JPH04162433A - Field-effect attenuator device capable of controlling electrical length - Google Patents

Abstract

PURPOSE: To enable high-speed digital control operation over wide bands by constituting a gate control line for operating a gate electrode and 1st and 2nd areas of a semiconductor material, so as to control the amount of attenuation applied between input and output electrodes. CONSTITUTION: Ohmic contacts 24 and 26 are arranged on a surface 22 of a semiconductor material 21 at a resistor element 20. An active N-channel semiconductor layer supports a rectifier Schottky gate electrode, including the ohmic contacts 24 and 26 and contacts 30 and 34. Because of the gate electrode, Schottky rectifier bonding, composed of the semiconductor 21 can be provided through a known method by using the gate electrode constituted, by suitably combining a metal such as tungsten, gold and titanium. According to the known method, an N+-region 44 is provided on the lower layer of electrode 24 and an N+-region 48 is provided on the lower layer of an electrode 26 respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates generally to attenuator circuits, and more particularly to variable attenuator devices using field effect technology.

BACKGROUND OF THE INVENTION Today's electronics applications often use attenuator circuits or components that change resistance in response to control signals. This attenuator uses an automatic gain control (AGC) circuit, position
It is useful in n locating systems, telephone systems, television systems, etc.

Traditional electronic variable solid-state attenuators for high frequencies (R') commonly used PIN diodes.P
IN diode attenuators can be assembled in a variety of network configurations. A bias control circuit having field effect transistors (FETs) is used to bias the PIN diode in response to various analog control signal levels, thereby causing the PTN diode network to respond to one of the various resistance levels.
I was able to give one. Discrete PIN diode attenuators were capable of superior performance, but required excessive power in some applications. In general, PIN diodes cannot be easily integrated into monolithic circuits.

Other conventional methods of providing electronic variable attenuators may utilize FETs, such as gallium arsenide (GaAS) metal semiconductor field effect transistors (MESFETs).

These devices can be configured with a variety of circuit networks, and each device can operate without bias, consuming little power outside of switching operations. Analog control signals are applied to the gates of these devices to adjust the attenuation level. The resistance provided by each FET is adjusted by controlling the depth of the device's non-depleted channel in response to the analog signal level. Unfortunately, varying the depth of the undepleted channel in this manner results in non-linear transfer characteristics and increases the level of intermodulation between the RF signals applied to these devices. Therefore, unnecessary frequency components are generated, resulting in distortion.

To minimize such distortion, some prior art attenuators have utilized separate circuitry, which undesirably increases cost, size and weight, and reduces reliability.

In general, conventional attenuators rarely require accurate control of analog control signal levels.

However, it is difficult to accurately control the analog control signal level because the analog signal level tends to change due to temperature, the age of the semiconductor, variations in process parameters due to individual differences in devices, etc. Also, the tuning range of some MESFE under-attenuator networks tends to be limited.

GaAS has high frequency response capability and small circuit size.
Monolithic microwave integrated circuit (Ht4IC) using semiconductor materials
Several integrated circuit applications are currently being developed. These applications require electronic variable attenuators that are compatible with currently available semiconductor manufacturing methods. The conventional attenuators described above may be very expensive to manufacture, take up too much space, and tend to have extremely high reject rates in some of these applications. Some conventional complex attenuators that utilize analog control signals operate very slowly and therefore tend not to take advantage of the inherent speed characteristics of MMIC circuits.

(Means for Solving the Problems) Accordingly, it is an object of the present invention to provide an electronic variable attenuator capable of high-speed digital control operation over a wide band.

Another object of the present invention is to be compatible with MMIC usage;
and to provide an electronic variable attenuator that can be assembled into a monolithic integrated circuit.

An electronically variable field effect attenuator according to one example of the invention has a semiconductor material that provides a signal path between an input electrode and an output electrode. Overlapping electrodes are arranged extending from different directions onto the semiconductor material. A gate electrode control line is selectively coupled to the Goo1~ electrode. Adjust these control lines and selectively apply a digital control signal as "1" or rOJ to the gate electrode to control the conductivity of the semiconductor region under the electrode to Goo 1 and connect the input and output electrodes. The level of attenuation provided by the attenuator is controlled by varying the length of the conductive path between the attenuators.

(Example) FIG. 1 shows a semiconductor resistance element 10 having a width 12, a length 14, and a depth 16. It is assumed that the current flows in the direction of the arrow between the front surface 15 and the hidden back surface. The resistance value of resistor element 10 is determined by the shape of the resistor element and the material properties of the active semiconductor material. Specifically, the resistance R is linear and follows the equation: (rho)*L R-(1'> where (rho) is the bulk resistivity of the active semiconductor material;
L is the electrical length of the conductive path and A is the cross-sectional area of the semiconductor active region. In FIG. 1, the cross-sectional area A of the element 10 is the width 12
and depth 16.

The bulk resistivity of the semiconductor material of device 10 can be calculated in first order as: (rho) - (2) uo N where q is the electron charge and uo is the low field transport of the semiconductor material. low-field mobility, N is the free carrier density of the semiconductor
density).

When a portion of the semiconductor material of the resistive element is depleted, the cross-sectional area A of the semiconductor active region is the product of the active region depth and the active region width.

FIG. 2 shows a top view of a configuration of an electronically variable field effect attenuator 20 in accordance with an example of the present invention. The discrete levels of attenuation are determined by the digital signal “1” applied to the gate electrode of device 20.
” and is provided by the device 20 depending on the configuration of the rOJ. Device 20 can be manufactured using one of a number of different technologies, including standard silicon or gallium arsenide (GaAS) manufacturing techniques. In many microwave circuit applications, it is useful to fabricate device 20 using depletion mode gallium arsenide (GaAs) MESFET technology. This technology is compatible with MMIC manufacturing methods.

Specifically, device 20 comprises a semiconductor material with a resistance generally determined by the principles described in device 10. The rectangular top surface 22 of the material 21 has first and second parallel ends 23.25 and first and second parallel sides 2.
7.29. An ohmic contact input electrode 24 is located near an end 23 of the semiconductor surface 22 and an output ohmic contact 26 is located near the other end 25 of the semiconductor surface 22.

Assuming no interaction with other control mechanisms, semiconductor material 21 generally provides a current path between electrodes 24 and 26 with a predetermined length Ll.

Parallel Schottky gate electrodes 28, 30, 32° 34.
36 is located on top of surface 22. The practical limit to the number of possible gate electrode arrangements is determined by the particular manufacturing technique used. The gate electrodes 28.32.36 form a first group of gate electrodes, which group extends halfway across the semiconductor surface 22 in a direction perpendicular to the normal current direction between the first side 27 and the electrodes 24.26. It is growing up to. Gate electrode 3
0.degree. 34 forms a second group of gate electrodes, which extends halfway across the semiconductor surface 22 from the second side 29. Gate electrode 30.34 is electrode 28.32.3
Arranged so that it falls between 6 (inter-digitate
d) has been done. The end 31 of the electrode X30 and the end 33 of the electrode 34 are the same as the ends 35, 37, .
39 is closer to the side portion 27. The electrodes from each electrode group are
They overlap each other as shown in the figure.

FIG. 3 is a cross-sectional view of device 20 along line 40 of FIG. Specifically, ohmic contacts 24 , 26 are arranged on the surface 22 of the semiconductor material 21 of the resistive element 20 .

The active N-channel semiconductor layer 21 has an ohmic contact 24.26
and supports a rectifying Schottky gate electrode including contact 30°34.

The Schottky rectifying junction formed with the semiconductor 21 by the gate electrode can be provided in a known manner by using a gate electrode made of a suitable combination of metals such as tungsten, gold, titanium, etc. By known methods,
The N ten area t144 is on the lower layer of the electrode 24, and the N ten area 10
, 48 are provided below the electrode 26, respectively.

For example, a portion of gate electrode 34 is a conductor that extends above surface 22. The remaining portion 41 of the conductor can be considered a gate control line.

Semiconductor material 21 is disposed on semi-insulating substrate 50 .

By simply placing a Schottky gate electrode on top of the semiconductor material 21, the semiconductor material 21 is directly below each contact of effective free charge.
The region is partially depleted. As shown in FIG. 3, the length of depletion region 54 of contact 34 is 52. barrier surface 22
This effect occurs because the Schottky metal 34 and the semiconductor 21 have different electron affinities. The formed depletion region automatically exists in a zero bias voltage state. These depletion □ regions do not affect the electrical length 1-1 of the conductive path between electrode 24 and electrode 26, but they do affect the depth of the conductive path.

The depth of the depletion region can be varied, for example, by varying the level of voltage applied to a gate electrode, such as electrode 34. Specifically, when a negative voltage of sufficient magnitude is applied to the gate electrode 34, the depletion region extends a length 61 in the direction of the top surface 58 of the semi-insulating substrate material 50, and the depletion region 6
0 and renders a portion of the semiconductor material 21 non-conductive. The voltage required to cause this condition is defined as the pinch-off voltage (Vp). When pinched off, ohmic contact 24.26 is created due to depletion in the carrier.
A region of extremely high resistance is formed between and adjacent to line 60 of semiconductor material 21 so that a portion of device 20 is substantially non-conductive.

Furthermore, when a positive voltage is applied to contact 34, for example, the depth 52 of zero bias depletion region 54 decreases, so that a portion of the current path under gate electrode 34 between ohmic contacts 24 and 26 decreases. The conductivity of at least a portion of device 20 is further increased. A similar explanation is
This also applies to the operation of other gate electrodes and underlying semiconductor materials.

The pinch-off voltage of a material is determined by the impurity concentration (d
opening density) and the depth of the active region. In gallium arsenide MESFET technology, the voltage level Vp can be approximately expressed as: q N D2 vp−”b i (,3>2 (eps) where D is the depth of the active semiconductor region, (esp) is the dielectric constant of the material, and ■bi is the built-in voltage of the rectifying contact, which is typically 0.8 volts.

Under normal operating conditions, each gate electrode is biased at one of two voltage levels at any given moment. One of these voltage levels corresponds to a digital "1". When the gate electrode receives this bias level, the underlying semiconductor material becomes "on". Once in the "on state", current can flow through the semiconductor material beneath the rectifying electrode. In depletion mode gallium arsenide MESFET technology commonly used in MMIC circuits, this digital "1" voltage is preferably O volts. Other voltage levels correspond to digital rOJ. When the gate electrode is at this bias level, the underlying semiconductor material “
It is in the "off state" and no current can flow under the electrode. In depletion mode gallium arsenide MESFET technology, this digital rOJ voltage is equal to the pinch-off voltage Vp or more negative than Vp.

In FIG. 4, one mode of operation of the attenuator 20 is illustrated. Assume that gate electrodes 34, 36 simultaneously receive a digital "O" voltage which causes the depletion region of each electrode to have the approximate top shape shown by dotted line 70, 72.

As mentioned above, these regions also extend down towards the substrate 50. Furthermore, another gate electrode 2B.

Assume that 30.32 receives a digital "1" control signal. Under these conditions, it can be assumed that the current flowing between the electrodes 24, 26 encounters a plurality of series resistances having different values, where the plurality of series resistances are combined into one composite resistance. The electrical length of first resistor R1 is measured by line 76 and the width of R1 is measured by line 78. The second resistor R2 is R at the dotted line 77.
1. The length of R2 is measured by line 79 and the width of R2 is measured by line 80 between dotted line 81 and side 27. The length of the third resistor R3 is the dotted line 81 and the side 2
9 and the width of R3 is measured by line 84. The length of the next resistor R4 is dotted line 8
7.89 and its width is measured by line 88. The length of the last resistor R5 is measured by line 90 and its width by line 78. Therefore, the total resistance between electrodes 24 and 26 is:
It is equal to the sum of R1+R2+R3+R4+R5. Therefore, by controlling the logic signal applied to the gate electrode to O or 1, the effective electrical length that the current encounters with electrode 24.26 can be adjusted to adjust the overall resistance to the desired value. Can be different values. The resistance value of the composite resistor is calculated using the formula (1
) can be used to calculate in the first order.

Also, different gate electrodes can be connected together, as shown in FIG. 5, where a gate control line 100 interconnects electrodes 32, 34, 36, and It shows that parts of the semiconductor material can be made conductive or non-conductive at the same time. When digital rOJ is applied to electrodes 36, 32.34 by control line 100, current generally flows along the path shown by line 102 in FIG. Path 102 is conductor 24.2
Since the path 102 is longer than the straight path between the path 102 and the path 102, the resistance increases.

FIG. 6 shows gate electrodes 106, 108, 110°112
．． 114 shows a top view of a variable attenuation device with 114. Gate control line 116 connects gate electrode 106 . ．． 110
, 114 are interconnected. From this □, the distance between parallel gate electrodes can be varied. Specifically, the minimum distance is the electrode 10
6 and electrode 108 by line 120. Electrode 10B.

The distance between electrodes 110 is longer as shown by line 122, and the distance between electrodes 1'' to 112 is even longer as measured by line 124.Finally, the distance between electrodes 112, 114 is maximum as shown by line 126. Additionally, gate electrode 106
The length extending in the width direction of the semiconductor material is shorter than that of the gate electrode. By varying the width of the conductive path around the electrode as it receives and receives rOJ, the length of the gate electrode can be adjusted to adjust the attenuation characteristics of the attenuator to meet specific criteria.

FIG. 7 shows the dimensions of the attenuator 130;
is similar to attenuator 20 except that gate electrodes 28, 32, 36 are interconnected by line 132. The resistance between the input electrode 24 and the output electrode 26 corresponding to the digital control signals shown must be as per the table below: Logic state (1-O volts, 0--Vp) rho = 50 ohms/sq. (28, 32, 36) 34 30 Overall R
(Ohms) 100 (Approx.) 250 Since the channel region under the gate electrode of the attenuator 20, 104, 130 above can be completely pinched-off or conductive, these devices can be partially Less intermodulation distortion compared to other attenuators operating in pinch-off mode. Therefore, the attenuator 20.10
No. 4,130 does not require a distortion correction circuit. Furthermore, the shapes of these attenuators can be configured to minimize the parasitic capacitance, and high speeds suitable for gallium arsenide MMIC circuits can be achieved.

Digital control of the attenuators 20, 104, 130 overcomes the problems described above with analog control. Additionally, these attenuators are capable of broadband operation and can be easily assembled in monolithic circuits, such as those required in MMIC applications.

Although the invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes in form and detail may be made without departing from the scope of the invention.

[Brief explanation of the drawing]

FIG. 1 shows a resistance element to which the present invention can be applied. FIG. 2 is a top view of a field effect attenuator which is an embodiment of the present invention. 3 is a cross-sectional view of the field effect attenuator of FIG. 1; FIG. FIG. 4 shows one mode of operation of the attenuator of FIG. FIG. 5 illustrates the mode of operation of an alternative embodiment attenuator configuration with interconnected gate electrodes. FIG. 6 shows another embodiment of an attenuator with non-uniformly placed gate electrodes. FIG. 7 shows a dimensional drawing of the attenuator. (Explanation of main symbols) 10. Resistive semiconductor element 20. Electronic variable field effect attenuator 21. Semiconductor material 23.25. ,,parallel ends, 27.29. ,,Parallel sides, 24,,,Input electrode (ohmic contact), 26,,,Output electrode (ohmic contact), 28',30,32,34.3610. Schottky gate electrode, 44.48. ,,N± region 50,,',semi-insulating substrate, 54,,depletion region, 60,,,depletion region,100,,,gate control line 1,104. ,,Variable Attenuator Device, 106.108
, 110, 112, 114. ,. gate electrode, 116. ,,gate control line, 130,,attenuator. Patent applicant Motorola Incorporated ↑ FIO-4 FIO-6

Claims (3)

[Claims] (1) A variable attenuator that provides different predetermined levels of attenuation in response to different control signals, comprising: an input electrode; an output electrode; coupled between the input electrode and the output electrode, typically the input electrode and the output electrode; a semiconductor material that provides a first continuous linear current path having a first direction between it and an electrode, the first current path having a predetermined electrical length; an angle with respect to the first direction; a first gate electrode extending part way across the semiconductor material in a second direction with an angle to the first direction; a second gate electrode extending part way across the semiconductor material in a third direction with a second gate electrode having a second region of the semiconductor material thereunder; energizing the first and second gate electrodes to simultaneously change the electrical conductivity of the first and second regions of semiconductor material to increase the electrical length of the current path between the input and output electrodes of the attenuator. a gate control line selectively varied to differ from the predetermined electrical length, the gate control line providing the gate electrode and the first and second regions of semiconductor material between the input and output electrodes; A variable attenuator comprising: a gate control line operated to control the amount of attenuation. (2) a variable field effect attenuator that provides different predetermined attenuation levels in response to different digital control signals, comprising: an input electrode; an output electrode; a first electrode coupled to the input electrode and the output electrode, respectively;
and a first current path in a first direction having a surface having a second parallel end and first and second parallel sides and having a predetermined electrical length between the input electrode and the output electrode. a plurality of first gate electrodes each extending halfway across the semiconductor surface from the first side of the semiconductor material surface in a direction at an angle to the first direction; At least one second gate electrode extends partway across the semiconductor surface from the second side of the material surface in a direction at an angle to the first direction, and extends part way across the semiconductor surface and intersects between the plurality of first gate electrodes. a gate electrode; and a current path between the input electrode and the output electrode by simultaneously energizing at least one of the plurality of first gate electrodes and the second gate electrode in response to a digital signal; selectively varying an electrical length to be different from the predetermined electrical length and controlling the amount of attenuation provided between the input electrode and the output electrode to achieve one of the predetermined attenuation levels. A variable field effect attenuator comprising: a gate control line for providing; (3) a plurality of second gate electrodes each extending halfway across the semiconductor surface from the second side portion and interposed between the plurality of first gate electrodes; 2. The variable field effect attenuator according to 2.