IXAN0016 IXBH40N160 BiMOSFETTM Developed for High Voltage, High Frequency Applications Ralph E. Locher IXYS Corporation Santa Clara, CA by Olaf Zschieschang IXYS Semiconductor GmbH Lampertheim, Germany ABSTRACT In the IXBH40N160, IXYS has developed an extremely fast, homogeneous base IGBT by the introduction of collector shorts. Since this modification made the device behave like a very low RDS(on) MOSFET, IXYS coined the acronym BiMOSFETTM to distinguish this new class of switches. Rated at 1600V, its RDS(on) is less than 10% of an equivalent voltage rated MOSFET yet it has a switching time of less than 200ns. Consequently it will supplant conventional IGBTs and MOSFETs in high voltage applications running at frequencies from 10kHz to 75kHz and higher using softswitching techniques. 1. INTRODUCTION Applications abound today for high voltage MOSFETs but which would also benefit from a better part. Examples are sweep circuits, radar pulse modulators, capacitor discharge circuits and high voltage switch-mode power supplies. MOSFETs are connected in series-parallel strings to overcome their voltage and high RDS(on) limitations. Conventional, high voltage IGBTs are just too slow. IXYS has developed a new 40A, 1600V, homogeneous base IGBT to fulfill this need for a faster and higher voltage switch. The conventional construction for both MOSFETs and IGBTs is commonly referred to as DMOS, double-diffused-metal-oxide-silicon, which consists of a thick layer of epitaxial silicon grown on top of a large, low resistivity silicon substrate. However, at voltages in excess of 1200V, the thickness of the N-silicon layer required to support those blocking voltages makes it attractive to construct a homogeneous-base IGBT. A cross-sectional view of this type of construction is shown in Figure 1. Referring to this figure, the typical pnpn-structure for the IGBT has been maintained but note that an N+ collector-short pattern has been introduced in order to reduce the current gain of the PNP transistor and consequently its turnoff switching behavior. However, now there is a "free" intrinsic diode, not unlike that found in a MOSFET. The turnoff behavior of the BiMOSFETTM is controlled by the amount of collector shorting. However, in order for the diode to be usable and not cause commutating dV/ Figure 1. BiMOSFETTM cross-sectional view. IXAN0016 dt problems, the lifetime of the minority carriers must be reduced by irradiation. The end result will be a device, which can be optimized for either high frequency or low frequency switching by Table 1: Electrical Performance Table PARAMETER IXBH 40N160 BiMOSFETTM IXSH 35N120A IGBT IXFH 12N100 MOSFET DC Parameters BVDSS @ 3mA 1600V 1200V 1000V VGE(th) @ 4mA 5-9V 4-8V 2-4.5V VCE(sat) @ I (125ºC) 7V @ 25A 4V @ 35A 13.9V @ 6A gFS @ I 20S 26S 10S CISS (25V) COES (25V) CRES (25V) 3275pF 210pF 28pF 3750pF 235pF 60pF 4000pF 310pF 70pF Qg(on) 121nC 150nC 122nC Ic(on) 110A 170A 48A td(on) (Rg = 5Ω) 50ns 80ns 21ns tri 195ns 150ns 33ns tfi (Rg = 22Ω) 240ns 1100ns 32ns Eoff /A (960V) 0.12mJ/A 0.26mJ/A 0.04mJ/A Switching (TJ = 125ºC) tailoring its collector short pattern along with suitable amounts of irradiation. 2. DC Electrical Performance We foresee that the BiMOSFETTM should find applications both as a high voltage switch as well as to increase the upper frequency performance of high voltage IGBTs. Table 1 offers a comparison of its electrical performance to that of an 1000V MOSFET (IXFH12N100) and a 1200V DMOS constructed, SCSOA rated IGBT (IXSH35N120A), all three parts being constructed using the same silicon chip size (7.11mm x 8.64mm). The comparison is conservative because both competing parts are lower-voltage rated. In examining this table and some of the figures below, we can note the following: 1. The threshold voltage of the BiMOSFETTM is the highest of all but its Qg(on) is comparable. This is due to its relatively low Miller gate capacitance resulting in low Miller gate charge as can be seen IXAN0016 VCE(sat) is also higher than the IGBT but its on-state voltage drop at 20A is only 15% of an 1000V MOSFET of equal silicon area. In actuality, the VCE(sat) of a 1500V rated MOSFET would go up by another factor of 2.4. 4. Figure 4 plots the temperature dependence of BVCES, VGE(th), and VCE(sat) normalized to their 25OC values. Figure 5 plots the forward voltage drop of the intrinsic diode at room and elevated temperatures. The behavior of BVCES and VGE(th) with temperature is the same as an IGBT. However, note that since both VCE(sat) and VF have a positive tem- VCE = 600V IC = 2 0 A 0LOOHUJDWH FKDUJHQ& V OW R 9 ( * 9 * 4 QDQRFRXORPEV Figure 2: Gate charge 7 - 2 9 *( & 9 9 *( 7 - 9 2 9 *( & 9 *( *( 9 9 V H U H S P $ , 9 *( 9 V H U H S P $ , & 9 9 9 & *( 9 9 *( 9 9 *( 9 &(9ROWV 9 9 Figure 3a. Output characteristics at 125 C. O in Figure 2. In one sense, a high threshold voltage can be considered as an advantage in electrically noisy environments. 2. Its transconductance and peak on-state current are lower than the IGBT, making the latter the preferred switch for low frequency applications. In order to survive short circuit testing at higher voltages, low transconductance is required so that the BiMOSFETTM can be used in applications where survivability to this type of fault is a must. 3. Figures 3a and 3b show the typical output characteristic of the BiMOSFETTM at 125OC. Its &(9ROWV Figure 3b. Extended output characteristics. U H W H P D U D 3 G H ] LO D 9 &(VDW %9 '66 9 P U R 1 *(WK 7 'HJUHHV&HQWLJUDGH - Figure 4. Temperature dependence of breakdown, threshold and saturation voltages. Values normalized to room temperature. IXAN0016 9 *( T J = 1 2 5 OC I C = 2 0A 9 RG = 22W 7 - V H U H S P $ , ) 2 & 7 - V H U H S P $ , 2 & 9 &( & 9 R OW V & W QV W 9 9ROWV & ) Figure 5. Forward voltage drop of the intrinsic diode. 7LPHQDQRVHFRQGV Figure 6. Turn-off current and voltage waveforms. perature coefficient, these devices will be much easier to parallel than IGBTs, which require very close matching to ensure equal current sharing when used in parallel. 3. Switching Performance The IXBH40N160 BiMOSFET does switch exceptionally fast for a 1600V rated part. Its total resistive turn-on time with a 5Ω gate resistor is typically 245ns. Figure 6 illustrates the part turning off a 20A inductive load into a 1000V clamp at the elevated temperature of 125OC. There is relatively little tail current so that the E(off) is 2.4mJ, which is be less than 50% of the comparative IGBT in Table 1. Figure 7 plots turn-off energy as a function of the series gate resistor RG. This resistor primarily determines the rate-of-rise of collector voltage, which increases as RG decreases and correspondingly E(off). Presently the one shortcoming of the BiMOSFETTM is that it is not completely latch-free at elevated temperatures. At TJ =125OC, the maximum dV/dt should be kept less than 10V/ns by either the RG selection or a simple snubber. To turn-off safely peak currents above 40A without a snubber, the minimum recommended RG resistor is 47Ω. However, it should also be remembered that the IXBH40N160 is only the first member of the BiMOSFETTM family and it is expected that further development will result in its being just as rugged as the short circuit rated IGBTs. TM 4. Conclusions In addition to high voltage, low frequency switching, the graphs in Figure 8 show the frequency bandwidth in which the BiMOSFETTM enjoys an advantage over the IGBT and MOSFET. Up to 8kHz, the IGBT can carry more current. Above that frequency, the BiMOSFETTM becomes the switch of choice until its switching losses force it to hand the baton to MOSFETs at around 50kHz. However, research continues in all aspects of power MOS switches so that continued improvements will be made in all these parts. The homogeneous-base IGBT does have inherent advantages, which will be exploited to optimize it for high voltage and high frequency switching. IXAN0016 PLOOLMRXOHV ( II R , & 9 2 - &/$03 , &3. $ &3. 0LQLPXPUHFRPPHQGHG5* 7 9 $ *DWHUHVLVWRU5 2KPV * Figure 7. Turn-off energy versus gate resistor RG. 7- R& 7& R& 921 9 92)) 9 ,;6+1$,*%7 3HDN&XUUHQW$PSHUHV ,;%+1%L026)(770 ,;)+1026)(7 )UHTXHQF\+] Figure 8. Comparison of the current carrying capability of the BiMOSFETTM to that of a 1200V IGBT and 1000V MOSFET. IXAN0016