Tapped integrated resistor

The invention relates to an integrated resistor formed in an epitaxial layer and provided with at least one tap. In order to reduce field effect action between the resistor and the epitaxial layer, the voltage on the two ends of the epitaxial layer underneath the resistor tracks the voltage on the two ends of the resistor. Moreover, the epitaxial layer is short-circuited by means of buried layers at the locations where the resistance layer also exhibits a short-circuit, such as underneath the contact area of the tap.

BACKGROUND OF THE INVENTION 
The invention relates to an integrated resistor having a substrate of a 
first conductivity type, a layer of a semiconductor material of a second, 
opposite conductivity type deposited epitaxially on said substrate, a 
region of the first conductivity type provided in said epitaxial layer, 
which region contains the resistance element of the integrated resistor 
and comprises a first and a second end contacted by a conductive layer and 
at least one tapping area which is also contacted by a conductive layer, 
the resistance element extending between the first and the second end, the 
tapping area being situated between the first and the second end from an 
electrical point of view, and a first contact connected to said epitaxial 
layer and disposed outside said region near the first end. 
A resistor having the features specified above is known from German patent 
application No. 27.20.653 which has been laid open to public inspection. 
A problem with integrated resistors is that the resistance value depends on 
the voltage on and across this resistor as a result of the field effect 
action of the epitaxial layer relative to the resistor. In said German 
patent application No. 27.20.653 it is proposed to reduce this effect by 
driving the epitaxial layer with a voltage derived from the voltage on or 
across the resistor, for example the voltage on one of the ends of this 
resistor. However, it is found that particularly for high signal voltages 
across the resistor this solution is not entirely satisfactory. 
SUMMARY OF THE INVENTION 
It is the object of the invention to improve the known solution, and for 
this purpose such a tapped resistor according to the invention is 
characterized by a second contact connected to said epitaxial layer and 
disposed adjacent said region near the second end, means for electrically 
coupling, at least for the signal current, the first contact to the first 
end and the second contact to the second end, a first buried layer which 
extends from beneath the first contact to beneath the first end, a second 
buried layer which extends from beneath the second contact to beneath the 
second end, and at least one further buried layer which extends underneath 
the tapping area, said buried layers being of the second conductivity 
type, being situated at least partly in the epitaxial layer and being more 
conductive than the epitaxial layer, and thus serving to short-circuit the 
epitaxial layer underneath the first and the second end, underneath the 
tapping area, and between the first contact and the first end and between 
the second contact and the second end. 
By driving the epitaxial layer at both ends of the resistor, so that the 
signal voltage across said epitaxial layer is the same as across the 
resistor, the voltage at any point of this epitaxial layer underneath said 
resistor more closely tracks the voltage at the corresponding point of 
said resistor so that substantially no field effect exists. Moreover, the 
compliance of the voltage variation along the epitaxial layer with the 
voltage variation along the resistor is improved substantially by said 
buried layers. 
It is to be noted that driving the epitaxial layer at both ends with the 
voltage across the resistor, but without the presence of a tapping area 
and buried layers, is known per se from Netherlands patent application No. 
72.01.965, which has been laid to public inspection, and where such a step 
is applied in order to reduce the effect of stray capacitances.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 schematically shows an integrated resistor in accordance with the 
invention. In an island 1 of semiconductor material, for example of the 
n-type, a resistance track 2 is formed by means of a p-type diffusion. On 
this track metal contacts 30 to 34 are arranged. At opposite ends of the 
resistance track 2 contacts 4 and 5 are arranged on the island 1. One end 
with the contact 30 is connected to the contact 4 via an emitter-follower 
transistor 7, while at the other end, the contact 34 is connected to the 
contact 5. 
FIG. 2 is a schematic sectional view of the resistor shown in FIG. 1, the 
metal contacts not being shown. On a p-type substrate P-SUB there is 
present an n-type epitaxial layer N-EPI, part of which is isolated from 
the surrounding material by means of a deep p-type isolating diffusion DP 
and thus forms the island 1. A p-type layer SP in this epitaxial layer 
forms the resistance track 2. On top of this an insulating layer is 
deposited, in which contact apertures CO are formed via which contact can 
be made with the resistance track in the epitaxial layer by means of the 
metal contacts 30 to 34, 4 and 5 (FIG. 1). Underneath the contacts 4 and 5 
contact is made with the epitaxial layer by means of a deep n-diffused 
region DN containing a shallow n-diffused region SN. Underneath the 
contacts 30 to 34 4 and 5, and underneath the connection between the 
contacts 4 and 30 and the contacts 5 and 34, readily conducting buried 
n-type layers BN are formed between the epitaxial layer N-EPI and the 
substrate P-SUB in order to short-circuit the epitaxial layer locally. 
FIG. 3 shows the electrical equivalent diagram of the resistor shown in 
FIGS. 1 and 2. The resistance track 2 may be represented by resistors 
between which short-circuits are present which corresponds to the contacts 
30 to 34. The epitaxial layer is also represented by resistors below the 
other resistors with short-circuits between them at the location of the 
buried layers BN. When a signal voltage is applied across the integrated 
resistor 2, a similar voltage is applied across the resistance of the 
epitaxial layer via the emitter-follower 7 and the connection 6. Since the 
pattern of resistors and short-circuits along the resistor 2 and the 
epitaxial layer 3 is the same, the voltage variation is also the same, so 
that the voltage on each point of the epitaxial layer follows the voltage 
on the overlying point of the integrated resistor, thereby eliminating any 
nonlinearity as a result of the field effect between the resistor and the 
epitaxial layer. 
The resistance track 2 need not be straight. For example, the contacts 31, 
32 and 33 may be formed by means of T-shaped lateral branches. Ladder 
structures are also possible. 
Moreover, the epitaxial layer contact 4 need not necessarily be driven via 
voltage follower 7. If the resistor 2 is energized from a low-impedance 
source, a direct electrical connection is possible.