Semiconductor switch with parallel LDMOS and LIGT

Abstract: A semiconductor switch comprising a lateral DMOS (Q MOS ) and a lateral IGT (Q IG ) both of which can be fabricated in a monolithic integrated circuit. In operation the lateral DMOS stays on (V GI ) while the lateral IGT (V G ) is switched off in order to reduce turn off power dissipation.

Description: The invention relates to a semiconductor switch including a lateral insulated gate transistor (LIGT) said LIGT being transferable between a conducting state and a nonconducting state and having a second gate, a cathode region and an anode region the switch including a lateral double diffused MOS transistor (QMOS) which is transferable between a conducting state and a nonconducting state and which has a first gate, a source region and a drain region, said source region and said cathode region being interconnected, said drain region and said anode region being interconnected, signal means connected to said second gate and capable of transmitting signals thereto for causing said lateral insulated gate transistor to transfer between its current conducting state and its nonconducting state, delay means connected to said first gate and capable of transmitting signals thereto to cause said double diffused MOS transistor to transfer between its current conducting state and its nonconducting state. More particularly it is an invention involving a semiconductor switch for power switching which can be readily fabricated in a monolithic integrated circuit. Various types of power switching semiconductor devices are known. Included among these are hybrid devices in which two different device structures are fabricated on a single semiconductor chip. One such hybrid device which has been proposed in, e.g. EP-A 0 144 909, combines a power MOS transistor with a vertical insulated gate transistor. This combination is acceptable in a separate device but cannot be fabricated in a monolithic integrated circuit together with numerous other devices because the vertical insulated gate transistor uses the substrate as one of its contacts. The power MOST, formed by a DMOST, is connected in parallel with the vertical IGT, for draining current when the IGT is switched-off. The gate of the DMOST is provided with delay means for delaying the turn-off of the DMOST. These delay means comprises a capacitor and a resistor, in series with the capacitor. In this known device, the delay time has a fixed value, determined by the RC-product. A lateral insulated gate transistor or LIGT is described in International Electron Devices Meeting 1985, Washington DC, pages 740-743. Means to reduce the power dissipation during turning-off are not indicated in this publication. In Patent Abstracts of Japan, Vol. 9, No. 19 (E292) (1742), JP-A 59-167119, a small-loss, high speed transistor, comprising a parallel arrangement of a MOS-transistor and a bipolar transistor, is disclosed. In accordance with the invention, there are provided means for producing a reference signal representative of current through said lateral insulated gate transistor, said delay means being responsive to a signal from said signal means when said signal cause said lateral insulated gate transistor to transfer to its nonconducting state said delay means being responsive to said reference signal representative of the current through said lateral insulated gate transistor to generate a signal to transfer said double diffused MOS transistor from its conducting state to its nonconducting state. An advantage of the invention is the low power dissipation during turn off compared to a lateral insulated gate transistor alone. In accordance with another aspect of the invention, there is provided a semiconductor device comprising a semiconductor substrate of a first conductivity type have a major surface. An epitaxial layer of a second conductivity type opposite to that of the first conductivity type is on the substrate forming part of the major surface. A major surface adjoining first channel region of the first conductivity type is in the epitaxial layer. A major surface adjoining drain region of the second conductivity type is also in the epitaxial layer spaced from the first channel region. A major surface adjoining source region of the second conductivity type is located in the first channel region. The first channel region together with the source region and the drain region form a lateral double diffused MOS transistor. The device also includes a major surface adjoining anode region of the first conductivity type in the epitaxial layer adjacent the drain region. A major surface-adjoining second channel region of the first conductivity type is also in the epitaxial layer spaced from the anode region. A major surface-adjoining cathode region of the second conductivity type is located in the second channel region. The anode region, the cathode region and the second channel region form a lateral insulated gate transistor. Other objects, features and advantages of the invention will be apparent to those skilled in the art from the following description when considered in conjunction with the appended claims and the accompanying drawing in which: Figure 1 is a diagram of an inductive circuit with a lateral insulated gate transistor; Figure 2 is a graph of some voltages applied to the circuit of Figure 1 and a resulting current therein; Figure 3 is a diagram of an inductive circuit with a lateral insulated gate transistor in parallel with a lateral double diffused MOS transistor; Figure 4 is a graph of some voltages applied to the circuit of Figure 3 and some resulting currents therein; and Figure 5 is a schematic, partial cross-sectional view of a semiconductor structure in accordance with the invention. Figure 6 is a schematic, partial cross-sectional view of part of the structure of Figure 5 with some changes in that structure. Figure 7 is a layout of part of the structure shown in Figures 5 and 6. In the drawing, equivalent elements are identified by the same reference characters. Also Figures 5, 6 and 7, it is to be understood, are not to scale. Referring specifically to Figure 1 there is shown a lateral insulated gate transistor Qig. In circuit with transistor Qig is an inductance L, rectifier D1, capacitance C and a load represented by resistor R. Such a circuit is intended to be representative of the input of a ballast lighting circuit. Voltage V cc is the operating voltage for the circuit. A signal source SS is connected to the gate of transistor Qig to apply a gate signal V G thereto. When gate voltage V G is applied to the gate of transistor Qig the voltage V Qig across transistor Qig drops substantially while the current to ground I Qig increases in a linear fashion. As can be seen from Figure 2, when gate voltage V G is removed inductance L causes the current to remain high for a significant period of time whereas the voltage V Qig across transistor Qig rises significantly. As a consequence both the current through the transistor and the voltage across it during turn off are high for some time thereby providing a significant power dissipation during this period. This limits the efficiency of the device. In order to provide a more efficient switching device than that shown in Figure 1, the circuitry of Figure 3 is disclosed herein. The same inductive circuit as in Figure 1 with its inductance L, capacitance C, rectifier D1, and load represented by resistor R is controlled by lateral insulated gate transistor Qig. In addition, a lateral double diffused MOS transistor OMOS is connected in parallel with the conducting path of transistor Qig. In this respect, the anode region of transistor Qig is connected to the drain region of transistor QMOS. The cathode region of transistor Qig is connected to ground to which the source region of transistor QMOS is also connected. Also connected in the circuit of Figure 3, is current source I with resistor R1 connected in parallel forming a reference signal source. The output of current source I is connected to one input of comparator COM. The other input of comparator COM is connected to a tap between resistor R2 and the series connected source of sensing transistor Qs. The gate of sensing transistor Qs is connected to the gate of transistor Qig and its drain is connected to the anode of transistor Qig. The gates of transistors Qig and Qs are also connected to signal source means SS. The source of sensing transistor Qs is connected to ground. The enable input of comparator COM is connected to the gate of transistor Qig. The output of comparator COM is connected to one input of "or" gate OR. The other input of the "or" gate is connected to the gate of transistor Qig. The output of "or" gate OR is connected to the input of buffer B whose output is connected to the gate of LDMOS transistor QMOS. As long as a gate voltage V G is applied to the gate of lateral transistor Qig by signal source means SS, comparator COM is disabled. When the gate voltage V G is removed comparator COM is enabled. This comparator functions to compare the current through transistor Qs with that from the reference signal source. As will be explained, because of its structure, the current through transistor Qs is indicative of the current through transistor Qig. As long as the current through transistor Qs is larger than the reference provided by current source I, "or" gate OR produces an output because it has an input either from gate voltage V G or from comparator COM. As a consequence buffer B provides a gate voltage V G1 to the gate of transistor QMOS to maintain it in the on condition. When gate voltage V G is removed from transistor Qig voltage V Qig across the transistor cannot rise as in the circuit of Figure 1 because during this period transistor QMOS is on. This condition continues as long as the signal from sensing transistor Qs is larger than the reference signal from current source I. This is illustrated in Figure 4 where voltage V Qig is shown to remain low at the removal of gate voltage V G and not to rise as in the case of the circuit of Figure 1, as shown in Figure 2. At the same time there is an increase in the current I QMOS through transistor QMOS but as shown in Figure 4 the power loss is not significant since the voltage V Qig remains low. When the current through transistor Q s falls to the reference set by current source I both inputs to comparator COM become equal and it ceases to produce an output. As a consequence both inputs to "or" gate OR are zero and it ceases to produce an output. The gate voltage V G1 to transistor QMOS goes to zero and the transistor shuts off. As can be seen from Figure 4 when this occurs the voltage V Qig across transistor Qig goes to its maximum but by this time there is little or no current through transistor Qig and transistor Qs. Moreover, that through transistor QMOS rapidly falls to zero. With the arrangement of Figure 3, significantly less losses occur during turn off than with the circuit of Figure 1. Moreover, because the losses in lateral DMOS transistor QMOS are small, it can be relatively small compared to a DMOS transistor of the same capacity as the combined lateral insulated gate transistor and the lateral DMOS transistor of this invention. It is contemplated that the circuitry shown in Figure 3 except for inductance L could all be placed in a monolithic integrated circuit. Figure 5 shows a representative structure for transistors Qig and QMOS as they would appear in such a monolithic integrated circuit. Shown in Figure 5 is a semiconductor substrate 10 of a first conductivity type, typically p-type, with an epitaxial surface layer 12 of a second conductivity type opposite to that of the first, typically n-type. The expitaxial layer is provided on the top surface of substrate 10 and provides part of a major surface 14. A major surface adjoining first channel region 16 of a first conductivity type is provided in the epitaxial layer and forms a p-n junction therewith. A major surface adjoining source region 18 of the second conductivity type is provided in first channel region 16. A major surface adjoining drain region 20 also of the second conductivity type is provided in the epitaxial layer at a location which is spaced apart from first channel region 16. First channel region 16 has a surface adjoining portion 22 located between the source and drain regions of the device which forms a channel of a lateral DMOS transistor. An insulating layer 24 is provided on the epitaxial surface layer 12 and covers at least that portion of the first channel region 16 located between the source and drain regions of the transistor. A gate electrode 26 is provided on the insulating layer over channel 22. Source electrode 28 and drain electrode 30 provide electrical connections respectively to the source and drain regions of the transistor. Buried layer 34 extends beneath first channel region 16 and part of epitaxial layer 12 between source and drain regions 18 and 20. Buried layer 34 extends across the entire area below gate electrode 26 and stops shortly thereafter. First channel region 16 and buried layer 34 extend down to substrate 10 to provide isolation. If the foregoing were all that was fabricated in the monolithic integrated circuit one would have a well known DMOS transistor, similar to that disclosed in U.S. Patent 4.300.150. However, in accordance with the invention an additional surface adjoining second channel region 35 of the first conductivity type is provided in epitaxial layer 12. A further surface adjoining cathode region 36 of the second conductivity type is provided in second channel region 35. This forms the cathode region of a further to be described lateral insulated gate transistor. A surface adjoining anode region 38 of the first conductivity type is provided in the epitaxial layer at a location which is spaced apart from second channel region 35. Second channel region 35 has a surface adjoining portion 39 located between cathode region 36 and anode region 38 which forms a channel for the lateral insulated gate transistor. An insulating layer 40 is provided on the surface of epitaxial layer 12 and covers at least that portion of second channel region 35 located between anode and cathode regions 36 and 38 of this lateral insulated gate transistor. As will be understood by those skilled in the art, a gate electrode 42 is provided on insulating layer 40 over second channel 35. The anode 38 of the lateral insulated gate transistor is connected to the drain region 20 of the lateral DMOS transistor by having common electrode 30 provide electrical connections to both. Cathode electrode 44 contacts both cathode region 36 and second channel region 35 providing a conventional cathode short therebetween. P-type buried region 45 is provided under channel region 35 and between the cathode and anode regions. As is shown buried layer 45 will typically extend across the entire area below the gate electrode and stop shortly thereafter. This buried layer provides field shaping for the lateral insulated gate transistor. Second channel region 35 and buried layer 45 extend down to substrate 10 to provide isolation. As those skilled in the art will understand the device shown in Figure 5 can be fabricated by well known techniques and will vary physically in accordance with the operating specifications desired for a particular application. For this reason no material or physical specifications have been included in order not to complicate the description. As stated earlier it is further contemplated that a monolithic integrated circuit of which the disclosed lateral DMOS transistor and lateral insulated gate transistor can form a part could also include the components shown in Figure 3 including resistors R, R1 and R2, capacitor C, comparator COM, diode D1, "or" circuit OR, buffer B and current source I. All of these would also be fabricated by standard techniques. Figures 6 and 7 show the manner in which transistor Qs is fabricated in the monolithic integrated circuit. A small section of the circuit is reserved for this purpose. Second channel region 48 and its surface adjoining portion 49 in the small section is separated from second channel region 35 and its surface adjoining portion 39 in the rest of the structure by n-type epitaxial region 12. Epitaxial region 12 also isolates second channel region 48 from p-type regions 45 and 10 in this small section. Gate contact 42 (Figures 5 and 6), however, is common for both channel region 35 and channel region 48. In addition, cathode region 52 in this small section is separated from cathode region 36 in transistor Qig and is isolated by channel region 48. Also contact 54 (Figure 6) is provided for cathode region 52 in this section. Contact 54 it is to be understood is separate from contact 44 (Figure 5) which contacts cathode region 36 in that part of transistor Qig not comprising transistor Qs. In this way resistor R2 can be connected between cathode contact 54 of transistor Qs and cathode contact 44 of transistor Qig as shown in Figure 3. Since transistor Qs is part of the same structure as transistor Qig it is to be understood that it conducts a current which is indicative of the current through transistor Qig.

Claims: A semiconductor switch including a lateral insulated gate transistor (Qig) (LIGT) said LIGT being transferable between a conducting state and a nonconducting state and having a second gate (42), a cathode region (36) and an anode region (20, 38) the switch including a lateral double diffused MOS transistor which is transferable between a conducting state and a nonconducting state and which has a first gate (26), a source region (18) and a drain region (20), said source region and said cathode region being interconnected, said drain region and said anode region being interconnected, signal means (SS) connected to said second gate and capable of transmitting signals (Vg) thereto for causing said lateral insulated gate transistor to transfer between its current conducting state and its nonconducting state, delay means (B, OR, COM) connected to said first gate and capable of transmitting signals thereto to cause said double diffused MOS transistor to transfer between its current conducting state and its nonconducting state characterized by means (Qs, R2) for producing a reference signal representative of the current through said lateral insulated gate transistor, said delay means being responsive to a signal from said signal means when said signal causes said lateral insulated gate transistor to transfer to its nonconducting state said delay means being responsive to said reference signal representative of the current through said lateral insulated gate transistor, to generate a signal (Vg1) to transfer said double diffused MOS transistor from its conducting state to its nonconducting state. A semiconductor switch as claimed in claim 1, characterized in that said double diffused MOS transistor and said lateral insulated gate transistor are both part of a monolithic integrated circuit A semiconductor switch as claimed in claim 1, characterized in that said lateral double diffused MOS transistor comprises a semiconductor substrate (10) of a first conductivity type, an epitaxial layer (12) of a second conductivity type opposite that of said first conductivity type on said substrate and comprising part of a major surface of said MOS transistor, a major surface-adjoining first channel region (16) of said first conductivity type in said epitaxial layer and forming a p-n junction therewith, said source region (18) being a major surface-adjoining region of said second conductivity type in said first channel region, said drain region (20) being a major surface adjoining region of said second conductivity type in said epitaxial layer and spaced apart from said channel region, an insulating layer (24) on said major surface and covering at least that portion (22) of said first channel region located between said source region and said drain region, said first gate (26) being a first gate electrode on said insulating layer over said portion of said first channel and electrically isolated from said major surface, and source and drain electrodes (28,30) connected to said source and drain regions. A semiconductor switch as claimed in claim 3, characterized in that said lateral insulated gate transistor comprises a major surface-adjoining second channel region (35) of said first conductivity type in said epitaxial layer forming a p-n junction therewith and spaced from said drain region, said cathode region being a major surface-adjoining region (36) of said second conductivity type in said second channel region, said anode region being a major surface adjoining region (38) of said first conductivity type in said epitaxial layer and spaced apart from said second channel region, an insulating layer (40) on said surface and covering at least that portion of said second channel region located between said cathode region and said anode region, said second gate (42) being a second gate electrode on said insulating layer over said portion of said second channel and electrically insulated from said major surface, and anode and cathode electrodes (30, 42) connected to said anode and cathode regions. A semiconductor switch as claimed in claim 1, 2 or 4, characterized in that said delay means includes sensing means for producing a signal representative of the current through said lateral insulated gate transistor, a reference signal source (I, R1) for producing a reference signal and a comparator (COM) for comparing said reference signal with said signal representative of the current through said lateral insulated gate transistor.

Inventors: BRUNING GERT WERNER, MUKHERJEE SATYENDRANATH, SINGER BARRY MANA, BRUNING, GERT WERNER, MUKHERJEE, SATYENDRANATH, SINGER, BARRY MANA
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H03K17
IPC codes: H03K17, H02M3, H01L29, H01L27