I have had a tough time with semiconductors, I cannot understand the working of a transistor in saturation, as a switch, and also the cases as to when which part is forward biased, I also tried google but found only about the working not specific cases

I need help understanding the following parts

1)working in saturation mode(for both NPN and PNP)

2)as switch - only the bias

3)also any logic with which I can deduce the type - reverse biased or forward biased for the junctions.

any help would be appreciated.

  • $\begingroup$ Would Electrical Engineering be a better home for this question? $\endgroup$
    – Qmechanic
    Commented Mar 6, 2017 at 19:38
  • $\begingroup$ For the behavior of a BJT in saturation, see this. $\endgroup$
    – The Photon
    Commented Mar 6, 2017 at 21:14

1 Answer 1


In saturation mode, the collector-base junction of a bipolar transistor is forward biased as opposed to reverse biased in normal operation. Thus the collector-base voltage is positive in a pnp-transistor and negative in a npn-transistor.This forward bias produces minority current injection from the collector into the base and thus causes a current component opposed to the normal emitter-collector current leading to a decreasing collector current with increasing collector-base forward bias. The operation as a switch can be easiest understood by considering the collector current vs collector-emitter voltage (output) characteristic with a resistive load line. (See, e.g., S.M Sze, Physics and Technology of Semiconductor devices, Chapter 5.) For a switch operation between saturation mode "on" (current high, voltage low) the resistive load line cuts the characteristics for high input base current in the saturation region of the transistor while the "off" state with low current and high voltage is the intersection of the load line with the output characteristics at low input base current. There exist logic circuits where the switching uses an "on" state in normal mode instead of saturation mode.

  • $\begingroup$ Thankyou for the help but I need to read more can u provide me with a link with this information ,a PDF will do ,since I don't have the text u mentioned $\endgroup$ Commented Mar 9, 2017 at 4:00
  • $\begingroup$ @Aditya Sher - A search in the internet yielded this page which explains the function of a bipolar transistor as a switch and illustrates this, corresponding to my above explanation, using the output characteristics of the bipolar transistor with a resistive load line. electronics-tutorials.ws/transistor/tran_4.html $\endgroup$
    – freecharly
    Commented Mar 9, 2017 at 18:04
  • $\begingroup$ @Aditya Sher - Here you can download the book mentioned. fulviofrisone.com/attachments/article/453/… $\endgroup$
    – freecharly
    Commented Mar 9, 2017 at 19:17

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