Active solid-state devices (e.g. – transistors – solid-state diode – With specified impurity concentration gradient – With high resistivity
Reexamination Certificate
2002-11-26
2004-03-02
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
With specified impurity concentration gradient
With high resistivity
Reexamination Certificate
active
06700180
ABSTRACT:
The invention relates to a rectifying diode, and in particular to a rectifying diode having an anode formed of a semiconductor of lower bandgap than the cathode.
A standard high voltage diode technology is a p-i-n diode structure with highly doped p+ and n+ regions sandwiching a low doped thin i region. The term “i region” is used in the present specification to refer to the low doped central region although in fact this region is not necessarily intrinsic and may be lightly doped n-type, or even perhaps p-type. The structure may be used, for example, for fast diodes with voltage ratings for example of 20V or more, e.g. around 600V.
In such p+-i-n+ structures, there are two components to the current through the diode when operated in a forward direction. Assuming the “i” region is n-type, one component is the hole current from holes in the p+ region entering the i region and recombining there, and the other component is the electron current from electrons in the i region entering the p+ region and recombining there. The hole current is generally dominant. Since the holes have a significant lifetime in the i region, there are an appreciable number of holes in the i region when the diode is conducting in the forward direction.
When the diode is switched from forward to reverse direction, these holes have to be removed, which takes time. In order to speed up the removal of the holes, the number of holes present during forward operation may be reduced. This may be achieved by doping the i region of the diode with recombination centres, also known as lifetime killers, such as gold or platinum. Alternatively, a region of different material may be introduced, for example by using an SiGe layer in a generally silicon structure. Misfit dislocations caused by the mechanical stress introduced between the layers of different material reduce the service lifetime of the minority carriers in a similar way to gold atoms.
When used for high voltage diodes rated at 600V this technology has a number of disadvantages, including a high forward voltage of 2.5V to 3.5V, and a much higher transient forward voltage during turn on, which may be as high as tens of volts. Reverse recovery includes heavy ringing (oscillations). Moreover, some dopants can deliver high reverse leakage at high temperature, leading to risks of thermal runaway.
A known approach to improving the reverse blocking characteristics of semiconductor structures is described in the fundamental patent U.S. Pat. No. 4,754,310 to Coe assigned to US Philips Corp. which describes the use of alternating p and n stripes as a region to be depleted to carry the high voltage across the device.
A further known approach is described in WO01/59844 to Philips. A p-i-n diode structure is formed with a plurality of trenches extending through the p and i layers into the n layer. Semi-insulating or resistive paths are formed in the trenches. When the device is reverse biased, an electric potential is generated along the reverse paths which causes a depletion region extending through the i layer. This increases the reverse breakdown voltage for a given doping level in the i-layer or equivalently increases the doping possible in the i-layer for a given breakdown voltage.
Each of the approaches of U.S. Pat. No. 4,734,310 and WO01/59844 is of particular benefit in unipolar devices such as MOSFETs and Schottky diodes, since it offers a way of increasing the doping in the middle i layer of a p-i-n structure and hence increasing conduction when the device is turned on whilst still ensuring that the i-layer can support a reverse voltage when the device is turned off.
For p-i-n diodes, the advantages of such field relief structures that enable the use of higher doping in the “i” region are less obvious. In such devices, it is always possible to use very low doped, close to intrinsic, “i” region to achieve maximum blocking voltage with minimum thickness. The low doped layer does not present a big disadvantage in the on-state since it is modulated by injection anyway, i.e. the number of carriers in the “i” layer during forward conduction is not simply the number of carriers from doping in the “i” layer but a much larger number of carriers, in particular holes injected from the p-anode.
As described in WO99/53553 to Philips, an ultrashort heterojunction rectifier structure may be used to reduce the reliance of a p-i-n diode on the injection of holes from the p-anode into the “i” region. This enables a very low reverse recovery time to be achieved without the use of recombination centres. A thin (short) region of low bandgap semiconductor, a silicon germanium alloy (SiGe) doped p-type, forms the anode of a diode in combination with a cathode formed of wider bandgap silicon (Si), doped n-type. The SiGe region is chosen to have a Si:Ge ratio that gives a lattice constant reasonably close to that of Si and is chosen to be sufficiently thin to largely avoid misfit dislocations.
Such diodes have a number of advantages compared to more conventional structures. In particular, in these diodes, a significant fraction of the current is carried by electrons injected from N-Si into the p-SiGe and recombining there. This reduces the level of injected holes in the n region when the diode is carrying current, as compared with conventional diode structures. This in turn means that the total voltage dropped in the diode during forward conduction is reduced and further that the reverse recovery time and charge transfer is low.
This technology has been successful in diodes intended for operation below 200V. The doping of 10
15
cm
−3
in the thin low doped layer appears to be of the same order of magnitude as the injected hole concentration during operation in that layer. This results in a very fast switching time.
However, experiments on short SiGe diodes to produce a diode capable of supporting a reverse voltage of 600V did not result in a product with a low enough reverse recovery time for commercial applications.
WO99/53553 itself addresses this problem and suggests reducing the reverse recovery time by a lifetime control method such as platinum doping or irradiation. However, this can be difficult to combine with commercial processing for manufacturing short SiGe diodes.
Thus, there remains a need for a diode with a short reverse recovery time capable of supporting a large reverse voltage. Such diodes are needed, for example, for power factor correction applications.
According to the invention there is provided a semiconductor diode including: a first region formed of a first semiconductor material doped to have a first conductivity type; a second region formed of a second semiconductor material having a lower bandgap than the first semiconductor material; and an intermediate region extending between the first and second regions; wherein the thin second region has a thickness and the lattice mismatch of the first and second semiconductor materials are selected such that the level of mechanical stress remains below a level at which misfit dislocations are formed; and the intermediate region includes a plurality of laterally spaced field relief regions providing vertical parallel paths extending between the first and the second region for depleting the intermediate region in an off state of the diode.
The inventors have realised that the reason for the difficulty in producing SiGe diodes capable of withstanding 600V is that low doping, of order 10
14
cm
−3
, is required in the low doped n region adjoining the p-type SiGe layer. This doping is much lower than the 10
15
cm
−3
doping that may be used in diodes operating at lower voltages. The n region also needs to be thicker to support the higher voltage. This thicker lower doped region stores more charge during forward operation than in low voltage devices, and the results show that the switching time is not a significant advance on conventional types. Thus, to extend the usability of ultrashort heterojunction rectifier structures to high voltage diodes such as those used in power
Ho Tu-Tu
Koninklijke Philips Electronics , N.V.
Nelms David
Waxler Aaron
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