DE2347067A1 - BIPOLAR TRANSISTOR - Google Patents

Description

BLUMBACH ■ WESER - BERGEN & KRAMER

PATENT LAWYERS IN WIESBADEN AND MUNICH

DlPL-ING. P. G. BLUMBACH · DIPL-PHYS. Dr. W. WESER DIPL-ING. DR. JUR. P. BERGEN DIPL-ING. R. KRAMER

WIESBADEN · SONNENBERGER STRASSE 43. TEL (06121) 562943, 561998 MÖNCHEN

Western Electric, Company, Poon 1-6

Incorporated

New York, N.Y., USA

Bipolar transistor

The invention relates to a transistor with a semiconductor wafer with emitter, base and collector zones, whereby the collector zone has an adjacent to the base zone, has relatively weakly doped part and a relatively heavily doped part separated by this from the base zone.

In broadband amplifiers the intermodulation distortion is third order is particularly important when many such amplifiers are used in series. For example can do hundreds in long-distance analogue transmission systems of amplifiers in cascade, and in such a case the intermodulation distortion tends to be third

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Order to prevail as a limiting factor for the effective system baud, as certain P-products third. In contrast to products of the second order, they tend to add up in phase.

The above problem is solved according to the invention with a bipolar Traneistor (junction transistor) solved, which is characterized by that the weakly doped part over a major part of its? "> icke of a practically linearly changing ootation concentration to reduce the non-linearity of the cutoff frequency properties of the transistor.

In the drawing show:

1 shows an Fpitaxietraneistor according to the

present invention; and

Fig. 2 A and

2 B as comparison doping profiles like them

for a typical known epitaxial transistor or one according to the invention Embodiment of an epitaxial transistor are characteristic.

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Fs v.atrde found that the third-order intermodulation distortion is linked to the limit frequency over output collector current behavior of the amplifier. ^{The more} linear this behavior is, the lower the distortion. Little attention has been paid to this behavior in the past and no special effort has been made to linearize it.

In addition, it has been found that in an emitter circuit transistor amplifier the linearity of this behavior can be improved if a lower basic feed rate is used transistor used in the amplifier. Basic feed is the term used to describe the phenomenon, is that the base zone expands when the emitter current increases

in takes. Accordingly, an improvement in linearity and third-order intermodulation distortion can be achieved in one Such an amplifier can be achieved in that an epitaxial transistor is used, which compared to the known type is modified in that a changing dopant profile is provided in the epitaxial collector zone, especially in the Neighborhood of the collector junction, which reduces the non-linearity of the cut-off frequency over collector current

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holding the transistor is suitable. In practice this will be best achieved by one in this zone practically linear dopant gradients with a value im

19 19 4

Range between 1x10 and 2.5 χ 10 atoms per cm provides.

Such a changing dopant profile can be implemented in various ways. It would be ideal if this could be achieved during the growth of the epitaxial layer by controlling the dopant content in the vapor used for epitaxial growth. While it is possible, it is currently quite cumbersome and much more convenient to achieve the desired dopant profile in the epitaxial region after it has been grown, by a subsequent introduction step such as ion implantation.

In addition, such a changing dopant profile appears to be additionally useful in reducing third order intermodulation distortion in transistor amplifiers of the common base and collector circuit design. For these built on, the distortion is reduced by the fact that the switch-off with occurring low-frequency non-linearities

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is improved. In particular, such a profile leads to a low collector current multiplication, the von'der Impact ionization originates, which is a suitable enhancement mechanism.

A further improvement is also achieved if the thickness of the epitaxial layer is reduced by a value that corresponds to yours is than he usually bein. ' Prior art would be used.

Reference is now made to the drawing. L = Ig. 1 shows an npn transistor 10 which is essentially a planar epitaxial transistor is similar to the species currently widely used. In particular, it has a single-crystalline platelet with an η-conducting emitter zone 11, a p-conducting <Jen base zone 12 and an η-conductive collector zone 13 with a thinner, lightly doped part 13A and a thicker, more heavily doped part 13B. Electrodes 15, 16 and 17 provide ohmic connections to the emitter, base and collector zones here. Typically the die includes a surface passivation layer (not shown) of insulating material such as silicon dioxide, which is provided with openings through which

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the electrodes are in contact with the semiconductor.

One usually begins to make such a transistor with a heavily doped η-conductive single crystal substrate (usually silicon), grows on such a substrate a more weakly doped one by precipitation from the vapor phase η-conductive epitaxial layer of the same material, an acceptor dopant diffuses into such an epitaxial layer to a diffuse a diffused p-type region that partially extends into the epitaxial layer and then diffuses a donor dopant in a limited surface zone of the last formed p-type zone to convert part of the surface back to η-conduction. Such a rearranged one again The n-surface area serves as the emitter region 11, the p-region as the base zone 12, and the remaining non-rearranged η-conductive part epitaxial layer and the original substrate serve as parts 13A and 13B of the collector zone 13. Thereafter, electrodes are provided on the different zones, as shown in FIG.

Oxide masking and photolithographic methods are used typically used to locate the various diffused zones and electrodes.

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Figure 2A shows the resulting concentration of the predominant dopant (the absolute value of the difference between the concentration of acceptors and donors) along line 2-2 shown in FIG. 1, if the Transistor is of typical known form.

In particular, it can be seen that in the zone, which of the weakly doped η zone 13A next to the collector junction and that Corresponds to part of the original epitaxial layer, which is essentially unaffected by the subsequent diffusions, the concentration level is practically constant. There is a certain increase in concentration at their interface with the original substrate 13B due to outdiffusion of the more heavily doped substrate during the growth the epitaxial layer and the subsequent diffusion steps, but such an increase is fairly localized. Typically in the past was the too deep penetration of dopants diffusing out of the substrate into the epitaxial layer undesirable because there was a tendency to increase the collector capacitance and the collector breakdown voltage reduce without bringing benefits as an exchange. Vice versa

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there may be a slight decrease localized at the interface with the p-zone 12 and of a particular one Overflow of diffused acceptors originates. Despite these localized non-uniformities, the resulting Doping substance concentration, however, over the main part of the zone 13A practically uniform. A typical value of such a level is about 5 10 donors per cubic centimeter, such as it is shown.

A typical construction of such a transistor would be about three micrometers wide, 100 micrometers long, and four tenths Emitter zone micrometers thick, about 30 micrometers wide, 125 micrometers long and four tenths of a micrometer thick base zone between the emitter and the collector junction, and before the collector zone, which consists of a six micrometer thick lightly doped and an approximately 250 micrometer thick heavily doped part is constructed. Such a transistor would normally operated with an emitter-collector bias of 15 volts and a collector current of 100 milliamps with a power loss of about 1.5 watts, and he would be a cutoff frequency for common emitter operation of 50 MHz.

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In Pig. 2B is the resulting concentration of the prevailing Doping in the various zones is shown along line 2-2 shown in Figure 1, which is typical for a transistor that has the property of the invention contains.

In particular, it can be seen that the resulting concentration of the predominant donor doping is practically linear over the main part of the weakly doped zone 13A changes (the line appears as a curve in the drawing because it is a semi-logarithmic representation).

In particular, the resulting concentration of increases

15th
a value of 2 10 donors per cubic centimeter at the interface with the base zone to a value of about 8x10 donors per cubic centimeter at its interface with the original substrate part. This linear part extends over a width of about three microns,

19 which is a linear concentration gradient of about 2 χ 10

4th
Atoms per cm. The lightly doped part of the collector is thinner by about a factor of 2 than in the typical known shape shown in FIG. 2A. The linear one

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Gradient allows a thinner portion 13A of the collector to be used without affecting the collector breakdown voltage or to influence the collector junction capacity significantly.

For better results it seems important that the concentration gradient in addition to being practically linear, a value in the range between 1 and 2.5 10 "'atoms per

4th
cm for a silicon transistor of the type described.

In other zones, the resulting concentration is the predominant one Doping practically the same for both types of transistors.

The desired concentration gradient of the predominant doping can be achieved in various ways. In particular the dopant concentration in the epitaxial layer can be controlled by changing the during the Growing the epitaxial layer in the vapor used dopant concentration present. On the other hand, the desired Gradient are realized by an ionic

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implantation with suitable donors, using the ion bombing energies changed as appropriate to achieve the desired distribution. as Another alternative can be the desired gradient by suitable diffusion of doping atoms from the substrate can be achieved in the epitaxial layer. Such an outdiffusion would generally involve a warming-up, which differs from those heating steps those normally used to grow the epitaxial layer and those used to form the emitter and base zones Diffusion steps are assigned.

The above is illustrative of a transistor that is primarily intended as a discrete device rather than a device in an arrangement in which a number of components are combined in a single monolithic die is. In the latter case, a more likely structure for the transistor is the so-called shape of the "buried." Collector ". In this form, the part of the collector corresponding to part 13B is simply a heavily doped surface

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zone of a substrate, the main part of which is opposite Conductivity type, and the connection to such a zone is typically established by a lightly doped, localized region or a terminal extending from the same surface that includes the emitter region. In other main respects the I ^ orin is similar to des buried collector, however, of the discrete shape. In particular, it comprises an epitaxial layer in which the emitter and Base zones are formed, and which adjacent to the collector junction a relatively lightly doped part of the same conductivity type as the relatively heavily doped "buried" Part includes, the two parts as a collector zone to serve. In addition, since the buried part usually does not have a uniform concentration of its predominant Doping will have, since it is normally produced by a diffusion step, its concentration becomes the prevailing doping consistently be considerably higher than in the epitaxial layer. In such an embodiment is the desired gradient of the predominant doping in the weakly doped part of the collector zone in the epitaxial

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layer arranged, which can be realized beforehand either by ion implantation or a special outdiffusion step can. It is characteristic of a component produced in this way that the two form the collector zone Parts (corresponding to FIGS. 13A and 13B) typically have more similar thicknesses. In addition, it is itself in this form of the buried collector possible to achieve a distribution in which the prevailing dopant concentration in the heavily doped collector part is relatively uniform, through the use of ion implantation, to the prevailing To introduce doping into such a buried zone either before or after the formation of the epitaxial layer.

As indicated above, the preferred distribution has the predominant Doping in the lightly doped part of the collector zone expediently has a practically linear gradient over at least half of the lightly doped part of the collector zone. In addition, the weakly endowed part is the one Part in which the prevailing dopant concentration

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1 ft

is less than 10 atoms per cubic centimeter and should be at least two micrometers thick. in the In contrast, the heavily doped part generally exhibits

18th

a predominant dopant concentration in excess of 10 atoms per cubic centimeter. Of course there are transition zones in which the prevailing iloting has an intermediate concentration level, but such Zones are typically thin.

The present invention allows an improved behavior with respect to the third order distortion for transistors, those in either emitter, base or collector circuit structures operate.

In particular, it was first used for emitter circuit construction analytically discovered that for frequencies above the cutoff frequency f_ divided by the current gain in the common emitter circuit the third-order intermodulation is determined by the non-linearity of the cutoff frequency. Special this distortion was found to be related to the second

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Derivation of the% -! «Curve, and that the distortion is so is lower, the more linear the curve is.

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Pies can be achieved if you have a lower base-advance effect in the transistor than in a Paper discussed is entitled "A Theory of Transistor Cutoff Frequency (f) Falloff at High Current Densities" IRE Transactions on Electron Devices ED-9, No. 2 March 1962, page 164. The dopant profile shown in Figure 2B is used particularly well for this goal.

In addition, it results that the same dopant profile also the third order intermodulation distortion for base and collector circuit designs is reduced, too if the physical circle is different. For the latter Structures, the changing collector doping profile described results in a sufficient amount of avalanche current., which is based on the impact ionization during normal operation, so that the current gain non-linearity the exponentially turns off emitter non-linearity. Accordingly, it is possible to use the same transistor with better ones Results in one of the three basic circuit layouts

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use, which adds to its versatility.

Of course, other structures are possible. In particular, the complementary pnp form can be used. as well any semiconductors useful for dipolar transistors can be used. The following also applies: although the invention is normally practiced in connection with epitaxial transistors is, in principle, this is not necessary if the desired dopant distribution is achieved in another way can be, such as by ion implantation.

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Claims (5)

APPLY FOR PATENT 1.) transistor with a semiconductor wafer with Emitter, base and collector zones, with the collector zone having a relatively weak one adjacent to the base zone has doped part and a relatively heavily doped part separated by this from the base zone, characterized, that the lightly doped part (L3A) has a main part its thickness a practically linearly changing doping concentration to reduce the non-linearity of the Has cutoff frequency properties of the transistor. 2. transistor according to claim 1, characterized, that the relatively lightly doped part (13A) is at least two micrometers thick and has a predominant dopant concentration of less than 10 atoms / cm. 40-98 13 / 0-9 26 3. Traneistor according to claim 2, characterized in that the dopant concentration over a major part of the Thickness of the weakly doped zone (13A) has a gradient 19 19 has between 1 χ 10 and 2.5 χ 10 atoms / cm. 4. Transistor according to claim 1, characterized in that the emitter zone (11), the base zone (12) and the weak doped part (13A) of the collector zone (13) are components of the epitaxial layer. 5. Transistor according to claim 1, characterized in that that the lightly doped part (13A) is about three microns thick and a predominant one over the major part of the thickness Has dopant concentration that changes practically linearly from about 2x10 atoms / cm at the base up to 8 χ 10 atoms / cm for the more heavily doped collector part (! 3B). 4098 1 3/0926 «Ο. Blank