The silicon bipolar transistor has been the device of choice for motor drive circuits, appliance controls, robotics and lighting ballasts. This is because bipolar transistors can be designed to handle relatively large current densities in the range of 40-50 A/cm.sup.2 and support relatively high blocking voltages in the range of 500-1000 V.
Despite the attractive power ratings achieved by bipolar transistors, there exist several fundamental drawbacks to their suitability for all high power applications. First of all, bipolar transistors are current controlled devices which require relatively large base currents, typically one fifth to one tenth of the collector current, to maintain the transistor in an operating mode. Proportionally larger base currents can be expected for applications which also require high speed turn-off. Because of the large base current demands, the base drive circuitry for controlling turn-on and turn-off is relatively complex and expensive. Bipolar transistors are also vulnerable to premature breakdown if a high current and high voltage are simultaneously applied to the device, as commonly required in inductive power circuit applications. Furthermore, it is relatively difficult to operate bipolar transistors in parallel because current diversion to a single transistor typically occurs at high temperatures, making emitter ballasting schemes necessary.
The silicon power MOSFET was developed to address this base drive problem. In a power MOSFET, the gate is used to provide turn-on and turn-off control upon the application of an appropriate gate signal bias. For example, turn-on in an N-type enhancement MOSFET occurs when a conductive N-type inversion layer is formed in the P-type channel region in response to the application of a positive gate bias. The inversion layer electrically connects the N-type source and drain regions and allows for majority carrier conduction therebetween. The power MOSFET's gate electrode is separated from the channel region by an intervening insulating layer, typically silicon dioxide. Because the gate is insulated from the channel region, little if any gate current is required to maintain the MOSFET in a conductive state or to switch the MOSFET from an on-state to an off-state or vice-versa. The gate current is kept small during switching because the gate forms a capacitor with the MOSFET's channel region. Thus, only charging and discharging current ("displacement current") is required during switching. Because of the high input impedance associated with the insulated-gate electrode, minimal current demands are placed on the gate and the gate drive circuitry can be easily implemented.
Moreover, because current conduction in the MOSFET occurs through majority carrier transport only, the delay associated with the recombination of excess minority carriers is not present. Accordingly, the switching speed of power MOSFETs can be made orders of magnitude faster than that of bipolar transistors. Unlike bipolar transistors, power MOSFETs can be designed to withstand high current densities and the application of high voltages for relatively long durations, without encountering the destructive failure mechanism known as "second breakdown". Power MOSFETs can also easily be paralleled, because the forward voltage drop of power MOSFETs increases with increasing temperature, thereby promoting an even current distribution in parallel connected devices.
These benefits are offset, however, by the relatively high on-resistance of the MOSFET's active region, which arises from the absence of minority carrier injection. As a result, the device's operating forward current density is limited to relatively low values, typically in the range of 10 A/cm.sup.2, for a 600 V device, as compared to 40-50 A/cm.sup.2 for the bipolar transistor. These and other features of the silicon power MOSFET are fully explained in a textbook by inventor B. J. Baliga entitled "Modern Power Devices", John Wiley & Sons, Chapter 6, pp. 263-343 (1987).
On the basis of these features of power bipolar transistors and MOSFET devices, hybrid devices embodying a combination of bipolar current conduction with MOS-controlled current flow were developed and found to provide significant advantages over single technologies such as bipolar or MOSFET alone. One example of a hybrid device is the Insulated Gate Bipolar Transistor (IGBT), disclosed in section 7.2 of the aforementioned Baliga textbook.
The IGBT combines the high impedance gate of the power MOSFET with the small on-state conduction losses of the power bipolar transistor. An added feature of the IGBT is its ability to block both forward and reverse bias voltages. One embodiment of an IGBT is disclosed in an article by inventor B. J. Baliga and M. S. Adler, R. P. Love, P. V. Gray and N. Zommer, entitled "The Insulated Gate Transistor: A New Three terminal MOS Controlled Bipolar Power Device," IEEE Trans. Electron Devices, ED-31, pp. 821-828 (1984), the disclosure of which is hereby incorporated herein by reference. Based on experimental results, on-state losses were shown to be greatly reduced when compared to power MOSFETs. This was caused by the conductivity modulation of the IGBT's drift region during the on-state. Moreover, very high conduction current densities in the range of 200-300 A/cm.sup.2 were also achieved. Accordingly, an IGBT can be expected to have a conduction current density approximately 20 times that of a power MOSFET and five (5) times that of an equivalent bipolar transistor. Typical turn-off times for the IGBT can be expected to be in the range of 10-50 .mu.s.
The basic structure of the IGBT is shown in cross-section in FIG. 1A, which is a reproduction of FIG. 1 from the aforementioned Baliga et al. article. In the IGBT, forward conduction can occur by positively biasing the collector with respect to the emitter and applying a positive gate bias of sufficient magnitude to invert the surface of the P-base region under the gate. By creating an inversion layer in the P-base region, electrons are allowed to flow from the N+emitter region to the N-base region. In this forward conducting state, the junction J2 is forward biased and the P+collector region injects holes into the N-Base region. As the collector forward bias is increased, the injected hole concentration increases until it exceeds the background doping level of the N-base. In this regime of operation, the device operates like a forward-biased P-i-N diode with heavy conductivity modulation of the N-base region.
Accordingly, the IGBT can operate at high current densities even when designed for operation at high blocking voltages. As long as the gate bias is sufficiently large to produce enough inversion layer charge for providing electrons into the N-base region, the IGBT forward conduction characteristics will look like those of a P-i-N diode. However, if the inversion layer conductivity is low, a significant voltage drop will begin to appear across this region like that observed in conventional MOSFETs. At this point, the forward current will saturate and the device will operate in its active or current saturation region, as shown in FIG. 1B, which is a reproduction of FIG. 2 from the aforementioned Baliga et al. article. As will be understood by those skilled in the art, high voltage current saturation is ultimately limited by avalanche induced breakdown. Finally, because the elimination of the inversion layer cuts off the supply of electrons into the N-base region and because there is no self-sustaining source of electrons to the N-base region, the IGBT will turn off even if the collector remains positively biased. Thus, regenerative (self-sustaining) conduction cannot be established in an IGBT.
It is recognized that although gate-controlled bipolar transistors, such as the IGBT, represent a significant improvement over using bipolar or MOSFET devices alone, even lower conduction losses can be expected by using a thyristor. This is because thyristors offer a higher degree of conductivity modulation and a lower forward voltage drop when turned on. Consequently, the investigation of thyristors is of great interest so long as adequate methods for providing forced gate turn-off can also be developed. As will be understood by one skilled in the art, a thyristor in its simplest form comprises a four-layer P1-N1-P2-N2 device with three P-N junctions in series: J1, J2, and J3, respectively. The four layers correspond to the anode (P1), the first base region (N1), the second base or P-base region (P2) and the cathode (N2), respectively. In the forward blocking state, the anode is biased positive with respect to the cathode and junctions J1 and J3 are forward biased and J2 is reversed-biased. Most of the forward voltage drop occurs across the central junction J2. In the forward conducting state, all three junctions are forward biased and the voltage drop across the device is very low and approximately equal to the voltage drop across a single forward biased P-N junction.
An inherent limitation to the use of thyristors for high current applications is sustained latch-up, however, arising from the cross-coupled P1-N1-P2 and N1-P2-N2 bipolar transistors which make up the four layers of the thyristor. This is because sustained thyristor latch-up can result in catastrophic device failure if the latched-up current is not otherwise sufficiently controlled by external circuitry or by reversing the anode potential. Sustained latch-up can occur, for example, when the summation of the current gains for the thyristor's cross-coupled P1-N1-P2 and wide base P1-N2-P2 transistors (.alpha..sub.pnp, .alpha..sub.pnp) exceeds unity. When this occurs, each transistor drives the other into saturation and provides the other with a self-sustaining (i.e., regenerative) supply of carriers to the respective transistor's base region. An alternative to providing external circuitry or reversing the anode potential to obtain turn-off, however, is to use a MOS-gate for controlling turn-on and turn-off.
Several methods for obtaining MOS-gated control over thyristor action, including latch-up, exist. For example, an original embodiment of a Base-Resistance Controlled Thyristor (BRT) is described in U.S. Pat. No. 5,099,300, to B. J. Baliga, and in an article entitled "A New MOS-Gated Power Thyristor Structure with Turn-Off Achieved by Controlling the Base Resistance," by M. Nandakumar, B. J. Baliga, M. Shekar, S. Tandon and A. Reisman, IEEE Electron Device Letters, Vol. 12, No. 5, pp. 227-229, May, 1991, both of which are hereby incorporated herein by reference. Another example of a BRT having single-polarity and dual-polarity turn-on and turn-off control is disclosed in U.S. Pat. No. 5,198,687 to B. J. Baliga, the disclosure of which is hereby incorporated herein by reference. Finally, a BRT having a common gate electrode 27 for providing both MOS-gated turn-on and turn-off is disclosed in U.S. Pat. No. 5,155,569 to Terashima. These embodiments of BRTs operate by modulating the lateral P-base resistance of the thyristor using MOS-gated control. Operational BRTs with 600 -volt forward blocking capability, such as the one shown in FIG. 2, have been developed. FIG. 2 is a reproduction of FIG. 7 from the aforementioned Baliga '300 patent.
As will be understood by those skilled in the art, the BRT of U.S. Pat. No. 5,099,300 can be turned-off by the application of a negative bias to a P-channel enhancement-mode MOSFET to thereby reduce the resistance of the P-base by shunting majority charge carriers to the cathode. The reduction in P-base resistance results in an increase in the device's holding current to a level above the operational current level and shuts-off the device. Unfortunately, although conventional BRTs are characterized by low on-state voltage drop, they are not designed to exhibit the high current saturation mode of operation exhibited by conventional IGBTs. Moreover, depending on the doping concentration in the P-base region and other aspects, conventional BRTs may still be susceptible to sustained parasitic latch-up.
Thus, notwithstanding the preferred characteristics of IGBTs and BRTs, there continues to be a need for a semiconductor switching device for high current and high blocking voltage applications which has both low on-state resistance, as with the BRT, and has high current saturation capability, as with the IGBT.