JP5639940B2 - Insulated gate bipolar transistor - Google Patents

Description

  The present invention relates to an insulated gate bipolar transistor (hereinafter referred to as “IGBT”).

  In an IGBT having a Schottky junction between a drain electrode and a semiconductor substrate, the amount of minority carriers (holes) injected from the drain electrode into the N-type semiconductor substrate is such that an IGBT having a PN junction without a Schottky junction is provided. Therefore, the switching speed in the turn-off operation can be increased (see Patent Document 1).

Japanese Patent No. 3895147

However, an IGBT provided with a Schottky junction has a problem that the switching speed can be increased while the drain-source voltage (on voltage Vce (sat)) for turning on the IGBT increases.
In order to reduce the ON voltage Vce (sat), it is effective to increase the Schottky barrier Φbn in the Schottky junction in order to increase the amount of holes injected from the Schottky junction to the semiconductor substrate during the ON operation. It is. However, when the Schottky barrier Φbn is increased, the breakdown voltage in the forward direction between the drain electrode and the source electrode in a state where no voltage is applied to the gate voltage (gate voltage = 0 V) is decreased.
This is because the depletion layer formed at the PN junction between the P-well region where the insulated gate transistor is formed and the semiconductor substrate increases toward the drain electrode as the drain electrode voltage is increased. This is due to the phenomenon of shortening the effective base length.

In an IGBT provided with a Schottky junction, a parasitic PNP bipolar formed using a drain electrode as a collector, a semiconductor substrate as a base, and a P-well region where an insulated gate transistor is formed as an emitter is used in order to suppress a forward breakdown. In the transistor, a method of increasing the impurity concentration of the base and reducing the current amplification factor in the bipolar transistor is conceivable. That is, by providing an N buffer layer (high concentration region) having an impurity concentration higher than that of the n type semiconductor substrate in a contact region between the n type semiconductor substrate and the drain electrode, the impurity concentration of the N buffer layer is made higher. The current amplification factor of the parasitic PNP transistor decreases, and the forward breakdown voltage can be increased.
However, increasing the concentration of the N buffer layer in contact with the drain electrode can increase the forward breakdown voltage, but this time, the amount of holes to the semiconductor substrate is reduced at turn-on, and the ON voltage Vce (sat ) Will rise.

As described above, the conventional IGBT cannot achieve both the reduction of the on-voltage Vce (sat) and the suppression of the decrease in the breakdown voltage in the forward direction. An object of the present invention is to provide an IGBT in which an ON voltage Vce (sat) is reduced and a forward breakdown voltage is suppressed from decreasing in an IGBT provided with a Schottky junction.
In addition, the said patent document 1 is disclosing only IGBT by which a drain electrode and a buffer layer contact in IGBT provided with Schottky junction.

The insulated gate bipolar transistor (IGBT) of the present invention is formed on the other surface side of the semiconductor substrate from the source electrode formed on the one surface side of the first conductivity type semiconductor substrate having the first impurity concentration. An insulated gate bipolar transistor that switches current flowing in the thickness direction of the semiconductor substrate toward the drain electrode by a voltage applied to a gate electrode of an insulated gate transistor formed on one surface of the semiconductor substrate. Injecting a second conductivity type carrier into the first conductivity type semiconductor substrate on the other surface of the first conductivity type semiconductor substrate when the insulated gate transistor is turned on to cause conductivity modulation. A Schottky junction is formed by the semiconductor substrate and the drain electrode, and the source electrode is formed from the drain electrode. To a location spaced, when the semiconductor substrate buffer layer having a second impurity concentration of the first conductivity type containing an impurity at a high concentration is formed from, an insulating gate bipolar transistor not having a buffer layer and a forward current In a state where a reference drain-source voltage for obtaining a predetermined current value is applied to the insulated gate bipolar transistor having a buffer layer, a separation distance between the drain electrode and the buffer layer, a width of the buffer layer And the second conductivity type carrier density obtained by multiplying the accumulated concentration of the second conductivity type carriers from the drain electrode to the buffer layer by the separation distance, and the second impurity concentration. first conductivity type carrier density multiplied by the width of the conductive type wherein the buffer layer to a carrier concentration of as larger than, especially that you set To.

  In the insulated gate bipolar transistor, the width of the buffer layer, the second impurity concentration, and the concentration of the second conductive carrier so that the second conductive carrier density is 7 times or more higher than the first conductive carrier density. The distance between the drain electrode and the buffer layer is set.

  In the insulated gate bipolar transistor, the separation distance is set to 5 μm or more.

According to the present invention, the buffer layer is disposed apart from the drain electrode. For example, the distance between the drain electrode and the buffer layer, the width of the buffer layer, and the impurity concentration of the buffer layer are set as follows. That is, a reference drain-source voltage for obtaining a predetermined current value when a forward current is applied to an IGBT having no buffer layer is applied to the IGBT having the buffer layer. In this state, the second conductivity type carrier density obtained by multiplying the accumulated concentration of the second conductivity type carriers from the drain electrode to the buffer layer by the separation distance is equal to the first conductivity type carrier concentration of the buffer layer and the width of the buffer layer. Is set to be higher than the first conductivity type carrier density multiplied by.
With this setting, in the IGBT, conductivity modulation in the semiconductor substrate is performed so that the injection of holes into the semiconductor substrate on the well region side where the insulated gate transistor is formed is hardly restricted (the amount of hole injection is reduced). Can be controlled.
Thereby, the forward breakdown voltage drop is suppressed by the buffer layer, and the injection amount of the second conductivity type carrier to the semiconductor substrate at the time of turn-on is almost limited (the injection amount of the second conductivity type carrier is reduced). Therefore, the turn-on voltage Vce (sat) can be lowered.

It is sectional drawing which shows the structure of IGBT100 of this invention. It is a figure which shows the relationship between Schottky barrier (PHI) bn in IGBT which does not have an N buffer layer, ON voltage Vce (sat), and the proof pressure of a forward direction. It is a figure which shows the relationship between the impurity concentration Nb of an N buffer layer, ON voltage Vce (sat), and a forward withstand pressure | voltage when not providing separation distance between an N buffer layer and a drain electrode. It is a figure which shows the relationship between the separation distance X between N buffer layer and a drain electrode, ON voltage Vce (sat), and the proof pressure of a forward direction. It is a graph which shows the dependence of the carrier density on the distance from the drain electrode when the impurity concentration of the N buffer layer is Nb = 2E + 14 [1 / cm2] and the separation distance is X = 0.33 μm. It is a graph which shows the dependence of the carrier density on the distance from the drain electrode when the impurity concentration of the N buffer layer is Nb = 4E + 16 [1 / cm 2] and the separation distance is X = 0.33 μm. It is a graph which shows the dependence of the carrier density on the distance from the drain electrode when the impurity concentration of the N buffer layer is Nb = 4E + 16 [1 / cm2] and the separation distance is X = 5 μm. It is a graph which shows the relationship between the separation distance X from the drain electrode of N buffer layer, and carrier density. It is a graph which shows the relationship between the separation distance X and a forward direction electric current value.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.
First, the structure of the IGBT 100 provided with a Schottky junction will be described with reference to FIG.
In the IGBT 100, a P-well region 116 is formed on the surface of the N-drift layer 114 (semiconductor substrate) by the DSA method so that the surface is exposed. Further, in the P well region 116, an N + source region 118 (also referred to as an N + emitter region, but in the present embodiment, hereinafter referred to as an N + source region) is formed so that the surface thereof is exposed.
A gate electrode 124 made of polysilicon is formed on the surface of the P well region 116 through a thin gate insulating film 122 such as SiO 2. The gate electrode 124 is arranged so as to straddle the P well region 116 and reach the N− drift layer 114 from the N + source region 118. The gate electrode 124 is connected to the gate terminal G.
The surface of the P well region immediately below the gate electrode 124 is referred to as a channel formation region 120. A source electrode 128 (also referred to as an emitter electrode in this embodiment) is formed so as to short-circuit the N + source region 118 and the P well region 116 at the surface. Connected to terminal S. The source electrode 128 is insulated by the gate electrode 124 and the interlayer insulating film 126.

On the other surface of the N-drift layer 114, a drain electrode 130 (also referred to as a collector electrode) made of a film of "metal that forms a Schottky contact by being in contact with the N-drift layer 114" may be used. Then, the drain electrode is formed, and the drain electrode 130 is connected to the drain terminal D.
Further, in the vicinity of the other surface of the N− drift layer 114, the N + buffer layer 132 is disposed away from the drain electrode in the direction of the P well region 116 by a separation distance X. The N + buffer layer 132 is formed as follows when manufacturing the IGBT 100. In the ion implantation process, which is a manufacturing process, N-type impurities such as arsenic or phosphorus are implanted into the ion implantation apparatus by using an ion implantation apparatus (projected range (Rp: Projected Range), dose) of the arsenic. , Implanted into the N-drift layer 114 (semiconductor substrate). In the subsequent heat treatment step, for example, an impurity implanted by an ion implantation method is activated by using an RTA (Rapid Thermal Annealing) apparatus. In this manner, the distance X, the impurity concentration, and the width of the N + buffer layer 132 from the drain electrode 130 are determined. In the present embodiment, the separation distance X is a distance from the position where the drain electrode 130 contacts the semiconductor substrate to the central portion of the N + buffer layer 132 determined by Rp in the ion implantation process.

  Subsequently, in order to explain the technical significance of disposing the N + buffer layer 132 apart from the drain electrode by the separation distance X in the IGBT 100, first, the IGBT that does not have the N + buffer layer 132 but has a Schottky junction. The operation (hereinafter referred to as IGBT 999) will be described. Subsequently, the problem of the IGBT having the N + buffer layer 132 and provided with the N + buffer layer 132 in contact with the Schottky junction will be described. The technical significance and effects of disposing the drain electrode by a separation distance X will be described.

The IGBT 999 is turned on by applying a positive voltage higher than a threshold value to the gate terminal G with respect to the source terminal S in a state where a positive voltage is applied to the drain terminal D with respect to the source terminal S. That is, when a positive voltage equal to or higher than the threshold is applied to the gate terminal G, an inversion channel is formed on the surface of the channel formation region 120 of the P well region 116 as in the MOSFET, and the N + source region 118 passes through the inversion channel and N Electrons flow into the drift layer 114.
In response to this, injection of holes (hereinafter also referred to as “holes”) from the Schottky junction into the N-drift layer 114 occurs, and the N-drift layer 114 undergoes conductivity modulation. For this reason, the IGBT 999 has a low on-resistance even if it is a high breakdown voltage element because the N-drift layer 114, which is originally set to a high resistance, has a low resistance due to conductivity modulation.

  The operation of this IGBT 999 without the N + buffer layer 132 has no Schottky junction, for example, the operation of the IGBT having a P + diffusion layer on the drain side and injecting holes through the PN junction, as described above. The basic operation is the same. However, in the IGBT 999 having a Schottky junction, the amount of hole injection is lower than that of an IGBT having no Schottky junction, and the number of holes remaining at turn-off can be reduced. As a result, the turn-off time is further shortened compared with the IGBT having no Schottky junction, and high-speed switching characteristics are realized.

However, the IGBT 999 provided with the Schottky junction has a problem that the switching speed can be increased while the drain-source voltage (ON voltage Vce (sat)) for turning on the IGBT is increased.
FIG. 2 is a diagram illustrating the relationship between the Schottky barrier Φbn, the on-voltage Vce (sat), and the forward breakdown voltage in the IGBT 999 that does not have an N buffer layer.
As shown in FIG. 2, in order to reduce the ON voltage Vce (sat), the Schottky barrier in the Schottky junction is increased in order to increase the amount of holes injected from the Schottky junction to the semiconductor substrate during the ON operation. It is effective to increase Φbn. However, when the Schottky barrier Φbn is increased, the breakdown voltage in the forward direction between the drain electrode and the source electrode in a state where no voltage is applied to the gate voltage (gate voltage = 0 V) is decreased.
This is because the depletion layer formed at the PN junction between the P-well region where the insulated gate transistor is formed and the semiconductor substrate increases toward the drain electrode as the drain electrode voltage is increased. This is due to the phenomenon of shortening the effective base length.

In an IGBT 999 provided with a Schottky junction, in order to suppress a decrease in breakdown voltage in the forward direction, a parasitic PNP bipolar formed using a drain electrode as a collector, a semiconductor substrate as a base, and a P well region where an insulated gate transistor is formed as an emitter. In the transistor, a method of increasing the impurity concentration of the base and reducing the current amplification factor in the bipolar transistor is conceivable. For example, an N buffer layer (high concentration region) having an impurity concentration higher than that of the n type semiconductor substrate is provided in the contact region between the n type semiconductor substrate (N-drift layer 114) and the drain electrode 130, and the impurity of the N buffer layer By increasing the concentration, the current amplification factor of the parasitic PNP transistor decreases, and the forward breakdown voltage can be increased.
FIG. 3 is a diagram showing the relationship between the impurity concentration Nb of the N buffer layer, the ON voltage Vce (sat), and the forward breakdown voltage when no separation distance is provided between the N buffer layer and the drain electrode. .
As shown in FIG. 3, in the IGBT having a Schottky junction, increasing the N buffer layer in contact with the drain electrode can increase the forward breakdown voltage, but this time the semiconductor substrate is turned on. As a result, the amount of holes is reduced and the on-voltage Vce (sat) is increased.

As described above, in the IGBT having a Schottky junction that does not provide a separation distance between the N buffer layer and the drain electrode, the ON voltage Vce (sat) is reduced and the decrease in the breakdown voltage in the forward direction is suppressed. It is impossible to achieve both.
Therefore, as shown in FIG. 1, in the insulated gate bipolar transistor (IGBT 100) provided with a Schottky junction, a separation distance X is provided between the N + buffer layer 132 and the drain electrode 130, and the ON voltage Vce (sat) is reduced. Achieving both lowering voltage and suppressing reduction in forward withstand voltage.

FIG. 4 is a diagram illustrating the relationship between the separation distance X between the N + buffer layer 132 and the drain electrode 130, the on-voltage Vce (sat), and the forward breakdown voltage.
FIG. 4 shows a width of 0.6 μm and an impurity concentration Nb = 4E + 16 [1 / in an N− drift layer 114 (first conductivity type semiconductor substrate) having a first impurity concentration of 1.4E + 14 [1 / cm3]. The on-voltage Vce (sat) and the forward breakdown voltage when the N + buffer layer 132 of cm3] is disposed at the separation distance X from the drain electrode 130 are shown.
As shown in FIG. 4, under the condition of the N + buffer layer 132 and the like, a shot that does not provide a separation distance between the N buffer layer and the drain electrode by setting the separation from the drain electrode 130 to a separation distance X = 5 μm. Compared to an IGBT having a key junction (conditions in the vicinity of X = 0), while suppressing a decrease in breakdown voltage in the forward direction, the amount of holes injected into the semiconductor substrate at turn-on is almost limited (reducing the amount of holes injected). ) And the on-state voltage Vce (sat) at the time of turn-on is reduced.
The separation distance X is not fixed to 5 μm, and may be, for example, 7 μm as long as it is equal to or higher than the forward breakdown voltage allowed for the IGBT as a product. Further, even if the N + buffer layer is enlarged in the range of the separation distance X shown in FIG. 4, the IGBT shown in the vicinity of the impurity concentration Nb = 0E + 16 [1 / cm 3] located at the left end of FIG. It is possible to ensure a breakdown voltage higher than the breakdown voltage in the forward direction of an IGBT having substantially the same impurity concentration as the IGBT 999 without the provision of 132.
Next, a mechanism for obtaining the effect of lowering the ON voltage Vce (sat) at the time of turn-on will be described.

5 to 7, the N + buffer layer 132 having the impurity concentration Nb is separated from the drain electrode 130 by the separation distance X, and the drain electrode 130 having the carrier concentration when the ON voltage Vce (sat) = 3.15 V is applied. It is a graph which shows the dependence of the distance from.
In FIG. 5, an N + buffer layer 132 having an impurity concentration Nb = 2E + 14 [1 / cm3] is arranged at a position 0.33 μm away from the drain electrode 130 (separation distance X = 0.33 μm), and an ON voltage Vce (sat) = The dependence of the carrier density on the distance from the drain electrode when 3.15 V is applied is shown.
The condition of the impurity concentration Nb of the IGBT indicates the condition of the IGBT (Nb concentration is minimal) shown in the vicinity of the impurity concentration Nb = 0E + 16 [1 / cm3] located at the left end of FIG. The conditions are almost the same as those of the IGBT 999 without the N + buffer layer 132 described above.
In FIG. 5, the hole density (accumulated concentration) of holes (second conductivity type carriers) from the drain electrode 130 to the N + buffer layer 132 is multiplied by the separation distance (0.33 μm). The second conductivity type carrier density) is 2.5E + 11 [1 / cm 2]. The electron density (first conductivity type carrier density) obtained by multiplying the electron concentration (first conductivity type carrier concentration) of the N + buffer layer 132 by the width of the N + buffer layer 132 is 8.2E + 9 [1 / cm 2]. is there.

Specifically, the hole density is divided into a plurality of widths from the drain electrode 130 to the front of the N + buffer layer 132 (up to a position where the concentration change hardly occurs). A value obtained by multiplying the hole concentration in is calculated, and the calculated value is added for all the divided widths. That is, the hole density is a value that can be obtained more accurately by integrating the hole concentration between the drain electrode 130 and the N + buffer layer 132.
Similarly, specifically, the electron density is obtained by dividing the N + buffer layer 132 into a plurality of widths, calculating a value obtained by multiplying each divided width by the electron concentration in the width, and dividing the calculated value. It is the value added for all the widths. That is, the electron density is a value that can be obtained more accurately by integrating the electron concentration between the widths of the N + buffer layers 132.

6, the N + buffer layer 132 having an impurity concentration Nb = 4E + 16 [1 / cm3] is placed from the drain electrode 130 to a position of 0.33 μm (separation distance X = 0.33 μm), similarly to the IGBT shown in FIG. The dependence of the carrier density on the distance from the drain electrode when the on-voltage Vce (sat) = 3.15 V is applied is shown.
This separation distance X = 0.33 μm is a point near the separation distance X = 0 from the IGBT shown by the impurity concentration Nb = 4E + 16 [1 / cm 3] in FIG. 3, that is, from the back surface of the N buffer layer in FIG. The same conditions as those of the IGBT shown in FIG. 5 (conditions where the N + buffer layer is in contact with the drain electrode) are shown.
In FIG. 6, the hole density (first density) is obtained by multiplying the hole density (accumulated density) of holes (second conductivity type carriers) from the drain electrode 130 to the N + buffer layer 132 by the separation distance (0.33 μm). 2 conductivity type carrier density) is 2.0E + 11 [1 / cm2]. The electron density (first conductivity type carrier density) obtained by multiplying the electron concentration (first conductivity type carrier concentration) of the N + buffer layer 132 by the width of the N + buffer layer 132 is 7.1E + 11 [1 / cm 2]. is there. Note that these hole density and electron density are values calculated in the same manner as in FIG. 5 described above.

In FIG. 7, an N + buffer layer 132 having an impurity concentration Nb = 4E + 16 [1 / cm3] is arranged at a position of 5 μm (separation distance X = 5 μm) from the drain electrode 130, and an ON voltage Vce (sat) = 3.15 V. It shows the dependence of the carrier density on the distance from the drain electrode when.
The separation distance X = 5 μm indicates the IGBT 100 in which the N + buffer layer is arranged 5 μm apart from the drain electrode under the same conditions as the impurity concentration Nb in FIG.
In FIG. 7, the hole density (second conductivity) obtained by multiplying the hole concentration (accumulated concentration) of the holes (second conductivity type carriers) from the drain electrode 130 to the N + buffer layer 132 by the separation distance (5 μm). The mold carrier density) is 5.3E + 12 [1 / cm 2]. The electron density (first conductivity type carrier density) obtained by multiplying the electron concentration (first conductivity type carrier concentration) of the N + buffer layer 132 by the width of the N + buffer layer 132 is 7.1E + 11 [1 / cm 2]. is there. Note that these hole density and electron density are values calculated in the same manner as in FIG. 5 described above.

Here, when the on-state voltage Vce (sat) = 3.15 V is applied, as shown in FIG. 7, in the IGBT 100, the left and right hole concentrations are about 6E + 15 [ 1 / cm3], the right side is about 4E + 15 [1 / cm3], which is lower than the left side. That is, the presence of the N + buffer layer 132 suppresses accumulation of holes from the drain electrode 130 on the back surface side to the insulated gate transistor side on the front surface side.
Next, comparing FIG. 6 with FIG. 7, the hole concentration on the surface side (insulated gate transistor side) from the N + buffer layer 132 is about 3.5E + 14 [1 / cm 3] in FIG. In FIG. 7, it is about 4E + 15 [1 / cm3] as described above.
In other words, in contrast to an IGBT having a Schottky junction that does not provide a separation distance between the N buffer layer and the drain electrode, the IGBT 100 in which the N buffer layer is arranged at a distance of 5 μm from the drain electrode suppresses a decrease in forward breakdown voltage. Therefore, it is shown that even if an N buffer layer is provided, the flow of holes toward the insulated gate transistor on the surface side can be increased (almost not limited).

5 and FIG. 7, the hole concentration on the surface side (insulated gate transistor side) from the N + buffer layer 132 is about 4E + 15 [1 / cm 3] in FIG. Thus, about 4E + 15 [1 / cm3], both are substantially the same concentration.
That is, in the IGBT 100 in which the N + buffer layer 132 is arranged apart from the drain electrode 130, even if an N buffer layer is provided in order to suppress a decrease in forward breakdown voltage, it is the same as an IGBT 999 having a Schottky junction without providing an N buffer layer. In addition, it is shown that the turn-on operation can be performed without restricting the flow of holes to the insulated gate transistor side on the surface side.
As described above, in the IGBT 100, the turn-on operation can be performed without almost limiting the amount of hole injection into the semiconductor substrate at the time of turn-on (decreasing the amount of hole injection).

Here, in the IGBT having the separation distance X = 0.33 μm (FIG. 6), the hole density (second conductivity type carrier density) is 2.0E + 11 [1 / cm 2], and the electron density (first conductivity type carrier density) is. 7.1E + 11, and the first conductivity type carrier density is higher than the second conductivity type carrier density.
On the other hand, in the IGBT 100 (FIG. 7) with the separation distance X = 5 μm, the hole density (second conductivity type carrier density) is 5.3E + 12 [1 / cm 2], and the electron density (first conductivity type carrier density) is 7.1E + 11. The second conductivity type carrier density is higher than the first conductivity type carrier density.
From the above results, the reason why the IGBT 100 of the present application can perform the turn-on operation without almost limiting the amount of hole injection into the semiconductor substrate at the time of turn-on (reducing the amount of hole injection) is as follows. is there.
A reference drain-source voltage (3.15 V in FIG. 5) for obtaining a predetermined current value when a forward current is applied to an insulated gate bipolar transistor (IGBT 999 described with reference to FIG. 5) without a buffer layer. Is applied to the IGBT 100 having the buffer layer (N + buffer layer 132). In this state, the second conductivity obtained by multiplying the accumulated concentration of carriers of the second conductivity type from the drain electrode 130 to the buffer layer (hole concentration in the IGBT 100 described with reference to FIG. 7) by the separation distance (5 μm in FIG. 7). The type carrier density (hole density) is higher than the first conductivity type carrier density (electron density) obtained by multiplying the buffer layer width by the first conductivity type carrier concentration (electron concentration) of the buffer layer. The distance from the drain electrode of the N + buffer layer 132, the width of the N + buffer layer 132, and the impurity concentration Nb of the N + buffer layer 132 are set.
As described with reference to FIGS. 5 to 7, this setting hardly restricts the injection of holes into the N-drift layer 114 on the P well region 116 side (decreases the hole injection amount). This is because the conductivity modulation in the N-drift layer 114 on the P well region 116 side can be controlled.

Subsequently, even if the impurity concentration Nb of the N buffer layer is changed, the ON voltage Vce (sat) at turn-on can be lowered by taking the separation distance from the drain electrode 130 by the above setting. Explain what you can do.
FIG. 8 is a graph showing the relationship between the separation distance X of the N + buffer layer 132 from the drain electrode 130 and the carrier density in the IGBT 100. FIG. 9 is a graph showing the relationship between the separation distance X and the forward current value in the IGBT 100.
FIG. 8 shows that when the impurity concentration Nb of the N + buffer layer 132 is 2E + 16, 4E + 16, 6E + 16 [1 / cm3], the separation distance X of the N + buffer layer 132 from the drain electrode 130 is 0.33 μm, 1.33 μm, 3 is a graph plotting the hole density from the drain electrode to the N buffer layer and the electron density of the N buffer layer at 3.33 μm and 5 μm.

Among these, when the distance X of the N + buffer layer 132 from the drain electrode 130 is 0.33 μm and 5 μm under the condition that the impurity concentration Nb of the N + buffer layer 132 is 4E + 16 [1 / cm 3], FIG. 6 and FIG. 7 is a calculation of the hole density from the drain electrode to the N buffer layer and the electron density of the N buffer layer based on the carrier concentration shown in FIG.
In FIG. 8, the separation distance X from the drain electrode 130 of the N + buffer layer 132 is changed as described above even under other conditions of the impurity concentration Nb of the N + buffer layer 132, and is shown in FIGS. 6 and 7. The hole density and electron density calculated based on the concentration corresponding to the carrier concentration are calculated and plotted. Further, “Nb = 4E + 16 wide” shown in FIG. 8 indicates the characteristics of the IGBT 100 in which the width of the N + buffer layer 132 is doubled (1.2 μm) under the condition of the impurity concentration Nb = 4E + 16 [1 / cm 3].

As shown in FIG. 8, the N buffer layer electron density increases as the impurity concentration Nb of the N + buffer layer 132 increases, but is substantially constant with respect to the separation distance X. On the other hand, the hole density from the drain electrode 130 to the N + buffer layer 132 increases as the separation distance X increases in each impurity concentration Nb of the N + buffer layer 132.
Therefore, in each impurity concentration Nb of the N + buffer layer 132 shown in FIG. 8, there is a separation distance X where the hole density from the drain electrode 130 to the N + buffer layer 132 is larger than the N buffer layer electron density.
Further, the hole density from the drain electrode 130 to the N + buffer layer 132 tends to be higher when the impurity concentration Nb of the N + buffer layer 132 is lower at the same separation distance X.
Therefore, the separation distance X at which the hole density from the drain electrode 130 to the N + buffer layer 132 is larger than the N buffer layer electron density is smaller than the higher concentration when the impurity concentration Nb of the N + buffer layer 132 is lower. Become.

FIG. 9 shows the relationship between the distance X from the drain electrode 130 of the N + buffer layer 132 in the IGBT 100 and the forward current value (Ice value). Here, as shown by the point of “Nb = 2E + 14 (very small)” in FIG. 9, the IGBT 999 has an ON voltage Vce of 3.15 V when a forward current of a predetermined current (200 A) flows. The forward current value shown in FIG. 9 is in the forward direction (from the drain electrode 130 to the source electrode 128) when a reference voltage is applied between the source and drain of the IGBT 100 with the on-voltage Vce = 3.15V as a reference voltage. This is the current value that flows (definition of forward current value).
FIG. 9 shows the forward current value Ice when the separation distance X is changed in each of the impurity concentrations Nb of the N + buffer layer 132 when this reference voltage (ON voltage Vce = 3.15 V) is applied to the IGT 100. Is shown.
That is, in FIG. 9, the reference drain-source voltage (3.15 V) for obtaining a predetermined current value (200 A) when a forward current is supplied to an insulated gate bipolar transistor (IGBT 999) having no buffer layer, The value of current flowing through the IGBT 100 in a state where it is applied to an insulated gate bipolar transistor (IGBT 100) having a buffer layer is shown.

  As shown in FIG. 9, in each impurity concentration Nb of the N + buffer layer 132, the current value when the same 3.15 V is applied increases as the separation distance X increases. Therefore, the ON voltage increases as the separation distance X increases. It can be seen that Vce is a small value. That is, the N + buffer layer 132 is arranged at a position separated from the drain electrode as compared with the IGBT having the smallest separation distance X of X = 0.33 (the IGBT in which the N + buffer layer is in contact with the drain electrode). Vce can be set to a small value.

8 and 9, the IGBT 100 is composed of an N− drift layer 114 (first conductivity type semiconductor substrate) having a first impurity concentration of 1.4E + 14 [1 / cm 3], and the on-voltage Vce is set to IGBT 999. In order to obtain a voltage value comparable to the above, it is desirable to increase the hole density (second conductivity type carrier density) by 7 times or more the electron density (first conductivity type carrier density). This is due to the following reason.
In the position of the separation distance X = 5 μm shown in FIG. 8, when the impurity concentration Nb = 2E + 16 [1 / cm3] of the N + buffer layer 132 is obtained, the hole density = 5.6E + 12 [1 / cm2] and the electron density = 3.6E + 11 [ 1 / cm 2], and the hole density is 16 times the electron density.
Further, when the impurity concentration of the N + buffer layer 132 is Nb = 4E + 16 [1 / cm3], the hole density is 5.3E + 12 [1 / cm2], the electron density is 7.1E + 11 [1 / cm2], and the hole density is It becomes 7.5 times the electron density.
Further, when the impurity concentration Nb of the N + buffer layer 132 is 6E + 16 [1 / cm3], the hole density is 4.7E + 12 [1 / cm2], the electron density is 1.1E + 12 [1 / cm2], and the hole density is It becomes 4.5 times the electron density.

Thus, in the IGBT 100, the ratio of the hole density to the electron density changes due to the difference in the impurity concentration Nb of the N + buffer layer 132, as described above, because the hole density from the drain electrode 130 to the N + buffer layer 132 changes. This is because the separation distance X, which is larger than the N buffer layer electron density, is smaller when the impurity concentration Nb of the N + buffer layer 132 is lower than when it is higher.
Therefore, at the impurity concentration Nb = 2E + 16 [1 / cm3], the ratio of the hole density to the electron density is 7 times or more. Therefore, as shown in FIG. 9, the ON voltage Vce is already set at the position where the separation distance X <5 μm. The voltage value is 3.15 V, which is comparable to that of IGBT 999.
In addition, at the impurity concentration Nb = 4E + 16 [1 / cm3], the ratio of the hole density to the electron density of about 7 times is almost 7 times. Therefore, as shown in FIG. The voltage Vce can be set to a voltage value of 3.15 V, which is comparable to the IGBT 999.
Further, in the case of the impurity concentration Nb = 6E + 16 [1 / cm3] and “Nb = 4E + 16 wide”, if the ratio of the hole density to the electron density is 7 times or more, the ON voltage Vce is set at the position where the separation distance X> 5 μm. There is a very high possibility that a voltage value comparable to that of IGBT 999 can be 3.15V.

As described above, the IGBT 100 has a hole density (second conductivity type carrier density) under the conditions of the buffer layer width = 0.6 μm, the impurity concentration Nb = 2E + 16 to 6E + 16 [1 / cm 3], and the separation distance X> 5 μm. Is 7 times or more of the electron density (first conductivity type carrier density), the on-voltage Vce can be set to a voltage value of 3.15 V or less, which is comparable to the IGBT 999.
Note that the IGBT 100 in which the N + buffer layer 132 is disposed apart from the drain electrode 130 has a large forward breakdown voltage as compared with the IGBT 999 (the IGBT indicated by the leftmost point in FIG. 3) without the N + buffer layer. Further, the IGBT 100 in which the N + buffer layer 132 is arranged apart from the drain electrode 130 is an IGBT formed by contacting the N + buffer layer with the drain electrode (the IGBT shown at the right end point in FIG. 3 or the left end point in FIG. 4). In comparison with the above, the forward pressure drop is suppressed.

As described above, in the IGBT of the present invention, the N + buffer layer 132 (buffer layer) is separated from the drain electrode 130.
At this time, for example, the separation distance between the drain electrode 130 and the N + buffer layer 132, the width of the N + buffer layer 132, and the impurity concentration Nb are set as follows.
That is, a reference drain-source voltage (3.15 V in the above example) for obtaining a predetermined current value (for example, 200 A) when a forward current is applied to an insulated gate bipolar transistor (IGBT 999) having no buffer layer is expressed as N + The voltage is applied to the insulated gate bipolar transistor (IGBT 100) having the buffer layer 132. In this state, the hole density (second conductivity type carrier density) obtained by multiplying the hole density (accumulated density) from the drain electrode 130 to the N + buffer layer 132 by the separation distance X is the electron density (N + buffer layer 132) The drain electrode 130 and the N + buffer layer 132 are increased so as to have an electron density (first conductivity type carrier density) obtained by multiplying the carrier concentration) by the width of the N + buffer layer (buffer layer width, 1.2 μm in the above example). , The N + buffer layer 132 width and the impurity concentration Nb are set.

With this setting, in the IGBT 100, the N − on the P well region 116 side is hardly restricted (the hole injection amount is reduced) so that the injection of holes into the N − drift layer 114 on the P well region 116 side is hardly restricted. The conductivity modulation in the drift layer 114 can be controlled.
Therefore, in the IGBT 100, the N + buffer layer 132 suppresses the forward breakdown voltage drop, and almost restricts the amount of holes injected into the N− drift layer 114 (semiconductor substrate) at the time of turn-on (hole injection). The ON voltage Vce (sat) at the time of turn-on can be reduced without reducing the amount.

The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes changes and the like without departing from the gist of the present invention.
For example, in the above-described embodiment, the IGBT in which the N buffer layer is provided in the N − drift layer has been described. However, the polarity may be reversed and an IGBT in which the P + buffer layer is provided in the P − drift layer may be used. In this case, the electron density (second conductivity type carrier density) obtained by multiplying the electron density (accumulated density) from the drain electrode to the P + buffer layer by the separation distance X is equal to the hole density (carrier density) of the P + buffer layer. The separation distance, the width of the P + buffer layer, and the impurity concentration of the P + buffer layer may be set so as to be higher than the hole density (first conductivity type carrier density) multiplied by the width of the buffer layer.

  114 ... N-drift layer, 116 ... P well region, 118 ... N + source region, 120 ... channel forming region, 122 ... gate insulating film, 124 ... gate electrode, 126 ... interlayer insulating film, 128 ... source electrode, 130 ... drain Electrode, 132 ... N + buffer layer, Vce ... on voltage, Ice ... forward current value, Nb ... impurity concentration, X ... separation distance, 100,999 ... IGBT

Claims (3)

A thickness direction of the semiconductor substrate from a source electrode formed on one surface side of the first conductivity type semiconductor substrate having a first impurity concentration toward a drain electrode formed on the other surface side of the semiconductor substrate. An insulated gate bipolar transistor that performs switching of a current flowing through a voltage by a voltage applied to a gate electrode of an insulated gate transistor formed on one surface of the semiconductor substrate,
The second conductivity type carrier is injected into the other surface of the first conductivity type semiconductor substrate when the insulated gate transistor is turned on to cause conductivity modulation by injecting the second conductivity type carrier into the first conductivity type semiconductor substrate. A Schottky junction is formed by the semiconductor substrate and the drain electrode,
A buffer layer having a second impurity concentration of a first conductivity type containing impurities at a higher concentration than the semiconductor substrate is formed at a position spaced from the drain electrode toward the source electrode ;
In a state where a reference drain-source voltage for obtaining a predetermined current value when a forward current is applied to the insulated gate bipolar transistor not having the buffer layer is applied to the insulated gate bipolar transistor having the buffer layer,
The separation distance between the drain electrode and the buffer layer, the width of the buffer layer, and the second impurity concentration,
The second conductivity type carrier density obtained by multiplying the accumulated concentration of the second conductivity type carriers from the drain electrode to the buffer layer by the separation distance is set to the first conductivity type carrier concentration of the buffer layer. to be larger than the first conductivity type carrier density multiplied by the width, insulated gate bipolar transistor, characterized in that you set. The buffer layer width, the second impurity concentration, and the separation distance between the drain electrode and the buffer layer so that the second conductivity type carrier density is 7 times or more higher than the first conductivity type carrier density. The insulated gate bipolar transistor according to claim 1 , wherein: The insulated gate bipolar transistor according to claim 2 , wherein the separation distance is set to 5 μm or more.

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