254 Chapter 5. Solid State Detectors
That is, not the whole intrinsic material is doped, rather the doping is done only up
to a certain depth. The rest of the material then remains intrinsic. We will learn
more about this technique and its advantages later in the Chapter. Let us first have
a closer look at the physical process involved in acceptor and donor doping.
D.1 Doping with Acceptor Impurity
If the element added has an electron less than the one in the semiconductor, it can
either form one bond less than the semiconductor atoms or captures an electron from
the semiconductor lattice to fit in the structure. In both the cases a net positive
vacancy orholeis created. If a large number of such impurity atoms are added,
the created holes outnumber the free electrons in the valence band. Interestingly
enough, if an external electric field is now applied to the material, these holes start
drifting and constitute an electric current. Such a semiconductor material with an
acceptor impurity is called a p-type semiconductor.
The net effect of acceptor impurity addition is the shifting of Fermi level towards
the valence band, which essentially means that the occupancy of free positive charges
in the bulk of the material is larger than that of the negative charges.
Si Si Si SiSi Si SiSi Si Si SiSi Si Si SiHole ElectronCovalent BondEnergy
(a)
(b)
Acceptor Levels
Valence BandConduction BandBFigure 5.1.2: (a)Addition of boron in silicon lat-
tice. Boron having three available electrons for
bonding leaves a hole, which can be filled by a free
electron in the lattice. (b)Addition of an acceptor
impurity in a semiconductor shifts the Fermi level
towards the valence band.Boron is a common example of an acceptor impurity that can turn silicon into a
p-type semiconductor. Fig.5.1.2 depicts the effect of this doping on a regular silicon
lattice. A silicon atom in the semiconductor lattice has 4 electrons making covalent
bonds with four other neighboring silicon atoms. If a boron atom having three outer