JP3245793B2 - Manufacturing method of thermoelectric conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates, regarding the method for manufacturing a thermoelectric conversion element.

[0002]

2. Description of the Related Art A thermoelectric conversion element is a thermoelectric power generation element that directly converts heat into electricity using a temperature difference, and conversely, an electronic cooling that cools the element by applying a temperature difference to both sides of the element by flowing electricity. There are two types of elements. Generally, the effect of generating electricity when a temperature difference is applied to both ends of a semiconductor device is called the Seebeck effect, and the effect of generating a temperature difference at both ends of the device when electricity flows through the semiconductor is called the Peltier effect.

[0003] At present, thermoelectric generators are used for energy saving technologies such as recovery of waste heat from factories and automobiles, and small generators using heating elements, or alternative energy sources, against the background of global environmental problems and energy resource problems. Is attracting attention.
In addition, the electronic cooling element has attracted attention as a cooling technology that does not use a refrigerant such as Freon, and is used for cooling and temperature adjustment of electronic components, a dehumidifier, a small refrigerator, and the like.

For example, thermoelectric conversion in the case of electronic cooling is represented by P
This is performed by passing DC electricity through a circuit formed by joining a mold and an N-type semiconductor with electrodes, whereby heat is absorbed on one side of the joint and heat is generated on the other side.
This phenomenon is reversed when the direction of electricity flowing through the circuit is changed.

A thermoelectric conversion element uses a plurality of pairs of P-type and N-type semiconductors, which are electrically arranged in series and thermally arranged in parallel.

[0006]

Conventional methods for manufacturing a thermoelectric conversion element include various crystal growth methods, hot-press sintering methods such as a normal pressure powder sintering method using a cold press, and a hot press method. There is. In addition, there are a thick film method, a thin film method, and the like. However, these methods are incomplete in terms of technology, and there are few examples currently used.

[0007] Generally, the crystal growth method provides better characteristics than the powder sintering method using a cold press or the hot pressing method. However, the crystal growth method has disadvantages in that it requires a long time for production, has high cleavage costs due to low mechanical strength due to cleavage, and has a low yield of devices.

On the other hand, the powder sintering method by cold pressing is a method in which the raw material powder is put into a mold and press-molded, and then heat-treated. The mechanical strength of the element is increased and the yield is improved. The characteristics of the device are inferior to those of the crystal growth method. Moreover, in order to perform the pressing, the powdered raw material must be re-granulated to an appropriate size or the fine powder must be removed, and the raw material yield is poor and productivity cannot be said to be excellent. Is the current situation. Furthermore, oxides, adsorbed gas, and the like mixed in the raw materials degrade the properties of the thermoelectric conversion element, so that it is necessary to remove them, and the management of the raw materials is important.

The hot press method is the production method closest to the plasma activated sintering method. However, since the raw material powder is directly pressed and simultaneously heated to be sintered, a very high pressure is required to increase the sintering density. -High temperature conditions are required, and a long sintering time is required. In addition, it is necessary to appropriately control the purity of the raw material powder.

[0010] Thermoelectric conversion elements have attracted attention from the viewpoint of environmental problems and energy saving. However, in order to spread thermoelectric conversion elements, it is necessary to solve the cost problem caused by the current low-productivity manufacturing method.

Conventionally, in the case of a thermoelectric conversion element used for thermoelectric power generation, a material used at a relatively low temperature of 300 ° C. or lower, such as a bismuth telluride compound or an anti-montelul compound, or a PbTe or GeTe type compound. Materials used at temperatures up to around 600 ° C, such as SiGe and iron silicide
Materials that can be used even around 1000 ° C are roughly classified into three categories. However, when these materials are put to practical use in thermoelectric power generation, the target materials are limited in view of power generation efficiency and cost.

The efficiency η of thermoelectric power generation is expressed by the following equation.

Η = η _o / [2 + η _o · {4 / (Z · ΔT) -0.5}] Here, η _o is the Carnot efficiency and is determined by the temperature condition used.

In this equation, the only way to increase the power generation efficiency η is to increase Z and ΔT. Z is a figure of merit and indicates the capability of the thermoelectric conversion material, and is a coefficient determined by the material. ΔT is the temperature difference between the heating side and the heat radiation side, and the greater the heat resistance of the material, the greater the ΔT.

BiTe, SbTe, and their compound systems have a very high figure of merit Z near normal temperature as compared with other materials, and Z = about 2.0 to 2.6 × 10 ⁻³ / K.
However, since the melting point of the material is around 600 ° C. and the characteristics are sharply lowered in a high temperature range, ΔT cannot be so large (ΔT =＝ 300 ° C.), and the power generation efficiency η is about 5-6%.

The iron silicide-based material has a heat resistance having a melting point around 1200 ° C., so that a very large ΔT can be obtained (ΔT = 〜
1000 ° C.), but the figure of merit Z is as small as 1/5 or less of that of the BiTe system, and the conversion efficiency η is as very small as several%.

On the other hand, a lead tellurium-based material has a melting point of 920 ° C.
The operating temperature range is around ~ 600 ° C, so the temperature difference can be 500 ° C or more. Also, the figure of merit Z is 1.5 ×× 10 ⁻³ /
Above K, the power generation efficiency can exceed 10%.

From these viewpoints, lead tellurium-based materials are the most practical thermoelectric power generation materials, but some practical problems must be solved for practical use. One is bonding with an electrode that has heat resistance (withstands use at 600 ° C or more), and the other is sufficient strength under conditions where the temperature cycle of heating and cooling is repeated at a temperature difference of about 500 ° C.・
This is the development of an element or unit structure having heat resistance.

Since the bismuth telluride-based material is used in a relatively low temperature range, the joining method is to use a normal solder or a heat-resistant solder, for example, a lead-based (melting point of 327 ° C.) or zinc-based (melting point of 419 ° C.) metal. It can be mounted relatively easily by using a method such as plating, vapor deposition, sputtering, etc. to reduce the device performance due to diffusion of the bonding material into the device (or diffusion in the opposite direction). Diffusion could be stopped by forming a metal barrier layer of nickel or the like.

In the case of FeSi ₂ (iron silicide), the melting point is high.
A high-temperature brazing material (50 to 850 ° C.) is used, or a method in which a P-type and an N-type are directly bonded and no electrode is used, as seen in many documents.

However, in the PbTe system, the operating temperature range is around 600 ° C. and the melting point of the material is 9%.
Because it is about 00 ° C, normal high-temperature solder cannot be used,
In addition, since the working temperature of the brazing material for high temperature is close to the melting point of lead tellurium, the brazing material is easily diffused into lead tellurium, and the film formed by ordinary plating or sputtering prevents diffusion at the working temperature. It is difficult. In addition, when rigidly hardened against a large temperature difference and a thermal stress caused by a temperature cycle, element destruction is likely to occur.

The cause is considered to be a stress caused by a difference in expansion coefficient between the substrate, the electrode and the element and a stress caused by a change in temperature difference. It can be considered that the same problem occurs when power generation is attempted with a large temperature difference, and the same problem occurs in the case of FeSi ₂ . For this purpose, a method of mechanically pressing a thermoelectric power generation element against an electrode with a spring such as a spring has been considered, but the apparatus is large and is not suitable for practical use.

A first object of the present invention is to provide a thermoelectric conversion element which can increase the productivity of a thermoelectric conversion element, improve the yield, reduce the production cost, and improve the characteristics of the element as much as the crystal growth method. An object of the present invention is to provide a manufacturing method and a thermoelectric conversion element manufactured by the manufacturing method.

A second object of the present invention is to provide a method for manufacturing a thermoelectric conversion element, which can improve the mechanical strength and the thermal strength of the thermoelectric conversion element to prevent the element from being destroyed, and a thermoelectric conversion element manufactured by the manufacturing method. It is to provide an element.

[0025]

According to the present invention, there is provided a means for solving], the first object, the semiconductor powder for thermoelectric conversion element was loaded into sintering jig in which is provided in the chamber, predetermined its powder powder pressed with a pressure pressurized by passing a pulsed current to said powder is accomplished by forming a semiconductor layer by sintering a semiconductor powder to each other while leaving the adsorbed gas adsorbed to the semiconductor powder.

According to the present invention, the second object is to provide a semiconductor device.
Body powder and metal material for electrodes, such as metal powder and metal plate
Is achieved by sintering together by arranging them in layers.
You .

The plasma activated sintering method is a type of powder sintering method generally called plasma activated sintering (PAS) or spark plasma sintering (SPS).

[0028]

According to the first manufacturing method of the present invention and the thermoelectric conversion element manufactured by the manufacturing method, a semiconductor for the thermoelectric conversion element is formed by using a plasma activated sintering method using plasma discharge as a heating method. Because it is manufactured, the raw material powder can be sintered in a short time at low pressure and low temperature, and by controlling the pressure and temperature at the time of molding, the crystal growth method can be performed using the conventional raw material powder or a lower-purity raw material. The same characteristics can be obtained, and the oxides on the surface of the raw material powder are removed by the plasma and the adsorbed gas is desorbed, so that the management of the raw material can be simplified, thereby increasing the productivity of the thermoelectric conversion element. Together with improving yield, reducing production costs, and
The characteristics of the device can be improved to the level of the crystal growth method.

According to the second manufacturing method of the present invention and the lead tellurium-based thermoelectric conversion element manufactured by the manufacturing method, the semiconductor powder and the metal powder for the electrode are arranged in layers, and the plasma activated sintering is performed. The electrodes can be bonded to the semiconductor in the temperature range of sintering conditions (650 ° C or higher) because they are integrally molded using the method. Therefore, electrode bonding with heat resistance can be performed. A porous portion can be formed between the substrate, the electrode, and the element, so that the stress caused by the difference in expansion coefficient between the substrate, the electrode, and the element and the stress caused by the temperature difference can be absorbed by the porous portion. Since the semiconductor powder enters the obstacle finely, the bonding strength between the semiconductor and the electrode can be increased, and as a result, the mechanical strength and the thermal strength of the thermoelectric conversion element can be improved and the element can be prevented from being broken.

In the plasma activated sintering method, a voltage is directly applied to the raw material powder to be molded to generate a discharge plasma between the powder particles, thereby activating the particle surface to remove an oxide layer or an adsorbed gas adhering to the surface. It is a method of removing and promoting sintering at a low pressure in a short time.

This plasma activated sintering method has been conventionally used for sintering metals, joining between different metals, or joining magnetic materials or ceramics. However, ordinary semiconductors requiring control of a trace amount of impurities are required. Sintering methods such as plasma sintering were not considered to be applicable to the production of
No examples have been used in the manufacture of semiconductors.

However, the thermoelectric conversion element is a semiconductor having a very high impurity concentration among semiconductors, and has a low sensitivity to impurities as compared with a general semiconductor and is easy to manage.
It was considered that the plasma activated sintering method could be applied.

[0033]

Focusing on short-time sintering at low pressure and low temperature, which is an advantage of the plasma activated sintering method, various moldings were performed using raw material powders of thermoelectric semiconductors, and the pressure and temperature during molding were controlled. It has been found that the same properties as those obtained by the crystal growth method can be obtained using the conventional raw material powder.

With respect to productivity, one general plasma sintering apparatus was able to obtain about 5 to 10 times the productivity of a crystal or powder pressing apparatus of the same price.

Another feature is that the raw material need not be strictly controlled due to effects such as removal of oxides from the raw material by plasma and desorption of adsorbed gas.

FIG. 1 shows a schematic configuration of the plasma sintering apparatus.

In the chamber 1, a sintering powder 3 as a raw material is put into a sintering jig 2, and the
And a lower punch 5, and a sintering power supply 9 is applied to the upper punch electrode 7 connected to the upper punch 4 and the lower punch electrode 8 connected to the lower punch 5 while applying a predetermined pressure from both of them. , A plasma current is generated between the particles of the powder 3 to perform sintering.

It is also possible to place a metal electrode, for example, a metal plate or a metal powder, on the sintering powder 3 that has been temporarily pressed and put it into the sintering jig 2 to integrally mold the electrode simultaneously with sintering of the semiconductor. It is.

The pressurizing mechanism 6 and the sintering power supply 9 are connected to a control device 10 for controlling the pressure and pulse current applied to the powder 3, and the control device 10 includes a position measuring mechanism 11, an atmosphere control mechanism 12, It is connected to a water cooling mechanism 13, a temperature measuring device 14, and the like. This plasma activated sintering method can be used not only for sintering a semiconductor and integrally molding a semiconductor and an electrode, but also for bonding a semiconductor to a conductor and bonding a semiconductor to an insulator.

As an example of thermoelectric conversion in a thermoelectric conversion element, the principle of thermoelectric conversion in the case of electronic cooling will be described with reference to FIG.

One end of the P-type semiconductor 15 and one end of the N-type semiconductor 16 are joined by an electrode 17, and the other end of the P-type semiconductor 15 is connected to the minus side of the power supply 19 via the electrode 18, and the other end of the N-type semiconductor 16 is The electrode
A circuit 21 is formed by connecting to the positive side of the power supply 19 via 20 and when a direct current is supplied from the power supply 19 to the circuit 21, heat is absorbed at the junction with the electrode 17 and the junction with the electrode 18 and the electrode 20 is formed. Generates heat.

The amount of heat Qab that can be absorbed when a direct current is supplied to the circuit 21 is calculated from the heat absorption Qp due to the Peltier effect as shown in the following equation. Heat returning to the side 1 / 2I ²・
R is the amount obtained by subtracting the amount of heat Qc that returns from the high-temperature side to the low-temperature side due to heat conduction due to the temperature difference between both ends of the thermoelectric conversion element.

[0043] ^{Qab = Qp -1 / 2I 2 ·} R-Qc Incidentally, when the direction of the DC current flowing through the circuit 21 to the opposite, heat absorption and release phenomenon described above is reversed.

As shown in FIG. 3, the thermoelectric conversion element uses a plurality of pairs of a P-type semiconductor 15 and an N-type semiconductor 16 arranged electrically in series and thermally in parallel.

The following is an example of the production of a thermoelectric conversion element using the plasma activated sintering method. Here, as a numerical value indicating the characteristic of the thermoelectric conversion element, the following figure of merit Z (figure) is used.
of merit).

[0046] As a material of the thermoelectric conversion element, a bismuth tellurium-based material (Bi · Sb) ₂ (Te · Se) ₃ was used, and this raw material powder was sintered by a sintering jig of a plasma sintering apparatus (Sumitomo Coal Industry Dr. Sinter). And fixed by temporary pressing, followed by plasma sintering. The ingot produced under different pressure and temperature conditions during sintering was cut and its properties were measured.

Table 1 shows the pressure and temperature conditions during sintering and the characteristic values of the thermoelectric conversion element.

Table 1 (Bi · Sb) 2 (Te · Se) 3 Sintering pressure Sintering temperature Figure of merit (kg / cm ² ) (° C) Z (10 ^-3 ) Plasma sintering method 1 250 300 0.7 2 300 300 1.3 3 350 300 2.14 400 300 300 2.05 500 300 1.8 1.8 300 300 250 0.67 300 350 2.2 8 300 400 1.9 9 500 400 400 0.8 Crystal growth Method (conventional method) 2.2 Powder pressing method (conventional method) 1.6.

Thus, it is understood that the sintering conditions for obtaining the predetermined characteristic values include an appropriate pressure and an appropriate temperature. These are also affected by the raw material composition, raw material shape, and the like. If the temperature and pressure are insufficient, the mechanical strength of the ingot also decreases, and problems such as delamination occur. Incidentally, the yield of the thermoelectric conversion elements from the ingot obtained in Sample 7 exceeds about 95%.

The size of the ingot that can be manufactured by the current plasma sintering apparatus is 35 mm in diameter × 30 mm in thickness or more.
One ingot can be manufactured in 10 minutes. In the case of the crystal growth method, a diameter of 35 mm × thickness of 200 mm requires about 40 to 50 hours. Therefore, the plasma activated sintering method has a productivity 40 times smaller than that of the crystal growth method.

Moreover, it is considered that this manufacturing method can be applied to sintering of all thermoelectric conversion element materials, not limited to bismuth tellurium-based materials.

Further, the present inventor tried sintering experiments of various thermoelectric materials by the plasma sintering method, and as a result, obtained a P-type or N-type powder of thermoelectric material, or a mixture of P-type and N-type powder. When simultaneously filling and sintering an electrically insulating oxide powder, such as glass powder, which separates, simultaneously sintering a metal material, such as metal powder, which becomes an electrode of a thermoelectric conversion element, has excellent heat resistance and very high strength In addition, the present inventors have found that a thermoelectric conversion element resistant to thermal stress can be obtained.

In this method, a predetermined amount of metal powder to be an electrode is first placed on a jig for plasma sintering, and P
Type or N-type powder, or P-type and N-type powders are simultaneously filled with a predetermined amount with glass or other electrically insulating powder sandwiched between them. And perform plasma sintering.

Here, the metal material used as the electrode has heat resistance, has a coefficient of thermal expansion similar to that of the thermoelectric conversion element, does not easily react with the thermoelectric conversion element at a high temperature, and may have a reaction product. The material must be stable under the conditions of use of thermoelectric generation. In addition, it must be a material that does not hinder the electrical characteristics of the thermoelectric conversion element when it is joined to the thermoelectric material.

In general, in the case of a thermoelectric conversion element used at room temperature, metals used as electrodes include copper, nickel, aluminum and the like. Metals are targeted. For example, iron, nickel,
Titanium, stainless steel, molybdenum, tungsten, or alloys of these metals containing trace amounts of lead and tellurium as components.

As a result of the examination, in the case of a lead tellurium-based thermoelectric material,
When simultaneously sintering metal powder to be an electrode, materials that can be sintered within the upper limit of sintering temperature (up to 750 ° C) of lead tellurium powder have melting points of iron, nickel, titanium, etc. of 1400 to 17
Materials near 00 ° C are suitable.

In the case of nickel, simultaneous sintering with lead tellurium forms a nickel / tellurium intermetallic compound, resulting in very good bonding. However, rapid sintering at 650 ° C. or higher results in a rapid reaction. Progresses and the joint melts,
If the joined body at about 600 ° C is kept at about 500 ° C for a long time, the joints will swell, making it unsuitable for use at 500 ° C or higher.

Titanium forms a titanium / tellurium compound, which is very stable;
No change was observed in the joint even with time.

Iron hardly reacts with lead tellurium,
The bonding strength is extremely weak with sheet iron. However, when iron powder is sintered at a sintering density of 70 to 98%, a porous iron electrode is formed, and lead tellurium penetrates finely into the iron barrier, increasing the bonding strength between the element and the electrode. Also, this porous portion effectively acts to absorb thermal stress caused by a difference in expansion coefficient.

When such different materials having different melting points are simultaneously sintered and joined, in plasma sintering, the sintering is easily performed with a temperature gradient by changing the material of the punch or the sintering material to different sizes. And the unique feature that the sintering density can be easily controlled with good reproducibility. Therefore, when sintering metal materials having different melting points simultaneously with sintering of thermoelectric materials, it is absolutely necessary to use plasma sintering.

An example of manufacturing a thermoelectric conversion element integrated with electrodes using plasma sintering will be described.

As shown in FIG. 4, with the lower punch 5 installed on the sintering jig 2, a metal material to be an electrode, for example, iron powder 22 (325 mesh electrolytic iron) in this case is fixedly placed. Then, a predetermined amount of lead tellurium powder 23 is filled thereon and flattened.

Further, iron powder 22 is put on it again. Then, it is mounted on the plasma sintering apparatus of FIG. 1 and sintered in a vacuum at a sintering temperature of 500 to 800 ° C., whereby an ingot of a thermoelectric conversion element having metal electrodes joined on both sides can be produced.

For use as a module, this element may be cut, and the substrate may be brazed with a high-temperature brazing material, for example, silver brazing, and used.

Another example of manufacturing a thermoelectric conversion element with an electrode using plasma sintering will be described.

As shown in FIG. 5, the jig 2 for plasma sintering is used.
Then, the metal powder 25 is filled in the same manner as above. Further, P-type and N-type lead tellurium powders 26 and 27 are filled with an insulating and low thermal conductive oxide powder 28 interposed between the powders 26 and 27, and further filled thereon. The metal powder 25 is placed and plasma sintering is performed.

The melting point of the insulating powder 28 is desirably 700 ° C. or more, and for example, a glass-based powder, or a mixed powder of alumina-silica-based, alumina-silica-magnesia-based powder, or the like is preferable.

The thickness of the insulating layer formed by sintering the insulating powder 28 is desirably 0.5 mm or less in consideration of the flow of heat. However, the insulating layer has a high melting point and can maintain a porous state under PbTe sintering conditions. There is no problem if the thickness is 1 mm or more.

By sintering the insulating powder 28 integrally, as shown in FIG. 6, an ingot 29 in which P-type and N-type lead tellurium are sintered and bonded to the same electrode is completed.

By slicing the ingot 29 to form a pair of P / N thermoelectric generators 30 and mounting only the low-temperature side of the thermoelectric generator 30 as shown in FIG. 7, the metal plate 25 becomes an electrode on one side. A bare module 31 can be created.

Similarly, by sintering a plurality of basic structures consisting of P-type and N-type powders and insulating powders,
It is also possible to obtain many element pairs at a time.

At the time of mounting, for example, an inorganic adhesive having high thermal conductivity is applied to the heat exchange portion on the high temperature side, and the electrode surfaces are bonded and used, or the module is bonded to an electrically insulating substrate by a similar method. Use as

[0073]

According to the first manufacturing method of the present invention and the thermoelectric conversion element manufactured by the manufacturing method, since the semiconductor for the thermoelectric conversion element is manufactured using the plasma activated sintering method, the raw material powder is used. Can be sintered in a short time at low pressure and low temperature, and by controlling the pressure and temperature during molding, it is possible to obtain the same characteristics as the crystal growth method using the conventional raw material powder,
In addition, the oxides in the raw material are removed by the plasma and the adsorbed gas is desorbed, so that the management of the raw material can be simplified, and as a result, the productivity of the thermoelectric conversion element is increased, the yield is improved, and the production cost is reduced. At the same time, the characteristics of the device can be improved to the same level as the crystal growth method, and therefore, the market for thermoelectric conversion devices that have been prevented from spreading because of high production costs can be expanded,
Can play a part in solving environmental problems.

According to the second manufacturing method of the present invention and the thermoelectric conversion element manufactured by the manufacturing method, the semiconductor powder and the metal material for electrodes having different melting points are integrated by using the plasma activated sintering method. Because of the molding, a porous portion can be formed between the semiconductor and the electrode, and the stress caused by the difference in expansion coefficient between the substrate, the electrode, and the element and the stress due to the temperature difference are absorbed by the porous portion. In addition, since the semiconductor powder finely penetrates into the obstacles of the metal powder, the bonding strength between the semiconductor and the electrode can be increased, and as a result, the mechanical strength and the thermal strength of the thermoelectric conversion element are improved. Can avoid destruction.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a plasma sintering apparatus used for manufacturing a thermoelectric conversion element of the present invention.

FIG. 2 is a diagram illustrating the principle of thermoelectric conversion in the case of electronic cooling.

FIG. 3 is a perspective view of a thermoelectric conversion element.

FIG. 4 is an explanatory diagram of a production example of a thermoelectric conversion element integrated with electrodes using plasma sintering.

FIG. 5 is an explanatory view of another production example of a thermoelectric conversion element integrated with electrodes using plasma sintering.

FIG. 6 is a perspective view of an ingot obtained by integrally sintering an insulating powder.

FIG. 7 is a perspective view of a module mounted with an ingot obtained by integrally sintering an insulating powder.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Chamber 2 Sintering jig 3 Sintering powder 4 Upper punch 5 Lower punch 6 Pressing mechanism 7 Upper punch electrode 8 Lower punch electrode 9 Sintering power supply 10 Controller 15 P-type semiconductor 16 N-type semiconductor 17 Electrode 18 Electrode 19 Power supply 20 Electrode 21 Circuit 22 Iron powder 23 Lead tellurium powder 25 Metal powder 26 P-type lead-tellurium powder 27 N-type lead-tellurium powder 28 Insulator powder 29 Ingot 30 Thermoelectric generator 31 Module

Claims (2)

(57) [Claims] 1. A loaded semiconductor powders for thermoelectric conversion elements sintering jig in which is provided in the chamber, pressurizing the powder powder at a predetermined pressure, by passing a pulsed current to said powder, said A method for manufacturing a thermoelectric conversion element, comprising sintering semiconductor powders to form a semiconductor layer while desorbing an adsorbed gas adsorbed on the semiconductor powders. 2. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein an electrode is integrally sintered at an end of the semiconductor layer in the direction of current flow.