Constructional Features of an IGBT
Constructional Features of an IGBT:
- The major difference with the corresponding MOSFET cell structure lies in the addition of a p injecting layer. This layer forms a pn junction with the drain layer and injects minority carriers into it. The n type drain layer itself may have two different doping levels.
- The lightly doped n- region is called the drain drift region. Doping level and width of this layer sets the forward blocking voltage (determined by the reverse break down voltage of J2) of the device.
- However, it does not affect the on state voltage drop of the device due to conductivity modulation as discussed in connection with the power diode. This construction of the device is called “Punch Trough” (PT) design.
- The Non-Punch Through (NPT) construction does not have this added n buffer layer. The PT construction does offer lower on state voltage drop compared to the NPT construction particularly for lower voltage rated devices.
- However, it does so at the cost of lower reverse break down voltage for the device, since the reverse break down voltage of the junction J1 is small. The rest of the construction of the device is very similar to that of a vertical MOSFETincluding the insulated gate structure and the shorted body (p type) – emitter (n type) structure.
- The doping level and physical geometry of the p type body region however, is considerably different from that of a MOSFET in order to defeat the latch up action of a parasitic thyristor embedded in the IGBT structure. A large number of basic cells are grown on a single silicon wafer and connected in parallel to form a complete IGBT device.
- The IGBT cell has a parasitic p-n-p-n thyristor structure embedded into it.The constituent p-n-p transistor, n-p-n transistor and the driver MOSFET are shown by dotted lines in this figure. Important resistances in the current flow path are also indicated.
- The top p-n-p transistor is formed by the p injecting layer as the emitter, the n type drain layer as the base and the p type body layer as the collector. The lower n-p-n transistor has the n type source, the p type body and the n type drain as the emitter, base and collector respectively. The base of the lower n-p-n transistor is shorted to the emitter by the emitter metallization.
- However, due to imperfect shorting, the exact equivalent circuit of the IGBT includes the body spreading resistance between the base and the emitter of the lower n-p-n transistor.
- If the output current is large enough, the voltage drop across this resistance may forward bias the lower n-p-n transistor and initiate the latch up process of the p-n-p-n thyristor structure. Once this structure latches up the gate control of IGBT is lost and the device is destroyed due to excessive power loss.
- A major effort in the development of IGBT has been towards prevention of latch up of the parasitic thyristor. This has been achieved by modifying the doping level and physical geometry of the body region. The modern IGBT is latch-up proof for all practical purpose.