阅读说明：本技术 场效应晶体管 (Field effect transistor ) 是由 渡边伸介 于 2018-10-03 设计创作，主要内容包括：本发明的场效应晶体管在半导体基板(1、2)的表面形成有栅极电极(3)、源极电极(4)以及漏极电极(5)。绝缘膜(6)在栅极电极(3)与漏极电极(5)之间的区域覆盖半导体基板(1、2)的表面。源极场板(7)形成于绝缘膜(6)之上,未与漏极电极(5)连接。二极管(8)的阴极与源极场板(7)连接,阳极为恒定电位。(A field effect transistor is provided with a gate electrode (3), a source electrode (4), and a drain electrode (5) formed on the surfaces of semiconductor substrates (1, 2). The insulating film (6) covers the surfaces of the semiconductor substrates (1, 2) in the region between the gate electrode (3) and the drain electrode (5). The source field plate (7) is formed on the insulating film (6) and is not connected to the drain electrode (5). The cathode of the diode (8) is connected to the source field plate (7), and the anode is at a constant potential.)

1. A field effect transistor, characterized in that,

the field effect transistor includes:

a semiconductor substrate;

a gate electrode, a source electrode, and a drain electrode formed on a surface of the semiconductor substrate;

an insulating film covering a surface of the semiconductor substrate in a region between the gate electrode and the drain electrode;

a source field plate formed on the insulating film and not connected to the drain electrode; and

a diode having a cathode connected to the source field plate and an anode at a constant potential,

the shortest distance between the gate electrode and the source field plate is less than 1 μm.

2. The field effect transistor according to claim 1,

the anode of the diode is connected to the source electrode or a ground pad having a ground potential.

3. The field effect transistor according to claim 1,

the anode of the diode is connected to a DC pad to which a direct voltage is applied from the outside.

4. The field effect transistor according to any one of claims 1 to 3,

the diode is a schottky diode or a PN diode.

5. The field effect transistor according to any one of claims 1 to 4,

the diode is also provided with a matching circuit connected to the anode or the cathode of the diode.

Technical Field

The present invention relates to a field effect transistor.

Background

As a material of the semiconductor substrate, silicon carbide, gallium arsenide, gallium nitride, indium phosphide, or the like can be used. There are field effect transistors using a gallium nitride substrate therein. In the present application, a normally-on high electron mobility transistor in which a drain-source current flows when a gate voltage is zero volts is described.

Gallium nitride has a large band gap and saturated electron velocity, high dielectric breakdown field strength, compared to silicon or gallium arsenide. Due to these characteristics, it is desirable that a high electron mobility transistor using gallium nitride can generate an output signal with high power and can be a high-frequency device with high efficiency and high gain.

However, when a high voltage of several tens of volts is applied to the drain electrode and a voltage close to zero volts is applied to the gate electrode in order to perform a high power operation, a strong electric field is generated in the semiconductor substrate near the gate electrode. Hot electrons generated by the strong electric field can be trapped by a trap (trap) present near the surface of the semiconductor or inside the semiconductor. In this way, accumulation of negative charges occurs, and variation in characteristics of the transistor occurs. When a high-frequency signal with large power is excessively input to the gate electrode, the drain voltage instantaneously exceeds 100 v, and the accumulation of negative charges occurs, and the concentration of the two-dimensional electron gas decreases. As a result, it is found that the maximum drain current is reduced or the drain current at the bias point is changed.

In order to suppress such characteristic variation, a source field plate may be used. The source field plate is connected to a source electrode or ground pad. When the source field plate having a potential of zero volts, which is grounded, is brought close to the semiconductor surface between the gate and the drain, the change in potential near the gate electrode is small, and the electric field can be relaxed. Therefore, temporary characteristic fluctuation of the transistor can be suppressed, transient response characteristics can be improved, and withstand voltage can be improved.

Fig. 15 is a diagram of load lines showing temporal changes in drain current and drain voltage that occur when a high-frequency signal is input to the gate. The state a is a state in which a positive gate voltage is instantaneously input, and a large drain current Id and a low drain voltage Vd are instantaneously obtained. The state B is a state where the drain current Id is weak and the drain voltage Vd is high instantaneously. A strong electric field is generated in the vicinity of the gate electrode due to the high drain voltage Vd in the state B, and the characteristics vary. On the other hand, a device in which a drain electrode and a source field plate are connected via a resistor or a voltage dividing circuit is disclosed (for example, see patent document 1). Accordingly, it is desirable to suppress the characteristic variation by applying a positive voltage to the source field plate even when the drain voltage Vd becomes a high positive voltage.

Patent document 1: international publication No. 2013/027722

However, resistance damage may occur by connecting a source field plate, which is typically zero volts, to a drain electrode, which is momentarily at a voltage of several hundred volts, through a resistor. In addition, a loop path of a high-frequency signal of high power is created by creating a path of the high-frequency signal from the drain electrode toward the source field plate in the vicinity of the gate electrode. Not only the characteristics of the high electron mobility transistor are deteriorated due to the formation of the loop of the high frequency signal, but also the transistor as a whole may be damaged due to oscillation. Therefore, there is a problem that reliability cannot be ensured.

The inventors have excessively input a high-frequency signal to a high-electron-mobility transistor under various conditions and studied characteristic variations of the transistor. Various loads are applied to the drain, the shape of the load line is changed, and the magnitude of characteristic variation of the transistor is examined. As a result, it was found that characteristic variations occur not only in the state B but also in the state a. In a high electron mobility transistor using gallium nitride in recent years in which a high bias voltage is applied to a drain, a high electric field is generated in the vicinity of a gate electrode even in the state a. When the carriers are in a high electric field, impact ionization (impact ionization) occurs, and holes that cause characteristic variations are generated. In the state a where a large number of carriers pass directly under the gate, it is considered that a large number of holes are generated. In the state a where the gate voltage is instantaneously positive, injection of holes from the gate electrode into the semiconductor substrate also occurs.

The inventors have found that if a source field plate having a size equal to or larger than a certain size is present, the variation in transistor characteristics increases. This is considered to be because the holes are accumulated in the insulating film or the semiconductor immediately below the source field plate. Therefore, it is desirable to reduce the electric field by setting the potential of the source field plate to zero volts in state B and to suppress the accumulation of holes by applying a positive voltage to the source field plate in state a. However, in the conventional device, since the potential of the source field plate is interlocked with the drain voltage, the behavior is opposite to the ideal potential variation. Therefore, there is a problem that the characteristic variation cannot be suppressed.

Disclosure of Invention

The present invention has been made to solve the above problems, and an object of the present invention is to obtain a field effect transistor capable of suppressing characteristic variations while ensuring reliability.

The field effect transistor according to the present invention includes: a semiconductor substrate; a gate electrode, a source electrode, and a drain electrode formed on a surface of the semiconductor substrate; an insulating film covering a surface of the semiconductor substrate in a region between the gate electrode and the drain electrode; a source field plate formed on the insulating film and not connected to the drain electrode; and a diode having a cathode connected to the source field plate and an anode at a constant potential, wherein the shortest distance between the gate electrode and the source field plate is 1 μm or less.

In the present invention, since the shortest distance between the gate electrode and the source field plate is 1 μm or less, the potential of the source field plate follows the gate voltage. A diode is provided in which the cathode is connected to the source field plate and the anode is at a constant potential. Therefore, the behavior of the potential of the source field plate becomes ideal, and thus the characteristic variation can be suppressed. In addition, since the source field plate is not connected to the drain electrode, a loop path of a high-power high-frequency signal is not created. Therefore, damage of the transistor due to deterioration of characteristics of the high electron mobility transistor and oscillation does not occur, and thus reliability can be ensured.

Drawings

Fig. 1 is a cross-sectional view showing a field effect transistor according to embodiment 1.

Fig. 2 is an equivalent circuit diagram showing a field effect transistor according to embodiment 1.

Fig. 3 is an equivalent circuit diagram showing a state where the source field plate is grounded.

Fig. 4 is a graph showing temporal changes in gate voltage in the case where the source field plate is grounded.

Fig. 5 is a diagram showing temporal changes in the potential of the source field plate in the case where the source field plate is grounded.

Fig. 6 is an equivalent circuit diagram showing a state where the source field plate is floating.

Fig. 7 is a graph showing temporal changes in gate voltage in the case where the source field plate is floating.

Fig. 8 is a graph showing temporal changes in the potential of the source field plate in the case where the source field plate is floating.

Fig. 9 is a diagram showing a temporal change in the gate voltage Vg of the field effect transistor according to embodiment 1.

Fig. 10 is a diagram showing temporal changes in potential of a source field plate of the field effect transistor according to embodiment 1.

Fig. 11 is a graph showing temporal changes in gate voltage when the shortest distance between the gate electrode and the source field plate is 2 μm.

Fig. 12 is a graph showing temporal changes in the potential of the source field plate when the shortest distance between the gate electrode and the source field plate is 2 μm.

Fig. 13 is a cross-sectional view showing a field effect transistor according to embodiment 2.

Fig. 14 is a cross-sectional view showing a field effect transistor according to embodiment 3.

Fig. 15 is a diagram of a load line showing temporal changes in drain current and drain voltage generated when a high-frequency signal is input to the gate.

Detailed Description

A field effect transistor according to an embodiment will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repeated description thereof may be omitted.

Embodiment mode 1

Fig. 1 is a cross-sectional view showing a field effect transistor according to embodiment 1. Fig. 2 is an equivalent circuit diagram showing a field effect transistor according to embodiment 1. The field effect transistor is a normally-on high electron mobility transistor that amplifies high frequency signals.

An AlGaN layer 2 is formed on the GaN substrate 1. A gate electrode 3, a source electrode 4, and a drain electrode 5 made of metal are formed on the surface of the AlGaN layer 2. The insulating film 6 covers the gate electrode 3 and the surface of the AlGaN layer 2. Accordingly, the insulating film 6 covers the surface of the AlGaN layer 2 in the region between the gate electrode 3 and the drain electrode 5.

A source field plate 7 made of metal is formed on the insulating film 6. The source field plate 7 is not connected to the drain electrode 5. The cathode of the diode 8 is connected to the source field plate 7. The anode of the diode 8 is at ground potential, and is connected to the source electrode 4 or a ground pad having ground potential, for example.

Two-dimensional electron gas is generated in a region near the AlGaN layer 2 inside the GaN substrate 1 by piezoelectric polarization due to lattice mismatch between GaN and AlGaN. The two-dimensional electron gas becomes a current between the drain electrode 5 and the source electrode 4. The two-dimensional electron gas changes according to the voltage applied to the gate electrode 3, and can switch between on and off of the current between the drain and the source. When a high-frequency signal is input to the gate electrode 3 by applying appropriate bias voltages to the gate electrode 3, the drain electrode 5, and the source electrode 4, a high-power high-frequency signal is generated in the drain electrode 5, and an amplifying action is obtained.

Consider a case where a high-frequency signal of high power is input to the gate electrode 3, and the drain voltage and the drain current change as shown in fig. 15. In the state B where the drain current is small, the gate voltage is negative. The source field plate 7 attempts to become negative following the gate voltage. However, since a forward voltage is applied to the diode 8, the diode 8 is in a short-circuited state. Therefore, the potential of source field plate 7 is forced to be zero volts, which is the ground potential.

On the other hand, in the state a where the power of the input signal is sufficiently large, the gate voltage changes to positive. At this time, the potential of the source field plate 7 also becomes positive following the gate voltage. The diode 8 is in a released state by applying a reverse voltage to the diode 8. Therefore, the source field plate 7 is in a floating state. In this case, the potential of the source field plate 7 becomes positive following the gate voltage regardless of the ground potential.

Therefore, in this embodiment, an ideal state is automatically realized, that is, the potential of the source field plate 7 is zero volts in the state B, and the potential of the source field plate 7 becomes a positive potential in the state a. Therefore, the source field plate 7 having a potential of zero volts when the drain voltage is high relaxes the electric field in the vicinity of the gate electrode 3. When the gate voltage is high and the drain current is large, accumulation of holes is suppressed by a positive potential applied to the source field plate 7. Thus, a plurality of mechanisms causing characteristic variations of the high electron mobility transistor are eliminated.

Temporal changes in the gate voltage Vg and the potential Vsfp of the source field plate 7 were simulated using the characteristics of the hemt and the diode 8 and the capacitance value between the source field plate 7 and the gate electrode 3, which were actually measured and studied. The frequency of the high-frequency signal input to the gate electrode 3 is 2.7 gigahertz. The power of the input high frequency signal varies from minus 10dBm to plus 25dBm in steps of 1 dBm.

Fig. 3 is an equivalent circuit diagram showing a state where the source field plate is grounded. Fig. 4 is a graph showing temporal changes in gate voltage in the case where the source field plate is grounded. The lines shown in the figure represent the case where the input power is changed by steps of 1 dBm. The moment the gate voltage Vg becomes high is a state a in which a large drain current flows in the transistor. The moment of the change to the low gate voltage is the state B where the drain current becomes minute.

Fig. 5 is a diagram showing temporal changes in the potential of the source field plate in the case where the source field plate is grounded. If the source field plate 7 is grounded, the potential Vsfp of the source field plate 7 is zero volts regardless of the gate voltage Vg.

Fig. 6 is an equivalent circuit diagram showing a state where the source field plate is floating. Fig. 7 is a graph showing temporal changes in gate voltage in the case where the source field plate is floating. Fig. 8 is a graph showing temporal changes in the potential of the source field plate in the case where the source field plate is floating. If the source field plate 7 is sufficiently close to the gate electrode 3 and the capacitance value between the two is large, the potential Vsfp of the source field plate 7 follows the gate voltage Vg. As a result, the time waveform of the gate voltage Vg and the time waveform of the potential Vsfp of the source field plate have the same shape, although the amplitudes do not necessarily match.

Fig. 9 is a diagram showing a temporal change in the gate voltage Vg of the field effect transistor according to embodiment 1. Fig. 10 is a diagram showing temporal changes in potential of a source field plate of the field effect transistor according to embodiment 1. As a result of the simulation, the potential Vsfp of the source field plate 7 becomes a positive voltage in the state a and becomes zero volt in the state B.

However, the source field plate 7 needs to be close enough to the gate electrode 3 so that the potential Vsfp of the source field plate 7 follows the gate voltage Vg. Specifically, the shortest distance between the gate electrode 3 and the source field plate 7 is set to 1 μm or less.

In the above simulation, a high electron mobility transistor having a capacitance value of 3pF between the gate electrode 3 and the source field plate 7 was used. On the other hand, if the overlap area of the gate electrode 3 and the source field plate 7 is 1560 μm²SiN having a dielectric constant of 7 is used as the insulating film 6, and the capacitance value is 0.05pF when the shortest distance between the gate electrode 3 and the source field plate 7 is 2 μm. Fig. 11 is a graph showing temporal changes in gate voltage when the shortest distance between the gate electrode and the source field plate is 2 μm. Fig. 12 is a graph showing temporal changes in the potential of the source field plate when the shortest distance between the gate electrode and the source field plate is 2 μm. Therefore, it is found that if the shortest distance is longer than 1 μm, the change in the gate voltage Vg is not reflected in the potential Vsfp of the source field plate 7.

As described above, in the present embodiment, since the shortest distance between the gate electrode 3 and the source field plate 7 is 1 μm or less, the potential Vsfp of the source field plate 7 follows the gate voltage Vg. A diode 8 having a cathode connected to the source field plate 7 and an anode at a constant potential is provided. Therefore, the behavior of the potential Vsfp of the source field plate 7 becomes ideal, and thus the characteristic variation can be suppressed. In addition, since the source field plate 7 is not connected to the drain electrode 5, a loop path of a high-power high-frequency signal is not created. Therefore, damage of the transistor due to deterioration of characteristics of the high electron mobility transistor and oscillation does not occur, and thus reliability can be ensured.

In addition, the diode 8 is a schottky diode or a PN diode. Since it is difficult to use a zener diode for intentionally lowering the breakdown voltage in combination with a high-power transistor, the zener diode is not used as the diode 8.

Embodiment mode 2

Fig. 13 is a cross-sectional view showing a field effect transistor according to embodiment 2. In embodiment 1, switching between the released state and the short-circuited state of the diode 8 is determined by the rising voltage of the diode 8, which is the characteristic of the diode 8. Depending on the diode 8 prepared, there may be a case where its boosted voltage is not suitable for the high electron mobility transistor. Therefore, in the present embodiment, the anode of the diode 8 is connected to the DC pad 9 to which the DC voltage is applied, not to the ground.

When a dc voltage is applied to the anode of the diode 8, the time when the diode 8 is in a short circuit state changes, and the time when the potential of the source field plate 7 becomes zero volts changes. That is, by changing the voltage applied to the DC pad 9, the time waveform of the potential of the source field plate 7 shown in fig. 10 can be adjusted to a more appropriate waveform. In the case where it takes longer to lower the potential of the source field plate 7, a forward voltage is easily applied to the diode 8 by applying a positive voltage to the DC pad 9. Conversely, when it takes a longer time for the potential of the source field plate 7 to become positive, a negative voltage is applied to the DC pad 9, and thus a reverse voltage is easily applied to the diode 8. This can more reliably suppress the characteristic variation.

Embodiment 3

Fig. 14 is a cross-sectional view showing a field effect transistor according to embodiment 3. The anode and cathode of the diode 8 are connected to matching circuits 10, 11, respectively. The matching circuits 10 and 11 are constituted by capacitors, inductors, resistors, signal lines, and the like.

The time waveforms of the potentials at the cathode and the anode of the diode 8 vary depending on a capacitor, an inductor, or the like provided in the periphery. Therefore, by adjusting the matching circuits 10 and 11 connected to the diode 8, the time waveforms of the voltage applied to the diode 8 and the potential of the source field plate 7 can be arbitrarily adjusted.

Description of the reference numerals

1. A semiconductor substrate; a gate electrode; a source electrode; a drain electrode; an insulating film; a source field plate; a diode; a DC pad; 10. a matching circuit.