A P-N Junction is essentially an excellent diode. A few of the factors considered when preparing a bipolar diode include the device size, the doping geometry and structure, and connections and packaging. PIN diodes exist in two types of designs - epitaxial design and diffused design.
Figure 1. Pin-epitaxial and pin-diffused diode a) basic structure b) doping profile diagram
The advantages of pin-diodes become effective from 100 V upwards. In diodes produced today, the middle region is not "i" (intrinsic), but n- type, with a much lower doping level than in the outer regions. In epitaxial PIN diodes (Figure 1) an n- region is first separated from the highly doped n+ substrate (epitaxy). Then the p-region is diffused. In this manner, very small base widths w B in the region of just a few μm can be obtained. By integrating recombination centers (mostly gold), ultra-fast diodes can be achieved. Owing to the small base width wB , the on-state voltage will remain low despite the recombination centers. That said, it will still always be greater than the diffusion voltage of the pn-junction (0.6 to 0.8 V). The main field of application for epitaxial (epi) diodes are applications with off-state voltages of between 100 V and 600 V; some manufacturers even produce epi-diodes for 1200 V.
Controlled axial lifetime (CAL) diodes
From 1000 V upwards, the n- region is enlarged to such an extent that a diffused PIN diode (Figure1) can be obtained. The p- and n+ regions are diffused into the n- wafer. Recombination centers are also used. Recombination center profiles similar to those shown in Figure 2 can be generated by implanting protons or He++ ions into silicon. Implantation requires particle accelerators performing up to 10 MeV.
Figure 2. Narrow region with a high concentration of recombination centers at the pn-junction, generated by light ion irradiation
The arrangement of the high recombination center density at the pn-junction (Figure 2) is an optimum set-up , . In  it is demonstrated that the closer the arrangement of recombination centers at the pn-junction, the better the relation between peak reverse recovery current and forward on-state voltage will be. In on-state condition, charge carrier distribution will be inverted, with a higher charge carrier density at the n -n+ junction. As shown in Figure 3, the peak of radiation-induced recombination centers is even placed in the p-region close to the pn-junction in a CAL diode, since this will result in lower off-state currents. He++ implantation is combined with an adjustment to the basic charge carrier lifetime, preferably achieved by electron beam radiation.
Figure 3. Recombination center profile in the CAL diode
The height of the recombination center peak can be adjusted by varying the dose of He++ implantation: The higher the peak, the smaller the peak reverse recovery current. The lion's share of the CAL diode storage charge occurs in the tail current. The tail current itself can be controlled by the basic recombination center density. A reduction in basic charge carrier lifetime will reduce tail current duration; however, this is to the detriment of the diode on-state voltage. The two parameters "basic charge carrier lifetime" and "He++ implantation dose" enable the recovery behavior be controlled to a large extent. In this way, the diode will display soft recovery behavior under any operating conditions, especially when low currents are applied. CAL diodes manufactured in this way boast excellent dynamic ruggedness. CAL diodes dimensioned for 1200 V and 1700 V have been tested under lab conditions at dI/dt up to 15 kA/cm²μs and did not result in diode destruction.
A comparatively narrow base width wB can be chosen for CAL diodes regarding the PT (punch-through) dimensioning. This provides a comparatively low on-state voltage or results in a better compromise between switching characteristics and on-state voltage. The base width wB also has a considerable impact on the turn-on behavior of the diode. The forward recovery voltage VFR rises in proportion to the increase wB . In contrast to conventional diodes, 1700 V-CAL diodes were shown to result in a more than a 50% reduction in V FR .
Freewheeling diodes for IGCT with high reverse voltage ratings, as well as snubber diodes  are manufactured in line with the CAL concept, since dynamic ruggedness is one of the most important requirements. Optimized dimensioning in the direction of PT dimensioning now becomes possible, resulting in improved cosmic ray stability. This also allows for a more favorable trade-off between diode on-state voltage and switching characteristics. In snubber diodes, this enables a minimum VFR to be reached. In addition, a lower reverse current can be obtained than is the case for the conventional gold-diffusion process.
In a common PIN diode, the pn-junction is flooded by more charge carriers than the n-n+ junction. The idea behind the emitter concept is to invert this charge carrier distribution: the n-n+ junction is to be flooded by more charge carriers than the pn- junction. This is achieved by reducing the injection quantity at the p-emitter.
Figure 4. p-emitter to improve soft recovery behavior: a) Emitter structures, e.g. merged PIN/Schottky diode b) Fully reduced p-doping
A number of emitter structures whose functions basically result in this effect have been proposed. One example is the "Merged PIN/Schottky diode", which consists of a series of p+ regions and Schottky regions (Figure 4a). A number of similar structures also exist, including structures with diffused p-regions and n-regions.
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