RF power transistor

Details RF power transistor RF power transistor

2000-05-16

2002-01-22

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Forming bipolar transistor by formation or alteration of...

C438S330000

Reexamination Certificate

active

06340618

ABSTRACT:

FIELD OF THE INVENTION
The inventions related to silicon bipolar RF-power transistors, particularly discrete transistors using high voltage supply for use in cellular base stations, TV-transmitters etc.
DESCRIPTION OF THE RELATED ART
Bipolar transistors for high-frequency power amplification are widely used in output parts of communications system. High-frequency transistors were first fabricated in germanium in late fifties but were soon replaced by silicon bipolar transistors in the beginning of the sixties, and have since then dominated the RF-power area. Por cellular radio, bipolar transistors are dominating in the base station output amplifiers, and can deliver great performance up to at least 2 GHz and 100 W output power, with good stability, availability and price. Other technologies of choice for this class of applications are GaAs MESFETs and laterally diffused MOS-transistors (LD-MOS). There is a strong driving force to further improve the existing technology, as well as to explore new types of devices, because of the rapidly expanding telecommunications market. Computer tools presently available are not yet capable to predict detailed behavior or performance in real applications, and performance optimization is made using mainly experimental methods.
Power transistors are especially designed to deliver high output power and high gain. Manufacturing process, device parameters, layouts and package have been carefully tuned for this purpose. The devices need to meet numerous detailed requirements for breakdown voltages, DC gain or transconductance, capacitance, RF gain, ruggedness, noise figure, input/output impedance, distortion etc. The operating frequency range from several hundred MHz into the GHz region. Power transistors operate at large signal levels and high current densities. About 1 W output power is a starting level where special considerations have to taken into account, and may serve as a loose definition of a power device, compared to a “normal”, IC-type of transistor.
A bipolar transistor is usually designed using only one n-type (i.e. NPN) device on a single die. A collector layer (n
−
epi) is epitaxially deposited on an n
+
substrate. The base and emitter are formed by diffusion or ion implantation at the top of the epitaxial layer. By varying the doping profiles, it is possible to achieve different frequency and breakdown voltage characteristics. The output power requirements range up to several hundred watts, sometimes even kilowatts, and the high output power is achieved by paralleling many transistor cells on a single die, and paralleling several dies in a package. The packages often have large gold-plated heat sinks to remove heat generated by the chip.
For the DC-data, the BV
CEO
(collector-emitter breakdown voltage with open base) is the most limiting parameters, traditionally designed to be higher than V
cc
(24-28 V supply voltage is a common range for this class of devices). A well-known empiric formula for the relationship of the transistor breakdown voltages and the current gain, &bgr; or h
FE
, states:
B
⁢
 
⁢
V
CEO
=
B
⁢
 
⁢
V
CBO
β
n
(
1
)
where BV
CEO
already has been defined, BV
CBO
is the collector-base breakdown voltage with open emitter, and n is an empirical constant, usually between 2.5 and 4.5, related to the nature of the BC-junction breakdown. For a given epi doping and device design (constant n), BV
CBO
will be constant, and then BV
CEO
and &bgr; are directly correlated: higher &bgr; gives lower BV
CEO
. n can be improved by different doping profile tricks, to ensure that nature of the BV
CBO
is as close as possible to the one-dimensional junction case.
To obtain a device capable of high output power, the doping of the collector layer should be selected as high as possible, thus suppressing high current phenomena, such as the Kirk effect. A highly doped collector layer also has the advantage of having a smaller depletion region, which makes it possible to select a thinner epi layer, with less parasitic resistance and better high-frequency performance, without being limited by thickness-limited breakdown. The problem is that increased collector doping inevitably leads to a low BV
CBO
and thus a low BV
CEO
, according to equation (1).
To obtain a device capable of high power gain, the &bgr; must not be too low. The power gain G
p
can be described by the following relationship:
G
p
⁡
(
f
)
≈
β
1
+
β
2
⁡
(
f
f
max
)
4
(
2
)
where &bgr; is the zero-frequency gain (h
FE
) and f
max
is the maximum oscillation frequency, or the frequency where the power gain is equal to unity. A plot olf equation (2), h
FE
versus G
p
, is shown in
FIG. 1
for different f
max
values at f=1 GHz. From this plot it can be concluded that a high f
max
and not too low &bgr; are detrimental for a good RF power gain.
Because of the relations between output power via collector doping, power gain via &bgr; and BV
CEO
, if a low BV
ceo
can be accepted, this will lead to significant improvements of the most important parameters for RF power transistors.
Because Of this, data sheets may specify BV
CER
instead of BV
CEO
. A small resistor is connected between the base and emitter when designing the amplifier, to assure that the base is never fully open. If the resistor is small enough, BV
CER
will approach BV
CES,
which is close (slightly lower) to BV
CBO
.
The characteristics for the different collector breakdown voltages are shown in FIG.
2
.
As apparent from the previous section, if BV
CEO
is lower than V
cc
, an external resistor, which occupies additional space on a circuit board, must be used to assure safe operation of the device. The value is dependent on the size of the device, and an optimal value may be problematic to find, and requires some experience to not destroy the device while finding the value. If, in any way, the resistor disconnects from the circuit, e.g. during evaluations, bad soldering etc., the transistor may be damaged.
SUMMARY
By integrating a resistor on the bipolar RF-power transistor semiconductor die, between base and emitter in accordance to the present invention, it will be assured that the conditions to obtain the BV
CER
always will be fulfilled.
Therefore, integrating the resistor necessary for BV
CER
into the semiconductor die results in that the use of transistors with an intrinsic low BV
CEO
is simplified.
A method according to the present invention is set forth by the independent claim
1
and the dependent claims
2
-
6
. Further a transistor device according to the present invention is set forth by the independent claim
7
and further embodiments are set forth by the dependent claims
8
-
10
.


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Erlandsson, T.; International-Type Search Report; Search Request No. SE99/00616; App. No. SE9901771-7; Apr. 5, 2000, pp. 1-3.
H. F. Cooke,Microwave Transistors: Theory and Design, PROC, IEEE, vol. 59, pp. 1163-1181, Aug. 1971.
S. M. Sze,Physics of Semiconductor Devices, 2nd Ed., pp. 150-151, John Wiley & Sons, Inc., 1981.
R. Allison,Silicon Bipolar Microwave Power Transistors, IEEE Trans. Microwave Theory & Techniques, vol. MTT-27 No. 5, pp. 415-422, 1979.

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