Device Structure and Current-Voltage Characteristics
The device structure for IGBT is similar to that of a double-diffused MOSFET (DMOS) with the exception of having the p+ substrate (anode) for the IGBT device as shown in Fig. 5.1. As such, it is a four-layer structure that resembles that of a thyristor. Unlike the thyristor where the device latches, an IGBT is designed to turn on without any regenerative action and the MOS-gate remains in control. A wide-base p-n-p structure is formed at the bottom three layers with the p+ substrate as the emitter, the n--drift layer as the base, and the p-body of the MOSFET as the collector region. As such, the thickness and doping density of the n--drift layer determines the breakdown voltage of the device. To follow the convention of the four-layer structure, the top n+ source is named as the cathode while the p+ substrate is the anode for the IGBT device in this chapter.
Except the leakage current, there is no visible current-flow when a negative voltage is applied to the anode with respect to the cathode because the junction between the p+-body and the n--drift layer is reverse-biased. The IGBT is now operating in its reverse blocking mode with the I-V curve as shown in Fig. 5.2. Most of the depletion region is extended into the lightly doped n--drift layer. The reverse blocking voltage is essentially the BVCBO of the p+ substrate/n--drift/p-body transistor. As such, the doping and thickness of the n- layer are chosen to yield the desired blocking voltage. It should be noted that a proper junction edge termination and passivation technique
Fig. 5.1. Device structure of an IGBT device.
Fig. 5.2. Forward and reverse blockings (dash lines), and forward conduction I-V curves (solid lines) of an IGBT device.
must be employed to achieve the optimum reverse blocking voltage. When a positive voltage is applied to the anode terminal, with the gate shorted to the cathode (ground) terminal, the IGBT is operating in its forward blocking mode since the junction between the p-body and n--drift region is reverse- biased. The IGBT device is said to operate in its forward conduction state if a gate voltage of greater than the threshold voltage is applied under positive anode-to-cathode bias condition. Similar to the MOSFET device, a conductive channel is induced underneath the oxide gate in the p-body region. Electrons flow from the n+-cathode to the n--drift region while the p+ substrate injects holes into the n--drift layer region to form the bipolar conductivity modulation in the drift region. The I-V curves for various gate voltages are shown in Fig. 5.2. The injected hole concentration increases as the anode-to-cathode voltage increases. Thus, the forward current of the IGBT increases similarly to that of a p-i-n diode. The forward current starts to saturate when a significant voltage drop develops across the MOSFET conducting channel in the p-body region. These current-voltage characteristics are similar to those of a power MOSFET, except for the presence of the diode knee voltage at the starting point of current conduction.
In applications where the IGBT device is not required to block a reverse voltage, an asymmetrical IGBT structure is formed with an n-buffer layer placed between the p+ substrate and the much lightly doped n--drift layer as shown in the lower part of Fig. 5.3. In the symmetrical structure as shown in the same figure, the doping density and thickness of the n- layer are chosen to prevent punchthrough to the p+-anode of the IGBT. In the case of the asymmetrical IGBT structure which is a classic punchthrough structure, the electric field distribution changes from the triangular shape to the rectangular-alike shape. Thus, the forward blocking capability of the asymmetrical device is
Fig. 5.3. Symmetrical (upper) and asymmetrical (lower) IGBT device doping profiles and the field distributions.
increased approximately by a factor of 2 more than that of the symmetrical structure if a similar n- -drift layer thickness was used. Therefore, with a shorter drift region length it can be used to enhance the forward conduction characteristics. And, due to less amount of excess-charge storage, this asymmetrical IGBT structure has a lower turn-off time compared to that of the symmetrical IGBT structure.
