BJT is nothing but the addition of two PN junction diodes.
There are two types of BJT= P-N-P or N-P-N
P-N N-P
+ or +
N-P = P-N-P P-N =N-P-N
SCR is a thyristor which is made adding two BJTs. Of course they
are made of sillicon.
Exempli gratia: P-N-P
+ +
N-P-N = P-N-P-N comparison between scr bjt and mosfet Check the
related link for further information.