High breakdown voltage diode and method of forming same

Abstract

A multiple layer overvoltage protection device is provided. The method begins by providing a substrate having a first impurity concentration of a first conductivity type to define a mid-region layer. A dopant of a second conductivity type is introduced into the substrate with a second impurity concentration less than the first impurity concentration. An upper base region having a second type of conductivity is formed on the upper surface of the mid-region layer. A lower base region layer having a second type of conductivity is formed on a lower surface of the mid-region layer. A first emitter region having a first type of conductivity is formed on a surface of the upper base region layer. A first metal contact is coupled to the upper base region layer and a second metal contact is coupled to the lower base region layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional two layer semiconductor junction diode.

FIG. 2 represents the impurity concentration profile of the pn junction of FIG. 1.

FIG. 3 shows one embodiment of a semiconductor junction diode device constructed in accordance with the present invention.

FIG. 4 shows impurity concentration profiles of the substrate for device shown in FIG. 3.

FIG. 5 shows one alternative embodiment of the invention that serves as a bidirectional overvoltage device.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein; rather, this embodiment is provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

The present inventors have recognized that the breakdown voltage of a semiconductor junction diode device can be increased by reducing the overall impurity concentration in the starting substrate or wafer. Without being bound to any particular theory or model, the following analysis is presented to facilitate an understanding of the present invention.

As discussed, for example, in chapter 5 of Solid State Electronic Devices, 5^th Edition, B. Streeman et al., Pearson Education, Inc., 2004, the contact potential V_o across a pn junction of width W₀ can be expressed as:

$V_0=\frac{1}{2}·{\varepsilon }_0·W_0$

where ε₀ is the electric field across the junction.

The breakdown voltage V_B that arises under a certain critical electrical field ε_C thus can be expressed as

$V_B=\frac{1}{2}·{\varepsilon }_c·W_B$

where W_B is the depletion width at device breakdown and W_B>W₀

As shown in Eq. (5-22) of Solid State Electronic Devices the equilibrium width W₀ of the junction can be expressed as

$W_0={\left[\frac{2\in V_0}{q}\left(\frac{1}{N_a}+\frac{1}{N_d}\right)\right]}^{\frac{1}{2}}$ $\mathrm{and}\mathrm{thus}$ $W_B={\left[\frac{2\in V_B}{q}\left(\frac{1}{N_a}+\frac{1}{N_d}\right)\right]}^{\frac{1}{2}}$

where N_a and N_d are the doping concentrations of acceptor ions and donor ions, respectively.

Accordingly, V_B can be expressed as:

$V_B=\frac{\in {\varepsilon }_c^2}{2q}\left(\frac{N_a+N_d}{N_aN_d}\right)$

In the case of a one sided abrupt junction in which one side is more heavily doped (i.e., P⁺N) N_a>>N_d then (1-3) and be approached by following Eq.

$V_B\approx \frac{\in {\varepsilon }_c^2}{2q}{\left(N_d\right)}^{-1}$

In the present invention the pn junction is defined by a more lightly doped substrate and a more heavily doped upper base region (described below). Accordingly, N_d represents the doping concentration of donor ions in the substrate, which may therefore be referred to as the bulk concentration of the substrate. The present invention reduces this bulk concentration in order to increase the breakdown voltage of the resulting semiconductor diode device.

FIG. 3 shows one embodiment of a semiconductor junction diode device 20 constructed in accordance with the present invention. The device 20 includes metal contacts 7 and 9, an upper base region 2 having a cathode or emitter region 4 separated by shorting dots 6. The upper base 2 is P+-type conductivity while the emitter regions 4 are N++-type conductivity. The shorting dots 6 are P+-type conductivity like the upper base region 2. The emitter region 4 and the shorting dots 6 are coupled to the metal contact 7. Upper base 2 and emitter region 4 are formed by implantation and diffusion into a substrate 1 of N−-type conductivity. A lower base or anode region 5 is formed on a lower surface of substrate 1 and is in contact with the metal contact 9. The lower base region is P+-type conductivity.

The P-type dopants (e.g., boron) add positive charges to the semiconductor material, while the N-type dopants (e.g., phosphorus) add negative charges to the semiconductor materials. Preferably, the P+ or N++concentration approximately encompasses the range of 1×10¹⁹ to 1×10²¹ atoms/cm³ and the P+ and N+ concentration approximately encompasses the range of 1×10¹⁵ to 1×10²⁰ atoms/cm³. The P− to P (or N− to N) range of dopant concentration approximately encompasses the range of 1×10¹⁴ to 1×10¹⁶ atoms/cm³. The concentration in the regions may vary depending on what depth of the region is examined. Thus, these concentrations are provided only in approximate ranges.

If the substrate 1 is initially doped so that it has a given impurity concentration of a given impurity type (the “background concentration” of the substrate), the present invention introduces into the substrate 1 a lower impurity concentration of the opposite impurity type. In this way the active bulk concentration N_B discussed above is reduced, thereby increasing the breakdown voltage V_B of the device. In contrast, in the conventional device of FIG. 1 the impurity concentration of the substrate is increased by introducing additional dopant of the same conductivity type as initially provided in the substrate to form higher impurity concentration regions such as buried region 3, while in the present invention an impurity is introduced that has a conductivity type that is opposite to the conductivity type initially present in the substrate 1. More specifically, since in the embodiment of the invention shown in FIG. 3 the substrate is of n-type conductivity, a p-type impurity is introduced. FIG. 4 shows the impurity concentration profile of the initial substrate (curve 302), the impurity concentration profile that is introduced into the substrate by implantation and diffusion (curve 304), and the net or effective concentration profile of the substrate after the additional dopant has been introduced (curve 306).

The semiconductor junction diode device of the present invention may be fabricated in accordance with well-known techniques including a variety of different growth or deposition techniques, as well as different photolithographic and implantation techniques. That is, the pn junctions defined by the substrate 1 and the upper base 2 and the lower base 5 may be formed in a conventional manner after introducing an impurity into the substrate 1 that has a conductivity type that is opposite to the conductivity type initially present in the substrate 1.

By way of example, computer simulations have demonstrated that a diode device having a breakdown voltage of 130V can be fabricated from a substrate initially doped with arsenic and having a resistivity of 0.5 ohms/sq (corresponding to an impurity concentration of 1.14×10¹⁶/cc). This result can be achieved by introducing boron into the substrate by ion implantation at a dosage of 1×10¹³ ions/cc with an energy of 20 Kev and a drive-in for about 1400 minutes at 1265° C. Without the introduction of boron, the diode device would have a breakdown voltage of only about 58V.

The embodiment of the invention depicted in FIG. 3 is an asymmetrical device. The present invention, however, also encompasses bidirectional devices such as the device 90 depicted in FIG. 5. In FIGS. 3 and 5 like elements are denoted by like reference numerals. The device 90 has two metal contacts 7 and 9. Contact 7 may be connected to terminal 92 and contact 9 may be connected to terminal 112. The device 90 possesses two four-layer devices having a bidirectional current flow. On the right side of the circuit configuration 90, the device includes an upper base region 2, substrate 1 and a lower base region 5. The left hand of device 90 has a current flow in the opposite direction to that of the right side of the device. The circuit configuration on the left side of the device can be treated as being formed by an upper base region 5, a mid-region layer defined by the substrate 1 and a lower base region 2. Also with respect to the left hand circuit of device 90, the upper base region 5 is shown possessing emitter region 4 and shorting dots 6 coupled to the lower metal contact 9. The bidirectional configuration allows for either positive or negative polarity overvoltage protection from a single device. The device is fabricated using substantially the same conductivity types and concentrations as that disclosed in FIG. 3.

Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, the method of the present invention may be used to form a semiconductor junction diode in which the conductivities of the various semiconductor regions are reversed from those described herein.

Claims

1. A method of making a multiple layer overvoltage protection device comprising the steps of: providing a substrate having a first impurity concentration of a first conductivity type to define a mid-region layer;introducing a dopant of a second conductivity type into the substrate with a second impurity concentration less than the first impurity concentration;forming an upper base region having a second type of conductivity on the upper surface of said mid-region layer;forming a lower base region layer having a second type of conductivity on a lower surface of said mid-region layer;forming a first emitter region having a first type of conductivity on a surface of said upper base region layer;coupling a first metal contact to said upper base region layer; andcoupling a second metal contact to said lower base region layer.

2. The method of making a multiple layer overvoltage protection device according to claim 1 further providing a plurality of discrete first shorting dot regions in said first emitter region

3. The method of making a multiple layer overvoltage protection device according to claim 1 wherein said first type of conductivity is an N-type and said second type of conductivity is a P-type.

4. The method of making a multiple layer overvoltage protection device according to claim 1 wherein said first type of conductivity is a P-type and said second type of conductivity is a N-type.

5. The method of making a multiple layer overvoltage device according to claim 1 further comprising the step of forming a second emitter region having a first type of conductivity on a surface of said lower base region layer.

6. The method of making a multiple layer overvoltage device according to claim 5 further comprising the step of providing a second shorting dot region in said second emitter region.

7. The method of making a multiple layer overvoltage protection device according to claim 5 wherein said first type of conductivity is a P-type and said second type of conductivity is a N-type.

8. The method of making a multiple layer overvoltage protection device according to claim 5 wherein said first type of conductivity is an N-type and said second type of conductivity is a P-type.

9. A multiple layer overvoltage protection device, comprising: a substrate having a first impurity concentration of a first conductivity type, said substrate defining a mid-region layer, wherein said substrate further includes a dopant of a second conductivity type with a second impurity concentration less than the first impurity concentration;an upper base region having a second type of conductivity located on the upper surface of said mid-region layer;a lower base region layer having a second type of conductivity located on a lower surface of said mid-region layer;a first emitter region having a first type of conductivity located on a surface of said upper base region layer;a first metal contact electrically coupled to said upper base region layer; anda second metal contact electrically coupled to said lower base region layer.

10. The multiple layer overvoltage protection device according to claim 9 further comprising a plurality of discrete first shorting dot regions located in said first emitter region

11. The multiple layer overvoltage protection device according to claim 9 wherein said first type of conductivity is an N-type and said second type of conductivity is a P-type.

12. The multiple layer overvoltage protection device according to claim 9 wherein said first type of conductivity is a P-type and said second type of conductivity is a N-type.

13. The multiple layer overvoltage device according to claim 9 further comprising a second emitter region having a first type of conductivity on a surface of said lower base region layer.

14. The multiple layer overvoltage device according to claim 13 further comprising a second shorting dot region in said second emitter region.

15. The multiple layer overvoltage protection device according to claim 13 wherein said first type of conductivity is a P-type and said second type of conductivity is a N-type.

16. The multiple layer overvoltage protection device according to claim 13 wherein said first type of conductivity is an N-type and said second type of conductivity is a P-type.