Darlington device with an ultra-light weight emitter speed-up transistor, and relative manufacturing method

Abstract: The emitter region of the speed-up transistor is created in the base of the final transistor of the Darlington device and has a very low dope concentration and thick­ ness.

Description: Field of the Invention The present invention relates to a Darlington device with an ultra-lightweight emitter speed-up transistor, and the relative manufacturing method. Background of the Invention Bipolar power transistors in the Darlington configuration have the drawback of being very slow to switch on and off. In fact, at the moment of switching, the speed-up current of the charges stored in the base region has an immediate effect only on the driver transistor (T1) and a slower effect on the final transistor (T2). The most commonly used means for speeding up the switching of a Darlington is to insert a speed-up diode between the base of the driver and that of the final transistor, as described e.g. in FR-A- 2 377 706 and as shown in the circuit diagram of Fig. 1. This diode can be made integral with a very low gain speed-up transistor (T3), as shown in the diagram of Fig. 2. In a known Darlington device and relative manufacturing method the base of the speed-up transistor T3 is highly-doped P++ in order to considerably lower the relative gain. This device and relative method entail a number of drawbacks: step of diffusing the base of T3 separate from the step of obtaining the bases of T1 and T2; unevenness in the gain of T3, due to the fact that it is not easy to control the quantity of charge (dope) of the relative base since it is deposited from a solid source; introduction of heavy metals due to the use of a solid source of dope, with a consequent reduction of the lifetime and current characteristics of the device; greater probability of damage to the surface of the silicon due to the deposition of very large doses of dope (boron). Object of the Invention The solution according to this invention aims at overcoming the aforesaid drawbacks. In particular, it offers the following advantages: it enables the bases of T1, T2 and T3 to be made in a single step; it ensures greater gain uniformity of T3, due to the possibility of using ion implantation for the formation of its base and of its emitter; it increases the lifetime due to the absence of metallic ions, as a result of the aforesaid possibility of using ion implantation. Summary of the Invention The Darlington device according to the invention, of the type comprising a driver bipolar transistor (T1), a final bipolar transistor (T2) and a speed-up bipolar transistor (T3), acting as a speed-up diode, with the emitter-base diode of the speed-up transistor (T3) connected between the base of the driver (T1) and the base of the final transistor (T2), is basically characterized by the fact that: the emitter region of the speed-up transistor (T3) is located in the base region of the final transistor (T2) which acts as a common base while the collector regions of the driver transistor (T1), of the final transistor (T2) and of the speed-up transistor (T3) are common, the emitter region of the speed-up transistor (T3) has a very limited dopant concentration and thickness, the base regions of said driver transistor (T1), final transistor (T2) and speed-up transistor (T3) have a surface concentration of dopant of approximately 10¹⁸ atoms/ cm³ and a depth of approximately 10 µm. the emitter region of the speed-up transistor (T3) has a concentration of dopant slightly higher than that of said base regions and a depth smaller than 1 µm. Methods of manufacturing a Darlington device according to claim 1 are defined in claims 3 and 4. Brief Description of the Drawings The features of the invention will be more clearly evident from the following description with reference to the accompanying drawings of a non-restrictive embodiment, in which: Fig.3: shows the structure of a speed-up transistor made according to the known technique; Fig.4: shows the structure of a speed-up transistor made according to the invention. Specific Description The method for manufacturing the known Darlington device of Fig. 3 comprises the following steps: diffusion on an N substrate of the P+ bases of the transistors T1 and T2, using solid sources or implanted boron with a dose of approximately 10¹⁵ atoms/cm², and diffusion of P++ base of T3 so as to obtain a deeper base than that of T1 and T2; forming N+ emitter regions simultaneously for T1, T2 and T3. The difference in doping between the P+ regions and the P++ region consists in a difference in depth between the emitters of T1 and T2 (< 5 µm) and of T3 (< 2 µm). In this way a T3 transistor is obtained with a gain ranging from 0.01 to 0.02. The structure of Fig. 4 refers to an NPN Darlington according to the invention, constructed by means of the planar technique, in which the emitter region 4 of the speed-up transistor T3 is produced in a region 5 which acts as a common base with the transistor T2, and has a very limited dope concentration and thickness. In particular, the base region 6 of T1 and the region 5 have a surface concentration of dopant of approximately 10¹⁸ atoms/cm³ and a depth of approximately 10 µm, while the emitter region 4 of T3 has a concentration of dopant slightly higher than that of the bases and a depth smaller by 1 µm. The metallization on the front is made with a Pt/Ti/W/Al alloy in order to prevent spikes of Al from short-circuiting the emitter and the base of T3. The gain of the transistor T3 made in this way is very low (ranging from 0.0002 to 0.001) due to the low concentration of dopant of the emitter and, consequently, to its very low efficiency. The structure of Fig. 4 can be obtained by means of a process which comprises the following consecutive steps: epitaxial growth of an N- type layer on an N+ type substrate; formation, in the N- layer, of the P+ bases of T1, T2 and T3, with an implantation of boron at a concentration of 7.8 · 10¹⁴ atoms/cm² and subsequent diffusion for 8 hours at 1200°C, which make it possible to obtain bases with a surface concentration of dopant of approximately 10¹⁸ atoms/cm³ and a depth of approximately 10 µm; formation of the emitter regions 8, 9 and 4 of T1, T2 and T3, the emitter of the transistor T3 being made with an implantation of As with dose 4.5 · 10¹³ atoms /cm² and subsequent diffusion for 20′ at 1100°C, which makes it possible to obtain an emitter region with a concentration slightly higher than the aforesaid concentration of the bases and with a depth smaller by 1 µm; oxidation of the front (portions indicated by 0 in the figure) and opening of windows in the oxide for the contacts; metallization of the front by means of a Pt/Ti/W/Al alloy. It can be seen from the foregoing description that the method according to the invention consists of creating the emitter of T3 in the same base as T2, thereby using a single process for forming the bases and eliminating the steps for forming a base with a higher concentration for T3. The solution has been illustrated with reference to a single-stage NPN Darlington, constructed by means of the "planar" technique. It can, however, be applied, with obvious variations, to multi-stage Darlingtons and to those obtained by means of "epibase" and "multi-epimesa" techniques. It can also be applied to a PNP Darlington, in which case, the steps for constructing the Darlington can be summarized as follows: epitaxial growth of a P- type layer on a P type substrate; forming the bases of the transistors T1, T2 and T3 by implantation of an N type dopant (e.g. phosphorus) at a concentration of 5·10¹⁴ atoms/cm² and subsequent diffusion for 6 hours at 1200°C; forming the emitters of T1, T2 an T3 with P type dopant (e.g boron); the ultra-lightweight and thin emitter of T3 is achieved by ion implantation of boron at a dose of approximately 4-5·10¹³ atoms/cm2 followed by diffusion of the boron for 20 minutes at a temperature of 1100°C.

Claims: Darlington device of the type comprising a driver bipolar transistor (T1), a final bipolar transistor (T2) and a speed-up bipolar transistor (T3), acting as a speed-up diode, with the emitter-base diode of the speed-up transistor (T3) connected between the base of the driver (T1) and the base of the final transistor (T2), characterized by the fact that: the emitter region (4) of the speed-up transistor (T3) is located in the base region (5) of the final transistor (T2) which acts as a common base while the collector regions of the driver transistor (T1), of the final transistor (T2) and of the speed-up transistor (T3) are common, the emitter region (4) of the speed-up transistor (T3) has a very limited dopant concentration and thickness, the base regions (5,6) of said driver transistor (T1), final transistor (T2) and speed-up transistor (T3) have a surface concentration of dopant of approximately 10¹⁸ atoms/ cm³ and a depth of approximately 10 µm, the emitter region (4) of the speed-up transistor (T3) has a concentration of dopant slightly higher than that of said base regions (5,6) and a depth smaller than 1 µm. Darlington device according to Claim 1, characterized by the fact that metallization on the front consists of a Pt/Ti/W/Al alloy. Method of manufacturing a Darlington device according to Claim 1, of the type comprising an NPN driver transistor (T1), an NPN final transistor (T2) and an NPN transistor (T3) acting as a speed-up diode, characterized by the following steps: epitaxial growth of an N- type layer on an N+ type substrate; forming in the N⁻ type layer the P+ bases (6,5) of said driver transistor (T1), final transistor (T2) and speed-up transistor (T3), by implantation of P type dopant at a concentration of 7-8·10¹⁴ atoms/cm² and subsequent diffusion; forming the emitters (8,9,4) of said driver transistor (T1), final transistor (T2) and speed-up transistor (T3), with N type dopant, the emitter (4) of the speed-up transistor (T3), being obtained by arsenic ion implantation into the P+ base (5) of said final transistor (T2) with a dose of 4-5·10¹³ atom/cm² and subsequent diffusion; metallization of the front by means of Pt/Ti/W/Al alloy. Method of manufacturing a Darlington device according to Claim 1, of the type comprising a PNP driver transistor (T1), a PNP final transistor (T2) and a PNP transistor (T3) acting as a speed-up diode, characterized by the following consecutive steps: epitaxial growth of a P- type layer on a P type substrate; forming in said P- type layer the bases (6,5) of said driver transistor (T1), final transistor (T2) and speed-up transistor (T3), by implantation of N type dopant at a concentration of 4-6·10¹⁴ atoms/cm² and subsequent diffusion; forming the emitter (8,9,4) of said driver transistor (T1), final transistor (T2) and speed-up transistor (T3), with P type dopant, the emitter (4) of the speed-up transistor (T3) being obtained by boron ion implantation into the base (5) of said final transistor (T2) with a dose of 3-6·10¹³ atoms/cm² and subsequent diffusion; metallization of the front by means of Pt/Ti/W/Al alloy.

Inventors: BALLARO' DAVID, FERLA GIUSEPPE, FRISINA FERRUCCIO, PATTI ALFONSO, BALLARO', DAVID, FERLA, GIUSEPPE, FRISINA, FERRUCCIO, PATTI, ALFONSO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1989
Main classification: H01L27
IPC codes: H01L29, H01L27, H01L21