IGBT Basic (7)

3-5. Forward Conduction Characteristics
Due to its structure, the IGBT is sometimes viewed as a serial connection of the MOSFET and PiN diode, and sometimes it is seen as a wide base PNP transistor driven by the MOSFET in Darlington configuration.
The former description can be used to understand the behavior of the device, but the latter better describes the IGBT.

Fig. 2 is a graph of the IGBT’s static characteristics. Even if a MOSFET channel of the input side is formed, the collector current does not flow if the anode-cathode forward voltage drop does not exceed approximately 0.7V as in the PiN diode. In addition, the current is saturated when the voltage across the MOSFET channel is greater than (VGE – Vth) and has an infinite output resistance, as in a power MOSFET. However, in a symmetrical IGBT, the collector current increases with the increase in collector voltage, and the rate of increase in the collector current also increases with the increase in collector voltage. Such finite output resistance is due to a shortening of the channel due to an increase in the collector voltage, and a secondary decrease in the drain output resistance due to bipolar transistor current flow. In order to increase the collector output resistance, an asymmetrical structure with a N+ buffer layer between the N- drift region and P+ substrate is used to prevent an increase in the bipolar transistor’s current gain with the increase in the collector voltage. In an asymmetrical structure, the width of the undepleted N- drift region does not change rapidly with the increase in the collector voltage due to the high concentration of the buffer layer, but it remains the same width as the N+ buffer layer for all collector voltages. This results in a constant value of the PNP transistor’s current gain. In addition to this, the N+ buffer layer reduces the injection efficiency of the P+ substrate / N+ buffer junction (J1). This reduces the current gain of the PNP transistor. As such, an IGBT with an asymmetrical structure has much superior output characteristics than a symmetrical type. In addition, collector output resistance can be increased with electron irradiation to shorten the minority carrier lifetime, which reduces the diffusion length. The following is the equation for obtaining the saturated collector current of the IGBT:

Transconductance at the active region can be obtained by differentiating the IC,sat with respect
to VGE.

The IGBT’s saturated collector current and transconductance are higher than those of the power MOSFETs of the same aspect ratio (Z/LCH). This is because the PNP transistor’s current gain (αPNP) is less than 1 (0.2 to 0.3 in general).

On-state voltage drop
Forward current-voltage characteristics and the conduction loss of a MOSFET are described as n-resistance. On the other hand, the characteristics of the IGBT are described as voltage drop at rated current, as is the case with the bipolar power transistor. On-state voltage drop is comprised of voltage drop of the forward biased P+ substrate / N- drift junction (J1), the voltage drop of conductivity modulated N- drift region and the voltage drop of MOSFET. Cut-in voltage for forward biased J1 is about 0.7V at room temperature. Cut-in voltage decreases due to a sharp increase in intrinsic carrier concentration as the temperature rises. The voltage drop of the N drift region can be obtained by integrating the electric field of the entire drift region, and it is generally less than 0.1V due to a strong conductivity modulation caused by injected holes from J1. The voltage drop of a MOSFET is the sum of the voltage drops from the channel region, JFET region and accumulation layer. Due to a decrease in drift layer resistance, the portion of JFET resistance and channel resistance is increased in the voltage drop between on-state collector-emitter. Hence, low JFET and channel resistance design are important factors in obtaining the best performance in an IGBT. The voltage drop at the channel is proportional to the channel length, gate oxide thickness. And it is inversely proportional to channel width, electron mobility and gate bias. The channel width can be increased by increasing the concentration of circuits by decreasing the size of each unit cell. But because of this the JFET resistance increases significantly, so the optimal size of the unit cell exists for each voltage rating.
The IGBT decreases the minority carrier lifetime with electron irradiation in order to improve the switching speed, and this increases the on-state voltage drop. Even in IGBTs with the same structure, the IGBT with a fast switching speed has a larger voltage drop, and the IGBT with a slower switching speed has a smaller on-state voltage drop depending on the condition of electron irradiation, which takes place after device fabrication.

High temperature characteristics
One must be aware of the changes in characteristics from changes in temperature, as the IGBT’s input characteristics are similar to a MOSFET, and output characteristics are similar to bipolar transistors. As temperature rises, the energy barrier of the P+ substrate / N- drift region junction (J1: emitter-base junction of PNP transistor) decreases, which leads to a lower cut-in voltage, and the threshold voltage decreases as in a MOSFET. As channel resistance increases, the amount of electron current (MOS current) decreases, which is injected to the Ndrift region. However, current gain, which is the ratio of the hole current (bipolar current) to the electron current, increases. In addition, N- drift region (base of the PNP transistor) resistance increases. Due to these characteristics, changes in cut-in voltage of J1 are larger than those in channel resistance and N- drift region resistance at low collector current level, so the IGBT has negative temperature coefficient similar to the bipolar transistor. On the other hand, channel resistance and N- drift region resistance determine the on-state voltage at high collector current, which results in a positive temperature coefficient similar to a power MOSFET. The crossover point for the two characteristics is different for each product, and the collector-emitter voltage drop is independent from temperature at the crossover point. In real applications, it is used in areas with negative temperature coefficient, and these factors must be considered in parallel application. Figs. 3~5 illustrate these characteristics with graphs from the data sheet.

to be continued…………..


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