The UC Berkeley document cited above indicates that the higher mobility also leads to higher transconductance, and higher transconductance means higher small-signal gain. (Note that this plot is specifically for silicon.)Īs you might have guessed, higher electron mobility gives NPN transistors a speed advantage over PNPs. More specifically, they have lower mobility.Īs shown in the following plot, electron mobility is always higher than hole mobility, though the doping concentration does influence the difference between the two. electron issue because holes are “slower” than electrons. But it turns out that we can’t simply ignore the hole vs. This fact might seem irrelevant to practical engineering since we really don’t care what type of charge carrier is used as long as the circuit works. This means that most of the charge carriers in a PNP are holes. HoleĪs shown above, a PNP transistor’s emitter and collector are formed via p-type doping. The rest of this article will discuss PNP characteristics and applications. So we can’t deny that PNPs are less common and, in general, less desirable-but that doesn’t mean we should ignore them.
The author of this 2009 UC Berkeley document, Chenming Hu, goes so far as to say that because of this situation-i.e., higher NPN performance and the general preference for MOSFETs-BJTs are “almost exclusively of the NPN type.” This has led to a particularly dominant position for the NPN because BJTs must compete with MOSFETs, and it’s easier for the BJT team to win when it sends an NPN into the match. NPNs are actually better than PNPs in important ways.
#PNP TRANSISTOR DRIVER#
These letters refer to the arrangement of positive and negatively doped semiconductor layers, as depicted in the following diagram: Though field-effect transistors currently dominate the electronics scene, the original transistor was a bipolar transistor, and this device was soon followed by the first bipolar junction transistor, or BJT.īJTs come in two fundamental flavors: NPN and PNP. These components function as both on/off switches and as amplifiers. You are probably well aware that modern electrical engineering, and in fact the entire modern world, is inextricably linked to devices known as transistors. The delay is very short, and probably not significant until you are switching at least 50kHz.This article helps you understand what PNP transistors are, how they’re used, and why they’re less common than NPN transistors. Q5 never enters saturation, because the emitter voltage is brought up to just the point where the transistor enters saturation, but not more. When you then turn it off, this capacitance has to discharge before Q1 really goes off, adding a bit of delay from when your MCU output goes low to when the diode gets switched off by Q1. The last subtle difference is that in circuit 1, Q1 enters saturation, which will charge the base-emitter capacitance. With R5 removed, you can pull the emitter up to \$V_ - 0.6V\$, so the LED current is a bit higher in circuit 1, assuming R1 and R4 are the same value.Ĭircuit 2 has the advantage that the base current goes towards powering the LED, but since the base current is small, this isn't a big effect. So, the emitter will be about 0.6V below the base if you forward bias it. Remember, the base-emitter junction is a diode. Essentially, the voltage at the emitter is the voltage at the base minus 0.6V, but the emitter current can be much more than the base current, because the gain of the transistor will draw more current from the collector. The configuration of Q5 is called common collector or emitter follower. Remove R5 and you will have what you describe.
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