Constant current source circuit

A constant current source circuit, in which first and second FET's are formed, is disclosed. The first FET has a source-drain path connected between power voltage lines and a gate connected to one of the power voltage lines with the source in common. The second FET is connected at its drain to a current output node of the constant current source circuit for supplying a constant current to a circuit coupled to the current output node, and at its gate to the drain of the first FET.

BACKGROUND OF THE INVENTION 
1. (Field of the Invention) 
The present invention relates to a constant current source circuit, and 
more particularly, to a constant current source circuit formed on a 
compound semiconductor substrate such as a semi-insulating gallium 
arsenide (GaAs) substrate. 
2. (Description of Related Art) 
As shown in FIG. 1A, a conventional constant current source circuit of this 
kind has the circuit construction wherein the source (S) and gate (G) of a 
field effect transistor (hereinafter called as FET) 31 are connected to 
the same power voltage line 310 which is connected to the terminal 
TP.sub.31, the gate-source voltage V.sub.GS of this FET 31 is fixed to 0 V 
and the drain (D) is connected to and used as a current output node 
TO.sub.31. Alternatively, as shown in FIG. 1B, a resistor element R.sub.31 
is inserted between the source (S) of an FET 32 and the power voltage line 
310, a predetermined reference voltage V.sub.RR, which is generated 
internally or supplied from an external power source, is applied to the 
gate (G) of this FET 32 and the drain (D) of the FET 32 is connected to 
and used as the current output node TO.sub.32. 
The conventional constant current source circuit described above, however, 
has the construction in which the gate-source voltage V.sub.GS is set to 0 
V or a predetermined fixed voltage V.sub.RR is applied to the gate. 
Therefore, if the threshold voltage V.sub.T of each FET 31, 32 deviates 
from the center of the design value due to the fabrication condition or 
the like, the supply current deviates from a design value, too, so that 
the noise margin of a logic circuit or an output circuit connected to this 
constant current source circuit drops or the output level deviates greatly 
from the design value. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a constant 
current source circuit capable of preventing the deviation of the supply 
current from the design value and also capable of supplying stably a 
constant current to a circuit connected to this constant current source 
circuit. 
According to a feature of the present invention, there is provided a 
constant current source circuit which comprises a first power voltage line 
supplying a first voltage, a second power voltage line supplying a second 
voltage lower than the first voltage, a third power voltage line supplying 
a third voltage lower than the first voltage, a current output node, a 
first resistor element connected at its one end to the first power voltage 
line, a first FET having a drain connected to the other end of the first 
resistor element, a source connected to the second power voltage line and 
a gate connected to the second power voltage line, and a second FET having 
a drain connected to the current output node, a source connected to the 
third power voltage line and a gate connected to the drain of the first 
FET. 
Further, a diode element as a level shift diode may be formed and connected 
between the first resistor element and the first power voltage line, and a 
second resistor element may be formed and connected between the source of 
the second FET and the third power voltage line. When the first voltage is 
a ground voltage (0 volts), the second and third voltages are negative 
voltages. Favorably, the second voltage (for example, -6.0 volts) is lower 
than the third voltage (for example, -5.2 volts). However, in some cases, 
the second and third voltages may be the same value (for example -5.2 
volts) by forming continuously the second and third power voltage lines in 
one line. The constant current source circuit may be connected to a 
differential logic circuit for supplying a constant current to the logic 
circuit, or the current output node and second FET of the constant current 
source circuit may constitute a portion of a level shift circuit of a 
logic circuit. 
Preferably, the constant current source circuit of the present invention is 
formed on a compound semiconductor substrate such as semi-insulating 
gallium arsenide substrate such that each of the first and second FET's 
includes an active impurity layer of one conductivity type (N-type) formed 
in a major surface of the substrate and having a pair of high impurity 
concentration portions serving as the source and drain and a low impurity 
concentration portion provided between and connected to the high impurity 
concentration portions, the low impurity concentration portion being used 
as a channel portion of the FET and being connected to the gate to form a 
Schottky barrier diode between the gate and the surface of the low 
impurity concentration portion. Also, the diode element may include an 
active impurity layer of the one conductivity type (N-type) formed in the 
major surface of the substrate and having a pair of high impurity 
concentration portions and a low impurity concentration portion provided 
between and connected to the high impurity concentration portions, an 
electrode formed on the low impurity concentration portion to form a 
Schottky barrier diode of the diode element between the electrode and the 
surface of the low impurity concentration portion, and a wiring connecting 
the high impurity concentration portions in common such that the electrode 
serves as the anode of the diode element and the active layer serves as 
the cathode of the diode element. The diode element of the FET type may be 
formed with the formation of the first and second FET's, simultaneously, 
so that the electrode of the diode element has the same material and 
thickness as those of the gate electrodes of the first and second FET's, 
and the high and low impurity concentration portions in the active layer 
of the diode element have the same impurity concentrations and depths as 
those of the high and low impurity portions in the active layers of the 
first and second FET's, respectively. Further, the electrode of the diode 
element may has the same plan figure as that of the gate electrode of 
every FET, and the active layer including the high and low impurity 
concentration portions may has the same plan figure as that of the active 
layer of every FET, if possible.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Referring to FIG. 2, a first embodiment of the present invention will be 
explained. The constant current source circuit comprises a first resistor 
element R.sub.11 connected at its one end to a first power voltage line 
110 to which a high voltage, for example, a ground voltage; 0 volts is 
applied through the terminal TP.sub.11, a first FET 11 connected at its 
drain(D)to the other end of the first resistor element R.sub.11 and at its 
source (S) and gate (G) to a second power voltage line 210 in common to 
which line 210 a low voltage (an negative voltage), for example, -6.0 
volts is applied through the terminal TP.sub.21, and a second FET 12 
connected at its drain (D) to a current output node TO.sub.11 of the 
constant current source circuit, at its source (S) to a third power 
voltage line 220 to which a low voltage (an negative voltage), for 
example, -5.2 volts is applied through the terminal TP.sub.22 and at its 
gate (G) to the drain of the first FET 11. The constant current source 
circuit functions as a constant current source of a differential logic 
circuit including FET's 13 and 14 connected to the current output node 
TO.sub.11. When the output terminals TO.sub.21 and TO.sub.22 are 
terminated by terminating resistors (not shown), respectively, an 
amplified output level determined by the value of the terminating resistor 
and by the current supplied from the second FET 12 is obtained at the 
output terminals TO.sub.21 and TO.sub.22. 
Next, the operation of this embodiment will be described. 
It will be assumed hereby that the threshold voltage value V.sub.T of each 
FET 11, 12 deviates from the design value towards the negative side due to 
fabrication conditions and environmental conditions. Since the gate-source 
voltage V.sub.GS of the first FET 11 is 0 V and constant, the current 
flowing into this FET 11 increases due to the deviation of the threshold 
value of FET 11 towards the negative side so that the voltage drop by the 
resistor R.sub.11 becomes great, and therefore, the drain potential of the 
first FET 11 drops. 
On the other hand, since the threshold voltage V.sub.T of the second FET 12 
deviates also towards the negative side, the current flowing into this FET 
12 tends to increase from the design value. However, since the drain 
potential of the first FET 11, or in other words, the gate potential of 
the second FET 12, drops, any influences resulting from the deviation of 
the threshold voltage value V.sub.T towards the negative side can be 
offset by designing the dimension of FET's 11, 12 and the values of the 
resistor R.sub.11 to suitable values. Consequently, the current flowing 
through the FET 12, or in other words, the current supplied to the 
differential logic circuit, can be kept at the design value. 
As a result, the output amplitude at the output terminals TO.sub.21, 
TO.sub.22 can be kept constant and the stable output level can be obtained 
even when the threshold voltage V.sub.T changes. 
If the threshold voltage V.sub.T deviates from the design value towards the 
positive side, on the contrary, the current flowing through the first FET 
11 decreases, the potential of the drain of this FET 11 rises and the gate 
potential of the second FET 12 rises, too, so that the current flowing 
through the second FET 12 can be kept at the design value. 
Referring to FIG. 3, a second embodiment of the present invention will be 
explained. In FIG. 3, the same components as those in FIG. 2 are indicated 
by the same reference numerals. In the second embodiment, a diode 21 is 
inserted between the first resistor element R.sub.11 and the first power 
voltage line 110, and a second resistor element R.sub.12 is inserted 
between the source (S) of the second FET 12 and the third power voltage 
line 220. The diode 21 functions as a level shift diode by which the 
change level of the drain voltage of the first FET 11, that is, of the 
gate voltage of the second FET 12 due to the deviation of the threshold 
voltage V.sub.T is adjusted to a proper one, and the second resistor 
element R.sub.12 functions as a source resistor by which the source 
voltage of the second FET 12 is adjusted so that a proper change of 
V.sub.SG of the second FET 12 in response to the deviation of the 
threshold voltage V.sub.T is conducted with the first FET 11, the first 
resistor element R.sub.11 and the diode 21. 
When, the FET's 11 and 12 have the same structure and characteristic each 
other, that is, the dimensions, impurity profiles and materials in every 
part are the same each other, and the design center of their threshold 
voltage V.sub.T is the same value of -0.4 volts, for example, both of the 
resistor elements R.sub.11 and R.sub.12 are formed to be 1 K.OMEGA. 
resistance value with the same dimensions and impurity profiles in every 
part each other. In this case, even if the threshold voltage V.sub.T of 
the respective FET's 11 and 12 deviates by .+-.0.2 volts from the center 
value (-0.4 volts), the fluctuation of the supply current through the 
current output node TO.sub.11 in FIG. 3 in accordance with this embodiment 
can be kept within 2% with respect to the design value, whereas the supply 
current flowing the current output node TO.sub.32 in FIG. 1B of the prior 
art circuit fluctuates by about 15% by the deviation of .+-.0.2 volts of 
the threshold voltage in the FET 32. 
Referring to FIGS. 4A to 4C, the device construction of the second 
embodiment shown in FIG. 3 will be explained. In a semi-insulating gallium 
arsenide (GaAs) substrate 40, pairs of N-type high impurity concentration 
regions 51-51', 52-52' and 53-53' and N-type low impurity concentration 
regions 61, 62 and 63 between and connected to respective high impurity 
concentration regions are formed from the major surface 41 of the 
substrate 40. Each of the high impurity concentration regions and each of 
the low impurity regions have the same dimensions and impurity profiles. 
Also, N-type impurity regions 81 and 82 are formed from the major surface 
41 in the substrate 40 with the same dimensions and impurity profile each 
other. In each section, the pair of N.sup.+ -type impurity regions and the 
N.sup.- -type impurity region constitute an active impurity layer. 
Stripe-like electrodes 71, 72 and 73 made of tungstein silicide (WSi) are 
formed on and contacted to the surface of the N-type low impurity 
concentration regions 61, 62 and 63 to form Schottky barrier diodes 
therebetween, respectively, and extend on the semi-insulating major 
surface of the substrate. Island-like electrodes 91, 91', 92, 92', 93, 
93', 94, 94', 95 and 95' each consisting of an Au-Ge (gold-germanium) film 
as a lower level film contacted to impurity regions and a Ni (nickel) film 
as an upper level film formed on the Au-Ge film, are formed on and ohmic 
contacted to respective impurity regions, and wirings 110, 210, 220, 230, 
240, 250 and 260 each consisting of a lower level titanium (Ti) film, a 
middle level platinum (Pt) film formed on the titanium film and an upper 
level gold film formed on the platinum film, are formed on an inter-ply 
insulating film 42 and connected to corresponding stripe-like and 
island-like electrodes through contact holes (in FIGS. 4B and 4C, and 71', 
72'and 73' in FIG. 4A) formed in the inter-ply insulating-film 42. 
Namely, the source electrode 91' connected to the N.sup.+ -type source 51' 
of the first FET 11 and the gate electrode 71 contacted to the N.sup.- 
-type channel region 61 of the first FET 11 for forming the Schottky 
barrier diode there are commonly connected to the wiring 210 of the second 
power voltage line supplying -6.0 volts. The drain electrode 91 connected 
to the N.sup.+ -type drain 51 (D) of the first FET 11, the gate electrode 
72 contacted to the N.sup.- -type channel region 62 of the second FET 12 
for forming the Schottky barrier diode there and the electrode 94' 
contacted to one end of the N-type region 81 of the first resistor element 
R.sub.11 are connected to the wiring 230. The drain electrode 92 contacted 
to the N.sup.+ -type drain 52 of the second FET 12 is connected to the 
wiring 250 which is extended to the current output node TO.sub.11 (not 
shown in FIG. 4), and the source electrode 92' contacted to the N.sup.+ 
-type source 52' of the second FET 12 and the electrode 95 contacted to 
one end of the N-type region 82 of the second resistor element R.sub.12 
are connected to the wiring 260. The electrode 95' contacted to the other 
end of the N-type region 82 of the second resistor element R.sub.12 is 
connected to the wiring of the third voltage line supplying -5.2 volts. 
The anode electrode 73 of the diode element 21 contacted to the N.sup.- 
-type portion 63 for forming the Schottky barrier diode using as the diode 
element is connected to the wiring 110 of the first power voltage line 
supplying 0 volts (ground voltage). The electrode 93, 93' contacted to the 
N.sup.+ -type regions 53, 53', respectively, are commonly contacted to the 
wiring 240 so that the N.sup.+ -type regions 53, 53' and the N.sup.- -type 
region 63 serve as the cathode of the diode element 21. The common 
connection wiring 240 connects to the electrode 94 contacted to the other 
end of the N-type region 81 of the first resistor element R.sub.11. 
Referring to FIG. 5, the third embodiment of the present invention will be 
explained. In FIG. 5, the same components as those in FIG. 3 are indicated 
by the same reference numerals. The constant current source circuit 
includes the diode 21, the first resistor element R.sub.11, the first FET 
11, a second FET 15, and a current output node TO.sub.12. As the second 
FET 12 in FIGS. 3 and 4, the second FET 15 in FIG. 5 has also the same 
construction such as dimensions, impurity profiles, materials and the same 
characteristics such as threshold voltage V.sub.T as of the first FET 11. 
The output node TO.sub.12 is connected to a BFL (Buffered FET Logic) 
circuit consisting of FET's 16, 17, 18 which are the same compound FET's 
(MESFET's) as FET's 11 and 15; each FET is formed on a compound 
semiconductor substrate such as semi-insulating gallium-arsenide (GeAs) 
substrate and has the Schottky barrier diode at the gate structure and a 
diode 22 as the diode 21. The BFL circuit is connected to the power 
voltage line 110 supplying 0 volts through the terminal TP.sub.11 , to a 
power voltage line 230 supplying -3.3 volts through the terminal TP.sub.23 
and to a power voltage line 240 supplying -2.0 volts through the terminal 
TP.sub.24, and outputs the output signal OUT.sub.3 from the current output 
node TO.sub.12. Input signals IN.sub.2 are inputted at the gate of the FET 
17. Apparently from FIG. 5, a portion consisting of the FET 15 and the 
node TO.sub.12 of the level shift circuit of the BFL circuit is a portion 
of the constant current source circuit. Even when the threshold voltages 
of the FET's 11, 15 deviate, the current flowing through the FET 15 can be 
kept constant by the same operation as in the first and second embodiments 
in accordance with this embodiment. Accordingly, this embodiment provides 
the advantages in that the delay time and noise margin of the BFL circuit 
can be kept constant irrespective of the change of the threshold voltage 
V.sub.T. 
Incidentally, in the second and third embodiment, the resistor R.sub.11 may 
be connected directly to the power source terminal TP.sub.11 and an FET 
may be used in place of the diode 21. 
In accordance with the present invention described above, the change of the 
current flowing through the first FET due to the deviation of the 
threshold voltage of the first FET is transmitted to the second FET so as 
to offset the change of the current flowing through the second FET due to 
the deviation of the threshold voltage of the second FET. According to 
this arrangement, even when the threshold voltage deviates, a stable 
constant current can be supplied as designed to the circuit connected to 
the current output terminals.