Patent Publication Number: US-9900002-B2

Title: Methods of operating a double-base-contact bidirectional bipolar junction transistor

Description:
CROSS-REFERENCE 
     Priority is claimed (as a continuation-in-part) from U.S. application Ser. No. 14/735,782, which is a continuation of Ser. No. 14/514,988 (now issued as U.S. Pat. No. 9,059,710), which is a continuation of Ser. No. 14/313,960 (now issued as U.S. Pat. No. 9,029,909), and therethrough from U.S. applications 61/838,578, 61/841,624, 61/914,491, 61/914,538, 61/924,884, 61/925,311, 61/928,133, 61/928,644, 61/929,731, 61/929,874, 61/933,442, 62/007,004, and 62/008,275. Priority is also claimed from U.S. applications 62/077,777, 62/182,878, and 62/194,167. ALL of these, singly and in combination, are hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present application relates to bidirectional bipolar junction transistors with two separate base contacts, and also to related methods. 
     The present application teaches, among other innovations, methods for operating bidirectional double-base-contact bipolar junction transistors, known as B-TRANs. In various preferred embodiments, the present application discloses each of the following, separately and in combinations: turn-on as a diode, before base current is allowed to bring the device into low-voltage-drop transistor operation; base drive from the collector-side base contact terminal; a two-stage turnoff, in which the first stage shorts each base contact region to the adjacent emitter/collector region; and a passive-off mode, in which a low-forward-voltage diode (e.g. a Schottky diode) is connected in parallel with each emitter-base junction, to prevent the emitter-base junctions from getting close to forward bias. 
     The present application adds additional data to the disclosure of the parent application, to further show the technical success of the various disclosed innovations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed inventions will be described with reference to the accompanying drawings, which show important sample embodiments and which are incorporated in the specification hereof by reference, wherein: 
         FIGS. 1A and 1B  show sample embodiments of B-TRANs according to the present inventions. 
         FIG. 2  shows one sample embodiment of a base drive circuit for a B-TRAN according to the present inventions. 
         FIGS. 3A, 3B, 3C, 3D, 3E, and 3F  show sample equivalent circuits for an exemplary device in various stages of operation. 
         FIG. 4A  shows a sample embodiment of a B-TRAN according to the present inventions. 
         FIG. 4B  shows a sample conventional transistor. 
         FIG. 5A  shows a sample proposed circuit symbol. 
         FIG. 5B  shows a circuit symbol for a conventional transistor. 
         FIG. 6  shows another sample embodiment of the present inventions. 
         FIG. 7A  shows a top view of a sample conventional transistor. 
         FIG. 7B  shows a top view of one sample embodiment of the present inventions. 
         FIGS. 8A, 8B, 8C, and 8D  show several sample embodiments of the present inventions. 
         FIG. 9  shows one sample embodiment of a B-TRAN base drive circuit according to the present inventions. 
         FIG. 10  shows sample currents and voltages for one sample embodiment of the present inventions. 
         FIG. 11  shows another sample embodiment of a B-TRAN base drive circuit. 
         FIG. 12  shows another sample embodiment of a B-TRAN base drive circuit. 
         FIGS. 13A, 13B, and 13C  show further sample embodiments of B-TRANs according to the present inventions. 
         FIGS. 14A-14B  show further sample embodiments of B-TRANs according to the present inventions. 
         FIG. 15  shows another sample embodiment of a B-TRAN according to the present inventions. 
         FIGS. 16A, 16B, 16C, 16D, and 16E  show one sample process for fabricating a B-TRAN according to the present inventions. 
         FIG. 17  shows one sample embodiment of a termination structure for a B-TRAN according to the present inventions. 
         FIGS. 18A, 18B, 18C, and 18D  show sample embodiments of pad arrangements according to the present inventions. 
         FIG. 19  shows a model npn B-TRAN which has been simulated. 
         FIG. 20  shows a close-up view of one emitter/collector region (which acts as an emitter or a collector depending on the conduction direction). 
         FIG. 21  shows a simulated B-TRAN hard turn-off at 100 A, 800 V, and 80° C. 
         FIG. 22  shows simulated collector voltage V C  and total current I total  for a hard B-TRAN turn-on for 100 A, 800 V, and 80° C. 
       In  FIG. 23 , a hard B-TRAN turn-on with reverse recovery is simulated under the same operating conditions as in  FIGS. 21-26 . 
         FIG. 24  simulates turnoff of a B-TRAN in a power-packet-switching power converter architecture (PPSA), under 200 A and 25° C. 
         FIG. 25  shows simulated emitter current I E  and collector voltage V C  for a B-TRAN used in PPSA turn-on for 200 A at 25° C. 
         FIG. 26  shows simulated currents for a B-TRAN in use in a PPSA converter for 200 A at 25° C., with a gain of 11 for a 1.5 cm 2  B-TRAN with a current density of 133 A/cm 2 . 
         FIG. 27  shows one sample embodiment of a voltage source drive for a B-TRAN. 
         FIG. 28  shows another sample embodiment of a B-TRAN base drive. 
     
    
    
     DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS 
     The numerous innovative teachings of the present application will be described with particular reference to presently preferred embodiments (by way of example, and not of limitation). The present application describes several inventions, and none of the statements below should be taken as limiting the claims generally. 
     An important general principle of the various inventive embodiments disclosed herein is that, in a power-packet-switching power converter, switching is accomplished synergistically by using merged double-base bidirectional opposite-faced devices which operate under conditions of high non-equilibrium carrier concentration. For maximum efficiency, it is preferable to use devices which provide bidirectional conduction with less than a diode drop (approximately 1 V in silicon) in each direction. 
     A power-packet-switching (PPS) converter needs fully bidirectional drive devices, due to the unique architecture. Efficiency is a key criterion in PPS converter designs—even more than in other converter designs, because PPS converters already achieve such high efficiencies. (An additional 1% of inefficiency is minor in a design which operates at e.g. 85% efficiency, but makes a drastic difference in a design which is achieving 98%.) 
     One element of efficiency is losses in the switching devices. IGBT devices are inherently liable to an on-state voltage difference caused by the forward “diode drop” voltage (in addition to any resistive losses). While FETs typically do not suffer from a diode drop, they must deal with the lack of conductivity modulation: their ON resistance cannot be lower than the intrinsic resistance of the base semiconductor. 
     The innovative devices of the present inventions overcome the diode drop problem of the IGBT, while also achieving very low ON resistance from conductivity modulation. Switching losses are low, at least in part by virtue of active turn-off from having base contacts on both sides. IGBTs have inherently slow turn-off since they turn-off open base, which forces those devices to be made with very short carrier lifetimes, which in turn increases the forward voltage drop. 
     The present application teaches a new approach to switching device design for PPS converters. By driving a fully-symmetric double-base bipolar into a state of high carrier concentration, the on-state voltage drop is made very low. This is contrary to conventional wisdom, because such a heavy drive current imposes greater demands on the base drive circuitry, and reduces the effective gain (beta) of the device. 
     The fully-symmetric double-base bipolar is implemented as a “collectorless” structure, in which emitter and base structures are formed on both front and back surfaces of a semiconductor wafer (typically a thinned wafer). Depending on the instantaneous direction of current, one of the two opposed emitter diffusions will operate as a collector. Heavy base current is applied to the emitter which is not acting as a collector, to achieve a high non equilibrium carrier concentration, and hence a low on-state voltage drop. 
     A number of alternative and improved device structures are disclosed, which individually and in various combinations reduce the required base drive (for a given power level). These include e.g. heterojunction emitters, tunnel oxides, field-shaping regions below the base contact, and vertical relations between the emitter and base depths. 
     The present inventions teach, inter alia a bidirectional bipolar transistor (B-TRAN), which is a vertically-symmetric, four-terminal, three-layer, vertical-current-flow semiconductor device. A B-TRAN is most preferably formed as an NPN device, but can also be e.g. a PNP device. 
     B-TRANs are high-level injection devices, and recombination thus occurs quite differently from in low level injection devices. Presently-preferred sample embodiments combine high level carrier injection with thick base regions. In one sample embodiment, the base region can be e.g. 60 μm. 
     An important realization is that the voltage drop under conditions of high level non-equilibrium carrier concentration will be low, even if the resistivity under small currents is large. Thus a device can be optimized to withstand high voltages (e.g. 1200V or more) while still achieving a forward voltage drop which is less than one diode drop. 
     The disclosed devices combine synergistically with the power-packet-switching (PPS) power converter architecture by providing fully-bidirectional switching with less than a diode drop, high voltage resistance, and high ruggedness. The disclosed devices can be 100% bidirectional, with low on-resistance at full current. 
     For reference, in this type of switching bidirectional bipolar, the side which is instantaneously acting as the emitter can be referred to as the anode, and the other side the cathode. 
     Basic Implementations 
     Simple NPN and PNP implementations will be described first. Improvements on this generation of sample embodiments, as discussed below, are more preferable, but this version helps to illustrate basic concepts and principles more clearly. 
     In the NPN sample embodiment of  FIG. 1A , p-type semiconductor base layer  102 A has N+ emitter/collector regions  104 A on an upper and a lower surface. Emitter/collector terminals  106 A and  106 B are connected on opposite sides of the B-TRAN to respective N+ emitter/collector regions  104 A. Similarly, base terminals  108 A and  108 B are connected on opposite sides of the B-TRAN to respective external portions of P type base/drift region  102 A. Each of base terminals  108 A and  108 B can be left “open” (e.g. not connected to anything), shorted to respective terminal  106 A or  106 B (e.g.  108 A to  106 A or  108 B to  106 B), or connected to a power source. 
     The PNP sample embodiment of  FIG. 1B  is similar to the NPN sample embodiment of  FIG. 1A , except that N-type emitter/collector regions  104 A become P-type emitter/collector regions  104 B, and P-type drift region  102 A becomes N-type drift region  102 B. 
     Methods of Operation 
       FIG. 2  shows a simplified schematic of one implementation of a base drive circuit, which can be used with  FIGS. 3A-3F  to illustrate the basic operation of B-TRAN  210 . 
       FIG. 3A  shows a sample equivalent circuit for one exemplary NPN B-TRAN. Body diodes  312 A and  312 B can correspond to e.g. the upper and lower P-N junctions, respectively. In, for example, the sample embodiment of  FIG. 1A , these can correspond to the junctions between emitter/collector regions  104 A and base regions  102 A. Switches  314 A and  314 B can short respective base terminals  108 A and  108 B to respective emitter/collector terminals  106 A and  106 B. 
     In one sample embodiment, a B-TRAN can have six phases of operation in each direction, as follows. 
     1) Initially, as seen in  FIG. 3B , voltage on emitter/collector terminal T 1  is positive with respect to emitter/collector terminal T 2 . Switches  314 A and  316 A are open, leaving base terminal B 1  open. Switch  314 B is closed, shorting base terminal B 2  to emitter/collector terminal T 2 . This, in turn, functionally bypasses body diode  312 B. In this state, the device is turned off No current will flow in this state, due to the reverse-biased P-N junction (represented by body diode  312 A) at the upper side of the device. 
     2) As seen in  FIG. 3C , the voltage on emitter/collector terminal T 1  is brought negative with respect to emitter/collector terminal T 2 . P-N diode junction  312 A is now forward biased, and now begins injecting electrons into the drift region. Current flows as for a forward-biased diode. 
     After a short time, e.g. a few microseconds, the drift layer is well-charged. The forward voltage drop is low, but greater in magnitude than 0.7 V (a typical silicon diode voltage drop). In one sample embodiment, a typical forward voltage drop (Vf) at a typical current density of e.g. 200 A/cm 2  can have a magnitude of e.g. 1.0 V. 
     3) To further reduce forward voltage drop Vf, the conductivity of the drift region is increased, as in e.g.  FIG. 3D . To inject more charge carriers (here, holes) into the drift region, thereby increasing its conductivity and decreasing forward voltage drop Vf, base terminal B 2  is disconnected from terminal T 2  by opening switch  314 B. Base terminal B 2  is then connected to a source of positive charge by switch  316 B. In one sample embodiment, the source of positive charge can be, e.g., a capacitor charged to +1.5 VDC. As a result, a surge current will flow into the drift region, thus injecting holes. This will in turn cause upper P-N diode junction  312 A to inject even more electrons into the drift region. This significantly increases the conductivity of the drift region and decreases forward voltage drop Vf to e.g. 0.1-0.2 V, placing the device into saturation. 
     4) Continuing in the sample embodiment of  FIG. 3D , current continuously flows into the drift region through base terminal B 2  to maintain a low forward voltage drop Vf. The necessary current magnitude is determined by, e.g., the gain of equivalent NPN transistor  318 . As the device is being driven in a high level injection regime, this gain is determined by high level recombination factors such as e.g. surface recombination velocity, rather than by low-level-regime factors such as thickness of, and carrier lifetime within, the base/drift region. 
     5) To turn the device off, as in e.g.  FIG. 3E , base terminal B 2  is disconnected from the positive power supply and connected instead to emitter terminal T 2 , opening switch  316 B and closing switch  314 B. This causes a large current to flow out of the drift region, which in turn rapidly takes the device out of saturation. Closing switch  314 A connects base terminal B 1  to collector terminal T 1 , stopping electron injection at upper P-N junction  312 A. Both of these actions rapidly remove charge carriers from the drift region while only slightly increasing forward voltage drop Vf. As both base terminals are shorted to the respective emitter/collector terminals by switches  314 A and  314 B, body diodes  312 A and  312 B are both functionally bypassed. 
     6) Finally, at an optimum time (which can be e.g. nominally 2 μs for a 1200 V device), full turn-off can occur, as seen in e.g.  FIG. 3F . Full turn-off can begin by opening switch  314 B, disconnecting base terminal B 2  from corresponding terminal T 2 . This causes a depletion region to form from lower P-N diode junction  312 B as it goes into reverse bias. Any remaining charge carriers recombine, or are collected at the upper base. The device stops conducting and blocks forward voltage. 
     The procedure of steps  1 - 6  can, when modified appropriately, used to operate the device in the opposite direction. Steps  1 - 6  can also be modified to operate a PNP B-TRAN (e.g. by inverting all relevant polarities). 
     B-TRAN with Deep Emitters on Both Faces 
       FIG. 4A  shows another sample embodiment of an epitaxial base B-TRAN (the circuit symbol of which can be seen in  FIG. 5A ) according to the present inventions. By contrast,  FIG. 4B  shows a conventional epitaxial base bipolar transistor (the circuit symbol of which can be seen in  FIG. 5B ). These two devices differ significantly in their structures, even beyond the presence of a second base contact region on the bottom surface of the B-TRAN. 
     In the sample embodiment of  FIG. 6 , base contact regions  636  are not connected to adjacent heavily doped N+ emitter/collector regions  604  and a relatively high reverse bias is applied across the nearby reverse-biased collector-to-base junction between emitter/collector regions  604  and base  602 . The depletion region of the reverse-biased collector-to-base junction will electrically isolate lower base contact regions  636  from the rest of base  602 . 
     This condition, that base contact regions  636  on one side of the device are electrically isolated from the remainder of base  602  when a sufficiently high reverse voltage is present across the associated reverse-biased base-to-collector junction, can be obtained, for example, through a combination of the following parameters. 
     1) Have a sufficiently-lightly-doped base region. This requirement can be easily met, since the doping concentration of the base region helps determine the breakdown voltage of the base-to-collector junction, and since the N+ emitter/collector regions are more heavily doped (as seen in e.g.  FIG. 4A ). 
     2) Have N+ regions extend deeper than P+ base contact regions so the depletion of the reverse-biased base-to-collector junction spreads across the region beneath the base contact. This condition can be met, e.g., by introducing the P+ doping species sufficiently long after the introduction of the N+ doping species, or by using a P+ doping species that diffuses more slowly than the N+ doping species, or by a combination of these two techniques. 
     3) Use a cell geometry wherein the depletion region of the reverse-biased base-to-collector junction isolates all of the associated base contact regions. This requirement is not met in a conventional epitaxial base NPN transistor, a top view of which can be seen in  FIG. 7A . For greatest current density, base contact regions will be present on both sides of each of the emitter regions in the device of  FIG. 7A . However, the exemplary B-TRAN of  FIG. 7B  has openings in the N+ emitter region where P+ base contact regions are formed. Heavily-doped N+ emitter regions surround each P+ base contact region with a junction that can form the needed depletion region, as seen in e.g.  FIGS. 8A-8D . 
     In  FIG. 8A , a low reverse bias is present across terminals B 2  and E 2 /C 2 . Separate depletion regions have formed around each reverse-biased N+ region  804 . As the reverse bias increases, the depletion regions begin to merge, as in e.g.  FIG. 8B . As the reverse bias continues to increase, the depletion region widens, as in e.g.  FIG. 8C . In  FIG. 8D , the reverse-bias voltage between emitter/collector terminal E 2 /C 2  and base terminal B 2  has continued increasing until it approaches the breakdown voltage. 
     Trench Isolation 
     Note that  FIG. 13A  shows oxide-filled trenches, on each surface, which laterally separate the emitter region (N+ for an NPN) from the adjacent base contact region. This structure, and its advantages, will be described further in the following sections. 
     Tunnel Oxide 
     As a result of the high level non-equilibrium carrier densities, in an NPN B-TRAN, it is typically desirable to maximize electron injection from emitter to base while minimizing hole injection from base to emitter. In some sample embodiments, high electron injection from emitter to base and low hole injection from base to emitter can be achieved using tunnel oxide, which can be on the order of e.g. 10 Å (1 nm). Electrons will typically have a much higher probability to tunnel through the thin tunnel oxide than will holes. 
     In one sample embodiment, this can be accomplished by providing a thin layer of tunnel oxide between emitter regions and emitter contacts. In the sample embodiment of  FIG. 13A , a thin layer of tunnel oxide  1324  is present between N+ emitter region  1304  and polysilicon layer  1322 . Poly layer  1322 , in turn, contacts emitter metallization  1326 . Oxide  1328  fills trench  1330 , in addition to separating metallization  1326  and poly  1322  from base poly layer  1332  and metallization  1334 . Oxide  1328  in trench  1330  further minimizes unwanted same-side carrier recombination between emitter region  1304  and otherwise-adjacent base contact region  1336 . 
     In the sample embodiment of  FIG. 13B , N+ emitter region  1304 B is significantly smaller than in, e.g., the sample embodiment of  FIG. 13A . This in combination with tunnel oxide layer  1324  can reduce opportunities for undesired hole injection and recombination in emitter region  1304 . 
     The N+ emitter regions can also be entirely absent, as in the sample embodiment of  FIG. 13C . Poly region  1322 B can instead act as an emitter, and is protected from undesired hole injection and recombination by tunnel oxide layer  1324  between base region  102 A and emitter  1322 B. (It will of course be understood that the sample embodiments of  FIGS. 13B-13C  show only one side of each device, which will generally be fabricated identically on both surfaces.) 
     Surface recombination velocity, which can be a factor in high-level-regime current gain, can be problematic due to recombination at the metal contacts, primarily on the emitter contacts. In some sample embodiments, tunnel oxide can be used to nearly eliminate this recombination in NPN B-TRANs by blocking holes from reaching the emitter contact. In the sample embodiment of  FIG. 14A , tunnel oxide  1424 A is disposed between emitter regions  1404  and base  102 A. As in  FIG. 13A , oxide-filled trench  1330  is disposed between N+ emitter regions  1404  and P+ base contact regions  1336 . Since N+ emitter regions  1404  are deposited on top of tunnel oxide  1424 A, N+ regions  1404  can be e.g. polycrystalline silicon, rather than being formed as part of the mono crystalline substrate. 
     In the sample embodiment of  FIG. 14B , tunnel oxide  1424 B continues along the edge of polycrystalline N+ emitter regions  1404  to the surface of the device. In the absence of the oxide-filled trenches of e.g.  FIG. 14A , P+ base contact regions  1436  are offset from emitter regions  1404  in order to not contact tunnel oxide  1424 B around N+ emitter regions  1404 . This separation fulfills a purpose similar to the oxide-filled trenches of  FIG. 14A  by minimizing undesirable direct electrical contact between base contact regions  1436  and emitter regions  1404 . This can help minimize unwanted same-side carrier flow and recombination between emitter and base contact regions. 
     The sample embodiment of  FIG. 14B  is somewhat less preferable than that of  FIG. 14A , however. In the sample embodiment of  FIG. 14A , the structure with sidewall  1330  can be more compact, and tunnel oxide  1424 A is only on the bottom of N+ regions  1404 . In the sample embodiment of  FIG. 14B , the necessary separation between N+ regions  1404  and P+ regions  1436  increases cell size. Tunnel oxide  1424 B must also be formed over a larger surface area than in  FIG. 14A , increasing fabrication complexity. 
     Devices with Heterojunction Emitter 
     In other sample embodiments, electron-hole discrimination can be achieved using heterojunction emitter regions. In the sample embodiment of  FIG. 15 , emitter regions  1504  can be e.g. amorphous silicon on a crystalline substrate. As amorphous silicon has a band gap of 1.4 V, compared to a band gap of 1.1 V for crystalline silicon, electrons injected from emitter  1504  to base  102 A are relatively energetic, and more electrons are injected from emitter  1504  to base  102 A than holes from base  102 A to emitter  1504 . 
     Base Drive 
     Base drive circuits according to the present inventions are preferably applied to each of the two base terminals of a B-TRAN to permit operation as described herein. In one sample embodiment, sample B-TRAN base drive circuits preferably permit a B-TRAN to turn on as a diode; transition to a very-low-forward-voltage saturation mode after initial turn-on; transition back to a no-longer-saturated-but-still-on state; achieve stored charge reduction prior to turn-off to reduce tail current; and then achieve full turn-off and block forward voltage. 
     In one sample NPN embodiment, a base drive circuit like that of  FIG. 9  can be applied to both base B 1  and base B 2 , and can operate e.g. as follows.
         a. Open base B 1 : base terminal B 1  is free to float, and negative voltage from opposite terminal T 2  is blocked. The expected floating base voltage can be, e.g., in the range of 0.7 V to less than 20 V. In this state, opposite base terminal B 2  is shorted to respective opposite terminal T 2 . In the sample embodiment of  FIG. 9 , base B 1  can be left floating by turning off MOSFET switches S 1  and S 2 .   b. Base B 1  shorted to terminal T 1 : base terminal B 1  is shorted to respective terminal T 1 , e.g. by turning switch S 1  off and turning switch S 2  on. Opposite base terminal B 2  is open, and positive voltage from opposite terminal T 2  is blocked. Alternatively, the base drive can be in this state while the B-TRAN conducts in forward-biased diode mode when opposite terminal T 2  is negative. In this latter state, the nominal forward voltage drop Vf can be, e.g., between 1 V and 3 V for silicon diodes.   c. Base B 1  connected to positive bias: Base terminal B 1  is connected to a source of positive charge, e.g. by turning on switch S 1  and turning off switch S 2 . In this state, opposite base B 2  is open, while the B-TRAN is conducting in forward-biased saturated NPN bipolar transistor mode. The nominal forward voltage drop can be, e.g., approximately 0.2 V. A large current flows to base B 1  from the positive bias immediately after connection. Subsequent current flow is lower.   d. Immediately following step (c), base terminal B 1  is connected to respective terminal T 1 , e.g. by opening switch S 1  and closing switch S 2 , while opposite base terminal B 2  is open. A large current briefly flows to terminal T 1  from base terminal B 1 . This rapidly depletes charge carriers in the drift region. The device comes out of saturation and reverts to forward-biased diode mode.   e. Immediately following step (d), opposite base terminal B 2  is shorted to respective opposite terminal T 2  by a similar mechanism. A small current flows to base terminal B 1  from respective terminal T 1 , and charge carriers are swept from the drift region, increasing forward voltage drop Vf   f. Immediately following step (e), base terminal B 1  is floated, e.g. by turning switches S 1  and S 2  off, while opposite base terminal B 2  remains shorted to respective opposite terminal T 2 . The B-TRAN turns off as the PN junction between base terminal B 1  and respective terminal T 1  becomes reverse biased.       

     In one sample embodiment, switch S 2  can be e.g. a GaN MOSFET. Since switch S 2  conducts and blocks voltage in both directions, and the largest positive voltage switch S 2  sees is +1.5 V, a GaN MOSFET can be preferable for switch S 2 , since the body diode of a GaN MOSFET does not conduct current at 1.5 V or less. 
       FIG. 10  shows plots of some sample currents and voltages during a process according to the present inventions, e.g. like that of steps (a)-(f) above, modified for a PNP sample embodiment like that of  FIG. 11 . 
       FIG. 11  shows one sample embodiment of a PNP B-TRAN base drive circuit which can include common source MOSFET pairs for base-emitter shorting. Isolated power supplies P 1  and P 2  are included in parallel with respective capacitors C 1  and C 2 . In one sample embodiment, isolated power supply P 1  can be at, e.g., −0.7 V with respect to emitter E 1 , and isolated power supply P 2  can be at, e.g., −0.7 V with respect to emitter E 2 . 
     Common source MOSFET pairs (Q 11 , Q 21 ) and (Q 22 , Q 12 ) are preferably used for base-emitter shorting between respective emitter-base pairs (E 1 , B 1 ) and (E 2 , B 2 ). 
     JFETs Q 31  and Q 32  are preferably used at startup to increase blocking voltage, and then are preferably turned off while the converter is running. 
     MOSFETs Q 41  and Q 42  are preferably used after device turn-on to reduce forward voltage drop Vf. 
       FIG. 12  shows a presently-preferred sample embodiment of a base drive circuit for NPN B-TRAN  1210 . Two common grounds are shown. Common ground  1222  connects together emitter/collector terminal T 1  and the base drive circuitry driving base B 1   108 A. Common ground  1220  connects together emitter/collector terminal T 2  and the base drive circuitry driving base B 2   108 B. 
     In one sample embodiment, this base drive circuitry can drive base terminal B 2   108 B in one of three modes. In passive off mode, base terminal B 2   108 B is preferably clamped (e.g. by Schottky diode D 22 ) to be no higher than e.g. about 0.3V relative to emitter  106 B, and is allowed to be lower in voltage than emitter  106 B. In passive off mode, only normally-on JFET S 52  is on. This can permit base B 2   108 B to float, as in e.g.  FIG. 3F . 
     In base-emitter shorting mode, only MOSFET switches S 42  and S 32  are on, thereby shorting base B 2   108 B to emitter T 2   106 B. 
     Injection mode, for NPN B-TRANs, injects current into the respective base terminal when the device is in active on mode. This can lower the forward voltage drop of the B-TRAN to less than a diode drop, e.g. 0.7 volts. In some sample embodiments, this step can lower the forward voltage drop Vf to e.g. 0.1-0.2 V. Switch S 42  is on, while switches S 32  and S 52  are off MOSFET switches S 12  and S 22  are controlled on and off to produce an appropriate current into base terminal B 2  in switch mode power supply configuration. Current swings can be controlled by inductors L 11  and L 12  , and current can be sensed by resistors R 11 and R 12 . A suitable control system (not shown) controls the switches, inductors, and resistors in order to control the base current, to thereby produce a low forward voltage drop Vf. 
     Fabrication 
     In one sample embodiment, the deposition of hydrogenated amorphous silicon (a-Si:H) or hydrogenated amorphous silicon carbide (a-SiC:H) can advantageously be used for fabricating B-TRANs having heterojunction emitters as described herein. This material can be sputtered, but is more preferably deposited using chemical vapor deposition (CVD) or plasma enhanced CVD. Since these materials are altered by high temperature processing, the emitter material needs to be deposited after the high temperature processing steps have been completed. 
     In one sample embodiment, tunnel oxide can be fabricated between base and emitter regions as described herein. The bare surface of the base region can be exposed to an oxidizing ambient long enough to grow a thin layer of a dielectric that is primarily e.g. silicon dioxide. In one sample embodiment, this thin layer of tunnel oxide can be e.g. in the range of 10 Å-30 Å. Following such an oxidation step, a layer of amorphous or polycrystalline silicon can be deposited, e.g. using low pressure chemical vapor deposition (LPCVD). In some sample embodiments, this can be followed by introducing a dopant such as e.g. arsenic to dope the polycrystalline silicon. 
     The present inventors have realized that physical and electrical performances can be made nearly equivalent on both sides of a B-TRAN semiconductor die. All dopant species are introduced into each side of the wafer, and then a single long high-temperature diffusion step is preferably performed. 
     In one sample embodiment, the long high-temperature diffusion step can be, e.g., at a temperature of 1100-1150° C. Long high-temperature diffusion processes can most preferably be used in conjunction with the two handle-wafer process described below, but can also be used independently of the two handle-wafer process. 
     Presently-preferred processing steps for fabricating a B-TRAN according to the present inventions include masking operations, thermal oxidation, etching, impurity introduction, chemical vapor deposition (CVD), and physical vapor deposition (PVD). 
     A sample fabrication sequence according to the present inventions most preferably includes both a high-temperature-handle-wafer bonding step and a medium-temperature-handle-wafer bonding step. These two handle wafers are preferably attached to different sides of the same wafer at different points in the fabrication sequence, and preferably in different temperature ranges. 
     “High temperature”, in the context of the present innovative processes, can mean, e.g., any temperature above the alloy/anneal temperature of aluminum or aluminum alloys. In one sample embodiment, “high temperature” can refer to, e.g., any temperature above approximately 450° C. 
     “Medium temperature”, in the context of the present innovative processes, can mean, e.g., any temperature between the melting temperature of solder and the alloying temperature of aluminum or aluminum alloys (inclusive). In one sample embodiment, “medium temperature” can refer to, e.g., any temperature between approximately 240° C. and approximately 450° C. (inclusive). 
     “Low temperature”, in the context of the present innovative processes, can mean, e.g., any temperature between approximately room temperature and the melting temperature of solder. In one sample embodiment, “low temperature” can refer to, e.g., any temperature between approximately 25° C. and 240° C. 
     In one sample embodiment, a B-TRAN fabrication sequence according to the present innovative processes can proceed e.g. as follows: 
     Step  1  through Step M: Perform all high temperature steps, such as thermal oxidation, some chemical vapor deposition (CVD) operations, and high temperature anneals, up to the contact mask step, on one side of the wafer. This stops just short of a high-temperature, relatively-long diffusion step designed to diffuse impurities to a desired junction depth on both sides of the wafer. 
     Step M+1: Deposit a protective layer (e.g. CVD silicon dioxide) or sandwich of protective layers (e.g. CVD silicon dioxide, CVD silicon nitride, and CVD silicon dioxide) on the first side of the device wafer. The protective layer can prevent unwanted changes to the first side. This protective layer can also serve as a stopping point for a thinning operation later used to remove material from the first side. Chemical-mechanical planarization (CMP) can be performed to flatten the top surface of the deposited protective layer (or deposited sandwich of protective layers). A sample of the device at this point can be seen in, e.g.,  FIG. 16A . 
     Step M+2: Attach handle wafer  1  to the first side of the device wafer at high temperature. Any acceptable high-temperature material can be used for handle wafer  1 , such as e.g. silicon, silicon dioxide, silicon carbide, or sapphire. Handle wafer  1  must bond to the first side of the device wafer. If silicon is used as the handle wafer, the surface of the silicon handle wafer will bond directly to the top of the protective layer or protective layer sandwich at high temperature, as seen in e.g.  FIG. 16B . 
     Step M+3: From a second side opposite the first side, thin the device wafer to the desired thickness. This can be done, e.g., by a combination of grinding, lapping, and polishing. 
     Step M+4 through Step N: Perform steps  1  through M on the second side of the device wafer. 
     Step N+1: Perform a relatively long high-temperature diffusion step to obtain the desired dopant junction depths and dopant distribution on both sides of the device wafer. 
     Step N+2 through Step P: Perform steps from contact mask through passivation layer deposition and a pad etch step on the second side of the device wafer. 
     Step P+1: Attach handle wafer  2  to the second side of the device wafer at a medium temperature. Handle wafer  2  can be any acceptable medium-temperature material, such as e.g. quartz, glass, silicon, silicon carbide, and sapphire. In one sample embodiment, the device at this point can be like that seen in e.g.  FIG. 16C . 
     Step P+2: Remove handle wafer  1 . This can be done, e.g., by grinding, lapping, chemical-mechanical planarization (CMP), and is most preferably continued up to, but not through, the “stopping point” layer deposited in e.g. Step M+1. As a result of this step, the device wafer can be like that seen in, e.g.,  FIG. 16D . 
     Step P+3: Remove the stopping layer (e.g. by etching or chemical-mechanical planarization (CMP)). 
     Step P+4 through Step Q: Perform the contact mask through the passivation deposition and the pad etching steps on the first side of the device wafer. At this point, the wafer has completed conventional wafer processing. 
     Step Q+1: Do nothing, apply tape to one side of the wafer, or mount the wafer on a substrate. 
     Step Q+2: Remove handle wafer  2  from the second side of the device wafer, resulting in a structure like that of e.g.  FIG. 16E . 
     Step Q+3 through end: Plate one or both sides of the device. 
     Final processing continues through dicing (chip separation) and testing as appropriate. 
     In one sample embodiment, the first side of the device can be plated before handle wafer  2  is removed from the second side of the device wafer. 
     In one sample embodiment, B-TRAN fabrication can start with double-side-polished starting wafers. Following the high-temperature bonding step, alignment marks can be placed on both exposed surfaces of the bonded wafer stack using an alignment algorithm. In another sample embodiment, front-surface-to-back-surface alignment can be obtained by, e.g., infrared alignment, which allows the features on one wafer surface to be “seen” through the wafer during alignment. In yet another sample embodiment, front-back alignment can be obtained by mechanical means, e.g. aligning to the first surface of the wafer while the mask is present on the second surfaces. Each of these front-back alignment techniques has advantages and disadvantages, including associated equipment costs. 
     In one sample B-TRAN fabrication embodiment, double-sided plating of a B-TRAN can use the same metal and pad pattern on each surface. Connection to the desired regions on each surface can then be made using a patterned layer. In one sample embodiment, contact can be made to the bottom surface using e.g. a patterned metallization on a ceramic, and to the top surface using e.g. a patterned copper lead frame. The ability to use the same pad mask on both surfaces can greatly simplify fabrication. 
     In one sample embodiment, contact to both the bottom and the top of the die is preferably obtained by a layer of solder. The solder is typically deposited on the patterned regions which have also been plated, and which are present on each surface of the die. 
     However, it can be advantageous from a manufacturing perspective to attach the bottom of a B-TRAN die to a metallized ceramic using solder, and to bond large-diameter wire to the metallized regions on the top of the die. 
     One complicating factor, however, is that large regions of plated material, e.g. nickel, have residual stress that can crack or otherwise damage thin die. 
     The present application teaches inter alia that the same metal and pad masks can be used on both top and bottom surfaces of a B-TRAN, subject to three conditions: 
     1) There are sufficient bonding pads of an appropriate size on the top surface of the die to accommodate the necessary number of large-diameter wire bonds needed. 
     2) There are enough plated regions on the bottom of the die to allow the formation of a low resistance contact. 
     3) The pattern of open plated regions does not result in stress large enough to damage the die. 
     Accordingly, pad masks are proposed for B-TRAN fabrication which have a mixture of (typically relatively large) open regions suitable for both wire bonds and a plated layer, and smaller open regions which can be plated but generally not bonded. 
     The design of the edge termination structure of a high voltage semiconductor device such as a B-TRAN is crucial to its long term operation. A termination structure designed to operate in a given set of conditions can exhibit a considerable reduction in voltage handling ability in the presence of unwanted positive or negative charge at or near the surface of the termination structure. A considerable decrease in breakdown voltage can occur in the presence of either positive or negative charge in the termination region (as demonstrated in “The Effect of Static and Dynamic Parasitic Charge in the Termination Area of High Voltage Devices and Possible Solutions”, T. Trajkovic et al., which is hereby incorporated by reference). 
     The present innovative B-TRAN fabrication methods also teach (inter alia) the use of novel and innovative structures to prevent any decrease in the breakdown voltage of a device by the presence of charge in the termination region. One sample embodiment of these innovative structures, as seen in e.g.  FIG. 17 , can consist of a P+ region that contains one or more n-type regions. 
     This combination of two doped regions, one within the other, is similar to the body and source of a DMOS transistor, or to the base and emitter of a bipolar transistor. 
     In one sample embodiment, a B-TRAN termination structure according to the present inventions can be fabricated through the use of two masks, one for each dopant type. In another sample embodiment, a B-TRAN termination structure according to the present inventions can be fabricated by using a single mask with both dopant types introduced through the same opening. In each of these cases, the required openings can be added to pre-existing masking layers, with the final dopant distributions being obtained using existing dopant-introduction and diffusion steps. 
     In some sample embodiments, innovative bidirectional devices of the present inventions can have pad metallizations which are not symmetric between the front and back of a device, as in e.g.  FIG. 18A .  FIG. 18B  shows how offset contact pads can significantly improve thermal dissipation by significantly reducing the distance from any point in the die to a region with low thermal resistance. In one sample embodiment, contact pads on each side can be spaced e.g. 1270 μm apart (in the x direction), and the die can be e.g. 100-200 μm thick (in the z direction). In one sample embodiment, contact pads on each side of the die can be offset to minimize distance to regions with low thermal resistance.  FIGS. 18C-18D  show alternative embodiments of pad metallization fractions. 
     Simulations of a model npn B-TRAN as seen in  FIG. 19  were performed in ATLAS by Silvaco. The simulated B-TRAN has a nominal base doping of 10 14  cm −3 , a base width of 120 μm, a trench depth of 5 μm, and a trench width of 1.5 μm. 
       FIG. 20  shows a close-up view of one emitter region (which acts as an emitter or a collector depending on the conduction direction). Doping densities in the emitter region vary from approximately 10 14 -10 15 /cm 3  near the emitter-base junction to greater than 3×10 19 /cm 3  at the surface of the device. The emitter-base junction has a depth of 6 μm Doping densities in the base region vary from approximately 10 12 -10 14 /cm 3  in the bulk region to approximately 2×10 19 /cm 3  at the surface. 
       FIG. 21  shows a simulated B-TRAN hard turn-off at 100 A, 800 V, and 80° C. During the interval shown, the voltage on the collector rises roughly exponentially from near zero to 800 V over the course of ˜160 ns, then remains steady at 800 V. When the collector voltage V C  reaches 800 V, the total current drops rapidly from 100 A to ˜1 A over the course of ˜10 ns, then decreases more gradually to zero over the next 10-15 ns. Energy dissipated in this transition, due to instants when nonzero current flows at nonzero voltage, is 4.1 mJ. 
       FIG. 22  shows simulated collector voltage V C  and total current I total  for a hard B-TRAN turn-on for 100 A, 800 V, and 80° C. Within a few nanoseconds of the total current increasing rapidly from zero to 100 A, the collector voltage V C  drops rapidly from 800 V, and then near-asymptotically approaches zero over the subsequent 40-50 ns. Energy dissipated in this transition, due to instants when nonzero current flows at nonzero voltage, is 0.29 mJ. 
     In  FIG. 23 , a hard B-TRAN turn-on with reverse recovery is simulated under the same operating conditions as in  FIGS. 21-22 . This Figure relates particularly to the pull-up device in the operating configuration of  FIGS. 19-26 , and will be discussed further below. 
     In the interval shown, current I high  decreases linearly from 100 A to ˜−4.5 A over ˜45 ns, then near-asymptotically approaches zero over ˜5 ns. Emitter voltage V e  decreases exponentially from 800 A, beginning to drop most noticeably at around the time current I high  crosses zero, reaching and maintaining ˜0 V when current I high  reaches its minimum and begins increasing back to 0 A. Energy dissipated in the high-side device during this transition is 1.1 mJ. (However, another 22.6 mJ is dissipated in the other B-TRAN device, during the operations described below.) 
       FIG. 24  simulates turnoff of a B-TRAN in a power-packet-switching power converter architecture (PPSA), under 200 A and 25° C. Collector current I C  generally decreases (with several aperiodic positive spikes) from roughly 150 A to a steady-state current of 0 A over the course of ˜1 μs. Collector voltage V C  meanwhile begins increasing linearly from 0 V to ˜650 V over the course of ˜5 μs. Energy dissipated in this transition, due to instants when nonzero current flows at nonzero voltage, is 0.7 mJ. 
       FIG. 25  shows simulated emitter current I E  and collector voltage V C  for a B-TRAN used in PPSA turn-on for 200 A at 25° C. From time t=0s, emitter current I E  increases to a peak of ˜350 A after roughly 150 ns, then near-asymptotically approaches 200 A until about t=2 μs. Collector voltage V C  rises to a maximum of about 10 V at ˜t=100 ns, roughly the same time as emitter current I E  crosses 200 A, then decays to ˜1 V by about t=2 μs. Energy dissipated in this transition is 0.4 mJ. 
       FIG. 26  shows simulated currents for a B-TRAN in use in a PPSA converter for 200 A at 25° C., with a gain of 11 for a 1.5 cm 2  B-TRAN with a current density of 133 A/cm 2 . Emitter current I E  spikes to about −350 A at ˜t=10 ns, then remains at a steady-state current of −200 A (with a brief spike to −350 A at t=2 μs) until t=7 μs, at which point it spikes sharply to ˜+550 A, drops rapidly back to −150 A by t˜8.5 μs, then reaches zero (after several aperiodic negative spikes) by t=9 μs. 
     Collector current I C  spikes to just over 200 A at t˜10 ns, then drops back below 200 A and gradually increases (with a brief spike at t=2 μs) to just under 200 A at t=7 μs, at which point it spikes momentarily to 300 A, decreases from 200 A to ˜180 A by t=8.5 μs, then reaches zero (after several aperiodic positive spikes) by t=9 μs. 
     Collector-base current I B-C  spikes to ˜100 A at t˜10 ns, decreases gradually (with a momentary spike to just over 200 A at t=2 μs) to ˜0.3 A at 7 μs, spikes momentarily to −20 A, increases gradually to ˜0.5 A at 8.5 μs, then drops sharply to zero by t=9 μs. 
     Emitter-base current I B-E  remains at 0 A until t=7 μs, spikes sharply to −750 A, decays to about −50 A by t=8.5 μs, then decays more sharply and reaches zero at t=9 μs. 
     A B-TRAN is in the “active off-state” when the e-base (base on emitter side) is shorted to the emitter, and the c-base (base on the collector side) is open. In this state with the NPN B-TRAN, the collector is the anode (high voltage side), and the emitter is the cathode (low voltage side). 
     The B-TRAN is also off when both bases are open, but due to the high gain of the B-TRAN in this state, the breakdown voltage is low. The series combination of a normally on JFET and a Schottky diode attached between each base on its respective emitter/collector, as previously disclosed, will significantly increase the blocking voltage in this “passive off-state”. The JFETs are turned off during normal operation. 
     The e-base is essentially at a constant voltage—it varies only about 0.1 V from a low drive to a high drive condition. The c-base, in contrast, is a nearly constant current drive, even as the voltage is varied from 0 V above the collector to about 0.6V. Instead of the c-base current changing with c-base voltage, V CE  changes. At c-base of 0V (c-base shorted to collector), there is a certain gain that depends on the emitter current density, and V CE  is nominally 0.9 V over a large range of current density. Raising c-base to 0.1 V above the collector does not change the gain, but it lowers V CE  by nominally 0.1 V. Raising c-base to 0.6V drops V CE  to about 0.2 or 0.3 V. Thus, when driving the c-base, a voltage source is advantageously used, as in the sample embodiment of  FIG. 27 , not a current source. 
     This is a very significant difference from the way BJTs are normally driven, which is from a current source into the e-base. 
     The impendence, in terms of di/dv, of the c-base is high. The c-base drive current changes very little with c-base to collector voltage (V CB ), until it gets close to forward biasing the collector/base junction (over 0.6V at 25 C in silicon). That&#39;s why it needs a voltage source type drive. C-base current changes with emitter current, increasing with increased emitter current even when V CB  remains constant, but not much with changing V CB . 
     The impedance of the e-base is very low, as it is at nearly constant voltage while the c-base is varied. 
       FIG. 28  shows one sample embodiment of another B-TRAN base drive circuit. 
     One presently-preferred sample embodiment for B-TRAN turn-on is to simultaneously, from the active off-state and blocking forward voltage, open the e-base to emitter short while shorting the c-base to the collector. This immediately introduces charge carries into the highest field region of the depletion zone around the collector/base junction, so as to achieve very fast, forward biased turn-on for hard switching, very similar to IGBT turn-on. 
     Another advantageous turn-on method, from the active off-state, is to have the circuit containing the B-TRAN reverse the B-TRAN polarity, which produces the same base state described in the hard turn-on method, but at near zero voltage. That is, the e-base which is shorted to the emitter becomes the c-base shorted to the collector as the B-TRAN voltage reverses from the active off-state polarity. Again, turn-on is fast. 
     In a third turn-on method from the active off-state, the e-base is disconnected from the emitter, and connected to a current or voltage source of sufficient voltage to inject charge carriers into the base region. This method is likely slower, since the charge carriers go into the base just below the depletion zone. Also, it is known that carrier injection into the e-base results in inferior gain relative to carrier injection into the c-base. 
     After turn-on is achieved with either of the methods using the c-base, V CE  is more than a diode drop. To drive V CE  below a diode drop, turn-on goes to the second stage of increased charge injection into the c-base via a voltage or current source. The amount of increased charge injection determines how much V CE  is reduced below a diode drop. Injection into the e-base will also reduce V CE , but the gain is much lower than with c-base injection. 
     Turn-off can be achieved by any of several methods. The most advantageous method is a two-step process. In the first step, the c-base is disconnected from the carrier injection power supply and shorted to the collector, while the previously open e-base is shorted to the emitter. This results in a large current flow between each base and its emitter/collector, which rapidly removes charge carriers from the drift region. This in turn results in a rising V CE  as the resistivity of the drift region increases. At some optimum time after the bases are shorted, the connection between the c-base and the collector is opened, after which V CE  increases rapidly as the depletion region forms around the collector/base junction. 
     Alternately, turn-off can be achieved by simply opening the c-base and shorting the e-base to the emitter, but this will result in higher turn-off losses since the drift region (base) will have a high level of charge carriers at the start of depletion zone formation. 
     Alternately, turn-off can be achieved by simply opening the c-base and shorting the e-base to the emitter, but this will result in higher turn-off losses since the drift region (base) will have a high level of charge carriers at the start of depletion zone formation. 
     Alternatively, turn-off can be achieved by simply opening the c-base and leaving the e-base open, but this will result in the highest turn-off losses and also a low breakdown voltage. 
     Additional general background, which helps to show variations and implementations, as well as some features which can be implemented synergistically with the inventions claimed below, may be found in the following US patents and applications. All of these have at least some common ownership, copendency, and inventorship with the present application, and all of them, as well as any material directly or indirectly incorporated within them, are hereby incorporated by reference: U.S. Pat. Nos. 9,029,909; 9,035,350; 9,054,707; 9,054,706; 9,059,710; 9,190,894; Ser. Nos. 14/566,576; 14/599,191; 14/918,440; 14/930,627; and 14/935,344; all of which are hereby incorporated by reference, singly and in combination. 
     Advantages 
     The disclosed innovations, in various embodiments, provide one or more of at least the following advantages. However, not all of these advantages result from every one of the innovations disclosed, and this list of advantages does not limit the various claimed inventions.
         On-state voltage drop is less than a diode drop.   Bidirectional operation is achieved with identical electrical characteristics in either direction.   Provides totally flat planar device.   Bidirectional bipolar transistors operate under conditions of high non-equilibrium carrier concentration.   High voltage resistance.   High ruggedness.   Innovative fabrication techniques enable two-sided bidirectional device fabrication.   Fully bidirectional conduction with less than a diode drop in each direction.   Enables fully-bidirectional switching in power-packet-switching power converters.   No double-diffused base necessary.       

     According to some but not necessarily all embodiments, there is provided: Methods, systems, circuits, and devices for power-packet-switching power converters using bidirectional bipolar transistors (B-TRANs) for switching. Four-terminal three-layer B-TRANs provide substantially identical operation in either direction with forward voltages of less than a diode drop. B-TRANs are fully symmetric merged double-base bidirectional bipolar opposite-faced devices which operate under conditions of high non-equilibrium carrier concentration, and which can have surprising synergies when used as bidirectional switches for power-packet-switching power converters. B-TRANs are driven into a state of high carrier concentration, making the on-state voltage drop very low. 
     According to some but not necessarily all embodiments, there is provided: A power-packet-switching power converter, comprising: a plurality of phase legs which each include two bidirectional switches which can connect a respective external line to either side of a link inductor which is paralleled by a capacitor, each said bidirectional switch comprising: first and second first-conductivity-type emitter regions on opposing faces of a second-conductivity-type semiconductor mass, and first and second second-conductivity-type base contact regions, in proximity to said first and second emitter regions respectively; control circuitry which, repeatedly, turns on a selected one or two of said bidirectional switches to drive energy from one or more input lines into said inductor, and then turns off all of said switches to disconnect said inductor, and then turns on a different selected one or two of said bidirectional switches to drive energy from said inductor onto one or two output lines; and a drive circuit which, when the control circuitry selects one of said bidirectional switches for turn-on, drives a base contact region of that switch, to forward bias the associated emitter-base junction and permit majority carriers to flow to the other emitter region on the opposing surface; wherein the drive circuit applies sufficient current to the selected base contact region to generate a nonequilibrium carrier concentration, in the interior of said semiconductor mass, which is more than thirty times as great as the off-state equilibrium majority carrier concentration in the semiconductor mass, to thereby lower the voltage drop across the switch. 
     According to some but not necessarily all embodiments, there is provided: A power-packet-switching power converter, comprising: a plurality of phase legs which each include two bidirectional switches which can connect a respective external line to either side of a link inductor which is paralleled by a capacitor, each said bidirectional switch comprising: first and second first-conductivity-type emitter regions on opposing faces of a second-conductivity-type semiconductor mass, and first and second second-conductivity-type base contact regions, in proximity to said first and second emitter regions respectively; control circuitry which, repeatedly, turns on a selected one or two of said bidirectional switches to drive energy from one or more input lines into said inductor, and then turns off all of said switches to disconnect said inductor, and then turns on a different selected one or two of said bidirectional switches to drive energy from said inductor onto one or two output lines; and a drive circuit which, when the control circuitry selects one of said bidirectional switches for turn-on, drives a base contact region of that switch, to forward bias the associated emitter-base junction and permit majority carriers to flow to the other emitter region on the opposing surface; wherein the drive circuit applies sufficient current to the selected base contact region to drive the beta down to less than one-quarter of its small-signal value. 
     According to some but not necessarily all embodiments, there is provided: A power-packet-switching power converter, comprising: a plurality of phase legs which each include two bidirectional switches which can connect a respective external line to either side of a link inductor which is paralleled by a capacitor, each said bidirectional switch comprising: first and second first-conductivity-type emitter regions on opposing faces of a second-conductivity-type semiconductor mass, and first and second second-conductivity-type base contact regions, in proximity to said first and second emitter regions respectively; control circuitry which, repeatedly, turns on a selected one or two of said bidirectional switches to drive energy from one or more input lines into said inductor, and then turns off all of said switches to disconnect said inductor, and then turns on a different selected one or two of said bidirectional switches to drive energy from said inductor onto one or two output lines; and a drive circuit which, when the control circuitry selects one of said bidirectional switches for turn-on, drives a base contact region of that switch, to forward bias the associated emitter-base junction and permit majority carriers to flow to the other emitter region on the opposing surface; wherein the drive circuit applies sufficient current to the selected base contact region to generate a nonequilibrium carrier concentration, in the interior of said semiconductor mass, which is more than thirty times as great as the off-state equilibrium majority carrier concentration, to thereby lower the voltage drop across the switch to less than half a diode drop. 
     According to some but not necessarily all embodiments, there is provided: A power-packet-switching power converter, comprising: a plurality of phase legs which each include two bidirectional switches which can connect a respective external line to either side of a link inductor which is paralleled by a capacitor; each said bidirectional switch comprising: first-conductivity-type emitter regions on opposing faces of a second-conductivity-type semiconductor mass, and second-conductivity-type base contact regions in proximity to respective emitter regions; control circuitry which turns on two of said bidirectional switches to drive energy from one or more input lines into said inductor, and then turns off all of said switches to disconnect said inductor, and then turns on a different two of said bidirectional switches to drive energy from said inductor onto one or two output lines; and a drive circuit which, when the control circuitry selects one of said bidirectional switches for turn-on: begins turn-on by shorting a first one of the base contact regions of that switch to the respective emitter region, while leaving the base contact regions on the opposing face of that switch floating; drives said first one of the base contact regions, to forward bias the associated emitter-base junction and permit majority carriers flow to the other emitter region on the opposing surface, thereby entering saturation mode; shorts said first one of the base contact regions to the respective emitter region, thereby exiting saturation mode; begins turn-off by shorting the base contact region on the opposing surface to the respective emitter region; and completes turn-off by causing said first one of the base contact region to float; wherein the drive circuit applies sufficient current to the selected base contact region to generate a nonequilibrium carrier concentration, in said semiconductor mass, to thereby lower the voltage drop across the switch. 
     According to some but not necessarily all embodiments, there is provided: A bidirectional power switching circuit, comprising: first and second first-conductivity-type emitter regions on opposing faces of a second-conductivity-type semiconductor mass, and first and second second-conductivity-type base contact regions, in proximity to said first and second emitter regions respectively; and a drive circuit which, when the control circuitry selects one of said bidirectional switches for turn-on, drives a base contact region of that switch, to forward bias the associated emitter-base junction and permit majority carriers to flow to the other emitter region on the opposing surface; wherein the drive circuit applies sufficient current to the selected base contact region to generate a nonequilibrium carrier concentration, in the interior of said semiconductor mass, which is more than thirty times as great as the off-state equilibrium majority carrier concentration, to thereby lower the voltage drop across the switch. 
     According to some but not necessarily all embodiments, there is provided: A method of operating a power-packet-switching power converter, comprising: driving a first array of bidirectional switches to drive power into a link inductor which is paralleled by a capacitor; driving a second array of bidirectional switches to draw power onto an output line from said inductor; wherein each said bidirectional switch comprises first-conductivity-type emitter regions on opposing faces of a second-conductivity-type base region, and second-conductivity-type base contact regions on said opposing faces of said second-conductivity-type base region; wherein driving each said bidirectional switch comprises: when one said face of one said bidirectional switch is desired to be in passive off mode, clamping a voltage of the respective base contact region to be less than or equal to a voltage of the respective emitter region plus a Schottky diode drop; when one said face of one said bidirectional switch is desired to be in active off mode or in diode mode, shorting the respective base contact region to the respective emitter region; and when one said face of one said bidirectional switch is desired to be in active on mode, injecting minority charge carriers into the respective base contact region. 
     According to some but not necessarily all embodiments, there is provided: A semiconductor device, comprising: first-conductivity-type emitter regions on opposing faces of a second-conductivity-type semiconductor mass; second-conductivity-type base contact regions on said opposing faces of said second-conductivity-type semiconductor mass; wherein said first-conductivity-type emitter regions and said second-conductivity-type semiconductor mass form a heterojunction therebetween; drive circuitry which applies sufficient current to the selected base contact region to generate a nonequilibrium carrier concentration, in the interior of said semiconductor mass, which is more than thirty times as great as the off-state equilibrium majority carrier concentration, to thereby lower the voltage drop. 
     According to some but not necessarily all embodiments, there is provided: A semiconductor device, comprising: first-conductivity-type emitter regions on opposing faces of a second-conductivity-type semiconductor mass; second-conductivity-type base contact regions on said opposing faces of said second-conductivity-type semiconductor mass; a thin layer of tunnel oxide between each said first-conductivity-type emitter region and said second-conductivity-type semiconductor mass, and which forms a differential between holes and electrons; drive circuitry which applies sufficient current to the selected base contact region to generate a nonequilibrium carrier concentration, in the interior of said semiconductor mass, which is more than thirty times as great as the off-state equilibrium majority carrier concentration, to thereby lower the voltage drop. 
     According to some but not necessarily all embodiments, there is provided: A semiconductor device, comprising: first-conductivity-type emitter regions on opposing faces of a second-conductivity-type semiconductor mass; second-conductivity-type base contact regions on said opposing faces of said second-conductivity-type semiconductor mass; a thin layer of tunnel oxide between each said first-conductivity-type emitter region and a respective emitter metallization, and which forms a differential between holes and electrons; drive circuitry which applies sufficient current to the selected base contact region to generate a nonequilibrium carrier concentration, in the interior of said semiconductor mass, which is more than thirty times as great as the off-state equilibrium majority carrier concentration, to thereby lower the voltage drop. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes both an n-type emitter/collector region, and also a p-type base contact region, on both first and second surfaces of a p-type semiconductor die, comprising: in the ON state, flowing base current through the base contact region which is nearer the more positive one of the emitter/collector regions, without flowing base current through the other of the base contact regions; wherein the base contact region on the first surface is not electrically connected to the base contact region on the second surface, except through the semiconductor die itself; and wherein the emitter/collector region on the first surface is not electrically connected to the emitter/collector region on the second surface; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes first emitter/collector and base contact regions on a first surface of a semiconductor die, and second emitter/collector and base contact regions on a second surface of the die; wherein the emitter/collector regions both have a first conductivity type, and the base contact regions and the semiconductor die all have a second conductivity type; comprising: in the ON state, when the applied voltage polarity is such that the first and second emitter/collector regions can act as collector and emitter respectively, flowing base current through the first base contact region, but substantially not through the second base contact region; and in the OFF state, not flowing external current through either of the base contact regions; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes distinct first and second first-conductivity-type emitter/collector regions which are not electrically connected together, and also includes distinct first and second second-conductivity-type base contact regions which are not electrically connected together, the emitter/collector regions and the base contact regions all lying within a common semiconductor die, comprising: in the ON state, when the applied voltage polarity is such that the first emitter/collector region can act as the collector of, and the emitter/collector region on the second of said surfaces can act as the emitter of, a bipolar transistor, driving the first base contact toward a voltage different from the first emitter/collector region by flowing base current through the first base contact region, while NOT flowing base current through the second base contact region; and in the OFF state, floating at least one of the base contact regions, while not applying external current to either of the base contact regions; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes both a respective p-type emitter/collector region, and also a respective n-type base contact region, on both first and second surfaces of an n-type semiconductor die, comprising: in the ON state, flowing base current through the base contact region which is nearer the more negative one of the emitter/collector regions, without flowing base current through the other of the base contact regions; wherein the base contact region on the first surface is not electrically connected to the base contact region on the second surface, except through the semiconductor die itself; and wherein the emitter/collector region on the first surface is not electrically connected to the emitter/collector region on the second surface; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes both an n-type emitter/collector region, and also a p-type base contact region, on both first and second surfaces of a p-type semiconductor die, comprising: at turn-on, shorting the more positive one of the emitter/collector regions together with the base contact region on the same one of the surfaces, to thereby conduct current with a diode voltage drop; and thereafter flowing base current through at least one of the base contact regions, to initiate conduction as a bipolar transistor with less than a diode voltage drop; wherein the base contact region on the first surface is not electrically connected to the base contact region on the second surface, except ohmically through the semiconductor die; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power bipolar semiconductor device which includes both first-conductivity-type emitter/collector regions and also second-conductivity-type base contact regions on both opposed surfaces of a semiconductor die, comprising the actions of: at turn-on, when the applied voltage polarity is such that the emitter/collector region on the first of said surfaces can act as the collector of a vertical bipolar transistor while the emitter/collector region on the second of said surfaces acts as the emitter of the same vertical bipolar transistor, beginning turn-on by shorting the base contact and emitter/collector regions on the first surface together, without driving the base contact region on the second surface, to thereby conduct current as a diode without external base current power; and thereafter applying base current to one but not both of the base contact regions. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power bipolar semiconductor device which includes distinct first and second first-conductivity-type emitter/collector regions which are not electrically connected together, and also includes distinct first and second second-conductivity-type base contact regions which are respectively close to the first and second collector/emitter regions and which are not electrically connected together, comprising: at turn-on, when the applied voltage polarity is such that the emitter/collector region on the first of said surfaces can act as the collector of a vertical bipolar transistor while the emitter/collector region on the second of said surfaces acts as the emitter of the same vertical bipolar transistor, beginning turn-on by shorting the first base contact and first emitter/collector regions together, to thereby conduct current between the first and second emitter/collector regions as a diode without external base current; and thereafter applying base current at one but not both of the base contact regions. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power bipolar semiconductor device which includes both a respective n-type emitter/collector region, and also a respective p-type base contact region, on both first and second surfaces of a p-type semiconductor die, comprising: in active OFF mode, when the emitter/collector region on the first surface is more negative than the emitter/collector region on the second surface, connecting the emitter/collector and base contact regions on the first surface together, while floating the base contact region on the second surface; and thereafter, when the polarity of applied voltage reverses, keeping the emitter/collector and base contact regions on the first surface connected, to thereby conduct current with a diode voltage drop; and thereafter driving one of the base contact regions, to initiate conduction as a bipolar transistor with less than a diode voltage drop. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power bipolar semiconductor device which includes both an n-type emitter/collector region, and also a p-type base contact region, on both first and second surfaces of a p-type semiconductor die, comprising: during the ON state, driving base current into one of the base contact regions; and during transition to the OFF state, temporarily shorting the base contact region on the first surface to the emitter/collector region on the first surface, while also shorting the base contact region on the second surface to the emitter/collector region on the second surface, and thereafter floating at least the base contact region on the first surface; wherein the base contact region on the first surface is not connected to the base contact region on the second surface; whereby currents of both polarities are controllably switched between the emitter/collector regions on opposite surfaces. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power bipolar semiconductor device which includes both first-conductivity-type emitter/collector regions and also second-conductivity-type base contact regions on both opposed surfaces of a semiconductor die, comprising the actions of: beginning turn-off by temporarily connecting the base contact and emitter/collector regions on the first surface together, while separately connecting the base contact and emitter/collector regions on the second surface together, without connecting the base contact region on the first surface to the base contact region on the second surface; and then floating one, but not both, of the base contact regions; whereby the total time required for turn-off is reduced. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power bipolar semiconductor device which includes both a p-type emitter/collector region, and also an n-type base contact region, on both first and second surfaces of an n-type semiconductor die, comprising: during the ON state, driving one of the base contact regions toward a voltage lower than the nearest emitter/collector region; and during transition to the OFF state, temporarily shorting the base contact region on the first surface to the emitter/collector region on the first surface, while also shorting the base contact region on the second surface to the emitter/collector region on the second surface, and thereafter floating the base contact region on the first surface, but not the base contact region on the second surface; whereby currents of both polarities are controllably switched between the emitter/collector regions on opposite surfaces. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes both first-conductivity-type emitter/collector regions and also second-conductivity-type base contact regions on both surfaces of a second-conductivity-type semiconductor die, comprising the actions of: a) using a control circuit to control the flow of current between the emitter/collector regions; and b) when the control circuit is inactive, automatically enabling both a first clamp circuit which clamps forward bias across the junction between the first base contact region and the first emitter/collector region to less than a diode drop, and also a second clamp circuit which clamps forward bias across the junction between the second base contact region and the second emitter/collector region to less than a diode drop, regardless of the voltage between the first and second emitter/collector regions; whereby breakdown voltage is not degraded by amplification of leakage currents. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes both an n-type emitter/collector region, and also a p-type base contact region, on both first and second surfaces of a p-type semiconductor die, comprising: at turn-on, shorting the more positive one of the emitter/collector regions together with the base contact region on the same one of the surfaces, to thereby conduct current with a diode voltage drop; and thereafter in the ON state, flowing base current through the base contact region which is nearer the more positive one of the emitter/collector regions, without flowing base current through the other of the base contact regions; wherein the base contact region on the first surface is not electrically connected to the base contact region on the second surface, except through the semiconductor die itself; and wherein the emitter/collector region on the first surface is not electrically connected to the emitter/collector region on the second surface; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes first emitter/collector and base contact regions on a first surface of a semiconductor die, and second emitter/collector and base contact regions on a second surface of the die; wherein the emitter/collector regions both have a first conductivity type, and the base contact regions and the semiconductor die all have a second conductivity type; comprising: at turn-on, when the applied voltage polarity is such that the emitter/collector region on the first of said surfaces can act as the collector of a vertical bipolar transistor while the emitter/collector region on the second of said surfaces acts as the emitter of the same vertical bipolar transistor, beginning turn-on by shorting the base contact and emitter/collector regions on the first surface together, without driving the base contact region on the second surface, to thereby conduct current as a diode without external base current power; and thereafter in the ON state, flowing base current through the first base contact region, but substantially not through the second base contact region; and in the OFF state, not flowing external current through either of the base contact regions; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes distinct first and second first-conductivity-type emitter/collector regions which are not electrically connected together, and also includes distinct first and second second-conductivity-type base contact regions which are not electrically connected together, the emitter/collector regions and the base contact regions all lying within a common semiconductor die, comprising: at turn-on, when the applied voltage polarity is such that the emitter/collector region on the first of said surfaces can act as the collector of a vertical bipolar transistor while the emitter/collector region on the second of said surfaces acts as the emitter of the same vertical bipolar transistor, beginning turn-on by shorting the first base contact and first emitter/collector regions together, to thereby conduct current between the first and second emitter/collector regions as a diode without external base current; and thereafter in the ON state, driving the first base contact toward a voltage different from the first emitter/collector region by flowing base current through the first base contact region, while NOT flowing base current through the second base contact region; and in the OFF state, floating at least one of the base contact regions, while not applying external current to either of the base contact regions; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes both a respective p-type emitter/collector region, and also a respective n-type base contact region, on both first and second surfaces of an n-type semiconductor die, comprising: in active OFF mode, when the emitter/collector region on the first surface is more positive than the emitter/collector region on the second surface, connecting the emitter/collector and base contact regions on the first surface together, while floating the base contact region on the second surface; and thereafter, when the polarity of applied voltage reverses, keeping the emitter/collector and base contact regions on the first surface connected, to thereby conduct current with a diode voltage drop; and thereafter in the ON state, flowing base current through the base contact region which is nearer the more negative one of the emitter/collector regions, without flowing base current through the other of the base contact regions; wherein the base contact region on the first surface is not electrically connected to the base contact region on the second surface, except through the semiconductor die itself; and wherein the emitter/collector region on the first surface is not electrically connected to the emitter/collector region on the second surface; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes both an n-type emitter/collector region, and also a p-type base contact region, on both first and second surfaces of a p-type semiconductor die, comprising: at turn-on, shorting the more positive one of the emitter/collector regions together with the base contact region on the same one of the surfaces, to thereby conduct current with a diode voltage drop; and thereafter in the ON state, flowing base current through the base contact region which is nearer the more positive one of the emitter/collector regions, without flowing base current through the other of the base contact regions; during transition to the OFF state, temporarily shorting the base contact region on the first surface to the emitter/collector region on the first surface, while also shorting the base contact region on the second surface to the emitter/collector region on the second surface, and thereafter floating at least the base contact region on the first surface; in the OFF state, automatically enabling both a first clamp circuit which clamps forward bias across the junction between the base contact region on the first surface and the emitter/collector region on the first surface to less than a diode drop, and also a second clamp circuit which clamps forward bias across the junction between the base contact region on the second surface and the emitter/collector region on the second surface to less than a diode drop, regardless of the voltage between the emitter/collector regions on the first and second surfaces; wherein the base contact region on the first surface is not electrically connected to the base contact region on the second surface, except through the semiconductor die itself; and wherein the emitter/collector region on the first surface is not electrically connected to the emitter/collector region on the second surface; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes first emitter/collector and base contact regions on a first surface of a semiconductor die, and second emitter/collector and base contact regions on a second surface of the die; wherein the emitter/collector regions both have a first conductivity type, and the base contact regions and the semiconductor die all have a second conductivity type; comprising: at turn-on, when the applied voltage polarity is such that the emitter/collector region on the first of said surfaces can act as the collector of a vertical bipolar transistor while the emitter/collector region on the second of said surfaces acts as the emitter of the same vertical bipolar transistor, beginning turn-on by shorting the base contact and emitter/collector regions on the first surface together, without driving the base contact region on the second surface, to thereby conduct current as a diode without external base current power; and thereafter in the ON state, flowing base current through the first base contact region, but substantially not through the second base contact region; beginning turn-off by temporarily connecting the base contact and emitter/collector regions on the first surface together, while separately connecting the base contact and emitter/collector regions on the second surface together, without connecting the base contact region on the first surface to the base contact region on the second surface; and then floating one, but not both, of the base contact regions; in the OFF state, automatically enabling a respective clamp circuit on each said surface between the respective base contact and emitter/collector regions; wherein each said clamp circuit comprises a voltage-limiting element and a normally-on switch in series, such that the anode of the voltage-limiting element is connected to the p side of a p-n junction between the respective base contact and emitter/collector regions, and the cathode is connected to the n side of the respective junction; wherein said voltage-limiting elements limit forward voltage on the respective junction to less than a diode drop; whereby leakage currents are not amplified when the control circuit is inactive, and breakdown voltage is not degraded by amplification of leakage currents; and whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes distinct first and second first-conductivity-type emitter/collector regions which are not electrically connected together, and also includes distinct first and second second-conductivity-type base contact regions which are not electrically connected together, the emitter/collector regions and the base contact regions all lying within a common semiconductor die, comprising: at turn-on, when the applied voltage polarity is such that the emitter/collector region on the first of said surfaces can act as the collector of a vertical bipolar transistor while the emitter/collector region on the second of said surfaces acts as the emitter of the same vertical bipolar transistor, beginning turn-on by shorting the first base contact and first emitter/collector regions together, to thereby conduct current between the first and second emitter/collector regions as a diode without external base current; and thereafter in the ON state, driving the first base contact toward a voltage different from the first emitter/collector region by flowing base current through the first base contact region, while NOT flowing base current through the second base contact region; beginning turn-off by temporarily connecting the base contact and emitter/collector regions on the first surface together, while separately connecting the base contact and emitter/collector regions on the second surface together, without connecting the base contact region on the first surface to the base contact region on the second surface; and then floating one, but not both, of the base contact regions, while not applying external current to either of the base contact regions; in the OFF state, automatically enabling a clamp circuit between each said emitter/collector region and the respective base contact region to thereby limit forward voltage on a junction between the respective base contact and emitter/collector regions to less than a diode drop; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A method for switching a power semiconductor device which includes both a respective p-type emitter/collector region, and also a respective n-type base contact region, on both first and second surfaces of an n-type semiconductor die, comprising: in active OFF mode, when the emitter/collector region on the first surface is more positive than the emitter/collector region on the second surface, connecting the emitter/collector and base contact regions on the first surface together, while floating the base contact region on the second surface; and thereafter, when the polarity of applied voltage reverses, keeping the emitter/collector and base contact regions on the first surface connected, to thereby conduct current with a diode voltage drop; and thereafter in the ON state, flowing base current through the base contact region which is nearer the more negative one of the emitter/collector regions, without flowing base current through the other of the base contact regions; during transition to the OFF state, temporarily shorting the base contact region on the first surface to the emitter/collector region on the first surface, while also shorting the base contact region on the second surface to the emitter/collector region on the second surface, and thereafter floating the base contact region on the first surface, but not the base contact region on the second surface; in the OFF state, automatically enabling clamp circuits on said first and second surfaces, each including a diode and a normally-on switch in series, with the anode of the diode operatively connected to the respective p-type emitter/collector region, and the cathode operatively connected to the respective n-type base contact region; wherein the base contact region on the first surface is not electrically connected to the base contact region on the second surface, except through the semiconductor die itself; and wherein the emitter/collector region on the first surface is not electrically connected to the emitter/collector region on the second surface; whereby bidirectional switching is achieved with low on-state voltage drop and reliable turn-off. 
     According to some but not necessarily all embodiments, there is provided: A switching circuit, comprising: a power semiconductor device which includes first and second first-conductivity-type emitter/collector regions separately defining first and second emitter junctions on two surfaces of a second-conductivity-type semiconductor die, and a first base contact region on the same surface as the first emitter/collector region, and a second base contact region on the same surface as the second emitter/collector region, both base contact regions separately making ohmic contact to the semiconductor die; a first clamp circuit, which connects a first voltage-limiting element across the first emitter junction through a first normally-on switch, and a second clamp circuit, which connects a second voltage-limiting element across the second emitter junction through a second normally-on switch; and a control circuit which drives the two base contact regions differently, during the ON state, to enable conduction, and which also drives the two base contact regions differently, during the OFF state, to block conduction; wherein the first voltage-limiting element limits forward voltage on the first emitter junction to less than a diode drop, and the second voltage-limiting element limits forward voltage on the second emitter junction to less than a diode drop; whereby leakage currents are not amplified when the control circuit is inactive, and breakdown voltage is not degraded by amplification of leakage currents. 
     According to some but not necessarily all embodiments, there is provided: A switching circuit, comprising: a power semiconductor device which includes first and second first-conductivity-type emitter/collector regions separately defining first and second emitter junctions on two surfaces of a second-conductivity-type semiconductor die, and a first base contact region on the same surface as the first emitter/collector region, and a second base contact region on the same surface as the second emitter/collector region, both base contact regions separately making ohmic contact to the semiconductor die; a first clamp circuit, which connects a first voltage-limiting element across the first emitter junction through a first normally-on switch, and a second clamp circuit, which connects a second voltage-limiting element across the second emitter junction through a second normally-on switch; and a control circuit which drives the first and second base contact regions differently, to control turn on and turn off of conduction; wherein the first voltage-limiting element limits forward voltage on the first emitter junction to less than a diode drop, and the second voltage-limiting element limits forward voltage on the second emitter junction to less than a diode drop; whereby leakage currents are not amplified when the control circuit is inactive, and breakdown voltage is not degraded by amplification of leakage currents. 
     According to some but not necessarily all embodiments, there is provided: A switching circuit, comprising: a power semiconductor device which includes first and second first-conductivity-type emitter/collector regions separately defining first and second emitter junctions on two surfaces of a second-conductivity-type semiconductor die, and a first base contact region on the same surface as the first emitter/collector region, and a second base contact region on the same surface as the second emitter/collector region, both base contact regions separately making ohmic contact to the semiconductor die; a first clamp circuit which, unless disabled, connects a first voltage-limiting element across the first emitter junction, and a second clamp circuit, which, unless disabled, connects a second voltage-limiting element across the second emitter junction; and a control circuit which drives the first and second base contact regions differently, to control turn on and turn off of conduction; wherein the first voltage-limiting element limits forward voltage on the first emitter junction to less than a diode drop, and the second voltage-limiting element limits forward voltage on the second emitter junction to less than a diode drop; whereby leakage currents are not amplified when the control circuit is inactive, and breakdown voltage is not degraded by amplification of leakage currents. 
     According to some but not necessarily all embodiments, there is provided: A switching circuit comprising: a two-base bidirectional npn semiconductor device which includes n-type emitter/collector regions and also p-type base contact regions on both opposed surfaces of a p-type monolithic semiconductor die; control circuitry which is connected separately to the first and second base contact regions on the opposed surfaces; and first and second distinct clamp circuits, each comprising a series combination of a low-voltage diode and a normally-on switch, connected so that, when the normally-on switch is on, the anode of the low-voltage diode is connected to the p-type base contact region, and the cathode of the low-voltage diode is connected to the n-type emitter/collector region; wherein the low-voltage diode turns on at a forward voltage which is less than the diode drop of the p-n junction between an emitter/collector region and the semiconductor die; whereby, when the normally-on switch is ON, the p-n junction between an emitter/collector region and the semiconductor die cannot ever be forward biased. 
     According to some but not necessarily all embodiments, there is provided: A switching circuit comprising: a double-base bipolar transistor, having distinct n-type emitter/collector regions on both first and second opposite surfaces of a p-type semiconductor die, each emitter/collector region defining a p-n emitter junction with respect to the die; first and second clamp circuits on said first and second opposite surfaces, each including a diode and a normally-on switch in series, with the anode of the diode operatively connected to the p side of the respective emitter junction, and the cathode operatively connected to the n side of the respective emitter junction; wherein the diodes have forward diode voltage drops which are substantially less than the forward diode voltage drop characteristic of the respective p-n emitter junction. 
     According to some but not necessarily all embodiments, there is provided: A switching circuit comprising: a double-base bipolar transistor, having distinct emitter/collector regions on both first and second opposite surfaces of an n-type semiconductor die, each emitter/collector region defining a p-n emitter junction with respect to the die; first and second clamp circuits on said first and second surfaces, each including a diode and a normally-on switch in series, with the anode of the diode operatively connected to the p side of the respective emitter junction, and the cathode operatively connected to the n side of the respective emitter junction; wherein the diodes have forward diode voltage drops which are substantially less than the forward diode voltage drop characteristic of the respective p-n junction. 
     According to some but not necessarily all embodiments, there is provided: 
     According to some but not necessarily all embodiments, there is provided: 
     Modifications and Variations 
     As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given. It is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. 
     In some embodiments, oxide trenches  1330  can be present between source regions and adjacent base contact regions. In other embodiments, this can be different. 
     In some heterojunction embodiments, emitter/collector regions can be amorphous silicon on a crystalline silicon substrate. In other embodiments, heterojunctions can be formed differently. 
     In most presently-preferred embodiments, base contact regions are more heavily doped than the majority of the base. However, in other embodiments, this can be different. 
     In some heterojunction sample embodiments, heterojunctions can be provided by junctions between e.g. amorphous and crystalline silicon. In other embodiments, heterojunctions can be e.g. provided by different materials. In still other embodiments, heterojunctions can be provided by e.g. crystalline-crystalline junctions, between e.g. crystalline silicon and a different crystalline semiconductor, provided e.g. that the resultant junction potentials are sharp enough to increase injection efficiency. 
     In one contemplated alternative embodiment, smaller emitter metallization fractions can be used to reduce hole recombination and thereby increase injection efficiency. 
     In some sample embodiments of heterojunction B-TRANs, heterojunction emitter/collector regions can be e.g. amorphous silicon. In other sample embodiments, heterojunction emitter/collector regions can be e.g. polycrystalline silicon. In still other embodiments, this can be different. 
     In some embodiments, one base can be driven more than the other base. 
     In some embodiments, other methods for increasing injection efficiency in high level non-equilibrium carrier densities can be used, singly or in combination with the innovative techniques disclosed herein. 
     In some alternative embodiments, the innovative B-TRAN fabrication techniques described herein can be applied to other two-sided bidirectional devices, such as e.g. bidirectional IGBTs. 
     In some alternative embodiments, the innovative devices of the present inventions can be advantageously applied to different sorts of power converters. In some alternative embodiments, the present innovative devices can be used for e.g. matrix converters. In other alternative embodiments, the present innovative devices can be applied to e.g. voltage reduction regulators as used for induction motor efficiency optimization and soft start. 
     In some alternative embodiments, emitter regions can be recessed emitters located in trenches. In other alternative embodiments, base contact regions can be recessed base contact regions located in trenches. 
     In some alternative embodiments, field-shaping regions can be present below base contact regions or emitter regions or both. 
     Additional general background, which helps to show variations and implementations, can be found in the following publications, all of which are hereby incorporated by reference: “The Effect of Static and Dynamic Parasitic Charge in the Termination Area of High Voltage Devices and Possible Solutions”, T. Trajkovic et al. 
     None of the description in the present application should be read as implying that any particular element, step, or function is an essential element which must be included in the claim scope: THE SCOPE OF PATENTED SUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none of these claims are intended to invoke paragraph six of 35 USC section 112 unless the exact words “means for” are followed by a participle. 
     The claims as filed are intended to be as comprehensive as possible, and NO subject matter is intentionally relinquished, dedicated, or abandoned.