An insulated gate bipolar transistor (IGBT) is a semiconductor structure of alternate layers of p-type and n-type doping. With the combination of an easily driven MOS gate and low conduction loss, IGBTs are quickly displaced bipolar transistors as these devices are preferred for high voltage and high current applications.

The IGBTs are classified into two types,

• Non-Punch Through or Symmetric IGBTs, and
• Punch Through or Asymmetric IGBTs.

Non-Punch Through IGBT (Symmetric IGBT) :

The below shows the structure of non-punch through IGBT which consists of four alternate PNPN layers. Under the forward blocking state, the junction J₂ i.e., the thickness of the drift (n^–) layer, decides the blocking capability of the device since it will be reverse biased.

When a reverse voltage is applied, junctions J₁ and J₃ will be reverse biased. However, due to the thin and moderately doped body layer (p layer) and heavily doped source layer (n⁺ layer), J₁ doesn’t have the capacity to block reverse voltage.

But junction J₃ will block all reverse voltage due to the presence of the drift layer, and thus the reverse voltage blocking capability of the device will be equal to the forward voltage blocking capability. However, the lightly doped drift layer increases the carrier lifetime which in turn increases the device switching time. Due to the high reverse breakdown voltage of symmetric IGBTs, they are well suited for ac applications.

Punch Through IGBT (Asymmetric IGBT) :

In punch-through or asymmetric IGBTs, there will be an addition of an n⁺ buffer layer introduced between the p⁺ injecting layer and the n^– drift layer as shown below. In the forward blocking state, the junction J₂ i.e., the thickness of the drift (n^–) layer, decides the blocking capability of the device since it will be reverse biased.

When a reverse voltage is applied, junctions J₁ and J₃ will be reverse biased. However, the addition of the buffer layer increases the number of charge carriers across junction J₃ and thus it fails to block large reverse voltages exhibiting an asymmetric voltage blocking capability.

Therefore, punch-through type IGBTs have less reverse breakdown voltage due to which they are used for dc circuits. Consequently, the total switching time and on-state loss of the device gets reduced.

Comparison Between Non-Punch Through and Punch Through IGBTs :

Features	Non-Punch Through IGBT	Punch Through IGBT
Thermal Stability	More	Less
Short Circuit Failure	More Rugged	Less Rugged
Turn OFF Loss	Less temperature sensitive	More temperature sensitive
Collector Doping	Lightly Doped	Heavily Doped
Temperature Coefficient of ON-state Voltage	Strongly positive	Small positive
Blocking Capability	Bidirectional blocking capability	Low reverse blocking capability
Forward Voltage Drop	Low	High
N-Buffer Layer	Absent	Present
Applications	Used for AC circuits	Used for DC circuits