Table of Contents
PNP Transistor Fabrication – Substrate or Vertical , Lateral or Horizontal and Triple Diffused
The base current that flows into the Collector of a PNP Transistor is Amplified. The base Typically Regulates the current flow. In the base, current moves in the opposite direction. The emitter of a PNP Transistors emits “holes,” which the Collector then collects.
The Integrated PNP Transistors are Fabricated in one of the following three structures.
1. Substrate or Vertical
2. Lateral or Horizontal
3. Triple Diffused
Substrate or Vertical PNP Transistor
The PNP Transistor’s P-substrate serves as the Collector, the N-epitaxial layer serves as the base, and the subsequent P-diffusion layer serves as the emitter region. Figure 1 depicts the physical makeup of Vertical Monolithic PNP Transistors Q1. Parallel to the PNP Transistor’s emitter region, the base region of an NPN Transistor structure is formed.
The Fabrication Method’s Collector is held at a constant negative potential, which is a Drawback. This is because the P-substrate of the Integrated circuit (IC) is Typically held at a negative potential to provide effective Isolation between the circuit Components and the Substrate.
Triple Diffused PNP Transistor
An Additional Diffusion process is added to the normal NPN Transistors processing steps to create this type of PNP Transistor. Due to the Additional P-region Diffusion in the second N-diffusion region of an NPN Transistor, this process is referred to as a triple Diffusion process. The Below-mentioned figure also Illustrates the structure of the triple Diffused Monolithic PNP Transistor Q2.
This has the Limitations of Requiring Additional Fabrication steps and Sophisticated Fabrication Assemblies.
Lateral or Horizontal PNP Transistor
This is the Integrated PNP Transistors Fabrication technique that is most Frequently used. This has the advantage of being Fabricated Concurrently with the NPN Transistor’s processing steps, Necessitating the use of an NPN Transistor as the PNP Transistor’s base. Two parallel P-regions, the emitter and Collector regions of the Horizontal PNP Transistors, are created during the P-type base Diffusion process of the NPN Transistors.
Comparison of Monolithic NPN and PNP
Normally, the NPN transistor is preferred in monolithic circuits due to the following reasons
1. The vertical PNP transistors must have his collector held at a fixed negative voltage.
2. The very wide base region of the lateral PNP transistor is constrained by the lateral diffusion of P-type impurities into the N-type base region. This makes the processes of creating photographic masks, aligning them, and etching them very challenging. As a result, lateral PNP transistors’ current gain is reduced to 1.5 to 30 compared to 50 to 300 for monolithic NPN transistors.
3. Before base and emitter diffusion, the collector region first forms. The collector impurities diffuse on either side of the specified collector junction during the following steps in the diffusion process. The N-type collector outperforms the P-type collector because the diffusion constant of N-type impurities is lower than that of P-type impurities. Due to the simpler process control, the NPN transistor is therefore preferred for monolithic fabrication.
4. Transistor with multiple emitters: Multiple emitters are necessary for applications like transistor-transistor logic (TTL). The circuit sectional view of three N-emitter regions diffused throughout the P-type base is shown in the figure below. With this configuration, the chip area is conserved and the IC’s component density is increased.