what is an n-p-n or a p-n-p transistor?

Question

in what way is it used to make alarms?

Answer 1

A transistor is made of two junctions on a substrate (usually silicon). The substrate is called the "base", the other junctions are called "emitter" and "collector".
Depending on the material used to make the junctions (the impurities in the silicon), we have two possibilities: a "+/-" junction of a "-/+" jonctions.
So we classify the transistors with 3 letters:
NPN transistor, Negative,Positive,Negative, and the
PNP transistor, Positive,Negative, Positive.
If you compare with diodes, it is like having:
--|<--->|---
or
-->|---|<---
(the "base" is the wire between the two diodes).

How does it make alarms?
Depends on the circuit you decide to use!

Answer 2

The n-p-n junction transistor consists of two n-type semiconductors (called the emitter and collector) separated by a thin layer of p-type semiconductor (called the base). The transistor action is such that if the electric potentials on the segments are properly determined, a small current between the base and emitter connections results in a large current between the emitter and collector connections, thus producing current amplification
The p-n-p junction transistor, consisting of a thin layer of n-type semiconductor lying between two p-type semiconductors, works in the same manner, except that all polarities are reversed.

and the design of alarn may vary.. one example is by using heating effect