Solid State Current Limiter

The phenomenon of "scattering limited velocity effect" in semiconductor material is known. The present invention relates to a practical application thereof.

An object of the present invention is to provide a solid state current limiter which may be fabricated as a discrete component or may be integrated on a common substrate with other components.

Another object of the invention is to provide a current limiter of the aforementioned character integrating in a single chip both a plurality of high power density active spot regions and adequate heat sink means to enable utilization of the phenomenon of "scattering limited velocity effect" for current limiting purposes above a designated applied electric field without destructive thermal breakdown.

Another object of the invention is to provide a current limiter of the aforementioned character having a relatively simple structure with relatively large dimensions affording ease of processing, and also affording the use of standard processing techniques.

Other objects and advantages will hereinafter appear.

FIG. 1 is a graph of carrier velocity versus electric field.

FIG. 2 is a top view, with metallization removed, of the preferred form of the present invention.

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 3.

FIG. 4 is a circuit diagram showing one of many advantageous uses of the current limiter of FIGS. 2 and 3.

The phenomenon of "scattering limited velocity effect" is present in all semiconductor material. At room temperature, free electrons (unbound electrons outside the outer shell of the atoms in the lattice) are randomly moving at high velocity. Application of an electric field across the semiconductor material changes the random velocity of the electrons and results in current flow.

The amount of current flow can be calculated by statistical distributions based on Newtonian mechanics of collisions between the electrons and the lattice. As the applied electric field increases, the velocity of the free electrons increases, and, assuming the distance between collisions remains constant, Ohm's law will still be valid, i.e. current (average velocity of free electrons) increases linearly with respect to the applied electric field.

At a certain value E.sub.1 of the applied electric field, the velocity of the free electrons no longer increases but rather remains constant, as shown in FIG. 1. This is the phenomenon of "scattering limited velocity effect." Further increase of the field above E.sub.1 does not increase the velocity of free electrons and hence does not increase the amount of current flow. Thus above E.sub.1, the material does not obey Ohm's law.

This phenomenon is theoretically explained in terms of quantum mechanical wave motion. Below E.sub.1, the electrons have energies corresponding to quantum mechanical waves of certain frequencies which have a high probability of colliding with atoms of a certain vibrational mode within the lattice to emit what is called an acoustical phonon. Above E.sub.1, the electrons have higher energies corresponding to higher frequency quantum mechanical waves which have the additional probability of colliding with other atoms of a different vibrational mode within the lattice to emit what is called a optical phonon. It is the additional collisions which limit the average velocity of the electrons and the amount of current flow. Increasing the applied electric field above the value E.sub.1 increases the probability of collision, and the increased number of collisions establishes an upper limit on the average velocity of the electrons in spite of the increased energy thereof.

FIGS. 2 and 3 show the preferred form of a current limiter 2 constructed in accordance with the present invention enabling utilization of the phenomenon of "scattering limited velocity effect." An N+ type substrate 3 has an N type epitaxial layer 4 formed thereon. A perforated P type layer 6 is diffused into layer 4 to form a plurality of N type spot regions or channels 8. Metallizations 10 and 12 and terminals 14 and 16 are added for circuit connection.

A positive potential applied to terminal 16 with respect to terminal 14 causes current flow through channels 8. Blocking PN junctions 18 formed by the interface of layers 4 and 6 prevent current flow through layer 6. There are thus formed a plurality of active spots regions or conduction channels 8 separated and surrounded by an encompassing inactive region 6.

Application of an electric field greater than E.sub.1 across terminals 16 and 14 results in extremely high current density in channels 8 which generate a high amount of heat that must be dissipated in order to avoid destructive thermal breakdown of the device. P type layer 6 acts as a heat sink to dissipate such heat. Instead of P type layer 6, simple dielectric isolation means could be used. Another variation would be the provision of a buried P layer, rather than surface P layer 6, to afford bipolar operation. Also, a P type substrate, rather than N type, may be used if desired because the "scattering limited velocity effect" is exhibited in P type material also.

The critical need is that of adequate heat dissipation. For example, if layer 6 were omitted from FIGS. 2 and 3, current would flow across the entire surface area of layers 3 and 4 of extremely high density for fields greater than E.sub.1, and the device would burn out because the area is too great to provide adequate conventional heat sinks, such as add-on cooling fins, etc. If the surface area were reduced to the size of a single spot, for example, the amount of current flow is too small for practical applications.

The embodiment of FIGS. 2 and 3 provides sufficient area to enable a usable amount of current flow and also provides necessary heat sink means. The present invention thus enables both of these heretobefore incompatible results, and does so in a single chip by integrating many high power density separated spots into a single structure. The power density in the channels is very high but the heat sinking action of layer 6 maintains the average power density along the surface at a moderate level whereby the device will not burn up. The cummulative current flow through the many channels provides a usable amount of current for practical applications.

A specific example will now be given. Using silicon semiconductor material, and for a reasonably high operating temperature, e.g. 200.degree. C., the lowest level of doping that will make silicon an extrinsic semiconductor is about

N (free electron density)=10.sup.15 electrons/cm.sup.3

One set of empirically determined values for velocity and electric field at point 20, FIG. 1, is

E=10.sup.5 volts/cm

velocity 32 10.sup.7 cm/sec.

(Further reference may be had to Physics of Semiconductor Devices, S. M. Sze, John Wiley and Sons, Inc., 1969, pp. 56-59, especially FIG. 29, for other values of electric field and carrier velocity.)

Given the above values, the following calculation can be made: ##EQU1##

In one particular embodiment for use in 30 volt DC aircraft power systems, a 3 micron thick layer was used. Given a spot area of 10.sup.-4 cm.sup.2, the following calculation can be made: ##EQU2##

A known device which operates at similar high power densities as the present invention is an impatt (impact-avalanche transit-time) diode. An impatt diode having an area of 10.sup.-4 cm.sup.2 can dissipate about 10 watts with a 200.degree. C. temperature rise. (Further reference may be had to Sze reference, pp. 256-257, especially FIG. 36). Based on the above calculation of 4.8 watts/spot, the specifically disclosed example of the current limiter will exhibit a 100.degree. C. spot temperature rise.

The above method of calculating local hot spot temperature is valid as long as the spots are placed at least several diameters apart. While not a limitation of the invention, such spacing is preferred, especially for high current ratings. In the specific example disclosed, the device limits current to 30 amps for 0.3 seconds at 30 volts (across terminals 16 and 14), and carries 10 amps for an unlimited period of time at 1 volt. In general, the spacing of the spots in the design of the current limiter is determined by the power and time requirements placed thereon. Wider spacing of the spots increases the length of time during which current is limited. Sufficiently wide spacing of the spots affords continuous limiting of the current.

A particularly useful application of the present invention is shown in FIG. 4. A power thyristor 22 having anode A and cathode K is provided for connection across a load (not shown) to control current thereto. A gate terminal G is provided for applying a triggering signal to fire thyristor 22 into conduction. A thermally sensitive thyristor 24 is electrically connected between G and K so as to be in parallel with the gate of the power thyristor 22. Tyristor 24 responds to a predetermined temperature by intrinsically switching states and may be like those disclosed in U.S. Pat. No. 3,971,056 or in U.S. patent application Ser. No. 652,192, filed Jan. 26, 1976. As taught in U.S. Pat. No. 4,050,083, thyristors 22 and 24 may be integrated in a single chip, the common substrate thermally coupling the two thyristors. Thyristors 22 and 24 may also be discrete components with thermal coupling means therebetween. Thyristor 24 senses the temperature of thyristor 22 and responds to a predetermined value thereof by intrinsically switching to its conductive state whereby to shunt gate current therethrough and away from the gate of thyristor 22. Variable resistor 26 adjusts the switching temperature of thyristor 24. The arrangement is thus a thermally self-protected power switching device since thyristor 24 automatically shunts gate current above a predetermined temperature of power thyristor 22.

When thyristor 24 shunts gate current away from thyristor 22, the latter will still remain conductive until the current flowing therethrough drops below its holding current. For example, if an alternating power source is applied across A and K, thyristor 22 will remain conductive until the end of the cycle when the applied voltage passes through zero or drops below some holding value; and thyristor 22 will not turn on in the next cycle. There is thus an unprotected fractional cycle portion.

The current limiter 2 may be connected in series with the power thyristor 22 to afford protection during the fractional remainder of the cycle by limiting the amount of current flow. Current limiter 2 need only limit the current for a period of time equal to the remainder of a single period of cycle of the power supply because thyristor 22 will remain off during subsequent cycles. For example with a 60 hertz A.C. source, the maximum length of time for which current must be limited is one-half cycle, or 0.008 sec. Current ratings of thyristor 22 determines values to which current must be limited by current limiter 2. The present invention in combination with the arrangement in FIG. 4 thus affords complete protection of thyristor 22, with no unprotected fractional cycle portion during which short-circuit, etc. current surges could otherwise damage thyristor 22 and/or a load thereacross.

An important feature of the present invention is its ease of integration, either as a single component or with other components. Though not limited thereto, the current limiter of the present invention is preferredly fabricated by using diffusion techniques because of the relatively simple structure requiring few diffusions. The epitaxial construction is preferred because it affords a uniform and high resistance in the active power dissipating spot. The diffusions are shallow and thus the diffusion times are short. Relatively large dimensions are involved, thus affording easy processing. Furthermore, standard processing techniques may be used.

The current limiter 2, FIG. 4, may be formed on the same chip with thyristors 22 and 24. The current limiter can be fabricated with many other integrated circuits for protective purposes, or may be used discretely as a single movement.

Another feature of the present invention is that since the "scattering limited velocity effect" is not dependent upon minority carrier lifetime, a broad range of materials and structures are possible. For example, multiple spots of GaAs (gallium arsenide) may be deposited onto a silicon wafer resulting in a lower voltage field threshold for current limiting than achievable with silicon. Another example would be GaAsP (gallium arsenide phosphide).