DE1050448B - - Google Patents

Description

Semiconductor arrangement with a barrier layer electrode system, in particular transistor or crystal diode

The invention relates to a junction electrode system, in particular a transistor or crystal diode, in which at least two semiconducting parts, of different types of conduction, intersect and so form a barrier. Such a barrier electrode system can for example be produced thereby be that a single crystal grows from a melt and by changing the impurities in the melt and / or by changing the growth rate parts of different Line type arise. By melting a metal with donor or acceptor properties a semiconducting crystal, in which latter part of the semiconducting material is in the metal dissolves and grows back on the original crystal lattice when it cools, but now with a certain amount Content of impurities, the grown part receives a conduction type that. different from that of the crystal is different. Diffusion or electrochemical processes can also be used will.

Various requirements can be placed on the barrier layer formed between such parts and depending on the application and the further construction of the system, one or a number of these Requirements are particularly emphasized. Often the requirement is made that if a Reverse voltage is applied to the junction, the resulting reverse current is very small and / or the breakdown voltage is high. Furthermore, it is often desirable that the capacity between the parts on either side of the barrier is small. Also, if one of the parts is at a Transistor must be effective as an emitter electrode, a high emitter efficiency must be sought. the Improvement of the barrier in one respect often has a deterioration in the other as a result. For example, a means for reducing the reverse current is known, namely the. use semiconducting material with a relatively low specific resistance. As a result of this Measure, however, the capacitance increases while the breakdown voltage decreases.

The capacitance of an emitter electrode can be reduced by turning the emitter electrode off Material is made with a high specific resistance. Jn. in this case, however, the Emitter efficiency, d. H. the ratio of the injected from the emitter electrode to the base electrode Current carried by minority charge carriers in the base electrode to the total current that flows over the barrier layer of the emitter electrode.

It has already been proposed in the case of transistors, the capacitance between the base and the collector

Applicant:

N. V. Philips' Gloeilampenfabrieken, Eindhoven (Netherlands)

Representative: Dr. rer. nat. P. Roßbach, patent attorney,
Hamburg 1, Mönckebergstr. 7th

Claimed priority:
Netherlands 21 April 1955

Frederik Hendrik Stieltjes
and Leonard Johan Tummers.
Eindhoven (Netherlands),
have been named as inventors

to make it small by inserting a layer of intrinsically conductive material. Here it is necessary to use a relatively high collector voltage so that the into the Base injected minority charge carriers can be quickly pulled through the intrinsic layer and the running time of these load carriers. The transistor is not useful at high frequencies adversely affected.

The present arrangement aims, among other things, to reduce the reverse current and the capacitance and an increase in the breakdown voltage or an increase in the emitter efficiency in such a way cause the above-mentioned difficulties not to arise. It is based on the knowledge that the capacitance between parts of different types of conduction is predominantly due to nature of the material is determined at the point of transition from one line type to another, during the Current of minority charge carriers passing through this junction is essentially through the Determines the nature of the material that is further away, within a distance that is equal to a multiple of the diffusion length of the minority charge carriers.

The invention relates to a semiconductor device with a barrier electrode system,

809 749/305

I 050

in particular transistor or semiconductor diode, in which at least two semiconducting parts of different Adjacent line types and thus form a p-n junction. According to the invention there is at least one of these semiconducting parts, which form a p-n junction, from two semiconductor layers, one of which is from a conductive type and with a higher specific resistance, also called a buffer layer, directly forms the p-n junction with the other semiconducting part of the other conductivity type and has a thickness which is at least twice the Debye length of this buffer layer, however is small in relation to the diffusion length of this buffer layer, and its other semiconductor layer with lower specific resistance, also called rector layer, on the one facing away from the p-n junction Side of the buffer layer is arranged, and it is the thickness of the reflector layer at least twice the buffer layer, and it is either the conductivity type of the buffer layer and the rector layer the same, and there are electrodes placed on one or both of these layers, or it the conductivity type of the buffer layer is different from the conductivity type of the reflector layer, and it is one Power supply is arranged on the buffer layer and possibly also one on the reflector layer, but in the latter case the reflector layer is electrically floating.

Debye length is used in accordance with the definition of this quantity in the journal »Bell System Technical Journal "of June 28, 1949, p. 441 (only expressed in a different system of units), a length dependent on the material of the buffer layer

Ll

B _r ■ S ₀ ■ k T

cf-C

understood, where t _{r is} the relative dielectric constant in the buffer layer, e ₀ the dielectric constant of the vacuum in the Giorgian system, k the Boltzmannsohc constant, T the absolute temperature, q the charge of the electron and C the sum of the equilibrium concentrations of the majority and minority charge carriers represents in the buffer layer.

To explain the meaning of this Debye length, it should already be noted that the strength χ of the space "charge area.es, which is responsible for the capacitance of the barrier layer; in the event that a very sharp transition is present, had the following value:

'2e _r . _B jy + V _d ) qC

= Ldii

2 (] (V + V _d ) kT 9 L _ie VV; V _d .

The last value applies at room temperature. Here, V represents the applied voltage is in the reverse direction and V _i the diffusion voltage. Because it is desired that the area of the space charge in the buffer layer Hegt should completely the buffer layer are more strongly dimensioned, the higher the voltage V + V 'is _d. On the emission side of a transistor, V is negative and low, as is V _d , so that the area of the space charge is very thin here and the buffer layer can also be thin.

Although a certain effect is already achieved if the buffer layer is thinner than the diffusion length in the material of this layer, it is desirable not to make the buffer layer thicker than necessary, for example with regard to the technique used in the piercing or the attachment of a Contact. By the expression that the thickness of the buffer layer is small in relation to the diffusion length, it is meant here that the layer should not be thicker than half this length. The thickness is preferably not more than a third of this length, but not less than twice the Debye length or so much more than is necessary with regard to the voltage V to be applied.

A certain effect is also achieved if the specific resistance of the reflector is lower than that of the buffer layer, but preferably this ratio becomes larger than three chosen.

To achieve the desired effect, the type of conduction of the reflector is irrelevant; she can do that the same as or different from that of the buffer layer. The equality or inequality However, this type of line is important with regard to the attachment of a current lead. When the reflector has the same type of line as the buffer layer, a current feeder can use an ohmic Contact can be attached to either of these two parts; if y-ödoch the two types of conduction are different from each other, the current feeder should have an ohmic contact with the buffer layer are connected.

There are PIalbleiteranrichtungen known which in certain respects, for. B. in the structure of the sequence of the semiconductor layers are similar to the semiconductor devices according to the invention. These well-known However, arrangements show this coincidental correspondence only partially, in such a way that they, since they aiming for a different purpose are different in respect of other essential characteristics, e.g. B. regarding the thickness ratio or the conductivity ratio of the buffer layer and the reflector layer and / or the attachment of the electrodes, e.g. B. with regard to the electrically floating state the reflector layer with different conductivity types of the buffer layer and the reflector layer.

The invention will be explained below by a description of some important to the invention Effects and explained in more detail by two exemplary embodiments.

1 to 4 are diagrams showing, among other things, the concentrations of charge carriers in the buffer layer and represent in the reflector;

Fig. 5 is a schematic section through a transistor.

Fig. 1 relates to the generally known case that a semiconducting part 1 of the η-type with a relatively low specific resistance merges at Tdnie 2 into a semiconducting part 3 of the p-type, which has a relatively high specific resistance, for reduction the capacity between the two parts. Plierbei results in a space charge region 4 which extends as far as line 5 . The depth to which this area penetrates into part 1 of the η type is much smaller due to the larger number of charge carriers in this area, so that this depth is negligible.

The equilibrium concentration P _{11 of} the majority charge carriers, ie the holes, in the space charge-free area 6 of the part 3, which is to the right of the

Line 5 extends is relatively low due to the liolien specific resistance. This concentration is shown schematically in the direction of the ordinates, not: on a scale, by the dashed line p _p . This concentration is, for example, 3 · 10 ^{14 Zcm 3} . The equilibrium concentration n _{p of} the minority charge carriers, c1, h. of electrons is relatively large in this area, although in the absolute sense of course much less than the concentration of corpses. It can be, for example, 3 · ^{10 12} Zcm '''and is indicated by the dashed line u _v .

If a voltage is now applied in the reverse direction between parts 1 and 2, i. H. so that the Part 1 is positive with respect to part 3, the electrons are attracted to part 3, which causes their Concentration, depending on the distance from the barrier layer and provided that the

Voltage V is large in relation to the value ------,

'J

can be represented by line 9. At room temperature the value mentioned is ¹ Z ₄₀ volts. The magnitude of the electron current is in the vicinity of the line 5 in each point proportional to the tangent of the angle which the tangent encloses with the abscissa at such a point. At the point of the boundary 5 of the space charge region 4, this is the tangent 10, the current is proportional to tga. The tangent of this angle is also equal to the quotient of the equilibrium concentration of the minority charge carriers divided by their diffusion length. The latter is indicated by the distance L in the figure. It can be seen from the figure that the reverse current I ₁ is greater, the greater the concentration of the minority charge carriers. One can thus write:

i _x - A tga = A γ,

where A is a constant.

FIG. 2 relates to the case that the part 1 of the η type is adjacent to a thin buffer layer 15 of the p type and the resistivity of the latter is equal to that of the part 3 of FIG. Here, too, there is a space charge region 4 which extends up to line 5; In the space-charge-free area 6, the equilibrium concentrations of the holes and the electrons are indicated schematically one after the other and not on a scale by the dashed lines p _p and n ". On the side of the buffer layer 15 facing away from the barrier layer there is now a semiconducting part 16 of the p-type, the so-called reflector, which has a much lower specific resistance than the buffer layer, such as. this is indicated by p + . The equilibrium concentration p _{p + of} the holes is here relatively high and, not on a scale, shown by the dashed line p _Pi . It can be, for example, ^{10 16} cm ³ . The equal weight concentration of the electrons is correspondingly low, for example IO ¹ Vcm ³ . it is indicated by the dashed line n _ßl . The boundary between the buffer layer 15 and the reflector 16 is indicated by the line 19. In the vicinity of the boundary 19, too, there is a space charge region which, however, is thinner than the layer 4 and is neglected below.

Ks it is now assumed that part 1 is made positive again with respect to the buffer layer IS. This can be done by means of ohmic contacts in these parts 1 and 15, with no contact with the reflector is produced, which is thus electrically floating.

So it goes over the border. 19 as many holes as electrons in the same direction between the I ⁰ outer layer and the reflector. The concentration of the minority charge carriers from the reflector is decisive for the magnitude of these currents, and this is indicated by the tangent of the angle a '. This is the angle which the abscissa forms with the tangent 20 on the curve 21, which represents the concentration of the electrons in the reflector. Field influences are again neglected here, while it is assumed that the diffusion length in the reflector is L again.

The concentration of the electrons in the buffer layer 15 is determined by a curve 22. The tangent on this curve at the point of the boundary 19 must be parallel to the tangent 20 because the electron flow is continuous. Because the buffer layer is very thin and the control of the electron flow is negligible despite the higher concentration, this is. Tangent, on curve 22 at point 5 at which the space charge region ends, also approximately parallel to tangent 20. The reverse current across the barrier layer is thus approximately determined by the value of the tangent of angle a ,

iy '- A tg a.

The reverse current at the point. der.Linie5 can go through

'i>, 19 "

A ■

where n _{IK ig represents} the concentration of electrons in the buffer layer at line 19 and Wp _{5 represents} the concentration at line 5, α is the strength of the part of the buffer layer free of space charges and Λ is a constant.

^For 'Vs S ^ilt:

and because the barrier layer is loaded in the reverse direction, V is negative and n _{p < 5} approximately equal to zero. As a result of the continuity, the same electrical current flows through the reflector at point 19

= A ■

<■ $ +, 19

Can be made clear, assuming that the reflector has a strength which is large in relation to the diffusion length. The equilibrium concentration of electrons in the / '! Part, the reflector, is represented by n _{p +.}
The following also applies approximately:

'"p , II)

these three calibrations result in:

a + L

n.

yy-d-

It has already been mentioned that the thickness α of the buffer layer is small in relation to the diffusion length L and, in addition, because n _{p is} much greater than n _{p +}

i 050 448

is, which latter value is shown much too large in the figure for the sake of clarity, so it applies

this value is thus by a factor

in the case shown in Fig. 1 without a reflector.

If the strength of the reflector is less than L , for example equal to b , as shown by the dashed line 23 , the equilibrium concentration n _{p + of} the minority charge carriers results at this limitation 23, while the curve 21 changes into a straight line 24 , so that:

less than

assumed that

As can be seen from the examples given, the combination of a buffer layer and a reflector to achieve an electrode system with a barrier layer that has a low Combines capacitance and a high breakdown voltage with a low reverse current. If the If voltage is across a barrier in the direction of flow, it may also be advantageous to have a low capacitance to be realized over the barrier layer. As an example, let the. Emitter electrode of a transistor mentioned. Assuming that the resistivity of the base is relatively low, it could can be achieved in that the emitter electrode is made of a material with high resistivity will be produced. In this case, however, the concentration of the minority charge carriers is in the Emitter electrode high, so that the current consisting of these charge carriers passes through the barrier layer passes over between the emitter and base electrodes, is also large. This stream is one for them Transistor effect of useless electricity. Because the emitter electrode consists of a buffer layer and a Reflector, this leakage current can be largely suppressed while maintaining the low capacity.

This case is explained in more detail with reference to FIG. It is assumed that a base zone 40 adjoins a “ ^νΛ ~ .buffer layer 41 , which is provided with a reflector 42

When an ohmic contact to the reflector 16 is provided. The base zone 40 consists of semiconductors, instead of being applied to the buffer layer 15 , the material of the η type, the buffer layer and the foregoing discussion remains unaffected. In a reflector made of material of the p-type, in which case the reflector receives a negative resistance of the former relatively high potential with respect to the buffer layer, but this and the latter two are relatively low. is unable to control the flow of electrons, which over the The limits are through the. Lines 43 and 44 crossed border 19 , because these give current 35. The equilibrium concentration of the minor is also now determined by tangent a . charge carrier in the buffer layer and the re-

As mentioned above, it is not flektor again to be n _p and n _{/ J +} . If now one

Voltage is applied in the direction of flow, d. H. the buffer layer positive with respect to the base electrode

b ■

In this case too, there is thus an improvement over that shown in FIG. 1. Case, so long

b

required that the reflector of the same
Line type like the buffer layer. In Fig. 3 it is assumed that the buffer layer 15 is made again from the 40, so the concentration of the Elekp type and with a reflector 30 of the η type increases with it
relatively low resistivity
is provided. The concentrations n _{n +} and p _{n +} der
Electrons and holes go through in sequence
the dashed lines and p, ₁₊ indicated. Also to calculate 45 limit 44 and from this also the

Tron at the location of the barrier layer 43 in the buffer layer up to a value "" ₄₃ . From the continuity of the electron flow across the limit 44 , the concentrations on both sides are in turn

in this case the flows of electrons and holes across the boundary 19 are equal to one another. In this case too, the magnitude of these currents is determined by that of the minority charge carriers,

Current.

The concentration of the electrons in the reflector is now represented by a curve 45 , the tangent at point 44 forms an angle γ with the abscissa.

which are now the holes. The concentration of these 50 The concentration of the electrons in the buffer layer is represented by the curve 46 .

Holes can be marked by a curve 21 similar to that shown in
Fig. 2 is drawn. The size,
of the hole flow. the place of the limit 19 is the
Tangent a "is proportional to the electron flow
is the same size, as a result, in the buffer layer 55
the concentration of electrons in turn depends on the
Curve 22 is shown, while the reverse current is in turn determined by the size of the angle a. It
it should be noted that the tangent a is not equal to the tangent a " ; these values are related to one another like the mobilities of the minority charge carriers.

In this case, however, the ohmic connection should be made with the buffer layer and not with the reflector, because if the connection with the reflector layer were made, the reflector would receive a negative potential with respect to the buffer layer, whereby the hole current no longer of the tangent a ", but rather would be determined by the number of holes that would be pulled out of the buffer layer by the field.

A ■ tgy =

also now approximately applies:

'j) +, 44

The designation a in turn represents the strength of the
Buffer layer. Furthermore applies

- n "e

kT

I 050 448

here V is positive. It follows

i = A

qV

A + L

η η

On the Kmitterseite the diffusion span and the control span are effective. If the first is set to au V ₄ volts and the second to ½ volts, then it becomes

11 +

This is thus the electron current which flows over the barrier layer between the buffer layer 41 and the base electrode 40.

It turns out again that this stream

decreases the larger the ratio is. Here too

the reflector can be made thinner than the diffusion length L.

Finally, two examples of calculated p-n-p transistors are given to illustrate some factors important for the invention.

In add. 5 the transistor is shown schematically, the base is denoted by 50, the collector buffer layer by 51, the collector reflector by 52, the emitter buffer layer by 53 and the emitter reflector by 54 .

Two cases are considered; ^ in which different strengths w of the base are assumed, namely 10 μ and 3 μ. It should be noted that it is very difficult to achieve the latter lower value, but that this is believed possible in view of the desire of the art to achieve such low base strengths and the results achieved in doing so.

Furthermore, a relatively low specific W value of 0.14 Ωcm is selected for the base material, while the specific resistance of the buffer layer is relatively high, namely 16 Ωcm ₁ . This results in the Debye length:

= 7 · IQ- « .fj ^ each = 1

μ ..

In order to ensure that the space charge region au: the Koilkktorseite up completely in the PufFerscbioht taken ίο, this should therefore sth in this case £ twenty times the Debye length to be strong, aui the emitter side a strength would suffice equal to four times this length .

Then the cut-off frequency f _{ca is} calculated in MHz, which follows from:

π W-

■ IO- ⁶ ,

where D represents the diffusion constant, which is set at 40 cm ² / see.

Then the base resistance r _b ~ is calculated in Ω

8π ■ w

where Q _{w is} the specific base range.

The diameters d _e and d _{c of} the emitter and collector electrodes are set at 0.250 and 0.375 mm, respectively. Now, because the strengths of the space charge layers x _e and x _{c are} known, the emitter and collector capacities must also be calculated:

k T c = ^F ■ ^i: »

q ² C

where the concentration of the majority charge carriers is used as C , which follows from the specific resistance of the material of the buffer layer, because

1 ■ C μ _τ

1800 cm ² /

where μ ,, represents the mobility of the holes Voltsec.

C is thus -2.2 ■ IO ⁺¹⁴ . At room temperature T = 290 ° K, L _m = 0.3 μ.

The strength χ of the space charge areas generated in the two buffer layers, one of which would be indicated by the number 4 in FIGS. 1 to 3, is derived from:

χ -

(V-gC

V ₁₁

and because

qC =

μ ρ

= Pe ₀ ■ ^ _r μ} f (V -f- V _d ) ρ 7 ■ 10- * V _d ) 'ρ.

If (V + V _d ) = 4 volts is set on the collector side, the strength of the space charge area is there

7 · ΙΟ- ⁵ ■] / 4 ■ 16 r = 5.6 μ.

where P is the area of the emitter or KaHector electrode.
Finally, the figure of merit M is according to the formula:

^M = idifc; ^MHz

calculated.

The results are summarized in the table below.

Basic strength in μ 10 3

Specific resistance of the

Base _Sw in Qcm 0.14 0.14

Specific resistance of the buffer

so Q _b in Qom 16 16

L _ie in μ 0.31 0.31

x _c in µ 5.6 5.6

x _e in μ 1 1

f _ca in MHz 13 140

r _b in Ω 6 18

d _e in mm 0.250 0.250

d _c in mm 0.375 0.375

C _e in pF 2.5 2.5

C _e in pF 7 7

M in MFiz 190 350

The strength of the reflectors is not given in the table, nor is their specific resistance. From the considerations in FIGS. 1 to 4 it emerged that the specific resistance must be selected to be low, for example one tenth of that of the buffer layer. The thickness is preferably dimensioned greater than the diffusion length L , whether-

809 749/305

Claims (9)

I 050 an improvement can also be achieved with a thinner layer. If the PuiTerschicht is dimensioned too strong on the emitter side, a capacitance will become noticeable, which results from the fact that the concentration of the charge carriers changes in each period of the current, which provides a quasi capacitive effect. This buffer layer should therefore not be made thicker than the base. Although the effect of a buffer layer and a reflector is calculated in the foregoing only in relation to a transistor, it can be seen, as was already apparent from FIGS Photocell can be used. P a τ ν. ν τ ans ρ h Och ε: 1. Semiconductor arrangement with a junction electrode system, in particular transistor or semiconductor diode, in which at least two semiconducting parts of different conductivity types adjoin one another and thus form a pn junction, characterized in that at least one of these semiconducting parts, which form a pn junction, consists of two semiconductor layers (53 and 54) , one of which (53) of one conductivity type and with a higher specific resistance, also called the buffer layer (53) , directly forms the pn junction with the other semiconducting part (50) of the other conductivity type and has a thickness, which is at least twice the Debye length of this buffer layer, but is small in relation to the diffusion length of this buffer layer, and its other semiconductor layer (54) with lower resistivity, also called reflector layer (54) , on the side facing away from the pn junction Buffer layer is arranged that the strength of the Re reflector layer is at least twice the buffer layer and that either the conductivity type of the buffer layer and the reflector layer is the same and electrodes are arranged on one or both of these layers or that the conductivity type of the buffer layer is different from the conductivity type of the reflector layer and one Power supply on the buffer layer and optionally also one on the reflector layer, arranged is, but in the latter case the .reflector layer is electrically floating. 2. Semiconductor arrangement according to Claim 1, characterized in that the buffer layer (53) and reflector layer (54) have the same conductivity type and an ear electrode is arranged on one or both of these layers. 3. Semiconductor arrangement according to claim!, Characterized in that the buffer layer (53) and the. Reilektorschicht (54) differ from each other by the type of conduction and an ohmic electrode is only arranged on the buffer layer (53) . 4. Semiconductor arrangement according to one of claims 2 ujkI 3, characterized in that the buffer layer (53) is thinner than a third of the diffusion length. 5. Semiconductor arrangement according to one of claims 2 to 4, characterized in that the specific resistance of the buffer layer (53) and the reflector layer (54) differ from one another by more than a factor of three. 6. transistor with base, emitter and collector zones according to one of the preceding claims, characterized in that only the collector zone consists of a buffer layer with a reflector. 7. transistor with base, emitter and collector zones according to any one of claims 1 to 5, characterized characterized in that only the emitter zone consists of a buffer layer with a reflector. 8. transistor with base, FImitter and collector zones according to one of the preceding claims, characterized in that the emitter and collector zones each consist of a buffer layer with one Consist of reflector. " 9. Transistor according to claim 7 and 8, characterized in that the emitter buffer layer is thinner than the base zone. Publications considered: German Patent Publication No. 814,487; U.S. Patent No. 2,654-59; Zeitschrift für Elektrochemie, Vol. 58, 1954, No. 5,. 283 to 321, Figs. 1, 22. 1 sheet of drawings © 809 749/305 2.