JP2017059697A - Cascode normally-off circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive cascode normally-off circuit which needs no external part, and which is arranged so that a normally-on switch can be turned off without breaking a normally-off switch when turning off a normally-off switch.SOLUTION: A cascode normally-off circuit comprises: a normally-on switch Q1 including a nitride semiconductor having a drain electrode, a source electrode and a gate electrode, in which a high voltage is applied to the drain electrode; a normally-off switch Q2 including a silicon semiconductor having a drain electrode, a source electrode and a gate electrode, in which the drain electrode is connected to the source electrode of the normally-on switch, the source electrode is connected to the gate electrode of the normally-on switch, and a voltage is applied to the gate electrode; and a leak current regulating part 1 for regulating a gate leak current of the normally-off switch so that the electric potential of the source electrode of the normally-on switch becomes equal to or larger than an electric potential which can turn off the normally-on switch under a breakdown voltage of the normally-off switch when the normally-off switch is turned off.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a cascode normally-off circuit configured by cascode-connecting a normally-on switch and a normally-off switch.

  As shown in FIG. 8, the cascode normally-off circuit is a circuit that operates as a normally-off transistor by cascode-connecting a normally-on switch Q1 and a normally-off switch Q2 made of MOSFETs (Patent Document 1).

  The normally-on switch Q1 and the normally-off switch Q2 are connected in series, the gate of the normally-on switch Q1 is connected to the source of the normally-off switch Q2, and an input signal is applied to the gate of the normally-off switch Q2. For example, several hundred volts is applied to the drain end of the normally-on switch Q1. The breakdown voltage of the normally-on switch Q1 is, for example, 600V.

  In the above configuration, when a voltage equal to or higher than the threshold voltage Vth is applied to the gate of the normally-off switch Q2, the cascode-connected switches Q1 and Q2 are turned on.

  However, at the moment when a voltage lower than the threshold voltage Vth is applied to the gate of the normally-off switch Q2, the normally-off switch Q2 is turned off, and the potential at the point X rises with respect to the source S of the normally-off switch Q2.

  Since the gate of the normally-on switch Q1 is connected to the source S of the normally-off switch Q2, the gate of the normally-on switch Q1 has a negative potential with respect to the point X (the source of the normally-on switch Q1). For this reason, the potential at the point X further increases, and when the negative voltage becomes larger than the threshold voltage Vth of the normally-off switch Q2, for example, −5V, the normally-on switch Q1 is turned off.

  For this reason, since a high voltage from the power supply is applied to the normally-on switch Q1, a large voltage is not applied to the normally-off switch Q2. Therefore, in order to reduce the conduction loss of the normally-off switch Q2, it is possible to select an FET having a low breakdown voltage, for example, a breakdown voltage of about 20V, for the normally-off switch Q2.

  However, if the normally-on switch Q1 has a large leakage current, the potential at the point X gradually increases even when the normally-on switch Q1 is off. If a high voltage is continuously applied to the point X, the normally-off switch Q2 will deteriorate.

  For this reason, as shown in FIG. 9, a resistor R2 is connected between the drain and source of the normally-off switch Q2 to suppress an increase in potential at the point X. In this case, the resistance value of the resistor R2 is selected according to the leakage current value of the normally-off switch Q2 and the potential at the point X. For example, if the normally-off switch Q2 has a leakage current of 1 μA and the potential at the point X is 10 V, the resistance value of the resistor R2 is 10 MΩ.

  Since the leakage current varies due to manufacturing variations and temperature dependence, the resistance R2 is designed to the maximum value of the assumed leakage current and the resistance value is reduced.

  In addition, as shown in FIG. 10, by connecting a diode D1 between the drain and source of the normally-off switch Q2 and clamping an overvoltage, an increase in the potential at the point X can be suppressed.

Japanese Patent No. 5697996

  However, when the variation in the leakage current is large and a small resistance value must be selected for the resistor R2, or when the leakage current is small, the normally-on switch Q1 can be turned off in the worst case without increasing the potential at the point X. Disappear. That is, unless a certain amount of leakage current flows, the potential at the point X does not become a potential at which the normally-on switch Q1 can be turned off.

  As shown in FIG. 10, when the diode D1 is provided, the potential at the point X is not actually stabilized due to the internal resistance of the diode D1. Further, both when the resistor R2 is provided and when the diode D1 is provided, an extra external component is required, resulting in an increase in cost.

  An object of the present invention is to provide a low-cost cascode normally-off circuit that does not break down a normally-off switch when the normally-off switch is turned off and can turn off the normally-on switch, and does not require external parts.

  A cascode normally-off circuit according to the present invention includes a normally-on switch made of a nitride semiconductor having a first drain electrode, a first source electrode, and a first gate electrode, and a high voltage applied to the first drain electrode; A silicon semiconductor having a drain electrode, a second source electrode, and a second gate electrode is connected, the second drain electrode is connected to the first source electrode of the normally-on switch, and the second source electrode is the first of the normally-on switch. A normally-off switch connected to the gate electrode and applying a voltage to the second gate electrode; and when the normally-off switch is turned off, the potential of the first source electrode of the normally-on switch is greater than or equal to a potential at which the normally-on switch can be turned off. And the normally-on switch is set so that it is less than the withstand voltage of the normally-off switch. Characterized in that it comprises a leakage current adjusting unit for adjusting the leakage current of the switch.

  According to the present invention, when the normally-off switch is turned off, the leakage current adjustment unit is configured such that the potential of the first source electrode of the normally-on switch is equal to or higher than the potential at which the normally-on switch can be turned off and less than the withstand voltage of the normally-off switch. Since the normally-on switch gate leakage current is adjusted, the normally-off switch is not destroyed and the normally-on switch can be turned off, and an inexpensive cascode normally-off circuit that does not require external components can be provided.

It is a figure which shows the basic circuit structure of the cascode normally-off circuit based on Example 1 of this invention. It is typical sectional drawing which shows the structure of the normally-on switch which has in the cascode normally-off circuit which concerns on Example 1 of this invention. It is a figure which shows the relationship between the drain voltage of the normally-on switch which has in the cascode normally-off circuit which concerns on Example 1 of this invention, and gate leakage current. It is a figure which shows the relationship between the drain voltage of the normally-on switch which has the conventional cascode normally-off circuit, and gate leak current. It is typical sectional drawing which shows the structure of the normally-on switch which has in the cascode normally-off circuit which concerns on Example 2 of this invention. It is typical sectional drawing which shows the structure of the normally-on switch which has in the cascode normally-off circuit which concerns on Example 3 of this invention. It is typical sectional drawing which shows the structure of the normally-on switch which has in the cascode normally-off circuit which concerns on Example 4 of this invention. It is a circuit diagram which shows the 1st example of the conventional cascode normally-off circuit. It is a circuit diagram which shows the 2nd example of the conventional cascode normally-off circuit. It is a circuit diagram which shows the 3rd example of the conventional cascode normally-off circuit.

  Hereinafter, a cascode normally-off circuit according to an embodiment of the present invention will be described in detail with reference to the drawings.

  1 is a diagram illustrating a basic circuit configuration of a cascode normally-off circuit according to a first embodiment of the present invention. The cascode normally-off circuit shown in FIG. 1 includes a normally-on switch Q1, a normally-off switch Q2, and a leakage current adjusting unit 1.

  The normally-on switch Q1 is made of a nitride semiconductor such as GaN or AlGaN, and has a first drain electrode D, a first source electrode S, and a first gate electrode G. It turns off when the potential becomes more negative than the potential of the first gate electrode G. The first drain electrode D of the normally-on switch Q1 is connected to a power source (not shown).

  The normally-off switch Q2 is made of a silicon semiconductor such as a MOSFET and has a second drain electrode D, a second source electrode S, and a second gate electrode G. The second drain electrode D is connected to the first source electrode S of the normally-on switch Q1, the second source electrode S is connected to the first gate electrode G of the normally-on switch Q1, and a voltage is applied to the second gate electrode G. The

  The leakage current flowing into the X point shown in FIG. 1, that is, the first source electrode S of the normally-on switch Q1, is the sum of the source current and the substrate current. Here, it is assumed that normally-on switch Q1 and normally-off switch Q2 are mounted on the substrate. In the device design stage, the gate leakage current leaking from the drain of the normally-on switch Q1 to the gate is dominantly adjusted in the entire leakage current of the normally-on switch Q1, so that the source leakage current of the normally-on switch Q1 and the substrate are adjusted. The total current with the current can be reduced. At this time, since the gate leakage current of the normally-on switch Q1 flows to the second source electrode S of the normally-off switch Q2, the potential at the point X does not change.

  The leakage current adjusting unit 1 is a means for adjusting the above-described gate leakage current, and is provided in the normally-on switch Q1. When the normally-off switch Q2 is turned off, the leakage current adjusting unit 1 is configured such that the potential of the first source electrode S of the normally-on switch Q1 is equal to or higher than the potential at which the normally-on switch Q1 can be turned off and less than the withstand voltage of the normally-off switch Q2. The gate leakage current flowing from the first drain electrode D to the first gate electrode G of the normally-on switch Q1 is adjusted.

  Next, a cross-sectional view showing the structure of the normally-on switch Q1 and a first specific example of the leakage current adjusting unit 1 will be described with reference to FIG.

  The normally-on switch Q1 is made of a nitride semiconductor device, and includes a source electrode 3, a drain electrode 4, and a gate electrode 5 disposed between the source electrode 3 and the drain electrode 4, as shown in FIG.

  The normally-on switch Q1 includes a nitride semiconductor layer 2 in which a carrier supply layer 22 and a carrier travel layer 21 that forms a heterojunction with the carrier supply layer 22 are stacked, and an insulating film 71 disposed on the nitride semiconductor layer 2 A HEMT in which a source electrode 3, a drain electrode 4, and a gate electrode 5 are disposed on the nitride semiconductor layer 2.

  The insulating film 7 is disposed so as to cover the source electrode 3 and the drain electrode 4, and the gate electrode 5 is disposed so as to fill the opening formed in the insulating film 71. An interlayer insulating film 72 is disposed on the gate electrode 5 and the insulating film 71, the source electrode wiring 31 is connected to the source electrode 3 through the openings formed in the interlayer insulating film 72, and the drain electrode wiring 41 is drained. It is connected to the electrode 4.

  The normally-on switch Q1 includes a gate field plate 50 that is electrically connected to the gate electrode 5. The gate field plate 50 disposed between the gate electrode 5 and the drain electrode 4 controls the curvature of the depletion layer at the drain side end of the gate electrode 5 and concentrates on the drain electrode side end of the gate electrode 5. Electric field concentration is reduced.

  The gate field plate 50 is connected to the upper end portion of the gate electrode 5 and is disposed on the insulating film 71 between the gate electrode 5 and the drain electrode 4. A buffer layer 11 is disposed on the substrate 10, and the nitride semiconductor layer 2 is disposed on the buffer layer 11.

  As the substrate 10, a semiconductor substrate such as a silicon (Si) substrate, a silicon carbide (SiC) substrate, or a GaN substrate, or an insulator substrate such as a sapphire substrate or a ceramic substrate can be used.

  The buffer layer 11 is formed by an epitaxial growth method such as a metal organic chemical vapor deposition (MOCVD) method. The nitride semiconductor layer 2 includes a carrier supply layer 22 made of a first nitride semiconductor layer, and a carrier traveling layer 21 made of a second nitride semiconductor layer having a band gap energy different from that of the first nitride semiconductor layer. Have.

  The carrier traveling layer 21 disposed on the buffer layer 11 is formed by, for example, epitaxially growing non-doped GaN to which no impurity is added by a metal organic chemical vapor deposition (MOCVD) method or the like. Here, non-doped means that no impurity is intentionally added.

The carrier supply layer 22 disposed on the carrier traveling layer 21 is made of a nitride semiconductor having a band gap larger than that of the carrier traveling layer 21 and a lattice constant smaller than that of the carrier traveling layer 21. Non-doped Al _x Ga _1-x N can be used as the carrier supply layer 22.

  The carrier supply layer 22 is formed on the carrier traveling layer 21 by epitaxial growth using MOCVD or the like. Since the carrier supply layer 22 and the carrier traveling layer 21 have different lattice constants, piezoelectric polarization due to lattice distortion occurs. Due to the piezoelectric polarization and the spontaneous polarization of the crystal of the carrier supply layer 22, high-density carriers are generated in the carrier traveling layer 21 near the heterojunction, and a two-dimensional carrier gas layer 23 as a current path (channel) is formed.

  The source electrode 3 and the drain electrode 4 are made of a metal capable of low resistance contact (ohmic contact) with the nitride semiconductor layer 2. For example, aluminum (Al), titanium (Ti), or the like is used for the source electrode 3 and the drain electrode 4. Alternatively, the source electrode 3 and the drain electrode 4 are formed as a laminate of Ti and Al.

  For the gate electrode 5, for example, nickel gold (NiAu) or the like can be used. For the source electrode wiring 31 and the drain electrode wiring 41, for example, Al, gold (Au), copper (Cu), or the like can be used.

  Next, a first specific example of the leakage current adjusting unit 1 that is a feature of the first embodiment will be described. The carrier supply layer 22 is made of AlGaN, and is characterized in that the gate leakage current is made larger than the predetermined current by making the Al composition ratio with respect to AlGaN larger than a predetermined ratio.

  The Al composition ratio of the conventional carrier supply layer to AlGaN is, for example, 0.25 (predetermined ratio). The Al composition ratio with respect to AlGaN in Example 1 is, for example, 0.26.

  That is, when the Al composition ratio of the carrier supply layer 22 is increased, the concentration of the two-dimensional carrier gas layer 23 is increased, so that the electric field strength in the vicinity of the gate electrode is increased and the gate leakage current is a predetermined current at an Al composition ratio of 0.25. Than to rise.

  Therefore, the total current of the source leakage current of the normally-on switch Q1 and the substrate current can be reduced. For this reason, when the normally-off switch Q2 is turned off, the potential of the first source electrode S of the normally-on switch Q1 can be equal to or higher than the potential at which the normally-on switch Q1 can be turned off and less than the withstand voltage of the normally-off switch Q2. . Therefore, the normally-off switch Q2 is not destroyed and the normally-on switch Q1 can be turned off, so that an inexpensive cascode normally-off circuit that does not require external components can be provided.

  FIG. 3 shows the relationship between the drain voltage of the normally-on switch Q1 of Example 1 and the gate leakage current. FIG. 4 shows the relationship between the drain voltage and gate leakage current of a conventional normally-on switch. 3 and 4, ID is a drain leakage current, IS is a source leakage current, IG is a gate leakage current, and ISUB is a substrate leakage current. In Example 1 shown in FIG. 3, it can be seen that the gate leakage current IG is large and the sum of the source leakage current IS and the substrate leakage current ISUB is small.

  On the other hand, in the conventional example shown in FIG. 4, it can be seen that the gate leakage current IG is small and the sum of the source leakage current IS and the substrate leakage current ISUB is large.

  Next, a second specific example of the leakage current adjusting unit 1 according to the second embodiment will be described with reference to a sectional view of a normally-on switch shown in FIG. The normally-on switch shown in FIG. 5 is characterized in that the gate leakage current is made larger than the predetermined current by making the thickness of the carrier supply layer 22a which is an epitaxial layer larger than the predetermined thickness.

  The thickness of the conventional carrier supply layer 22 is, for example, 25 nm (predetermined thickness), whereas the thickness of the carrier supply layer 22a of the second embodiment is, for example, 35 nm.

  As described above, when the thickness of the carrier supply layer 22a is larger than the predetermined thickness, the concentration of the two-dimensional carrier gas layer 23 is increased, so that the electric field strength in the vicinity of the gate electrode is increased and the gate leakage current is increased in the carrier supply layer 22. It rises above a predetermined current at a predetermined thickness.

  Therefore, also in the cascode normally-off circuit of the second embodiment, the same effect as that of the cascode normally-off circuit of the first embodiment can be obtained.

  Next, a third specific example of the leakage current adjusting unit 1 according to the third embodiment will be described with reference to a sectional view of a normally-on switch shown in FIG.

  The normally-on switch shown in FIG. 6 is characterized in that the gate leakage current is made larger than the predetermined current by making the length L2 of the gate field plate 50a shorter than the predetermined length.

  The length of the conventional gate field plate 50 is, for example, 2 μm (predetermined length), whereas the length of the gate field plate 50a of the third embodiment is, for example, 1 μm.

  As described above, when the length of the gate field plate 50a is made shorter than the predetermined length, the electric field relaxation is slightly weakened, so that the gate leakage current increases from the predetermined current at the predetermined length.

  Therefore, also in the cascode normally-off circuit of the third embodiment, the same effect as that of the cascode normally-off circuit of the first embodiment can be obtained.

  Next, a fourth specific example of the leakage current adjusting unit 1 according to the fourth embodiment will be described with reference to a sectional view of a normally-on switch shown in FIG. In the normally-on switch shown in FIG. 7, the gate leakage current is made larger than the predetermined current by making the inclination angle of the insulating film inclined portion 71a formed in the insulating film 71 at the end of the gate electrode 5 larger than the predetermined angle. It is characterized by that.

  The inclination angle of the insulating film inclined portion 71a formed in the conventional insulating film 71 is, for example, 30 degrees (predetermined angle), whereas the inclination angle of the insulating film inclined portion 71a of the fourth embodiment is, for example, It is 45 degrees.

  As described above, when the inclination angle of the insulating film inclined portion 71a is made larger than the predetermined angle, the electric field relaxation is slightly weakened, so that the gate leakage current increases from the predetermined current at the predetermined angle.

  Therefore, also in the cascode normally-off circuit of the fourth embodiment, the same effect as that of the cascode normally-off circuit of the first embodiment can be obtained.

  Note that the present invention is not limited to the cascode normally-off circuit of the first to fourth embodiments. For example, any number of embodiments of the cascode normally-off circuit according to the first to fourth embodiments may be used in combination. In this way, the effect is further increased.

Q1 Normally-on switch Q2 Normally-off switch R1, R2 Resistor 1 Leakage current adjusting unit 2 Nitride semiconductor layer 3 Source electrode 4 Drain electrode 5 Gate electrode 10 Substrate 11 Buffer layer 21 Carrier running layer 22, 22a Carrier supply layer 23 Two-dimensional carrier gas Layer 30 Source field plate 31 Source electrode wiring 41 Drain electrode wiring 50, 50a Gate field plate 71 Insulating film 71a Insulating film inclined portion 72 Interlayer insulating film

Claims (5)

A normally-on switch comprising a nitride semiconductor having a first drain electrode, a first source electrode, and a first gate electrode, wherein a high voltage is applied to the first drain electrode;
A silicon semiconductor having a second drain electrode, a second source electrode, and a second gate electrode is connected, the second drain electrode is connected to the first source electrode of the normally-on switch, and the second source electrode is connected to the normally-on switch. A normally-off switch connected to the first gate electrode and applying a voltage to the second gate electrode;
The gate leakage of the normally-on switch is such that when the normally-off switch is turned off, the potential of the first source electrode of the normally-on switch is greater than or equal to the potential at which the normally-on switch can be turned off and less than the withstand voltage of the normally-off switch. A leakage current adjustment unit for adjusting the current;
A cascode normally-off circuit comprising: The normally-on switch is
A nitride semiconductor layer in which a carrier supply layer made of AlGaN and a carrier travel layer that forms a heterojunction with the carrier supply layer are stacked;
An insulating film disposed on the nitride semiconductor layer;
The first source electrode disposed on the nitride semiconductor layer;
The first drain electrode disposed on the nitride semiconductor layer and spaced apart from the first source electrode;
The first gate electrode disposed on the nitride semiconductor layer between the first source electrode and the first drain electrode;
2. The cascode normally-off circuit according to claim 1, wherein the leakage current adjusting unit makes the gate leakage current larger than a predetermined current by making an Al composition ratio of the carrier supply layer to AlGaN larger than a predetermined ratio. . The normally-on switch is
A nitride semiconductor layer in which a carrier supply layer and a carrier travel layer that forms a heterojunction with the carrier supply layer are stacked;
An insulating film disposed on the nitride semiconductor layer;
The first source electrode disposed on the nitride semiconductor layer;
The first drain electrode disposed on the nitride semiconductor layer and spaced apart from the first source electrode;
The first gate electrode disposed on the nitride semiconductor layer between the first source electrode and the first drain electrode;
2. The cascode normally-off circuit according to claim 1, wherein the leakage current adjusting unit makes the gate leakage current larger than a predetermined current by making a thickness of the carrier supply layer larger than a predetermined thickness. The normally-on switch is
A nitride semiconductor layer in which a carrier supply layer and a carrier travel layer that forms a heterojunction with the carrier supply layer are stacked;
An insulating film disposed on the nitride semiconductor layer;
The first source electrode disposed on the nitride semiconductor layer;
The first drain electrode disposed on the nitride semiconductor layer and spaced apart from the first source electrode;
The first gate electrode disposed on the nitride semiconductor layer between the first source electrode and the first drain electrode;
A gate field plate electrically connected to the first gate electrode and disposed on the insulating film between the first gate electrode and the first drain electrode;
2. The cascode normally-off circuit according to claim 1, wherein the leakage current adjusting unit makes the gate leakage current larger than a predetermined current by making a length of the gate field plate shorter than a predetermined length. The normally-on switch is
A nitride semiconductor layer in which a carrier supply layer and a carrier travel layer that forms a heterojunction with the carrier supply layer are stacked;
An insulating film disposed on the nitride semiconductor layer;
The first source electrode disposed on the nitride semiconductor layer;
The first drain electrode disposed on the nitride semiconductor layer and spaced apart from the first source electrode;
The first gate electrode disposed on the nitride semiconductor layer between the first source electrode and the first drain electrode;
2. The leakage current adjusting unit according to claim 1, wherein the gate leakage current is made larger than a predetermined current by making an inclination angle of the insulating film at an end of the first gate electrode larger than a predetermined angle. The cascode normally-off circuit described.

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