PNP transistor operates as a semiconductor device where the roles of electrons and holes are swapped compared to an NPN transistor, and the collector region is made up of a p-substrate. The use of this configuration, known as “substrate-PNP,” is limited due to the presence of substrate currents, which can cause ground voltage variations due to IR drops.
In semiconductor design, the need for vertical PNP transistors is generally not justified, as PNP transistors are less commonly used compared to MOSFETs and NPN transistors. However, it is possible to create horizontal PNP transistors without additional layers by employing a unique design approach.
Construction of Horizontal PNP transistor
The construction of a horizontal PNP transistor involves the creation of concentric circles within an N-well region with p-doping, resembling the structure depicted in Figure 6.18 (left). In this design, the inner ring serves as the emitter, while the outer ring functions as the collector. The lightly doped n-well forms the base, and the width of the base region is determined by the difference in radii between the concentric circles. The primary current, which consists of holes, flows radially from the emitter to the collector. It is essential that there are no shallow trench isolations (STI) separating the doped areas between the emitter and collector.
PNP Transistor Characteristics
These horizontal PNP transistors exhibit relatively high current gains, denoted as B, typically reaching values around B = 50. Higher values of B, such as 98 to 99 holes out of 100 reaching the collector from the emitter, can also be achieved. This impressive performance is due to the fact that the emitter injects charge carriers across the entire interface of the base-emitter transition. Despite the apparent challenge posed by the diffusion of carriers into the base, a key element known as the n-type buried layer (NBL) plays a crucial role. Despite both the Nwell and NBL being n-doped zones, a small space-charge region forms at their interface due to significant differences in dopant concentrations. This space-charge region effectively repels holes diffusing downward from the top, guiding them toward the collector ring.
When resizing the emitter or collector regions of a horizontal PNP transistor, its electrical behavior, especially the current gain B, is significantly altered. Therefore, the fundamental layout, including the ring radii, must remain unchanged if the transistor needs to be sized for larger currents, typically achieved through replication of the basic layout.
How does a PNP transistor differ from an NPN transistor?
A PNP transistor operates similarly to an NPN transistor but with a fundamental difference in the charge carriers involved. In a PNP transistor, holes are the majority of charge carriers, and the transistor’s operation relies on the movement of holes. However, the roles of electrons and holes are swapped compared to an NPN transistor, where electrons are the majority carriers.
The collector in a PNP transistor is typically made up of a p-substrate. This configuration is known as “substrate-PNP.” However, substrate-PNP transistors are seldom used in modern semiconductor design due to the presence of substrate currents. These substrate currents can lead to undesirable effects, including changes in the ground voltage due to resistive voltage drops (IR drops). As a result, substrate-PNP transistors are not widely employed, especially with the availability of MOSFETs and NPN transistors, which offer more favorable characteristics.
How can a horizontal PNP transistor be constructed without additional layers?
A horizontal PNP transistor can be constructed without the need for additional semiconductor layers. The basic structure of this transistor involves creating concentric circles in the Nwell (n-type well) with p-doping, as shown in Fig. 6.18 (left). The key components of this structure are as follows:
Emitter: The inner ring of the concentric circles serves as the emitter of the transistor.
Collector: The outer ring of the concentric circles forms the collector region.
Base: The lightly doped n-well acts as the base of the transistor.
The base width of this transistor is determined by the difference in the radii of the concentric circles. The main current, consisting of holes as charge carriers, flows radially from the emitter to the collector. It’s important that these doped areas (emitter and collector) are not separated by STI (shallow trench isolation) to enable proper charge carrier flow. Furthermore, the base width is not affected by alignment tolerances since both the emitter and collector are patterned with the same mask.
What is the significance of the n-type buried layer (NBL)?
The n-type buried layer (NBL) plays a crucial role in the operation of the horizontal PNP transistor, facilitating the movement of charge carriers, which are primarily holes in this case. Despite both the Nwell and NBL being n-doped regions, a small space-charge region forms at the interface between them due to the significant difference in dopant concentrations.
This space-charge region repels the holes that are diffusing down from the emitter. As a result, most of these holes are prevented from diffusing directly into the base region. Instead, they are redirected towards the collector ring. The NBL’s presence and the resulting space-charge region effectively guide the majority of charge carriers (holes) from the emitter to the collector, ensuring the transistor’s proper operation.
How does the resizing of the emitter or collector affect the electrical behavior of the horizontal PNP transistor?
Resizing the emitter or collector of the horizontal PNP transistor has a significant impact on its electrical behavior, especially on the current gain (B). The electrical behavior is highly sensitive to the size of these regions. If the emitter or collector regions are resized, it can lead to substantial alterations in the transistor’s characteristics.
However, there is a limitation when it comes to resizing this type of PNP transistor. The basic layout of the transistor, including the radii of the concentric circles that define the emitter and collector regions, must remain unchanged. Therefore, the transistor can only be sized for larger currents by replicating the fundamental layout without altering its geometrical configuration. This limitation is due to the sensitivity of the transistor’s behavior to the size of these regions.
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