BI-DIRECTIONAL SEMICONDUCTOR SWITCH, AND SWITCH CIRCUIT FOR BATTERY-POWERED EQUIPMENT

Abstract: The invention provides a trench-gate bi-directional semiconductor switch (101), suitable for connecting and disconnecting a battery (103) in a battery-powered equipment, for example a portable computer. The switch has plane p-n junctions (31, 32) which are formed by a channel-accommodating body region (3) between first and second regions (1, 2) and are separated from respective first and second main electrodes (11, 12) by these first and second regions (1, 2). The trench-gate (13) extends in a trench (33) through this sandwiched region structure (1, 3, 2). The body region (3) has a substantially uniform first-conductivity-type doping concentration (P) that is of smaller magnitude than the doping concentrations (N +) of the second conductivity type of the first and second regions (1, 2). The dimensions (X, Y) and doping concentration (P) of the body region (3) are sufficiently large that the body region (3) retains an undepleted first-conductivity-type area (3a) of floating potential in all bias conditions of the first and second main electrodes (11, 12) and of the trench-gate (13) so as to give the switch (101) in an off-state a blocking voltage capability between the first and second main electrodes (11, 12) that is determined by the plane p-n junctions (31, 32).

Description: DESCRIPTION BI-DIRECTIONAL SEMICONDUCTOR SWITCH, AND SWITCH CIRCUIT FOR BATTERY-POWERED EQUIPMENT This invention relates to bi-directional semiconductor switches, and more particularly to three-terminal trench-gate switches suitable for connecting and disconnecting a battery in a battery-powered equipment, for example a portable computer or an electric razor. The invention also relates to switch circuits comprising such trench-gate bi-directional semiconductor switches and to battery-powered equipment comprising such switches and switch circuits. In portable applications, a battery is used to power electrical equipment and becomes discharged in normal use. The expression "battery" as generally used nowadays, and as specifically used in the present text, is understood as including within its scope a single power cell, as well as relating to combinations of power cells. When a re-chargeable battery is used, it may be re-charged from an external power source, during which time it may be disconnected from the load power line in the equipment. Semiconductor power switches are normally used for connecting and disconnecting the battery. The switch has to block in both directions, for example to prevent overcharging and/or to prevent operation into a short-circuited load under a fault condition. It may also have to conduct in both directions, for example depending on whether the cell is being charged or discharged. The normal solution is to use as the switch a back-to-back arrangement of two low voltage MOSFETs (insulated-gate field-effect transistors) in series connection. This solution requires a large semiconductor area (for the layout of two separate MOSFETS) and has four external terminals (or six terminals if the two MOSFETs are not in a single packaged device). United States patent specification US-A-5,682,050 discusses such background situations for the switch circuits and devices, together with the difficult requirements in different circuit conditions. It also proposes a particular bi-directional semiconductor switch as an improvement over a back-to-back arrangement of two MOSFET devices. This bi-directional switch is a four-terminal MOSFET in which there is no source-body short. It comprises a semiconductor body in which a channel-accommodating body region of a first conductivity type is present between first and second regions (source and drain regions) of an opposite second conductivity type. The body region may have a substantially uniform doping concentration of the first conductivity type which is of smaller magnitude than the doping concentrations of the second conductivity type of the source and drain regions. The source and drain regions form p-n junctions with the body region and separate these p-n junctions from respective first and second main electrodes (source and drain electrodes) in electrical contact with the source and drain regions, i.e. the p-n junctions are not shorted by these electrodes. A gate is capacitively coupled to the body region for inducing therein a conduction channel of charge carriers of the second conductivity type between the source and drain regions in an on-state of the switch. The fourth terminal of this switch is an electrode connection to the body region. Published PCT International Application WO 97/44828 discloses a three-terminal bi-directional semiconductor switch in the form of a trench-gate device, comprising a semiconductor body in which the channel-accommodating body region forms plane p-n junctions with the first and second regions of the second conductivity type. As in US-A-5,682,050, the first and second regions separate the p-n junctions from respective first and second main electrodes in electrical contact with the first and second regions. In this device of WO 97/44828, however, the first region and first main electrode are present adjacent a first major surface of the body from which a trench-gate extends through the first region and through the channel-accommodating region and into the second region. This trench-gate is capacitively coupled to the channel- accommodating region for inducing therein a conduction channel of charge carriers of the second conductivity type between the first and second regions in an on-state of the switch. The channel-accommodating region has a substantially uniform doping concentration of the first conductivity type which is of smaller magnitude than the doping concentrations of the second conductivity type of the first and second regions. The whole contents of both US-A-5,682,050 and WO 97/44828 are hereby incorporated herein as reference material. The thickness, width and uniform doping concentration of the body region in the switch of WO 97/44828 are chosen such that the body region is fully depleted by the combined effects of the gate and of the p-n junctions in an off-state of the switch. The uniform doping concentration of the body region is sufficiently high to avoid punch-through of the depletion layers from the p-n junctions. However WO 97/44828 teaches (for example at page 4 line Iff) that, first and foremost, the body region must have a well-defined potential in order to prevent the threshold voltage of the MOSFET from drifting upward and downward uncontrollably. In the absence of a shorting electrode connection or separate electrode connection to the body region, WO 97/44828 teaches (for example at page 6 line 4ff, and page 7 lines 3 to 34 ) that this is achieved by fully depleting the body region so that it does not float in potential. Thus, although the body region of the switch proposed in WO 97/44828 is not fully depleted in the absence of the effect of the gate, the width of the body region (as determined by the mesa width of its cellular trench-gate structure) is made sufficiently narrow that the added effect of the gate potential in the off-state of the switch effectively depletes the entirety of the body region. The relevant criteria for the dimensions and doping concentration of the body region are taught at page 10 line 15 to page 13 line 18. These criteria are not easy to implement in a high-volume manufacturing context. It is an aim of the present invention to provide an improved trench-gate bi-directional semiconductor switch which can be implemented more easily in a high-volume manufacturing context with well-defined blocking voltage characteristics, and to provide an improved switch circuit comprising such a switch, suitable for connecting and disconnecting a battery in a battery-powered equipment. According to the present invention, there is provided a trench-gate bi-directional semiconductor switch having the features set out in Claim 1, and there are also provided switch circuits having the features set out in Claims 5 and 6. In switches and switch circuits in accordance with the present invention, the dimensions and doping concentration of the body region are sufficiently large that an area of the body region remains undepleted (retaining its first conductivity type as an area of floating potential) in all bias conditions of the first and second main electrodes and of the trench-gate. The realisation of this switch structure (in terms of the relevant criteria for the dimensions and doping concentration of the body region) is easier to implement in a high-volume manufacturing context, than that of the switch of WO 97/44828. In particular, the body region does not need to be made so narrow, and its precise width does not have to correspond exactly with the laterally depleting effect of the gate. Although the resulting undepleted area of the body region of a switch in accordance with the invention does float in potential, its potential does not drift uncontrollably. Its potential moves between well-defined states, which the Applicant finds to be acceptable for use in switch circuits for battery-powered equipment. Thus, the present invention permits the realisation of a three- terminal switch with an advantageous structure for manufacture. Furthermore, due to the presence of the undepleted area of the body region, the blocking voltage capability of the switch in the off-state is determined by breakdown voltages of the plane p-n junctions (together with the gain of the parasitic bipolar transistor formed by these two p-n junctions). These breakdown voltages can be reproducibly controlled in a high-volume manufacturing context. Embodiments of the present invention are now described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a cross-sectional view of an active central part of a trench- gate bi-directional semiconductor switch in accordance with the invention; Figure 2 is a circuit diagram of the switch of Figure 1 in a switch circuit of a battery-powered equipment in accordance with the invention; Figure 3A is a simplified cross-sectional view of the region structure of the switch part of Figure 1 , when the trench-gate bi-directional semiconductor switch is in an on-state; Figure 3B is a simplified cross-sectional view of the region structure of the switch part of Figure 1 , when the trench-gate bi-directional semiconductor switch is in an off-state; Figure 4A depicts a change in gate voltage Vg with time t, when switching the trench-gate bi-directional semiconductor switch in a switch circuit; and Figure 4B illustrates the corresponding variation in potential Vf of the channel-accommodating body region of the trench-gate bi-directional semiconductor switch. It should be noted that all the Figures are diagrammatic. Especially in Figures 1, 3A and 3B, relative dimensions and proportions of parts of the drawings have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and different embodiments. The trench-gate bi-directional semiconductor switch 101 of Figure 1 is a low-voltage 3-terminal device comprising a semiconductor body 10 which accommodates an array of parallel device ceils between first and second main electrodes 11 and 12. These electrodes 11 and 12 provide the terminals S/D and D/S of the current-carrying path through the switch. Each cell of the switch comprises a channel-accommodating body region 3 of a first conductivity type (p-type in this example) which is sandwiched between first and second regions 1 and 2 of an opposite second conductivity type (n-type in this example) to form plane p-n junctions 31 and 32 with the regions 1 and 2. These first and second regions 1 and 2 (hereinafter termed "source and drain regions" respectively) separate the p-n junctions 31 and 32 from the respective first and second main electrodes 11 and 12 (hereinafter termed "source and drain electrodes" respectively) which are in electrical contact with the regions 1 and 2. The source region 1 and source electrode 11 are present adjacent a first major surface 10a of the body 10. The source electrode 11 is common to all the device cells of the array and connects in common the individual source regions 1 of the cells. The switch 101 has a trench-gate 13 present in a trench 33 which extends from the surface 10a through the source region 1 and through the body region 3 and into the drain region 2. The trench-gate 13 is insulated from the source electrode 11 by an intermediate insulating layer 20 on the trench-gate 13 and is contacted outside the plane of the Figure 1 drawing to provide the control, third terminal G of the switch. The plane p-n junctions 31 and 32 terminate at the sidewalls of the trench 33. The trench-gate 13 is capacitively coupled to the portion of the body region 3 adjacent the sidewall (across a gate insulating layer 23 in the Figure 1 embodiment). This capacitive coupling serves in known manner for inducing in the body region 3 of the first conductivity type a conduction channel 12 of charge carriers of the second conductivity type (electrons in the Figure 1 example) between the source and drain regions 1 and 2 in an on-state of the switch, as illustrated in Figure 4A. Thus, the control voltage Vg applied on the trench-gate 13 serves in known manner for controlling current flow in this conduction channel 12 between the main terminals S and D of the switch. The body region 3 has a substantially uniform doping concentration (P) of the first conductivity type which is of smaller magnitude than the doping concentrations (N+) of the second conductivity type of the source and drain regions 1 and 2. However, the dimensions X and Y and doping concentration P of the body region 3 are sufficiently large that there is always an inner area 3a of the body region 3 that remains undepleted. This undepleted area 3a retains its first-conductivity-type characteristics in all bias conditions Vs/d, Vd/s, Vg of the source and drain terminals S/D and D/S and of the trench-gate terminal G, and it has a floating potential Vf. Thus, in an off-state of the switch as illustrated in Figure 3B, this floating undepleted area 3a gives the switch a blocking voltage capability between the source and drain electrodes 11 and 12 that is determined by breakdown voltages of the plane p-n junctions 31 and 32 and the gain of the parasitic N+/P/N+ transistor 1 ,3,2. This floating undepleted area 3a gives the switch an important distinction as compared with the switch in WO 97/44828. Typically, the body 10 is of monocrystalline silicon, the trench-gate 13 is of conductively doped polycrystalline silicon on a gate insulating layer 23 of silicon dioxide, and the electrodes 11 and 12 are of, for example, aluminium. The doped trench-gate 13 is usually of opposite conductivity type (i.e. n-type in the Figure 1 example) to the conductivity type of the body region 3 in order to achieve a low threshold voltage. The switch typically comprises many thousands of parallel device cells in the semiconductor body 10 adjacent to the body surface 10a. The number of cells is dependent on the desired current-carrying capability of the switch. Switches in accordance with the invention may have any one of a variety of known cell geometries, for example an hexagonal close-packed geometry, or a square geometry, or an elongate stripe geometry. The trench-gate 13 extends laterally around the boundary of each cell, as a common insulated-gate electrode of the array. Thus, the trench-gate 13 laterally bounds the regions 1 and 3 in these cells, and part of the drain region 2. The drain region 2 is common to all the cells. By way of example, Figure 1 shows a vertical switch configuration in which the drain region 2 may be a silicon substrate of high conductivity, on which the body region 3 is epitaxially deposited. In this vertical configuration, the substrate (which forms the drain region 2) is contacted at the bottom major surface 10b of the device body 10 by the drain electrode 12. Thus, the device structure of the Figure 1 switch in accordance with the present invention is similar to that of a conventional trench-gate MOSFET and may be manufactured using known MOSFET technologies, except that there no epitaxial drain-drift region, and no source-body region short, and the body region 3 is an epitaxial layer. The active cellular area of the switch may be bounded around the periphery of the body 10 by any one of a variety of known MOSFET peripheral termination schemes, which are not shown in Figure 1. Such termination schemes normally include the formation of a thick field-oxide layer around the peripheral area of the body surface 10a, before the cell fabrication steps. Generally both the n-type regions 1 and 2 will have much higher donor doping concentratrations than the acceptor doping concentration of the p-type body region 3 and preferably substantially the same doping concentrations as each other, at least adjacent to their p-n junctions 31 and 32 with the body region 3. In this way, the switch can have a substantially symmetrical blocking voltage capability between its first and second main electrodes 11 and 12. Although the electrode 11 has been described as the source electrode and the electrode 12 as the drain electrode, it will be clear that these terms "source" and "drain" are interchangeable, particularly in this case of symmetrical characteristics. For this reason, in Figure 1 , the "source" terminal is designated S/D and the "drain" terminal is designated D/S. Similarly in Figures 3 and 4, the "source" voltage is designated Vs/d and the "drain" voltage is designated Vd/s. Figure 2 illustrates a specific example of a switch circuit for a battery-powered equipment, for example a portable computer. The circuit includes a trench-gate bi-directional switch 101 which comprises a semiconductor body 10 with the Figure 1 device structure. The switch 101 is represented by a special compound symbol of an insulated gate controlling conduction through back-to-back diodes, the insulated gate being the trench-gate 13 and the back-to-back diodes being the p-n junctions 31 and 32 formed with the p-type base region 3, at least the part 3a of which remains undepleted. The compound symbol also includes a third diode corresponding to the p-n junction which the p-type base region 3 also forms with the induced n-type channel 12 as a floating "back gate" of the switch. The switch circuit also comprises a rechargeable battery 103, a connectable battery charger 104, a control circuit 105, a power line 106 for powering a load 107 forming part of the equipment, and a return line 102. The battery 103 is coupled to the power line 106 via a trench-gate bi-directional semiconductor switch 101 in accordance with the invention. Thus, one main electrode 11 (terminal S/D) of the switch 101 is coupled to a first terminal 103A of the battery 103, and its other main electrode 12 (terminal D/S) is coupled to the power line 106. The control circuit 105 has an output line 111 to the switch 101 and also has sense lines 112, 113 and 114 respectively from the power line 106 and from the battery terminals 103A and 103B. The output line 111 is coupled to the trench-gate 13 (terminal G) of the switch 101 so as to apply a control signal Vg to the switch 101 for powering the power line 106 from the battery 103 when the conduction channel 12 is induced between the first and second regions 1 and 2 in the on-state of the switch 101 and for disconnecting the battery 103 from the power line 106 in the off-state of the switch 101 when the blocking voltage capability between the first and second main electrodes 11 and 12 is determined by the plane p-n junctions 31 and 32. Although not shown in Figure 2, another battery or an alternative power supply (for example, a mains ac/dc converter) may also be coupled to the power line 106 (for example via another switch, not shown), for powering the load 107 when the battery 103 is disconnected. Figure 2 does however show (in broken outline) a battery charger unit 104, which may additionally provide an alternative power supply to the load 107 when the battery 103 is being recharged. In the case of a portable personal computer, the battery 103 is usually re-charged using a charger unit 104 which is separate from the computer, as illustrated by its broken outline in Figure 2. When the charger unit 104 is plugged into the computer, it is coupled to the power line 106 via a terminal 106A and to the return line 102 via a terminal 102A. The sense line 112 may be used to detect the inclusion of the charger 104 in the switch control circuit, whereas the sense lines 113 and 114 may be used to sense the charge state of the battery 103. The control circuit 105 may be a known type of integrated circuit, separate from the switch 101. However, the control circuit 105 may be integrated with a switch 101 in known manner, for example in and on a p-type region of the body 10, adjacent the surface 10a. In a typical application, the bi-directional switch 101 only has to block about 10 volts in its off-state. In its on-state, the on-resistance through the switch 101 should typically not exceed 40 milliohms. These characteristics are achievable with the device structure of Figure 1 in accordance with the present invention. The switch 101 effectively replaces two back-to-back conventional MOSFETs, so offering at least a factor four reduction in silicon area. The Figure 1 device structure will first be compared with a conventional trench-gate MOSFET not in accordance with the present invention, in which an implanted/diffused p-body region 3' is shorted to the source region 1'. This short ensures that the potential of the p-body region 3' is precisely defined, and that the action of the parasitic transistor formed by the N+ source region 1 ', p-body region 3' and N+ drain region 2' is well suppressed. The shorting of the p-body region 3' to the source region 1' means that the MOSFET can only block in one direction, i.e. with D' (electrode 12') positive with respect to S' (electrode 11'). If the drain region 2' is made negative, then the p-n junction 32' between the body region 3' and the drain region 2' is forward biased and current flows. By contrast therewith, the trench-gate structure of Figure 1 has a p-body region 3 which is at a floating potential Vf and which is formed as a uniformally doped epitaxial layer rather than by diffusion. The Figure 1 device can therefore block in both directions. The blocking voltage is BVceo, the common-emitter breakdown voltage of the n-p-n parasitic transistor 1,3,2 with open base. The magnitude of BVceo is typically only a third of that of BVces (the collector- emitter breakdown voltage of the n-p-n parasitic transistor with its base shorted to the emitter), but this is not a limitation for a low voltage device such as the switches 101 and 102. Thus, the doping level of the p-body region 3 can be chosen to give a value of BVces which is high enough to give an adequate BVceo, without degrading the on-resistance of the switch. For such low voltage devices, the on-resistance is dominated by the channel 12 rather than the source and drain regions 1 and 2. Using an epitaxial layer for the region 3, together with a high doping level for the regions 1 and 2, gives a symmetrical structure which is optimum for bi-directional operation. Typical p-body region doping levels and dimensions are now discussed with reference to Figures 3A and 3B for a 10 volt device, assuming the BVceo is a third of the BVces. Assuming a one-sided approximation (in which the N+ doping of the regions 1 and 2 is much higher than the P doping of the region 3), then the P doping concentration should be no more than 3x10 16 cm "3 for the p- body region 3 to withstand 30 volts. Punch-through has to be prevented at the working voltage of 10 volts. In the p-body region 3 of 3x10 16 cm "3 , the width of the depletion region 30 at 10 volts is 0.6 μm (micrometres), and so a thickness X of, for example, 1.0 μm will be quite adequate for the region 3, leaving a floating undepleted area 3a of thickness x of about 0.4 μm. With this p-body thickness X, the specific on-resistance of the switch arising from the channel length will be at least as good as, for example, 50mΩ.mm 2 , which is a typical on-resistance achievable for a 30 volt conventional trench-gate MOSFET with a cell pitch H of 9μm and a width Y of 6μm for the body region 3'. The depth Xs of the source region 1 may be, for example, about 0.2μm. The cell pitch H in the bi-directional switch of the present invention can be quite small, as there is no space needed for short-circuiting the source region 1 to p-body region 3. This small cell pitch will also substantially reduce the on-resistance as compared with a conventional trench-gate MOSFET. However, the base region 3 must be given a sufficient width Y that the area 3a does not become depleted by the effect of the gate potential Vg. Typically, the width Y may be about 4μm or more (for example up to at most 6μm or so), in which case the undepleted width y of the area 3a is at least about 4μm in the off-state of the switch. The manner in which this switch 101 , 102 operates with a floating p-body region 3 will now be discussed. It is assumed that the switch is operated with a conventional polarity of positive drain voltage Vd/s (for example +10v), with the source voltage Vs/d at zero volts or at a reference voltage, and with a varying gate voltage Vg as illustrated in Figure 4A. The magnitude of Vg in Figure 4A is with respect to Vs/d, and so is the floating potential Vf of the p- body region 3 illustrated in Figure 4B. It is also assumed that the polysilicon doping of the trench-gate 13 and the thickness of the gate oxide 23 are such that the threshold voltage for channel conduction would be about 0.8v if the p- body region 3 were shorted to the source region 1 as in a conventional MOSFET device, and that the device would then be turned on hard at 5v. In these circumstances: with Vg at zero voltage. The switch is in the off-state of Figure 3B. The drain to p-body junction 32 is reverse biased with a depletion region 30 over part of the p-body region 3, and only a small leakage current flows across the junction 32. The leakage current has to flow through to the source region 1, so putting a small forward bias on the source to p-body junction 31, for example of 0.4v. The p-body region 3 is therefore slightly more positive than the source region 1, and so the area of the p-body region 3 under the influence of the trench-gate 13 is less depleted than normal. with Vg at 0.8v (the normal threshold voltage for a conventional MOSFET with short). The p-body region 3 is more positive than normal (effectively similar to a "back-bias" situation), and so the gate voltage Vg is not sufficient to form an inversion channel 12, and the device is still in its off-state. • with Vg at about 1.2v. The extra voltage on the trench-gate 13 is now sufficient to counteract the "back-bias" of the p-body region 3 and an inversion channel 12 is formed. with Vg at about 5.0v. The channel 12 is formed and has low resistance. In normal operation the load 107 would now drop most of the voltage, and the drain terminal D would therefore be at a low potential, given by the IR drop, where I is the current through the switch and R is the on-resistance of the switch. In a particular example, with R of 40 milliohm, the drain is at 80 mV with a current I of 2 Amps. The floating potential Vf of the p-body region 3 must now be intermediate between the source and drain voltages Vs/d and Vd/s, and so Vf is close to zero. Hence the Figure 1 device is turned on hard, just as it would be if the source region 1 was short-circuited to the p-body region 3. The operation with the drain and source polarities inverted is exactly the same as above, since the device is symmetrical. In this case however the depletion region 30 spreads in the body region 3 from the p-n junction 31 which is reversed biased, and the undepleted area 3a is adjacent to the p-n junction 32 which is forward biased. A non-trivial point is how quickly the device can switch, since large capacitative currents flow into and out of the gate terminal G during switching. In this application the device does not have to operate at high frequencies, but will have to be able to respond to fault conditions in, for example, a few microseconds. Considering now the flow of this current when the device turns on, again with a positive voltage Vd/s on the drain terminal D/S: As Vg becomes more positive, but below the threshold voltage, the depletion region 30 in the p-body region 3 increases in width. The holes which are released have to escape from the p-body region 3. They do this by flowing across the forward-biased source to p-body junction 31. If the current is large the bias will increase to, for example, 0.7v in this process; but, since a forward-biased junction can carry large current densities, there will be no problem. As the channel 12 is formed the extra charge goes into the inversion region which forms the channel 12, rather then into the depletion region 30. This channel 12 has an ohmic connection to the source region 1 , so again there is no problem. Considering now the situation when the device turns off: As long as the inversion channel 12 is present there is a good connection. The number of electrons in the channel 12 decreases as the gate voltage Vg falls; the excess electrons flow to the positive drain region 2. Once the inversion channel 12 has become weak, the change in the charge comes from the collapsing depletion region 30 in the p-body region 3. This collapse requires holes to flow in to neutralise the acceptors which make up the space charge of the depletion region 30. However there is no source of holes, apart from a small leakage current across the drain p-body junction 32, which is reverse biased. Thus, the p-body region 3 is only coupled by capacitance to the gate 13. Thus, as the gate 13 moves down in potential from the weak inversion point towards zero, the potential Vf of the p-body region 3 will be dragged down by the same amount, since there is no path for the capacitor to discharge. This will take the p-body potential Vf down by e.g. 1.0v, and so Vf may end up negative with respect to the source region 1. Slowly the leakage current will discharge the capacitor (as depicted in the right-hand side of Figure 4B) until eventually the p-body region 3 is at a potential Vf slightly higher than the source potential Vs/d, with the leakage now flowing through the forward biased p-body to source junction 31 once more. (This returns to the start). If the device is switched at a high frequency, the p-body region 3 will alternate between a clamped positive potential of about 0.7v as the gate/p-body capacitor charges up and a high current flows across to the source, and a negative potential (dependent on the load impedance) of a fraction of 1 volt. Because of this, the "threshold voltage" will show some hysteresis or time constant, but as long as the drive voltage Vg is high, for example 5v, compared to this varying substrate bias, the device will switch satisfactorily. The turn off of the device will show a tail of current as the weak inversion region is approached and the increasing channel resistance R, combined with the capacitance C being discharged, give an increasingly large time constant RC. Operation with the reverse polarity (Vs/d positive with respect to Vd/s) is identical as the switch is symmetrical. However the depletion region 30 spreads now from the p-n junction 31 which is reversed biased, whereas the undepleted area 3a is adjacent to the p-n junction 32 which is forward biased. Thus, as illustrated in the embodiments of Figures 1 to 4, the present invention permits the construction and use of a very simple switch of small area, and with an on-resistance of one quarter or less of the conventional back- to-back MOSFET pair solution. It may be used in switch circuits for all sorts of portable equipment which uses a low voltage rechargeable battery 103 or single cell for power. The switch is a simple device to drive, since it has only one control terminal G, which may be biased with reference to either of the other two terminals S/D and D/S By contrast, the conventional back-to-back MOSFET pair has two gate terminals which complicate the drive circuitry. The Figure 1 switch is inherently a high cell density device, since no short between the source and p-body has to be provided in each cell. This reduces the on-resistance further. It is also a simple device to make with its body region 3 as a thin epitaxial layer, which requires only an N+ implant for region 1 , the formation therein of a trench gate 13, and top electrode connections to the region 1 and trench-gate 13. Furthermore, the realisation of this device structure of a switch in accordance with the present invention (in terms of the relevant criteria for the dimensions X and Y and doping concentration P of the body region 3) is easier to implement in a high-volume manufacturing context, than that of the switch of WO 97/44828. In switches and switch circuits in accordance with the present invention, these dimensions and doping concentration are sufficiently large that the body region 3 retains an undepleted first-conductivity-type area 3a of floating potential in all bias conditions Vs, Vd, Vg of its three terminals S, D, G. In particular, the width Y of the body region 3 of a switch in accordance with the invention does not need to be made so narrow as that of WO 97/44828, whose width needs to correspond exactly with the laterally depleting effect of the gate in WO 97/44828. As described above, the undepleted inner area 3a of the body region 3 of a switch in accordance with the invention does float in potential, but its potential Vf does not drift uncontrollably. Its potential Vf floats between well-defined states, which the Applicant finds to be acceptable for use in switch circuits for battery-powered equipment. Furthermore, due to the presence of the undepleted inner area 3a of the body region 3, the blocking voltage capability of the switch in the off-state is determined by breakdown voltages of the plane p-n junctions 31 and 32, and these breakdown voltages can be reproducibly controlled in a high-volume manufacturing context. An n-channel switch has been described with reference to Figures 1 to 4. However, a p-channel switch is also possible in accordance with the invention, in which the regions 1 and 2 are p-type, the base region 3 is n-type and the conduction channel 12 is of holes. A vertical discrete device has been described with reference to Figures 1 to 4, having its second main electrode 12 contacting the region 2 at the back surface 10b of the body 10. However, an integrated switch is also possible in accordance with the invention. In this case, the region 2 may be a doped buried layer between a device substrate and the epitaxial body region 3 and may be contacted by electrode 12 at the front major surface 10a via a doped peripheral contact region which extends from the surface 10a to the depth of the buried layer. 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 switch circuits for battery-powered equipment, and component parts thereof, and which may be used instead of or in addition to features already described herein. Although Claims have been formulated in this Application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention 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 relates to the same invention as presently claimed in any Claim and 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 Application derived therefrom.

Claims: CLAIMS 1. A trench-gate bi-directional semiconductor switch comprising a semiconductor body in which a channel-accommodating body region of a first conductivity type is sandwiched between first and second regions of an opposite second conductivity type to form plane p-n junctions with the first and second regions, wherein the first and second regions separate the p-n junctions from respective first and second main electrodes in electrical contact with the first and second regions, the first region and first main electrode are present adjacent a first major surface of the body from which the trench-gate extends through the first region and through the body region and into the second region, the trench-gate is capacitively coupled to the body region for inducing therein a conduction channel of charge carriers of the second conductivity type between the first and second regions in an on-state of the switch, the body region has a substantially uniform doping concentration of the first conductivity type which is of smaller magnitude than the doping concentrations of the second conductivity type of the first and second regions, characterised in that the dimensions and doping concentration of the body region are sufficiently large that the body region retains an undepleted first-conductivity-type area of floating potential in all bias conditions of the first and second main electrodes and of the trench-gate so as to give the switch in an off-state a blocking voltage capability between the first and second main electrodes that is determined by the plane p-n junctions. 2. A semiconductor switch as claimed in Claim 1, further characterised in that the first and second regions have substantially the same doping concentrations as each other, at least adjacent to their p-n junctions with the body region, so as to give the switch a substantially symmetrical blocking voltage capability between the first and second main electrodes. 3. A semiconductor switch as claimed in Claim 2, further characterised in that the thickness of the body region is at least 1.Oμm (one micrometre) between the plane p-n junctions with the first and second regions, and the uniform conductivity-type-determining doping concentration of the body region is at least 1x10 16 cm "3 . 4. A semiconductor switch as claimed in Claim 2 or Claim 3, further characterised in that width of the body region is at least 4.0 μm between neighbouring sections of the trench-gate, and the uniform conductivity-type-determining doping concentration of the body region is at least 1x10 16 cm 3 . 5. A switch circuit for a battery-powered equipment comprising a power line for powering a load forming part of the equipment, and a battery coupled to the power line via a trench-gate bi-directional semiconductor switch as claimed in anyone of the preceding Claims, wherein one of the first and second main electrodes of the switch is coupled to a first terminal of the battery, the other of the first and second main electrodes of the switch is coupled to the power line, and a control circuit is coupled to the trench-gate of the switch so as to apply a control signal to the switch for powering the power line from the battery when the conduction channel is induced between the first and second regions in the on-state of the switch and for disconnecting the battery from the power line in the off-state of the switch when the blocking voltage capability between the first and second main electrodes is determined by the plane p-n junctions. 6. A switch circuit as claimed in Claim 5, wherein the battery is rechargeable and the power line has a battery-charger terminal for coupling a battery charger to the battery via the trench-gate bi-directional semiconductor switch. 7. A battery-powered equipment comprising a load coupled to the power line of a switch circuit as claimed in Claim 5 or Claim 6.

Inventors: GROVER RAYMOND J, GROVER, RAYMOND, J.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H01L29
IPC codes: H01L29