Abstract:
A semiconductor rectifier includes an intermediate semiconductor region ( 29 ) extending between anode ( 9 ) and cathode ( 7 ) contacts. A trenched gate ( 19 ) with insulated sidewalls ( 15 ) and base ( 17 ) can deplete the intermediate region. However, a shield region ( 23 ) acts to shield the intermediate region ( 29 ) from the gate ( 19 ) to allow current to flow in dependence on the polarity of the voltage applied between anode and cathode contacts ( 9, 7 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a semiconductor rectifier, and in particular to a semiconductor rectifier using a trench structure. 
     BACKGROUND OF THE INVENTION 
     A wide variety of different semiconductor rectifiers are known. The most widely adopted are conventional bipolar diodes and Schottky diodes. 
     A particularly desirable rectifier parameter is a good blocking characteristic in the off-state, including a high breakdown voltage. It would accordingly be desirable to provide a rectifier with such enhanced blocking capability. 
     One method of providing a rectifier is to use a conventional MOSFET as a rectifier. In this case, in the on state the channel is essentially resistive giving some conduction even at very low forward voltages. The disadvantage is that the gate of the MOSFET needs to be controlled synchronously with the voltage across the source and drain of the MOSFET in order to provide rectifying action. This requires additional control circuitry and is generally undesirable. 
     There thus remains a requirement for an improved rectifier. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided a semiconductor rectifier, comprising: an anode contact and a cathode contact; an intermediate semiconductor region of first conductivity type extending between the anode and the cathode contacts; at least one trenched gate electrode flanking the intermediate semiconductor region; a shield semiconductor region of second conductivity type opposite to the first conductivity type between the at least one trenched electrode and the intermediate semiconductor region, the shield semiconductor region being connected to one of the anode and the cathode contacts; a gate insulating layer between the shield semiconductor region and the trenched electrode; and a separate gate contact for applying a voltage to the gate to deplete the intermediate semiconductor region; wherein the intermediate semiconductor region can be depleted by applying a voltage to the gate and a voltage of a first polarity between anode and cathode and when a voltage of opposite polarity to the first polarity is applied between anode and cathode the shield semiconductor region conducts to shield the intermediate semiconductor region from the voltage applied to the gate to allow a conductive path through the intermediate semiconductor region between the anode and the cathode. 
     The invention works by applying a constant voltage to the gate sufficient to deplete the intermediate region. However, the shield region is filled with carriers when a voltage of one polarity is applied between the anode and cathode. For example, with an n-type intermediate region and a p-type shield region connected to the anode, the p-type shield regions will be filled with holes when the anode is positive with respect to the cathode. In this state, the shield regions will shield the intermediate regions from the bulk of the effect of the applied gate voltage, so that the anode and cathode are connected through the intermediate semiconductor region. The on-resistance of this structure can be quite low. 
     Keeping to the same example, when the anode is negative with respect to the cathode, the shield regions will be depleted and no longer shield out the effect of voltage applied to the gate electrode. The gate electrodes are then free to deplete the intermediate region and thus prevent a current flow between anode and cathode. 
     Preferably, the anode contact to the intermediate semiconductor region is a Schottky contact. Using such a structure, the Schottky contact causes rectification at low anode-cathode voltages. At higher forward voltages, the device conducts. At higher reverse voltages, the shield semiconductor region is depleted, as is the intermediate semiconductor region, and some of the voltage is dropped across the intermediate region. In this way, higher breakdown voltages can be obtained than simply using the Schottky contact itself for any given doping of the intermediate region. Preferably, the Schottky contact is a low barrier Schottky to minimise voltage drops in the structure. Preliminary modelling results suggest that the device according to the invention may deliver a greater improvement in breakdown voltage than using a trenched Schottky device where the gate is connected to the anode rather than to a more negative constant voltage source as in the present invention. 
     Conveniently, the intermediate semiconductor region may be a semiconductor body having opposed first and second faces, with the anode contact on one face and the cathode on the opposite face. The trenched gate electrode may then be provided in an insulating trench extending through the body towards the second face from the first face. The wall and base of the trench may be insulated by the gate insulating layer. 
     The shield semiconductor region may then extend along the outside of the side walls of the trench. 
     In an alternative structure, the shield semiconductor region may extend not merely along the side walls of the trench, but also underneath the base of the trench. In this way, the effect of any regions of high ionisation integral and hence high risk of breakdown that might otherwise be present at the corner between the intermediate semiconductor region, the shield semiconductor region and the gate insulating layer can be reduced. The alternative arrangement accordingly can provide a higher breakdown voltage. 
     The most similar device of which the applicants are aware is a static induction thyristor (SIT). This relies on depletion layers spreading from buried p-type regions to join up and prevent current flow between anode and cathode. Accordingly in an SIT the p-regions will deplete the path between anode and cathode regardless of the voltage between anode and cathode. The arrangement of the invention works in a similar but different way. Again, a depletion region extends to pinch off conduction but it will do this depending on the voltage applied to the p-region. The polarity of the voltage applied between anode and cathode causes the shield regions to conduct or not to conduct, and hence allows or prevents conduction between anode and cathode. 
     The structure according to the present invention responds spontaneously to changes in the anode bias to go into a conducting or a blocking state depending on the polarity. Thus, the conducting and blocking states come about automatically when the anode and cathode are biased, without the need to drive the gate “synchronously” with changes in anode biased plurality, as would be required using a MOSFET. 
     The only additional circuitry required is a simple negative voltage source to permanently bias the gate. This is available in many application circuits and does not present a significant drawback. 
     It will be appreciated the invention could be applied to either p-type or n-type circuits, in a variety of different materials, dopings or structures. 
     Other advantageous technical features of the present invention are set out in the attached dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Specific embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings in which 
     FIG. 1 shows a first embodiment of a semiconductor rectifier according to the invention; 
     FIG. 2 shows part of the structure of FIG. 1 used for modelling; 
     FIG. 3 shows a comparative example similar to the structure of FIG. 2; 
     FIG. 4 shows a second embodiment of a semiconductor rectifier according to the invention; 
     FIG. 5 shows the breakdown voltage as a function of mesa width for a number of gate voltages; 
     FIG. 6 shows the voltage contours in the second embodiment of the invention; 
     FIG. 7 is a graph showing the ionisation integral as a function of the anode-cathode voltage; 
     FIG. 8 is a graph showing the breakdown voltage as a function of doping concentration in the intermediate layer; 
     FIG. 9 is a graph showing the voltage contours in a device according to a third embodiment of the invention having a 2 μm thick gate oxide layer; 
     FIG. 10 is a graph of ionisation integral against anode voltage for a number of different gate voltages; and 
     FIG. 11 is a graph of current against forward bias for a device according to fourth embodiment of the invention. 
    
    
     It should be noted that all of the Figures are diagrammatic and not to scale, and that the relative dimensions of the parts have been exaggerated and reduced in size for clarity and convenience. Like reference numbers are generally used to refer to corresponding or similar features. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a semiconductor body  1  of a first conductivity type has a first face  3  and an opposed second face  5 . A low barrier Schottky contact  9  is provided on the first face  3  and an ohmic contact  7  on the second face  5 . In the described embodiment the first face  3  may be considered to be the top face and the first contact  9  on the first face  3  is an anode contact. The second face  5  is in the example the bottom face and carries the cathode contact  7 . 
     A plurality of trenches  11  define at least one semiconductor mesa  13  between the trenches. The mesa shape may be, for example, hexagonal, square, or curved. A plurality of mesas may be defined as illustrated in FIG. 2, which shows a hexagonal array of mesas  13  and corresponding hexagonal network of trenches  11 . In this example, the trenches are all linked together. The trenches may be, for example, between 1 μm and 10 μm deep. Modelling suggests that this range of depths is sufficient for the device to work without requiring trenches of excessive depth. 
     The trenches have insulated side walls  15 , an insulated base  17  which together form a gate insulator  15 , 17 , and contain conductive material forming a gate  19 . The material of the side walls  15  and base  17  may be, for example, oxide, nitride, or other convenient insulating material. The gate may be formed of any suitable conductor or semiconductor, for example, doped polysilicon or amorphous silicon. 
     A gate contact  21  is provided to connect to the conductive gate  19 . 
     The central region  29  of the mesas  13  is doped n-type, and indeed in the example is integrally formed with the material of the semiconductor body  1 . As the skilled person will appreciate, it is of course also possible that the mesas are formed in an epitaxial layer. The point is that there is a conductive path from the first electrode  9  to the second electrode  7  through a semiconductor intermediate region  29 , here the central region of the mesa  13  which may be depleted by applying a voltage to the gate. In the example, the intermediate region  29  which is controlled by the gate  11  is connected to the cathode contact  7  through body  1 . 
     A shield region  23  of semiconductor material is provided in the mesa  13  between the trench side walls  15  and the central region  29  of the mesa  13 . The shield region  23  is doped to be of opposite conductivity type to the intermediate semiconductor region  1 , 29  but more lightly doped. Thus, the intermediate semiconductor region  1 , 29  may be doped n-type and the shield region  23  p − -type. Alternatively, the intermediate semiconductor region  29  and body  1  may be p-type and the shield region  23  n − -type. In this latter case, the top electrode  9  is more properly referred to as the cathode and the lower electrode  7  the anode. 
     The shield region  23  is connected to the anode  9 , which extends over both shield region  23  and intermediate region  29  to contact both regions. 
     In use, a negative gate voltage is applied to the gate electrode  11  on gate contact  21 . Voltages in the present description are voltages referred to the voltage applied to the lower electrode  7  (cathode). 
     When a positive voltage is applied to the anode  9 , the p-type shield regions  23  are filled with holes and accordingly conduct when the anode  9  is positive with respect to the cathode  7 . In this state, the shield regions  23  can shield the intermediate regions  29  from the bulk of the effect of the applied gate voltage, so that the anode  9  and cathode  7  are connected through the intermediate semiconductor region  29 , which is doped sufficiently to conduct. 
     Conversely, when a negative voltage is applied to the anode, the shield region  23  becomes depleted of holes and is no longer able to shield the intermediate region  29  from the effects of the voltage applied to the gate electrode  11 . The voltage applied to the gate electrode  11  accordingly depletes the intermediate region  29  and blocks conduction between anode  7  and cathode  9 . 
     A specific structure according to the invention was modelled, purely by way of example. The shield region  23  in the modelled structure was doped p − -type with a doping concentration of 1.5×10 17  cm −3 . The doping concentration in the intermediate semiconductor region was set to be 4×10 16  cm −3 . The gate oxide thickness was 0.04 μm, and the width of the shield region  23  0.3 μm. These values apply to all the modelled results below except where stated otherwise. 
     The results for this example according to the invention were compared with a structure according to a comparative example illustrated in FIG. 3 in which the anode contact  9  and the gate contact  21  are replaced by a single Schottky contact  25 . The remaining layers and doping concentrations were the same as in the modelled structure. 
     Initial modelling was carried out for trench depths between 1 and 2.5 μm, but the properties of the devices did not vary much in this range. The single Schottky contact device (FIG. 3) gave a breakdown voltage of around 25 volts. With the example according to the invention, a gate voltage of −18 volts applied to the gate electrode  11  gave an anode-cathode breakdown voltage of 30 volts. Similar results were obtained with −20 volts applied to the gate  11 . Accordingly, the device according to the invention gave a better breakdown voltage than was obtained with the Schottky structure. 
     An alternative structure according to a second embodiment, illustrated in FIG. 4, differs from the arrangement of FIG. 1 in that the shield region  23  is provided not only adjacent to the side walls  15  of the trench but also under the base  17 . The depth of the shield region below the trench was fixed at 0.4 μm. This minimises any possibility of breakdown at the corner between the intermediate semiconductor region  1 , the shield region  23  and the sidewalls  15  and base  17  of the trench. 
     Results were then modelled taken for the comparative example of FIG.  3  and the device according to the invention of FIG. 4, and these results are presented in FIG.  5 . The modelling was carried out for three different gate voltages applied to the gate  11 , −22 V (curve  53 ), −24 V (curve  55 ) and −26 V (curve  57 ), together with the comparative example of FIG. 3 (curve  51 ). The modelling was carried out with the same doping levels as previously, for a number of different mesa  13  widths. 
     As will be apparent, the invention delivers significantly greater breakdown voltages for small mesa widths than the Schottky trench structure according to FIG.  3 . It is of course to be expected that the benefits of the invention are observed for small mesa widths, since the width of the mesa  13  should be sufficiently small so that the semiconductor in the mesa  13  can be depleted by the voltage applied to the gate electrode. It will be noted that the performance of the rectifier according to the invention is accordingly much better at low mesa widths, particularly below 3 μm. Furthermore, the greater the magnitude of the negative voltage applied to the gate electrode  11 , the better the breakdown voltage of the resulting structure. 
     FIG. 6 illustrates the voltage contours  61  when the device according to FIG. 4 is blocking 40 volts with a negative gate voltage of −24 volts applied to the gate electrode  11 . The horizontal axis shows the distance from the centre of the mesa  13 ; accordingly this device has a mesa width of 2 μm. The figure clearly shows the absence of voltage contours  61  in the trench region  11 . 
     FIG. 7 is a plot  71  of the ionisation integral as a function of anode voltage with a negative gate voltage of −24 volts. Breakdown occurs when the ionisation integral exceeds  1 . As can be seen, the breakdown occurs at around −40 volts. 
     The effect of varying the doping concentration in the n-type intermediate region  1  was then determined. A plot of the breakdown voltage against concentration in the n-type intermediate region varying between 3×10 16 cm −3  and 5×10 16  cm −3  for a number of different gate voltages is provided in FIG.  8 . Curve  81  is for a gate voltage of −24 V, curve  83  for −26 V, curve  85  for −28 V, curve  87  for −30 V and curve  89  for −32 V. Again, a comparison curve  80  for a Schottky trench device (FIG. 3) is also shown. As can be seen, the device according to the invention produces some improvement over a standard Schottky device even with voltages from −28 volts to −32 volts applied to the gate electrode  11 , but the best results are obtained with voltages of −24 volts and −26 volts applied. 
     Results were then obtained using a much thicker gate oxide, 0.2 μm thick rather than 0.04 μm as previously. In this case, with a gate voltage of −30 volts applied, the breakdown voltage was 56 volts between anode and cathode. FIG. 9 illustrates the voltage contours. 
     It should not be thought that the gate voltage on the gate can be increased without limit to improve the performance. FIG. 10 illustrates the ionisation integral as a function of anode voltage for different gate voltages for a mesa width of 1.5 μm and a doping concentration in the n-type intermediate region of 5×10 16 cm −3 . As will be seen, a voltage of −30 volts applied to the gate delivers a plateau in the ionisation integral largely below the thresheld value of 1.00. A good gate voltage to choose is a gate voltage that has a high magnitude (i.e. is more negative), since that increases the break down voltage, but in which the plateau in the ionisation integral occurs at a value less than 1.00 Higher magnitude gate voltages (more negative) have a plateau at an anode voltage where breakdown has already occurred, and so deliver worse breakdown voltages. Accordingly, in this case a gate voltage of −30 V gives a good breakdown performance. 
     The doping in the p-region may also be adjusted to affect the plateau. In general, when the doping concentration in the p-region is decreased, the plateau will shift downwards to allow a higher negative gate voltage to be applied. 
     FIG. 11 shows the current in amps for a forward voltage supplied between anode and cathode for the case of a Schottky contact from the anode contact  9  to the top face  3  of the semiconductor body. This forward plot, unlike the reverse bias plot, is exactly the same as would be expected for a conventional Schottky diode. 
     Suitable methods of making the device will be readily apparent to the skilled reader from the description of the device itself. In particular, the trenches  11  may be formed by conventional anisotropic dry etching and the gate oxide  15 , 17  formed by thermal oxidation. 
     Although the specific embodiment shown has an n-type body with p-type shield layers, the skilled person will appreciate that these can readily be reversed. Moreover, the doping levels and mesa sizes may be adjusted to achieve the desired results. 
     From reading the present disclosure, other variations and modifications will be apparent to persons skilled in the art. Such variations and modifications may involve equivalent and other features which are already known in the design, manufacture and use of semiconductor devices and which may be used in addition to or instead of features described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of disclosure also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to any such features and/or combinations of such features during the prosecution of the present application or of any further applications derived therefrom.