Patent Application
20090140388
Multiple semiconductor devices are known, which use emitter structures to emit charge carriers into semiconductor areas or volumes which are neighbored to the emitter structure.
For example, bipolar transistors comprise an emitter-volume, a base-volume, and a collector-volume of semiconductor material with alternating dopants. The emitter structure often has a high concentration of dopants, such as to provide charge carriers with high efficiency. Further examples for devices having an emitter structure are Electrostatic Discharge (“ESD”)-protection devices, used to protect electronic equipment, when an electrostatic discharge occurs. To this end, the charge provided by the electrostatic discharge event is transferred via the ESD-device, rather than via the protected device, which could eventually be destroyed by the high current produced by the deposited charge.
ESD-protection devices are designed to operate at a predetermined threshold voltage, i.e. when the threshold voltage is exceeded, the ESD-protection device typically connected in parallel to the protected device, becomes conductive with a relatively low resistance, such as to transport the current and prevent it from flowing through the protected device. Such, an ESD-protection device could be compared with a thyristor, which starts conducting at a predetermined threshold voltage. However, ESD-devices stop conducting when the voltage drops below a switch-off voltage, which depends on the specific design of the ESD-protection device. In some of the devices, the switch-off-voltage is significantly lower than the threshold voltage of the device.
Some embodiments discussed in further detail below comprise an emitter structure for emitting charge carriers of a first conductivity type. The emitter structure may emit the charge carriers in a vertical direction and comprise emitter volumes of first conductivity type material as well as semiconductor volumes of second conductivity type material, which neighbor the emitter volumes in a lateral direction perpendicular to the vertical direction. In some embodiments, the lateral dimension in the lateral direction of the emitter volume may be chosen such that an emitter efficiency with which charge carriers are emitted from the emitter volume is, in a predictable manner, limited for high currents. This may be the case, when the lateral dimension is chosen such that space charges created within the emitter volume by lateral diffusion of second conductivity type carriers from the neighboring semiconductor volumes limit a maximum charge carrier density of first conductivity type carriers within the emitter volume by more than 20% as compared to identical emitter volume not being neighbored by a semiconductor volume of the second conductivity type.
The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles of the invention. Other embodiments of the present invention and many of the intended advantages of the present invention will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.
Several embodiments will in the following be discussed referencing the enclosed figures, wherein:
In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.
Embodiments of the invention may be implemented, for example, partially integrated circuits or fully integrated circuits.
According to some embodiments, the lateral dimension is appropriately chosen to match a predetermined lateral dimension for the reasons set forth below. Charge carriers of the second conductivity type diffuse (using the electron-hole-model) laterally into the emitter volume 10, such that space charge regions 18a and 18b are created at the borders to the semiconductor volumes 16a and 16b of the second conductivity type material. The extension of the space charge regions in the lateral dimension depends also on the dopant concentration of the emitter volume 10 and the semiconductor volumes 16a and 16b. By a variation of the lateral dimension 12, the volume or area 20 within the emitter volume 10, which is not populated by space charges, may be varied. That is, making the lateral dimension 12 bigger, the unpopulated area would increase, i.e. a region 20 between the space charge volumes 18a and 18b will become wider. By choosing the lateral dimension 12, the charge transport properties of the emitter volume 10 may be varied as detailed below. This effect may also be seen as a variation of the emitter efficiency of an npn-transistor formed using the emitter structure 2, the base volume 6 and the collector volume 8.
For the following considerations, the structure in
Assuming there are no space charge areas 18a and 18b present, electrons are emitted from the emitter volume 10 into the base volume 6, from where they are transferred to the collector volume 8. This situation may be compared to the conditions within the central area 20 of the emitter volume 10, where no space charges are present.
However, electrons intended to travel in the vertical direction within the space-charge areas 18a and 18b have a higher probability of recombining with the space charges, i.e. with the holes causing the space charges in volumes 18a and 18b. When the current density within the emitter volume 10 is low, the generation of the space charge areas 18a and 18b by the semiconductor volumes 16a and 16b is of minor influence to the device performance, as npn-operation can be achieved with the electrons emitted or transported through the central area 20. However, the more the current increases, the more of the emitter volume 10 will be used, i.e. the charge carrier density within the emitter volume 10 increases. That is, in a simple picture, while being operated with relatively low currents, the central volume 20 of the emitter volume 10 is used to transport charge carriers or electrons. The more the current increases, the more area (more of the complete lateral dimension 12) will be used for charge transport. The more area used, the more electrons travel along paths having a high recombination probability, that is pass through areas or volumes of high space charge. The higher the current, the higher the probability that the single electron recombines or is transferred to the laterally neighbored semiconductor volumes 16a or 16b. When discussing a bipolar transistor, the effect could also be described as a dynamical variation of the emitter efficiency of the transistor, caused by the previously described effects.
In other words, the lateral dimension of the emitter volume 12 of the emitter structure of
For example, limiting a maximum density of first conductivity type carriers by more than 20% as compared to an undisturbed emitter volume of the same geometrical shape may be implemented. This may be achieved by choosing the lateral dimension 12 in the regime of microns. Thus, for example, some embodiments may have lateral dimensions smaller than 5 μm, 1 μm or 0.8 μm.
At low current densities, the transmission probability is close to an upper threshold value 34, which is close to the equivalent value of an emitter structure having an infinitely extending emitter volume. That is, the value of the threshold is mainly due to the inherent material properties or the dopant profiles of the emitter volume.
If, however, the current density increases, the transmission probability approaches a lower threshold 36, which depends on the extension of the space charge areas into the emitter volume. By varying the lateral dimension 12, the attenuation of the transmission probability may be varied. The solid line 38 may be an example for an emitter structure with a first lateral dimension, the dashed line 40 may be a transmission probability resulting from an emitter structure using similar doping concentrations, but smaller lateral dimensions 12 of the emitter volumes.
To be able to switch a high overall current having emitter volumes of limited lateral dimension, the generation of multiple emitter volumes within an emitter structure may be foreseen, each of the emitter volumes fulfilling the constraint with respect to the lateral dimension. Each of the emitter volumes may be electrically short circuited with each other to form a common terminal area. This may, on the one hand, provide the dynamical variation of the current transport probabilities and, on the other hand, provide the possibility of switching a high overall current.
The adjustment of the charge transport properties may be achieved by a fairly simple modification of the layout, for example, by appropriately designing the geometry of the photo-lithographic masks. This is simpler to be controlled as, for example, the adjustment of dopant profiles within tight tolerances.
The embodiment of an ESD-protection structure 54 includes an emitter structure 56, which is, in a vertical direction 58, neighbored by a base volume 60. In the particular embodiment of
The base volume is furthermore neighbored, in the vertical direction 58, by a collector volume 62. The emitter structure 56 includes multiple emitter volumes of first conductivity type material, as for example the emitter volumes 64a and 64b, which are n-doped.
In a lateral direction 65 perpendicular to the vertical direction, the emitter volumes are neighbored by semiconductor volumes of second conductivity type material, as for example by semiconductor volumes 66a and 66b. The base volume 60 is made up of second conductivity type material, i.e. of p-doped semiconductor material. The collector volume 62 is made up of n-doped material, i.e. of the first conductivity type material. To provide for a low loss electrical contact, the collector volume 62 furthermore includes a highly doped contact area 68, which could for example, be contacted by a metallization or similar measures.
The semiconductor structure of
For the explanation of the functionality of the ESD-protection device, several discrete circuit elements are illustrated within the 2-dimensional projection of
The fact that the emitter volumes 64a and 64b are, in the lateral direction, neighbored by the semiconductor volumes 66a and 66b, dynamically limits or alters the charge transport properties of the emitter volumes, as previously described.
The purpose of an ESD-protection device is to efficiently transfer ESD-charges, when a threshold voltage is exceeded. The threshold voltage may be higher than an operating voltage of the device to be protected. Thus, when an electronic discharge occurs, the threshold voltage is exceeded and the ESD-protection device becomes conductive, such that a charge deposited during the ESD event can be transferred to ground.
The ESD-protection structure may be connected to an external circuitry such that the positive potential is applied to the collector volume 62 (via contact area 68). Thus, when the geometrical dimensions of the collector and base volumes are appropriately chosen, the EDS-protection structure of
If, however, the voltage rises due to an ESD-event, an avalanche breakthrough may occur at diode 70, such that current flows through the diode 70. As the substrate itself has an inherent resistance, the rising avalanche current through the substrate causes a voltage gradient or a potential gradient within the structure. The higher the resistivity of the material, the steeper the gradient per unit length. At a certain current flow through the diode 70, the potential difference between the base volume and the emitter volume of the transistor 72 will be as high as to switch the transistor on, such that current is also transported by the transistor structure, i.e., through emitter volumes 64a and 64b. Once the transistor structure 72 is switched on, a resistance of the device decreases, and current is efficiently transported through the structure.
This principle behavior is illustrated by the dashed line of
Thus, using the structure of
By geometrically structuring the emitter structure 56 of the ESD-protection device in an appropriate manner, for example by varying the lateral dimension of the emitter volume 64a and 64b, the charge transport efficiency or the charge transfer capability of the emitter volumes 64a and 64b can be adjusted. With embodiments of an ESD-protection device having an emitter structure 56, the hold-voltage may be varied, and in particular, be chosen such that the ESD-protection structure 54 shows the behavior of the solid curve 88, where the hold-voltage 90 is close to the threshold voltage 80. The ESD-protection device may be operative with a characteristic close to the characteristic of a switch. This characteristic may be obtained and the threshold voltage 80 and the hold-voltage 90 may be arbitrarily chosen using an embodiment of the ESD-protection structure of
Some embodiments may, for example, be dimensioned such that the threshold voltage is around 50 V, whereas the hold-voltage is 40 V or more. Thus, devices having an operation voltage of somewhat less than 40 V, say for example, 35 V, may be reliably protected. To achieve a hold-voltage close to the trigger voltage, some embodiments may have base volumes, extending in a lateral dimension perpendicular to the vertical direction of the device by no more than 5 μm. Further embodiments may have emitter volumes with a lateral dimension smaller than 1.0 μm, or even as small as 0.8 μm. In further embodiments, the dopant concentrations of the emitter volumes and the semiconductor volumes is greater than 1*1019 1/cm3 and smaller than 5*1021 1/cm3. As the previous discussion indicated, the precise geometrical shape in the lateral dimension is of minor concern. It is, instead, a lateral dimension of the emitter volumes which is to be adjusted. The emitter volumes, or, to be more precise, the lateral shape of the emitter volumes of the emitter structure 56, do not necessarily have to be rectangular. For example, the emitter volumes could be also be circular, six-cornered, or of any geometrical shape, provided the lateral dimension is chosen such as to achieve the desired dynamical variation of the efficiency of the emittance of charge carriers into the base volume.
In the embodiment of
A vertical charge transport volume 108 of first conductivity type material extends, in the lateral device direction 102, from the terminal border 106 to a lateral device border 110. The vertical charge transport volume 108 includes, on a main surface 111 of the ESD-protection structure, a highly doped contact volume 112 of first conductivity type material. The highly doped contact volume 112 is used to electrically contact the vertical charge transport volume 108 using, for example, a metallization. The ESD-protection structure further includes a lateral charge transport volume 114, which is, in the vertical direction 58, neighbored to the collector volume 62 and which extends in the lateral device direction 102 up to the lateral device border 110. As such, the vertical charge transport volume 114 and the lateral charge transport volume 108 form a common volume with a high dopant concentration. This volume is intended to transport the charge carriers transmitted through the vertical components of the ESD-protection structure to the laterally displaced contact volume 112. To be more precise, charge carriers, or electrons travel through the vertical and lateral charge transport volumes 108 and 114 after having passed the transistor structure 72 or the diode 70.
The ESD-protection structure of
The electrostatic discharge protection device of
To be more precise, with the lateral ESD-protection structure of
Although most of the previously discussed embodiments relate to ESD-protection devices, or ESD-protection semiconductor structures, further embodiments of emitter structures may be applied to different semiconductor applications in which charge carriers are emitted or injected into neighboring semiconductor volumes. This could, for example, be applied within vertical FETs, such as JFETs, IGBTs or other vertical power transistors such as for example trench-transistors or the like. Furthermore, bipolar transistors using embodiments of emitter structures may be used.
The previous embodiments have mainly been described in terms of n-doped emitter volumes. It goes without saying that further embodiments may be implemented in a complementary technology, namely, using p-doped emitter volumes and n-doped semiconductor volumes laterally neighboring the emitter volumes.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments illustrated and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.
