Composite semiconductor device

1. A composite semiconductor device, comprising:

a first terminal receiving a first voltage;

a second terminal receiving a second voltage lower than said first voltage;

a third terminal selectively provided with any one of a third voltage and a fourth voltage higher than the third voltage;

a normally-on first field effect transistor having a drain connected to said first terminal and a gate connected to said second terminal;

a normally-off second field effect transistor having a drain connected to a source of said first field effect transistor, a source connected to said second terminal, and a gate connected to said third terminal, rendered non-conductive when said third voltage is provided to said third terminal, and rendered conductive when said fourth voltage is provided to said third terminal; and

a protection circuit connected in parallel to said second field effect transistor, for protecting said second field effect transistor by maintaining a voltage across the drain and the source of said second field effect transistor to a voltage not higher than a withstand voltage of said second field effect transistor,

said protection circuit including N (N being a natural number) unipolar rectifier elements connected in series between the drain and the source of said second field effect transistor in a forward direction and rendered conductive when a voltage across the drain and the source of said second field effect transistor exceeds a predetermined voltage not higher than a withstand voltage of said second field effect transistor.

2. The composite semiconductor device according to claim 1 , wherein in a case where a voltage of said third terminal varies from said fourth voltage to said third voltage, a time period from start of increase in voltage across the drain and the source of said second field effect transistor until conduction of said N unipolar rectifier elements is set to at most half a time period from start of increase in voltage across the drain and the source of said second field effect transistor until non-conduction of said first field effect transistor.

3. The composite semiconductor device according to claim 1 , wherein each of said N unipolar rectifier elements is a Schottky diode.

4. The composite semiconductor device according to claim 3 , wherein each of said N unipolar rectifier elements is formed of a material greater in energy band gap than Si.

5. The composite semiconductor device according to claim 4 , wherein said material is AlxGa{tilde over (1)}xN (0≦x≦1), SiC, diamond, AlxGa{tilde over (1)}xAs (0≦x≦1), InxGa{tilde over (1)}xP (0≦x≦1), or AlxInyGa{tilde over (1)}{tilde over (x)}yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1).

6. The composite semiconductor device according to claim 3 , comprising a first semiconductor chip including first and second nitride-based semiconductor layers successively stacked on a first semiconductor substrate, wherein said N Schottky diodes are formed in N regions on a surface of said first semiconductor chip, respectively,

an anode electrode and a cathode electrode of a corresponding Schottky diode are provided at a distance from each other in each region,

a recess portion penetrating said second nitride-based semiconductor layer and reaching said first nitride-based semiconductor layer is formed in each region,

said anode electrode is formed in said recess portion, and

said cathode electrode is formed on a surface of said second nitride-based semiconductor layer.

7. The composite semiconductor device according to claim 6 , comprising a second semiconductor chip including third and fourth nitride-based semiconductor layers successively stacked on a second semiconductor substrate, wherein

said first field effect transistor is formed on a surface of said second semiconductor chip,

a gate electrode, a source electrode, and a drain electrode of said first field effect transistor are formed on a surface of said fourth nitride-based semiconductor layer, and

the gate electrode is provided between the source electrode and the drain electrode.

8. The composite semiconductor device according to claim 7 , wherein said third nitride-based semiconductor layer is formed of GaN, and said fourth nitride-based semiconductor layer is formed of AlxGa{tilde over (1)}xN (0<x≦1).

9. The composite semiconductor device according to claim 3 , comprising a semiconductor chip including first and second nitride-based semiconductor layers successively stacked on a semiconductor substrate, wherein

said N Schottky diodes are formed in N first regions on a surface of the semiconductor chip, respectively,

said first field effect transistor is formed in a second region on the surface of said semiconductor chip,

an anode electrode and a cathode electrode of a corresponding Schottky diode are provided at a distance from each other in each first region,

a recess portion penetrating said second nitride-based semiconductor layer and reaching said first nitride-based semiconductor layer is formed in each first region,

said anode electrode is formed in said recess portion,

said cathode electrode is formed on a surface of said second nitride-based semiconductor layer,

a gate electrode, a source electrode, and a drain electrode of said first field effect transistor are formed on the surface of said second nitride-based semiconductor layer in said second region, and

the gate electrode is provided between the source electrode and the drain electrode.

10. The composite semiconductor device according to claim 9 , wherein said first nitride-based semiconductor layer is formed of GaN, and said second nitride-based semiconductor layer is formed of AlxGa{tilde over (1)}xN (0<x≦1).

11. The composite semiconductor device according to claim 1 , further comprising a first resistor element interposed between the gate of said first field effect transistor and said second terminal.

12. The composite semiconductor device according to claim 1 , further comprising a second resistor element interposed between said first terminal and the drain of said first field effect transistor.