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Competitive Analysis of IGBTs Reveals Their Internal Workings

Contributed by St.J. Dixon-Warren

Vertical Insulated Gate Bipolar Transistors (IGBTs) are one of the most important types of discrete power transistors. Structurally, they are similar to discrete, vertical N-type MOSFET devices, except the N-type drain region of the MOSFET is replaced by a P-type collector or anode region. The result is a PNP bipolar structure, where the current to the N-type transistor base is controlled by a gate. The gate connection is typically formed at the edges of the transistor array; with interconnect poly and metal connecting to a gate pad.

Vertical MOSFET Structure (on state)   Vertical IGBT Structure (on state)

IGBTs can operate at much higher currents than MOSFET devices. The electrical current in a MOSFET is limited by the majority carrier concentration in the drift region across the thickness of the substrate to the drain. IGBTs, by contrast, are minority carrier devices. IGBTs with an N-type substrate operate with a positive bias applied the to the collector side. If a bias is applied to the gate, then electrons are injected into the base from the source and are then swept across to the collector. At the same time, holes are injected from the collector into the base. These holes sweep across the base to the emitter. Since charge neutrality is maintained, very large current densities can be obtained, much larger than is possible with the majority carriers (electrons) present in the N- substrate.

In general, for power transistors, the two key parameters are blocking voltage, namely the maximum voltage the device can block in the off-state, and the on-state resistance, which should be as low as possible. The on-state resistance determines the maximum on-state operating current. IGBTs are favoured over MOSFETs when both high current and high blocking voltage is required, while MOSFETS are preferred for lower voltage and higher frequency operation. Due to the involvement of minority carriers, IGBTs do not operate well at high frequencies.

Both IGBTs and MOSFETS are commonly fabricated with trench gates. The trench gates allow increases in both the blocking voltage and the on-state operating current. Chipworks has analyzed several trench gate IGBT devices, including, most recently, the IGBT die found inside the Mitsubishi Electric PSS35S92F6-AG IGBT intelligent power module. The main function of the PSS35S92F6-AG is as a 3-phase DC/AC inverter. It is rated for collector-emitter voltage of up to 600 V at 35 A. The ton switching time is 1.6 µs. This Mitsubishi device contains six separate IGBT dies. Each IGBT die is provided with a separate freewheeling diode die, since IGBTs are not able to conduct current in the reverse direction.

Mitsubishi Electric PSS35S92F6-AG IGBT Intelligent Power Module Package   Mitsubishi Electric PSS35S92F6-AG IGBT Intelligent Power Module Package X-Ray

Cross-sectional SEM analysis of the IGBT die inside the PSS35S92F6-AG shows the trench gate transistors with emitter/source contacts placed between each gate. IGBTs require separate connection to the P-type emitter and to the N+ source of the control transistor. This can be achieved in the same way as for planar gates by placing the source implant adjacent to the edge of the gate. The contact then makes an electrical connection with both the P-type emitter and the N+ source. The PSS35S92F6-AG uses an alternate strategy, which can be most easily seen in plan view.

The emitter/source contact runs as a strip across the entire transistor gate array, with alternate doped P-emitter and N+ source regions.  The alternate doped regions can be seen in plan-view SEM, but are more dramatically demonstrated in plan-view scanning capacitance microscopy (SCM), where N-type materials shows up as yellow and P-type as purple.

Trench Gate IGBT Structure (on state)    Mitsubishi Electric PSS35S92F6-AG Trench Gate Array Plan View – SEM and SCM

The cross-sectional structure of the Mitsubishi trench gate device can be seen in SEM and SCM cross section through the source and emitter, denoted as cross sections 1 and 2, respectively. The first cross section shows the N+ source implant over the body of the trench gate transistor, while the second cross section shows the emitter connecting directly to the tungsten contact strip. The tungsten contacts appear to have been back-etched, rather than subjected to chemical mechanical polish (CMP).

Mitsubishi Electric PSS35S92F6-AG Trench Gates Source Contact (cross section #1) – SEM and SCM    Mitsubishi Electric PSS35S92F6-AG Trench Gates Emitter Contact (cross section #2) – SEM and SCM

Competitive analysis gives deep insight into the operational principles of this IGBT device. Detailed structural analysis complemented by accurate measurements of the dopants, by methods such as spreading resistance profiling (SRP) and secondary ion mass spectrometry (SIMS), allow for a complete characterization of the device. Details can be found in our recent Process Review Reports, listed below.

Related Chipworks Reports