JPH09321265A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To almost completely prevent crosstalk between photodiodes in a photo IC containing a plurality of photodiodes. SOLUTION: A n<-> -type epitaxial growth layer 2 is formed on a p-type semiconductor substrate 1, and a p<+> -type isolation regions 3 that extend to the p-type semiconductor substrate 1 are formed therein to partition the epitaxial growth layer into a plurality of regions. A p<+> -type buried layer 4 is formed under at least one part of the n<-> -type epitaxial growth layer 2 partitioned by p<+> -type isolation regions 3 in such a manner that the buried layer is connected with the p<+> -type isolation regions 3. Thus a photodiode is constituted of the n<-> -type epitaxial growth layer 2, the p<+> -type buried layer 4, and the p<+> -type isolation regions 3. A n<+> -type semiconductor region 6 is formed under the p<+> -type buried layer 4 in the photodiode in order to absorb floating carriers that cause crosstalk.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device, and is particularly suitable for being applied to a semiconductor device having a photodiode for receiving light.

[0002]

2. Description of the Related Art In recent years, research on integrating a light-receiving photodiode and an electronic element such as a transistor into one chip has been advanced, and has been used for an optical pickup section of an optical disk player such as a compact disk player. Such an optical pickup photodiode generally receives a laser beam reflected by the disk surface and outputs an RF signal, as well as a tracking error signal for causing the laser beam to follow the groove of the disk and a laser beam. It has a function of outputting a focus error signal for focusing the beam on the disk surface. Therefore, this photodiode needs to have a structure divided into a plurality of parts.

An example of a pattern of such a photodiode for an optical pickup is shown in FIG. As shown in FIG.
This photodiode for an optical pickup has six photodiodes a, b, c, d, e and f which are separated from each other. Here, these photodiodes a, b,
The outputs of c, d, e, and f are A, B, C, D, E, and
If it is F, in an optical system using this photodiode, as shown in FIG.
2 and L3 are irradiated, and the RF signal is detected by the output of A + B + C + D, the focus error signal is detected by the output of (A + C)-(B + D), and the tracking error signal is detected by the output of EF.

By the way, when a photodiode and an IC are produced in one chip, the photodiode must be produced by the IC wafer process. For this reason, it is difficult to optimally design the photodiode, and the characteristics of the photodiode itself are inferior to those of the case where a single photodiode is manufactured.

4 to 7 show a conventional photo I for an optical pickup in which a photodiode and an IC are formed in one chip.
A structural example of the photodiode in C is shown together with an npn transistor forming the IC.

In the conventional photo IC shown in FIG. 4,
An n ⁻ type epitaxial growth layer 102 is provided on a p type semiconductor substrate 101. In the p-type semiconductor substrate 101, an n ⁺ -type buried layer 103 is selectively provided with its upper part intruding into the n ⁻ -type epitaxial growth layer 102. In addition, the n ⁻ type epitaxial growth layer 10
2 is selectively provided with ap ⁺ type isolation region 104 having a depth reaching the p type semiconductor substrate 101, whereby an n ⁻ type epitaxial growth layer 102 is provided under each n ⁺ type buried layer 103. Is divided into a plurality of divided areas.

In the light receiving portion of this photo IC, p ⁺
A p-type semiconductor region 105 and an n ⁺ -type semiconductor region 106 are provided in the n ⁻ -type epitaxial growth layer 102a in the part divided by the type isolation region 104. Then, the p-type semiconductor region 105 and the n ⁻ -type epitaxial growth layer 102a and the n ⁺ -type buried layer 103 constitute a photodiode having a pn ⁻ n ⁺ structure. Reference numeral 107 indicates a depletion layer formed at the junction of the photodiode when the photo IC operates. n ⁺
The type semiconductor region 106 is used as a contact layer.

In the integrated circuit portion of this photo IC,
The p-type semiconductor region 108 and the n ⁺ -type semiconductor regions 109 and 110 are formed in the n ⁻ -type epitaxial growth layer 102b in the part divided by the p ⁺ -type isolation region 104.
Is provided. Then, the n ⁺ type semiconductor region 109,
The p-type semiconductor region 108 and the n ⁻ type epitaxial growth layer 102b form an npn transistor. The n ⁺ type semiconductor region 110 is used as a collector contact layer.

[0009] Then, in the conventional photo-IC shown in FIG. 5, n photodiode portion ^- -type n ⁺ -type buried layer 103 on the lower side of the epitaxial growth layer 102a is not provided, also, the n ^- type Epitaxial growth layer 10
The p-type semiconductor region 105 is not provided in 2a. In this case, the n ^-- type epitaxial growth layer 102a and the p-type semiconductor substrate 101 form a photodiode having an n ^-- p structure. Others are the same as the photo IC shown in FIG.

Next, in the conventional photo IC shown in FIG. 6, the n ⁺ type buried layer 103 is not provided below the n ⁻ type epitaxial growth layer 102a in the photodiode section. In addition, the n ⁻ type epitaxial growth layer 102a
Between with p-type semiconductor region 105 is provided connected to the p ⁺ -type isolation region 104 on one side, a p ⁺ -type isolation region 104 of the p-type semiconductor region 105 and the other side in Of the n ⁺ type semiconductor region 10 in the n ⁻ type epitaxial growth layer 102a
6 are provided. In this case, the n ⁻ type epitaxial growth layer 102 a, the p type semiconductor region 105, the p ⁺ type isolation region 104, and the p type semiconductor substrate 101 form a photodiode. Others are the same as the photo IC shown in FIG.

Next, in the conventional photo IC shown in FIG. 7, in the p-type semiconductor substrate 101 of the photodiode portion, it is connected to the p ⁺ -type isolation region 104, and the upper portion thereof is an n ⁻ -type epitaxial growth layer. The p ⁺ -type buried layer 111 is provided in a state where the p ⁺ -type buried layer 111 enters the inside of 102a. In this case, the n ⁻ type epitaxial growth layer 102
The a, the p ⁺ -type buried layer 111, and the p ⁺ -type isolation region 104 form a photodiode having an n ⁻ p ⁺ structure. Reference numeral 112 denotes a lower p ⁺ -type isolation region provided separately from the p ⁺ -type isolation region 104. Others are the same as the photo IC shown in FIG.

By the way, as a photodiode in a photo IC for an optical pickup, high sensitivity and high speed response are strongly demanded from the viewpoint of high-speed reading and improvement of signal-to-noise ratio (S / N ratio). However, according to the conventional photo IC shown in FIG. 4 and the conventional photo IC shown in FIG. 7, the high speed response of the photodiode is good, and in particular, the photo IC shown in FIG. 7 is a conventional photo IC.
Among them, the fastest response is the best, but both have the problem of low sensitivity. In order to improve the sensitivity, it is necessary to increase the width of the depletion layer 107 of the photodiode, which requires increasing the resistivity of the n ⁻ type epitaxial growth layer 102 and increasing the thickness thereof. By doing so, the cut-off frequency f _T of the npn transistor of the integrated circuit portion is lowered, and the depth of the p ⁺ type isolation region 104 is increased, so that the integration degree is lowered. Further, according to the conventional photo IC shown in FIG. 5 and the conventional photo IC shown in FIG. 6, contrary to the conventional photo IC shown in FIG. 4, there is a problem that the sensitivity is high but the high-speed response is poor. . The high-speed response is poor because floating carriers generated outside the depletion layer 107 of the photodiode reach the depletion layer 107 by diffusion and become a signal current.

As described above, it is difficult for the above-described photodiode having the conventional structure to satisfy the requirements for both high sensitivity and high speed response. Therefore, it is necessary to properly use the photodiode structure depending on the required function. There is.

A conventional photo IC for an optical pickup in which the structure of the photodiode is properly used according to the function as described above.
8 shows an example of the structure of the photodiode section in FIG. FIG. 8 schematically shows a cross section taken along line VIII-VIII in FIG. As shown in FIG. 8, in this example, RF that requires high-speed reading is used.
Photodiodes a and b for receiving signals (similar to the photodiodes c and d, although not shown in FIG. 8)
For this, a structure as shown in FIG. 7 is used. On the other hand, the photodiodes e and f for receiving the tracking error signal, which are required to have high sensitivity in order to increase the sensitivity to the deviation of the laser beams L2 and L3, are shown in FIG.
The structure shown in is used. However, as the photodiodes e and f, those having a structure as shown in FIG. 5 may be used.

[0015]

However, in the optical pickup photodiode shown in FIG. 3, an RF signal is obtained by the central laser beam L1 and a tracking error signal is obtained by the left and right laser beams L2, L3. Therefore, the photodiode a,
If the crosstalk between the photodiodes b, c, d and the photodiodes e, f is large, an inconvenience such as an RF signal being mixed with the tracking error signal occurs.

Here, the crosstalk between the photodiodes means that, when a certain photodiode is exposed to light, a signal current is also generated from the adjacent photodiode which is not exposed to the light. The mechanism of this crosstalk generation will be described with reference to FIG. 9 showing a part of FIG.
It is as follows. That is, in FIG. 9, for example, light of a wavelength used for reading a disc in a compact disc player reaches a depth of several tens μm from the surface of the photodiode and is photoelectrically converted, whereby floating carriers (electrons-holes) are generated. Pair) occurs. This floating carrier has a long life and diffuses in each direction. By the way, in the case of a photo IC in which a photodiode and an IC are formed in one chip, a photodiode having a depletion layer 107 extended to a sufficiently deep position cannot be manufactured in order to improve device characteristics and integration. In the photodiode having the structure as shown in FIG. 9, many floating carriers are generated even in the p-type semiconductor substrate 101. That is, in FIG. 9, for example, when the photodiode a is irradiated with the laser beam L1, floating carriers are generated in the p-type semiconductor substrate 101 below the p ⁺ -type buried layer 111, and the floating carriers are generated. As a result of reaching the adjacent photodiode e by diffusion, a signal current is also generated from the photodiode e not irradiated with the laser beam L1.

Therefore, in order to reduce such crosstalk between photodiodes, as shown in FIG.
The applicant of the present invention has proposed a photo IC in which an n-type semiconductor region for absorbing floating carriers is provided between the photodiodes a and e (JP-A-7-183563).
In FIG. 10, reference numeral 113 indicates an n ⁺ semiconductor region used as a contact layer. According to this photo IC, the crosstalk between the photodiodes can be improved to some extent, but in this case as well, some floating carriers reach the adjacent photodiode side, so that the crosstalk can be reduced. It was insufficient.

Therefore, an object of the present invention is to provide a semiconductor device capable of almost completely preventing crosstalk between photodiodes even when a plurality of photodiodes and an integrated circuit are formed in the same chip. It is in.

[0019]

In order to achieve the above object, the present invention provides a semiconductor substrate of a first conductivity type and a plurality of photodiodes provided on the semiconductor substrate of the first conductivity type separately from each other. A second conductivity type semiconductor layer for configuration, and a first conductivity type provided in a lower portion of at least one second conductivity type semiconductor layer and having a higher impurity concentration than that of the first conductivity type semiconductor substrate. The first semiconductor region of the second conductivity type and the first semiconductor region of the second conductivity type in the portion of the semiconductor layer of the second conductivity type in which the first semiconductor region of the first conductivity type is provided. In a semiconductor device in which a photodiode is formed by the first semiconductor region, a second conductivity type second semiconductor region for absorbing floating carriers is provided in a lower portion of the first conductivity type first semiconductor region. It is characterized by being provided .

In the present invention, the semiconductor device is typically a second semiconductor provided with a first semiconductor region of the first conductivity type.
A second conductive type third semiconductor region provided between the conductive type semiconductor layer and at least one other second conductive type semiconductor layer adjacent to the second conductive type semiconductor layer, The second conductive type third semiconductor region and the second conductive type second semiconductor region are electrically and / or structurally connected to each other. Also, typically, a pair of second adjoining ones
The conductive type semiconductor layers are separated from each other by a first conductive type element isolation region. Further, in the present invention, the semiconductor device has, for example, a fourth semiconductor region of the first conductivity type provided in another semiconductor layer of the second conductivity type and connected to the element isolation region on one side thereof. However, in the portion of the other semiconductor layer of the second conductivity type, the second semiconductor layer of the second conductivity type, the fourth semiconductor region of the first conductivity type, the element isolation region, and the semiconductor substrate of the first conductivity type are used for the photo. A diode is constructed.
Here, it is preferable that the second conductivity type second semiconductor region has a higher impurity concentration than the first conductivity type semiconductor substrate and a lower impurity concentration than the first conductivity type first semiconductor region. Is. In the present invention, typically, the first conductivity type is p-type and the second conductivity type is n-type.

In the semiconductor device according to the present invention configured as described above, the photodiode in the portion of the second-conductivity-type semiconductor layer in which the first-conductivity-type first semiconductor region is provided is irradiated with light. The floating carriers generated in the lower part of the first semiconductor region of the first conductivity type sometimes suck the floating carriers provided in the lower part of the first semiconductor region of the first conductivity type. Is absorbed by the second semiconductor region of the second conductivity type for preventing the floating carriers from reaching the photodiodes adjacent to the photodiodes by diffusion, thereby preventing crosstalk between the photodiodes. It can be prevented almost completely.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a photo IC for an optical pickup according to an embodiment of the present invention. The pattern of the photodiode of the light receiving portion of this photo IC is the same as that shown in FIG. 3, but in FIG.
Of the photodiodes a, b, c, d, e, and f shown in FIG. 3, only the photodiode a and the photodiode e are shown together with the npn transistor of the integrated circuit section. The RF signal receiving photodiodes a, b, c, d have the same structure as that shown in FIG. 7, and the tracking error signal receiving photodiodes e, f have the same structure as that shown in FIG. Have.

As shown in FIG. 1, in the photo IC according to this embodiment, a p-type substrate such as a p-type Si substrate is used.
Type semiconductor substrate 1 on the example n ^- type Si n such as epitaxial growth layer ^- -type epitaxial layer 2 is provided. In the n ⁻ type epitaxial growth layer 2, p
The p ⁺ type isolation region 3 having a depth reaching the type semiconductor substrate 1 is selectively provided, whereby the n ⁻ type epitaxial growth layer 2 is divided into a plurality of regions.

In the portion of the photodiode a of the light receiving portion of this photo IC, it is connected to the p ⁺ type isolation region 3 in the p type semiconductor substrate 1, and the upper part thereof is in the n ⁻ type epitaxial growth layer 2. P while in
^{A +} type buried layer 4 is selectively provided. And
The n ⁻ type epitaxial growth layer 2 and the p ⁺ type isolation region 3 and the p ⁺ type buried layer 4 constitute a photodiode a having an n ⁻ p ⁺ structure. Reference numeral 5 indicates an n ⁺ type semiconductor region used as a contact layer.

In this case, the n ⁺ type semiconductor region 6 is provided over the lower portion of the p ⁺ type buried layer 4 and one side wall thereof. As will be described later in detail, the n ⁺ type semiconductor region 6 is for absorbing and removing floating carriers generated when the photodiode a is irradiated with light. The impurity concentration of n ⁺ type semiconductor region 6 is preferably set higher than that of p type semiconductor substrate 1 and lower than that of p ⁺ type buried layer 4.

Here, the p ⁺ type buried layer 4 is, for example,
It may be formed at the same time in the same step as the step of forming the lower p ⁺ type isolation region 10 used in the integrated circuit section described later, or may be formed in the vertical pnp transistor used in the integrated circuit section ( It may be formed at the same time as the step of forming the p ⁺ -type buried layer of the collector (not shown). By doing so, the manufacturing process of the photo IC can be simplified. It goes without saying that only the p ⁺ type buried layer 4 may be separately formed. The impurity concentration of the p ⁺ -type buried layer 4 may be at least higher than the impurity concentration of the p-type semiconductor substrate 1. However, as described above, the p ⁺ -type buried layer 4 is formed in the p ⁺ -type isolation region 10 or the vertical pnp. When it is formed at the same time as the p ⁺ type buried layer of the transistor, it is, for example, 10 ¹⁷ to 10 ²⁰ cm ⁻³ .

The n ⁺ type semiconductor region 6 is formed simultaneously in the same step as the step of forming the n ⁺ type buried layer 12 of the collector of an npn transistor used in an integrated circuit section described later, for example. Or it may be formed at the same time in the same step as the step of forming a so-called n ⁺ type pocket layer provided in the vertical pnp transistor used in the integrated circuit section,
By doing so, it is possible to suppress an increase in the number of photo IC manufacturing steps due to the additional provision of the n ⁺ type semiconductor region 6. It goes without saying that only this n ⁺ type semiconductor region 6 may be separately formed.

In the photodiode e portion of the light receiving portion of this photo IC, the n ^-- type epitaxial growth layer 2 is formed.
A p-type semiconductor region 7 is provided therein so as to be connected to the p ⁺ -type isolation region 3 on one side thereof, and
An n ⁺ type semiconductor region 8 is provided in the n ⁻ type epitaxial growth layer 2 in a portion between the p type semiconductor region 7 and the p ⁺ type isolation region 3 on the other side. In this case, the p-type semiconductor region 5, the p ⁺ -type isolation region 3, the p-type semiconductor substrate 1 and the n ⁻ -type epitaxial growth layer 2 form a photodiode. n
^{The +} type semiconductor region 8 is used as a contact layer.

In the light receiving portion of this photo IC, a floating carrier absorption region is further formed by the n ⁻ type epitaxial growth layer 2 provided via the p ⁺ type isolation region 3 in the portion between the photodiode a and the photodiode e. Is configured. The n ⁻ type epitaxial growth layer 2 forming the floating carrier suction region is electrically and structurally connected to the n ⁺ type semiconductor region 6 forming the floating carrier suction region provided in the photodiode a. In addition, in the n ⁻ type epitaxial growth layer 2 forming the floating carrier suction region, an n ⁺ type semiconductor region 9 used as a contact layer is formed.
Is provided.

Reference numeral 10 indicates a depletion layer formed at the junction of the photodiodes a and e during the operation of this photo IC.

In the integrated circuit section of this photo IC,
An n ⁺ type buried layer 11 is selectively provided in the p type semiconductor substrate 1 in a portion surrounded by the p ⁺ type isolation region 3 with its upper part intruding into the n ⁻ type epitaxial growth layer 2. . Also, n ^{− of} this part
In the epitaxial growth layer 2, a p-type semiconductor region 12 is formed.
And n ⁺ type semiconductor regions 13 and 14 are provided.
The n ⁺ type semiconductor region 13, the p type semiconductor region 12 and the n ⁻ type epitaxial growth layer 2 form an npn transistor. The n ⁺ type semiconductor region 14 is used as a collector contact layer. Further, reference numeral 15 denotes a lower p ⁺ type isolation region provided separately from the p ⁺ type isolation region 3.

In the photo IC according to this embodiment, since the n ⁺ type semiconductor region 6 is provided below the p ⁺ type buried layer 4, crosstalk between the photodiodes a and e is prevented. It can be prevented almost completely. That is, in FIG. 2 showing a part of FIG. 1, it is assumed that the photodiode a is irradiated with the laser beam L1 and floating carriers are generated in the lower portion of the p ⁺ type semiconductor region 4. At this time, of the floating carriers, the n ⁺ type semiconductor region 6 provided in the lower part of the p ⁺ type buried layer 4 is provided.
What has been generated therein has a high impurity concentration and has a short carrier lifetime in the n ⁺ type semiconductor region 6, so that it recombines and disappears immediately after generation. Further, the floating carriers that have reached the n ⁺ type semiconductor region 6 by diffusion are also disappeared by recombination. Moreover, among the floating carriers generated in the lower portion of the n ⁺ -type semiconductor region 6, to diffuse into the n ⁺ type semiconductor region 6 are those that do not disappear by recombination, the lower side of the n ⁺ -type semiconductor region 6 Since it is captured by the depletion layer 11 in a part, it does not reach the photodiode e side. The floating carriers trapped in the depletion layer 11 are finally sucked into the n ⁻ type epitaxial growth layer 2 which is connected to the n ⁺ type semiconductor region 6 and constitutes the floating carrier suction region.

[0033] Is this way, stray carriers generated in the lower portion of the p ⁺ -type semiconductor region 4 in the portion of the photodiode a, disappear by recombination in the n ⁺ type semiconductor region 6, n ⁺ -type semiconductor Depletion layer 10 below the region 6
Therefore, it is prevented from reaching the photodiode e side. As a result, the photodiodes a and e
Crosstalk between them can be almost completely prevented, and inconveniences such as the RF signal being mixed in the tracking error signal can be prevented.

Although the embodiment of the present invention has been specifically described above, the present invention is not limited to the above-described embodiment, and various modifications based on the technical idea of the present invention are possible. .

[0035]

As described above, according to the semiconductor device of the present invention, the second conductivity type second semiconductor layer for sucking floating carriers into the lower portion of the first conductivity type first semiconductor region is obtained. By providing the semiconductor region, even when a plurality of photodiodes and an integrated circuit are formed in the same chip, crosstalk between the photodiodes can be almost completely prevented.

[Brief description of drawings]

FIG. 1 is a sectional view showing a photo IC according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view for explaining the reason why crosstalk between adjacent photodiodes is prevented in the photo IC according to the embodiment of the present invention.

FIG. 3 is a plan view showing a pattern of a conventional photodiode for an optical pickup.

FIG. 4 is a cross-sectional view showing a main part of a first conventional photo IC.

FIG. 5 is a cross-sectional view showing the main parts of a second conventional photo IC.

FIG. 6 is a cross-sectional view showing the main parts of a third conventional photo IC.

FIG. 7 is a cross-sectional view showing a main part of a fourth conventional photo IC.

FIG. 8 is a cross-sectional view showing a structural example of a conventional optical pickup photodiode.

9 is a cross-sectional view for explaining a problem of crosstalk occurring between adjacent photodiodes in the conventional photodiode for an optical pickup shown in FIG.

10 is a cross-sectional view showing a structural example of a photodiode proposed to reduce crosstalk between adjacent photodiodes that occurs in the conventional photodiode for an optical pickup shown in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... p-type semiconductor substrate, 2 ... n ^- type epitaxial growth layer, 3,15 ... p ⁺ type isolation region, 4 ... p ⁺ type buried layer, 6 ... n ⁺ type semiconductor Region, 7 ... p-type semiconductor region, 10 ... depletion layer, 1
1 ... n ⁺ type buried layer

Claims (6)

[Claims] 1. A first-conductivity-type semiconductor substrate, and a plurality of second-conductivity-type semiconductor layers for photodiode configuration, which are provided on the first-conductivity-type semiconductor substrate and are separated from each other. A first conductive type first semiconductor region provided in a lower portion of the second conductive type semiconductor layer and having a higher impurity concentration than that of the first conductive type semiconductor substrate; In the portion of the second conductivity type semiconductor layer in which the first conductivity type first semiconductor region is provided, a photodiode is formed by the second conductivity type semiconductor layer and the first conductivity type first semiconductor region. In the configured semiconductor device, a second conductive type second semiconductor region for absorbing floating carriers is provided in a lower portion of the first conductive type first semiconductor region. Semiconductor device. 2. A semiconductor layer of the second conductivity type provided with the first semiconductor region of the first conductivity type, and at least one other second conductivity type adjacent to the semiconductor layer of the second conductivity type. A third semiconductor region of the second conductivity type provided between the third semiconductor region of the second conductivity type and the second semiconductor region of the second conductivity type. Target and /
Alternatively, the semiconductor device according to claim 1, wherein the semiconductor devices are structurally connected to each other. 3. A second semiconductor region of the second conductivity type has a higher impurity concentration than a semiconductor substrate of the first conductivity type,
The semiconductor device according to claim 1, wherein the impurity concentration is lower than that of the first semiconductor region of the first conductivity type. 4. The semiconductor device according to claim 1, wherein a pair of the second conductive type semiconductor layers adjacent to each other are separated from each other by a first conductive type element isolation region. 5. A fourth semiconductor region of the first conductivity type is provided in the other semiconductor layer of the second conductivity type and connected to the element isolation region on one side thereof, and the fourth semiconductor region of the first conductivity type is provided. In the portion of the second conductivity type semiconductor layer, the second conductivity type semiconductor layer, the first conductivity type fourth semiconductor region, the element isolation region, and the first conductivity type semiconductor substrate are used to form a photo. The semiconductor device according to claim 4, wherein a diode is formed. 6. The first conductivity type is p-type, and the second conductivity type is p-type.
The semiconductor device according to claim 1, wherein the conductivity type is n-type.