51
Review of Basic Device Physics
conditions, we can solve them for any arbitrary device structure. Generally,
we will be able to simplify them based on physical approximations.
2.3 Theory of n-Type and p-Type Semiconductors in Contact
We have discussed the basic theory of intrinsic, n-type, and p-type semiconductors in Section 2.2. In this section, we will discuss the underlying physics
of a semiconductor substrate when one region is n-type and the immediate adjacent region is p-type, forming a junction called the pn-junction or
pn-junction diode or simply diode. In reality, a silicon pn-junction is formed by
counter doping a local region of a larger region of doped silicon as shown in
Figure 2.14. The pn junctions form the basis for all advanced semiconductor
devices. Therefore, understanding their operation is basic to the understanding of most advanced IC devices.
2.3.1 Basic Features of pn-Junctions
A silicon pn-junction structure is an alternating type of p-type and n-type
doped silicon layers. The pn-junctions can be fabricated in a variety of
techniques on a silicon substrate using photo mask → Implant → Drive-in.
A typical final impurity profile along the active region can be simplified as
an erfc or Gaussian as shown in Figure 2.14b and c.
As shown in Figure 2.14a, the basic structure includes an n-region doped
on a p-type substrate. The vertical cross section of the intrinsic or active
pn-junction is shown in Figure 2.14a by a vertical cutline A. The 1D-doping
A: Vertical cutline along the
active region of pn-junction
p-substrate
n
A
N d
N d
N a
Step junction
Linearly graded
junction
N a
x
x
N(x)
N(x)
(a)
(b)
(c)
X j
X j
X j
FIGURE 2.14
A typical pn-junction: (a) 2D cross section showing the cutline along the depth of the structure
to obtain 1D doping profiles, (b) 1D-doping profile of an abrupt junction, and (c) 1D-doping
profile of a graded junction.