Improving IGBT light load efficiency

An apparatus comprising an insulated gate bipolar transistor and a super junction metal-oxide semiconductor field effect transistor wherein the insulated gate bipolar transistor and the super-junction metal-oxide semiconductor field effect transistor are electrically and optionally structurally coupled.

FIELD OF THE INVENTION

Aspects of the present disclosure generally relate to transistors and more particularly to insulated gate bipolar transistors.

BACKGROUND OF THE INVENTION

A variety of modern applications use electronic switches to perform different functions during operation. While there are many different types of electronic switches including relays, transistors and vacuum tubes. Currently solid-state transistors are predominantly used in electronic circuits today. Two major types of transistors are Insulated Gate Bipolar Transistors (IGBTs) and metal-oxide semiconductor field effect transistors (MOSFETs).

IGBTs have excellent high current conductance attributes compared to MOSFETs. The ‘on’ state conductance of a MOSFET is linear at a standard temperature and can be modeled as a resistor using RDSon. On the other hand, the conductance of an IGBT at a standard temperature is non-linear and is better modeled as diode. Additionally IGBTs are superior in handling higher current densities compared to MOSFETs and also have a significantly simpler/lower cost fabrication process compared to a Super-Junction MOSFET. Thus, IGBTs are ideal for high current application because of their relatively reduced resistance and relative reduced cost.

While there are many positive characteristics of IGBTs compared to MOSFETS, there are also some significant drawbacks. One drawback is that IGBTs at low current have an ‘on’ state voltage threshold Vthand do not begin conducting until the voltage is above the threshold. This means that for low amperage and voltage applications traditional IGBTs have significantly higher conduction losses compared to MOSFETs, which begin conducting in the ‘on’ state at a non-zero voltage without any diode knee in their output characteristics. Another drawback of the IGBT is that due to its construction, it does not conduct current in the reverse current direction whereas MOSFETs have a built-in body diode that allows reverse current direction conduction.

To overcome this problem a diode may be placed anti-parallel to the IGBT commonly referred to as a freewheeling diode. Freewheeling diodes resolve the problem of reverse current direction conduction but do nothing to solve the voltage threshold issue. Thus, it would advantageous to configure an IGBT package that could conduct at low amperages and have good reverse current conduction characteristics.

It is within this context that aspects of the present disclosure arise.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Additionally, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a thickness range of about 1 nm to about 200 nm should be interpreted to include not only the explicitly recited limits of about 1 nm and about 200 nm, but also to include individual sizes such as but not limited to 2 nm, 3 nm, 4 nm, and sub-ranges such as 10 nm to 50 nm, 20 nm to 100 nm, etc. that are within the recited limits.

In the following discussion of the illustrated examples, the first conductivity type is typically N and the second conductivity type is P. However, it should be noted that substantially similar devices may be fabricated using a similar process but with conductivity types opposite those shown and described. Specifically, aspects of the present disclosure include implementations like those shown and described herein in which N is substituted for P and vice versa.

IGBTs generally have better high amperage conductance characteristics than MOSFETs. IGBT's generally are constructed similar to MOSFETS except they have an extra doped layer. Thus where a MOSFET may have a doping organization of N-doped layer, P-doped layer, N-doped layer. An IGBT will have a doping organization of P, N, P, N or N, P, N. P.

FIG. 1shows a prior art layer configuration of an IGBT. As shown the IGBT has a lightly doped drift region of a first conductivity type (e.g. N-doped)107. The doping concentration of the drift region may be between 1e13 cm−3and 5e14 cm−3depending on the desired breakdown voltage. Lower doping concentrations of the drift region result in higher breakdown voltages. A more heavily doped buffer region of the first conductivity type112underneath the drift region107. The doping concentration of the buffer can range from 1e15 cm−3to 5e16 cm−3. A lightly doped layer113of either conductivity type, coming from the starting substrate material exists under the buffer. The doping level of layer113is typically below 1e15 cm-3. A heavier doped layer114of the second conductivity type is underneath the lightly doped region. The heavier doped layer114forms the IGBT collector and can be implanted from backside or from frontside during epitaxial growth. Its doping levels range from 1e17 cm-3 to 1e19 cm-3. A collector contact metallic layer is formed on the bottom of the collector115.

On top of the lightly doped drift region107is a heavily doped region of the first conductivity type106. A body region105is located on top of the heavily doped region. The body region may be doped with the second conductivity type. The body region may be may have a doping concentration in the range of 1e17 cm−3to 1e18 cm−3. The body region105may have a heavily doped emitter region of the first conductivity type formed on top of it104. The doping concentration of the source region may be above around 2e19 cm−3.

A shield trench may be formed in the substrate and terminate at the depth of the lightly doped drift region107. The shield trench may be lined with a dielectric (e.g. an oxide layer)111. A shield trench electrode110is disposed on top of the dielectric and may be at emitter voltage. The shield trench electrode may be for example a polycrystalline silicon layer. A planar gate comprising a planar insulating layer (e.g. an oxide layer)108and a gate insulating layer109is formed on top of the shield electrode and extends over the emitter regions. The gate electrode109is formed on top of the gate insulating layer and more insulating layer108is formed around the gate electrode to isolate the gate electrode109from the emitter metal101. The gate electrode may be created using for example and without limitation a polycrystalline silicon layer.

As shown, not every shield electrode is covered by a gate. A gate oxide layer and gate electrode do not cover shield trench dielectric102and shield trench electrode103. The shield trenches serve to compensate N+ regions and to keep the breakdown voltage high.

FIG. 2shows a circuit diagram of an IGBT201and a diode202. Due to the construction of the IGBT, reversed bias, reverse current does not flow through the IGBT. The IGBT is configured such that current flows from the collector C to the emitter E when a voltage greater than the voltage gate-emitter threshold (Vge(th)) is applied to the gate G. A reverse bias applied from to the collector will not result in current being conducted across the IGBT. To overcome this issue prior IGBT circuit designs place a diode202antiparallel with the IGBT201. In the context of the present disclosure, antiparallel means that the device is connected in parallel but configured to conduct when a reverse bias is applied to the collector. Referring the diagram, the anode of the diode202is connected to the collector of the IGBT and the cathode of the diode is collected to the emitter of the IGBT.

According to aspects of the present disclosure IGBT, designs may be improved using a super-junction MOSFET arranged in parallel with the IGBT instead of freewheeling diode.FIG. 3shows an aspect of the present disclosure wherein the IGBT301is structurally to a super-junction MOSFET302. Additionally as shown, the IGBT may be conductively coupled to the super-junction MOSFET by way of sharing the same contact layer. The IGBT depicted is an N-channel IGBT and the super-junction MOSFET shown is an N-channel super-junction MOSFET. The gate G of the IGBT301and the gate G of the super-junction MOSFET302are conductively coupled303. The IGBT and the super-junction MOSFET are linked and the VGE(th)) for the IGBT and the VGS(th)) for the super-junction MOSFET should be within a similar range, e.g., within ±2 volts of each other. Due to the conductive coupling of the gates and the similar activation threshold values for the IGBT and the super-junction MOSFET, when a sufficient voltage is applied to the IGBT to put the device in the ‘on’ state, the super-junction MOSFET should also be in the ‘on’ state. The collector C of the IGBT301and the drain D of the super-junction MOSFET302may also be conductively coupled304. The emitter E of the IGBT and the source S of the super-junction MOSFET may be conductively coupled305as well. The super-junction MOSFET used for this purpose may include a fast recovery body diode which is created by reducing minority carrier lifetime of the super-junction MOSFET using methods such as electron irradiation.

Additionally, the super-junction MOSFET302is configured so that when arranged as described the body diode of the super junction MOSFET is antiparallel with the IGBT. As such during operation in reverse bias and reverse current mode, the body diode of the super-junction MOSFET acts as a freewheeling diode for the IGBT.

As used herein conductively coupled may mean an electrical connection between two elements that allows electrons to flow from one element to the other. The electrical connection may be through any conductive material such as wire, metallic leads, conductive gel, metallized glass, metallized plastic and the like. Structurally coupled may mean that two elements are affixed to each other or to the same structure or surface, where the affixation may be flexible or rigid. The structure or surface may be any surface known in the art for example and without limitation a PCB, an integrated circuit package, a metal surface, a plastic surface, a wooden surface or similar.

FIG. 4depicts an embodiment of the present disclosure where the IGBT and super-junction MOSFET that are structurally coupled by the same substrate, epitaxial layers and contact layers. As shown, the substrate and epitaxial layers includes both an IGBT401and super-junction MOSFET402. Additionally, the two switches share a contact metal417and a substrate contact411. By way of sharing a contact metal417and a substrate contact411, the IGBT401and the Super-junction MOSFET402are electrically coupled. As shown the contact metal417for the super-junction MOSFET402is the source metal contact and is in electrical contact with the Source region407. The source region may be doped with the first conductivity type and located in the surface of an epitaxial layer403. A body region408of the second conductivity type formed deeper in the epitaxial layer403and underneath the source region407. A doped column of the second conductivity type409is located under the body region408in the epitaxial layer403. The range of doping concentrations for the source region407and body region408may be as discussed above. By way of example, and not by way of limitation, the source doping concentration may be of order 2e19 and the body doping concentration may be of order 1-5e17. Vthcan be tuned by adjusting body dose and gate oxide thickness.

A drift region of the first conductivity type406may be located in the epitaxial layer between the two columns doped with the second conductivity type409. Above the drift region may be the gate insulator404, which may be for example and without limitation an oxide layer. A gate electrode405is located above the gate insulator404and protected from the contact metal417by the gate insulator. The gate electrode405may be for example and without limitation a polysilicon layer. When a voltage is applied to the gate electrode405at or above a voltage threshold (Vgs(th)) current applied to the drain (For an N-channel MOSFET) at the substrate layer411will be conducted vertically through the drift region406, the body region408and source region407to the contact metal417. The drift region406and columns409are sized and doped such that their charges balance out horizontally with adjacent columns. The concentrations of the columns and drifter region can be made higher than that of just a drift region in a typical transistor so that during the ON state they conduct with lower ‘on’ resistance. Additionally the VGS(th)) of the Super-junction MOSFET402should be chosen such that it is the same or within ±2 Volts of the Voltage threshold (VGE(th)) for IGBT401.

Under the drift region406is a heavily doped bottom layer410of the first conductivity type. Finally, in conductive contact with the layer410is the backside contact411or drain contact for the super-junction MOSFET. The heavily doped bottom layer may act as the drain for the device with current flowing from the backside contact411through the bottom layer410and eventually to the contact metal417.

An IGBT is formed from the same substrate and epitaxial layers401as the super-junction MOSFET402. As shown a shield trench may separate the IGBT401from the super-junction MOSFET402. The shield trench may be lined with a shield trench dielectric418which may be made of, without limitation, an oxide layer, as discussed above. A shield trench electrode419may be disposed on top the shield trench dielectric418and insulated from the epitaxial layer and substrate by the dielectric. The shield trench electrode may be made from a conductive material for example and without limitation, polycrystalline silicon.

The IGBT has a lightly doped epitaxial drift region412of a first conductivity type. The doping concentration of this region may be lower than the doping concentration of the Super-junction MOSFET402. A more heavily doped buffer region413of the first conductivity type is formed underneath the epitaxial drift region412. Under the buffer region413is a lightly doped layer414of either conductivity type and an implanted bottom layer415at the bottom of second conductivity type that forms the IGBT collector. A backside contact411is formed on the bottom of the implanted bottom layer415. The backside contact411may be a metal layer, which may be made from copper, aluminum or gold deposited on the back surface.

On top of the lightly doped epitaxial412, drift region is a heavily doped region416of the first conductivity type. A body region420is located on top of the heavily doped region. The body region may be doped with the second conductivity type. The body region420may have a heavily doped region emitter region421of the first conductivity type formed on top of it.

A shield trench may be formed in the substrate and terminate at the depth of the lightly doped epitaxial drift region412. The shield trench may be lined with a dielectric424. A shield trench electrode425is disposed on top of the dielectric and may be at emitter voltage. A gate comprising a gate insulating layer423is formed on top of the shield electrode and extending over the emitter regions. A gate electrode422is formed on top of the gate insulating layer and more insulating layer423is formed around the gate electrode to isolate the gate electrode422from the contact metal417.

Similar to Super-junction MOSFET402the VGE(th)of the IGBT401is configured to be within ±2 Volts of the VGS(th)for the Super-junction MOSFET. The implanted bottom layer415acts as a collector for the IGBT401and when a voltage is applied to the Gate electrodes422, current at the backside contact411flows vertically through the implanted layer415and epitaxial layers to the emitter region421finally to the contact metal417.

FIG. 5shows a bottom view of the device having an IGBT and Super-junction MOSFET structurally coupled by way of sharing back metal and epitaxial layers. In the shown embodiments, the back side of the chip is being described. In the IGBT portion, the shown region is the collector and in the Super-junction MOSFET, the region is the drain. The majority of the substrate space is occupied by the IGBT, implanted substrate of the second conductivity type501. The Super-junction MOSFET substrate regions of the first conductivity type502are interspersed regularly. In the shown embodiment, the Super-junction MOSFETS are circular regions separated by IGBTs.

FIG. 6depicts an alternative embodiment of the present disclosure. In this alternative embodiment, the shield trenches have been eliminated in the IGBT section601and Super-junction-like doped columns606are created underneath the body regions605and extend into the drift region607. The super-junction-like doped columns may be of the second conductivity type as the body region605. Compared to the IGBT inFIG. 4, the relative doping concentration of the first conductivity type for the epitaxial/drift region is greater in the alternative embodiment shown inFIG. 6. Additionally the drift region607extends all the way to the buffer implant layer610. Below the buffer is the lightly doped layer611of either conductivity type, and the implanted layer612of the second conductivity type that forms the IGBT collector.

The IGBT portion601also includes a gate insulating layer608formed on the epitaxial layer. The gate insulating layer608protects the gate electrode609from current flowing through epitaxial layer and contact metal603. The gate insulating layer may be for example and without limitation a silicon oxide layer. The gate electrode609is formed on the surface of the gate insulating layer608and the insulating layer encompasses the gate electrode to electrically isolate the gate electrode from the metal contact layer603. The gate electrode may be for example and without limitation a layer of polycrystalline silicon. When a voltage at or exceeding VGE(th)is applied to the gate electrode current flows from the substrate contact layer611through a vertical channel formed in the substrate implant region610, the drift region607, the body region605, the emitter layer604to the contact metal603.

The Super-junction portion602is largely unchanged from the portion described inFIG. 4. It should be noted that in this embodiment the Super-junction portion602and the IGBT portion601share a drift region607. The shared epitaxial/drift region may be at the same doping concentration for both the super-junction portion602and the IGBT portion601. The IGBT portion601and Super-junction portion602share a backside contact613.

FIG. 7depicts another alternative embodiment according to aspects of the present disclosure. Here, the IGBT701and the super-junction MOSFET702are physically separate but structurally coupled by way of electrical connections between the gate electrodes, and contacts. As shown, the construction of the IGBT portion701and Super-junction MOSFET702is similar to that ofFIG. 4. Unlike the embodiments shown inFIGS. 4 and 6, the IGBT portion has a separate emitter contact metal layer703, drift region708, buffer716, lightly doped region of either conductivity type717and implanted layer of second conductivity type718that forms IGBT collector and collector contact711. Likewise, the super-junction MOSFET includes a separate source contact metal layer704, epitaxial/drift region709, substrate layer712and drain contact710.

The operation of the two portions shown is similar to the previous embodiments because the gate electrode of the super-junction MOSFET portion714is electrically coupled to the gate electrodes of the IGBT portion715through the gate electrode leads705. Additionally in some embodiments the emitter contact metal layer703of the IGBT portion701is electrically coupled to the source contact metal layer704through the emitter contact leads706. Similarly, the collector contact layer711of the IGBT portion701is electrically coupled to the drain contact layer710through the collector contact leads707. This electrical coupling of areas of the two device portions allows the portions to operate together without sharing a common substrate or epitaxial layer. Specifically, the electrical coupling of the gate electrodes for the IGBT portion and the super-junction MOSFET portions means that during operation, IGBTs and Super-junctions MOSFETS with closely similar gate voltage thresholds will operate in synchronized fashion when switching to the ‘on’ state. Additionally the IGBT portion701and the Super-junction MOSFET portion701may be structurally coupled by way of being for example and without limitation, in the same integrated circuit package, on the same printed circuit board, or attached to the same surface.

Function

FIG. 8Ashows the function of the IGBT structurally coupled to a super-junction MOSFET at 25 C803according to aspects of the present disclosure. Also shown is the function of a lone IGBT801and a lone Super-junction MOSFET802. The graphs ofFIGS. 8A and 8Bshow current vs voltage for the different devices. As discussed above, the lone IGBT curve801exhibits a diode like voltage threshold where the current conducted across the device does not rise until the voltage is ˜0.6 Volts at 25 C. At 125 C, the current through the lone IGBT804does not rise until ˜0.45 volts. The lone Super-junction MOSFET curve802on the other hand shows a linear rise in current conducted across the device starting at 0 volts. Similarly at 125 C the rise in current through the lone super-junction device has linear characteristics805and is flatter than the curve at 25 C802. On the other hand after the voltage threshold the lone IGBT at both 25 C801and 125 C804device exhibits non-linear behavior. This behavior can be interpreted as the majority of current being conducted through the Super-junction MOSFET at currents below 0.6-1 amps and due to the non-linear behavior of the IGBT at currents above 0.6-1 amps, the majority of current is conducted through the IGBT portion of the device.

The IGBT structurally coupled and electrically coupled to a super-junction MOSFET curve803exhibits behavior of both a lone IGBT and a lone Super-junction MOSFET. As shown, the device exhibits linear behavior at low voltages, below 0.6 volts at 25 C and below 0.4 volts at 125 C. At higher voltages the device exhibits a non-linear relationship between current and voltages, this non-linear relationship persists from 25 C803to 125 C806. Thus the curves clearly show that the IGBT structurally coupled and electrically coupled to a super-junction MOSFET resolves the voltage threshold problem in prior art IGBT devices because at >0 volts the device begins to conduct current. The device also maintains the positive aspects of the IGBTs because after the voltage threshold, the device exhibits the typical non-linear IGBT behavior.

FIGS. 9A and 9Bshow the reverse current and reverse bias function of the IGBT structurally coupled and electrically coupled to a super-junction MOSFET current vs voltage curves at 25 C901and 125 C903respectively according to aspects of the present disclosure. The current vs voltage graphs also show the function of a normal IGBT co-packaged with an anti-parallel Fast Recovery Diode at 25 C902and 125 C904. The graph shows that for a normal IGBT902at low voltages, no current is conducted across the device. The curve indicates that in the reverse bias and reverse current direction conductance across the device is dominated by conductance through the body diode of the super-junction MOSFET portion of the device. The body diode of the super-junction MOSFET could be considered acting as a freewheeling diode for the device. Thus, the device also fulfills the need for a freewheeling diode in lone IGBT devices and has lower conduction losses compared to co-packaged FRD.