 and a rectangular emitter region 303 and a rectangular emitter contact 306 located thereon. Emitter stabilizer 307 is connected between emitter contact 306 and emitter terminal 308. The base ballast resistor 309 is connected between the base contact 305 and the base terminal 310 for DC bias input. A coupling capacitor 311 shunted to the base stability resistor 309 is connected between the base contact 305 and the coupling capacitor terminal 312 for the RF input.

Based on the compact layout shown in FIG. 2, the HBT cell can also be designed as a multi-finger device. 4 is a third preferred embodiment of the invention in which the HBT layout is designed as a multi-finger device. Similar to the second preferred embodiment, the multi-finger HBT device shown in FIG. 4 also includes a collector region 401, a base region 402, and an emitter region 403. Collector region 401 includes two collector contacts 404 and a rectangular base mesa 402. Base mesa 402 includes a rectangular base contact 405 and two rectangular emitter regions 403 located on either side of the base contact 405. Each emitter region 403 is located next to the emitter contact finger 406 and is electrically separated from the base region 402. Each emitter contact 406 is connected to emitter ballast resistor 407 and to emitter terminal 408. Base stability resistor 409 is connected between base contact 405 and base terminal 410 for DC bias input. A coupling capacitor 411 shunted to the base stability resistor 409 is connected between the base contact 405 and the coupling capacitor terminal 412 for the RF input.

The shapes of the emitter and base contact electrodes are obviously not limited to rectangular shapes. The contact electrodes can be designed in any shape as long as the shape of these contact fingers excludes the possibility of sharp corners. FIG. 5 illustrates a configuration that includes a circular emitter region 501 over the base mesa 502 and allows the shape of the outer collector region 503 and the collector contact 506 thereon to remain rectangular. Circular emitter contact 504 is located above emitter region 501. The emitter contact is further connected to the emitter stable resistor 507, to which the emitter terminal 508 is connected. A general circular shape base contact 505 is positioned around the circular emitter contact 504 above the base mesa 502. The base contact is further connected to a base stabilizer resistor 509 for the DC input and to a coupling capacitor 510 for the RF input that is connected in a shunt.

6A and 6B show the device performance of two different designs (designs A and B). The total emitter area of these two designs is 7680 μm ² , and both devices use 3Ω emitter stabilizers and 100Ω base stabilizers in series at their base contacts. The main difference between these two devices is their transistor layout, design A uses the prior art layout as shown in Figure 1 and design B uses the layout of the second preferred embodiment of the present invention. In Figure 6, the device performance of these two designs is compared. Even though the two designs have the same emitter and base stabilizer, the power amplification efficiency (PAE) of design B is shown to be nearly 10% higher than that of design A. From our ruggedness test, design A passes only the 10: 1 voltage standing wave ratio (VSWR) at V _{CE =} 4V (collector-emitter voltage), and design B passes the 10: 1 voltage standing wave at V _{CE =} 5V. Pass the rain (VSWR). 7A and 7B show the collector current-voltage characteristics (I _C -V _C ) of the other two designs (designs C and D), and improved device performance can be achieved by optimizing the coupling capacitors connected to the stabilizer resistor and shunt. It is clearly shown that there is. Design C uses a 75Ω base ballast resistor and a 3Ω emitter ballast resistor in series with the base contact without any coupling capacitors. In Design D, a 125Ω base ballast resistor is shunted to a 1.3㎊ coupling capacitor without the use of any emitter ballast resistors. As shown in FIG. 7, premature current collapse is found in design C when Vc> 4. Moreover, because of the use of shunt-connected base stability resistors without the use of emitter stability resistors, design D typically shows PAE ˜10% higher than design C. In addition, it can be found that Design D is more thermally stable than Design C. Our rigidity test shows that design C fails at V _CE = 3.6V for 8: 1 VSWR and design D passes 10: 1 VSWR at V _CE > 5V. From this comparative analysis, we conclude that by applying the appropriate ballast resistor design and device cell design, better performance and ruggedness can be achieved simultaneously.

The invention has been described with reference to the described embodiments, which are not intended to be construed in a limited sense. Various modifications and combinations of the described embodiments as well as other embodiments of the invention will be apparent to those skilled in the art by reference to the description. Therefore, the appended claims include any modifications or embodiments.

The present invention relates to a heterologous bipolar transistor (HBT) with improved ruggedness. The optimized design of the HBT power transistor combines the use of stable resistors, coupling capacitors and a new layout of the transistor cell, and avoids the problem of thermal runaway while maintaining the performance of the HBT power transistor.

Claims (8)

A collector region, a base region positioned above the collector region in the form of a base mesa, and an emitter region included in the base mesa; Two rectangular collector contacts located above the collector region; A rectangular base contact positioned over the base mesa; And A rectangular emitter contact located above the emitter region, And said rectangular emitter contact electrode is electrically separated from the base region and has a long edge facing one of the long edges of the rectangular base contact. A collector region, a base region positioned above the collector region in the form of a base mesa, and an emitter region included in the base mesa; Two rectangular collector contacts located above the collector region; A rectangular base contact positioned over the base mesa; And Includes two rectangular emitter contacts located on either side of the rectangular base contact over the emitter region, And wherein each of said rectangular emitter contact electrodes is electrically separated from the base region and has a long edge facing one of the long edges of the rectangular base contact. A collector region, a base region positioned above the collector region in the form of a base mesa, and an emitter region included in the base mesa; Rectangular collector contacts located above the collector region; A circular emitter located above the base mesa; And A circular shaped base contact positioned about the circular emitter contact over the base area, Wherein the circular emitter contact electrode is electrically isolated from the base region and the curvatures of the emitter and base contacts are designed such that the current is essentially drawn in and even to the emitter. The method according to any one of claims 1 to 3, Additionally, the base contact is a heterojunction bipolar transistor (HBT) power transistor electrically connected to the base stable resistor. The method according to any one of claims 1 to 3, Additionally, the emitter contact is a heterojunction bipolar transistor (HBT) power transistor electrically connected to the emitter ballast resistor. The method according to any one of claims 1 to 3, In addition, the base contact is a heterojunction bipolar transistor (HBT) power transistor electrically connected to a base capacitor and a coupling capacitor connected in a shunt. The method according to any one of claims 1 to 3, In addition, the base contact is electrically connected to the base stable resistor; Additionally, each emitter contact is a heterojunction bipolar transistor (HBT) power transistor electrically connected to the emitter ballast resistor. The method according to any one of claims 1 to 3, In addition, the base contact is electrically connected to a base capacitor and a coupling capacitor connected to the shunt; Additionally, each emitter contact is a heterojunction bipolar transistor (HBT) power transistor electrically connected to the emitter ballast resistor.