Appendix B Diodes; Signal Diodes - FIGnition FUZE Hardware Reference Manual

Appendix B Diodes

Diodes are the simplest semiconductors. FIGnition uses a simple low-power signal diode
in two different ways and a different type of diode (a Zener diode) in one way. Here's how
they work.
B.1 Signal Diodes
!
! (acceptor)!! ! (donator)
A diode is constructed from silicon (which doesn't normally conduct) with tiny amounts of
impurities. For example, adding about 1 in 108 atoms of Boron makes silicon good at ac-
cepting electrons whereas adding Arsenic makes Silicon good at donating electrons.
By themselves, either type of 'doped' Silicon will conduct electricity in either direction, but if
you put them together; a natural electrical barrier appears: the N-type Silicon donates
some electrons to the P-type (which accepts them) and this causes the N-type Silicon to
become positively charged (because it's lost some electrons) and the P-type Silicon to be-
come negatively charged (because it's gained some electrons).
This barrier quickly reaches equilibrium: the more electrons that reach the P-type Silicon,
the more likely they are to be attracted to the positively charged N-type; but the more they
cross back the other way, the more opportunities there are for the N-type to donate elec-
trons to the P-type.
If we apply a small negative voltage 11 to the N-type Silicon the barrier gets smaller, be-
cause electrons fill the positively charged N-type region. A few electrons will actually make
it all the way across, causing a tiny current to flow.
11 In this appendix we'll talk about negative voltages instead of positive voltages, because electrons actually
flow from the negatively charged end of a circuit (which we normally call ground) to the positively charged
side - it would look odd to talk about a positive voltage and have to show electrons being pumped in from the
opposite side.