Active solid-state devices (e.g. – transistors – solid-state diode – With means to increase breakdown voltage threshold – Field relief electrode
Reexamination Certificate
2000-03-13
2001-05-29
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
With means to increase breakdown voltage threshold
Field relief electrode
C257S565000, C257S578000
Reexamination Certificate
active
06239475
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a vertical bipolar power transistor and to a method of manufacturing a bipolar power transistor, said power transistors being primarily intended for high frequency applications, especially radio frequency applications.
STATE OF THE ART
Bipolar transistors for power amplification at high frequencies must, for a given supply voltage and operation frequency, fulfill a large number of detailed requirements concerning power amplification, ruggedness, breakdown voltage, noise, distortion, capacitance, input- and output impedance, etc. The operation frequencies for modern telecommunications electronics vary within the radio wave and microwave area. The requirements on the output power vary from a few watts to several hundred watts, where, in the latter case, several components connected in parallel in a casing may be used. Power transistors operate at high signal levels and high current densities. The computer tools currently available are not capable of simulating, in a detailed manner, the behaviour or performance in practical applications.
The semiconductor material most frequently used for power transistors, at least at frequencies below 3 GHz, is silicon. Also, because of the higher mobility of electrons compared to holes, primarily power transistor of npn type are used. The transistor structure is normally vertical, with the collector contact at the back of the silicon substrate. A collector layer is epitaxially deposited on the substrate, and field oxide can then be formed on top of the collector layer outside the active areas. The base and the emitter are formed through diffusion or ion implantation from the top down in the active areas of the epitaxial layer. Metallic interconnecting layers are formed higher up in the structure.
By varying the degree of doping in the collector, the base and/or the emitter, it is possible to obtain different types of frequency and breakdown characteristics. Different horizontal geometries give transistors with different current capacities.
The field oxide in transistors normally serves two purposes: isolation of components and reduction of parasitic capacitances to the substrate. For a bipolar power transistor normally no isolation of the components is needed, as the collector is constituted by the substrate. The purpose of the field oxide then becomes to isolate the metal layers from the substrate so that the parasitic capacitances between them are minimized.
One of the most critical parameters for power transistors at very high frequencies is the signal amplification. The signal amplification may be expressed as (see for example R. Allison,
Silicon bipolar microwave power transistors
, IEEE Trans. Microwave Theory and Techniques, Vol. MTT-27, No. 5, s 415, 1979):
G
(
f
)
≈
G
0
1
+
G
0
2
(
f
f
max
)
4
where G
0
is the so called beta value, that is the zero frequency amplification, f is the frequency and f
max
is the maximum oscillation frequency (for power amplification).
When the beta value and the frequency are high, that is, when
G
0
(
f
f
max
)
4
⪢
1
the amplification G (f) may be written as
G
(
f
)
≈
(
f
max
f
)
2
=
f
T
8
π
R
b
C
bc
1
f
2
where f
T
is the maximum border frequency (for current amplification), R
b
is the base resistance and C
bc
is the base collector capacitance.
The three key factors for obtaining a high power amplification thus are maximum border frequency, minimum base resistance and minimum base collector capacitance. See also, for example, H. F. Cooke,
Microwave transistors: theory and design
, Proc. IEEE, Vol. 59, p. 1163, 1971.
By using polysilicon in the emitter, a higher cut-off frequency may be obtained without the base resistance becoming too high compared to an emitter that has only been ion implanted.
The base resistance may be reduced by reducing the vertical dimensions of the power transistor.
The base-collector capacitance consists of both junction capacitance and metal-substrate capacitance. The junction capacitance is determined by the degree of doping on the least doped side, that is, the collector side, and cannot be adjusted much because of the relation between collector doping and base-collector breakdown voltage.
The metal-substrate capacitance can, in ways known in the art, be reduced, both by increasing the thickness of the field oxide as pointed out above and also by minimizing the metal area.
The maximum practical field oxide thickness is achieved at approximately 2.5-3 &mgr;m, depending on the thermal budget, on junction leaks caused by mechanical stress and on limitations in the process integration. By using, for example, HIPOX (High Pressure Oxidation) a thicker oxide, approximately 3 &mgr;m thick, may be obtained in a shorter time, said thicker oxide fulfilling the requirements on the thermal budget. To avoid large differences in height in the obtained surface topography and facilitate further processing, a transistor having a thick field oxide may be manufactured by etching back the epitaxial collector layer by approximately half of the desired field oxide thickness outside the base and emitter areas, and then thermally oxidizing said etched back surfaces to obtain a substantially plane surface topography. An elevated area is, however, formed in the border area between the field oxide and the silicon, and this elevated area must be etched.
SUMMARY OF THE INVENTION
The object of the present invention is to obtain a vertical bipolar power transistor with high performance, especially improved amplification, said power transistor comprising a substrate, an epitaxial collector layer on said substrate, a base and an emitter formed in the epitaxial layer.
This is achieved by reducing the parasitic base metal-collector substrate capacitance, which is achieved according to the invention by introducing a field shield between, an interconnecting layer connected to the base and the field oxide, said field shield being electrically connected to the emitter.
The field shield is to be located in the passive area of the power transistor, that is, outside the component area.
The transistor can further preferably comprise a thick field oxide between the epitaxial collector layer and metallic interconnecting layers located above it.
It is feasible to manufacture the inventive bipolar power transistor by a deposition step, two masking steps, and two etching steps being added to a conventional process. The field shield is made by depositing an electrically conductive layer, followed by masking and etching. The electric connection to the emitter is achieved by the masking and etching of a contact hole in an isolating layer laying on the field shield, and subsequently filling said contact hole with a interconnecting layer connected to the emitter.
An advantage of the invention is that the amplifying characteristics of the power amplifier are significantly improved when an inventive field shield is introduced. The field shield reduces the base-collector capacitance at the expense of the base-emitter and the emitter-collector capacitance. The latter ones are less important as regards the amplifying characteristics of the power transistor.
The performance of the power amplifier is further improved when a thick field oxide is used in combination with the field shield.
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pat
Johansson Ted
Leighton Larry Clifford
Burns Doane Swecker & Mathis L.L.P.
Chaudhuri Olik
Pham Hoai
Telefonaktiebolaget LM Ericsson (publ)
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