Page 323 - Semiconductor For Micro- and Nanotechnology An Introduction For Engineers

Interacting Subsystems
tunneling current through the barrier is exponential in the barrier
height, this correction is important. Another important effect is the
presence of surface defects [7.17]. In fact, we find many additional
“disturbances” at the band edges, mainly due to impurity states and
surface reconstruction states, all of which contribute to modify the
curvature of the band and height of the barrier at the metal-semicon-
ductor interface. We will not consider these modifications here, but
refer the reader to [7.17].

Metal-like • We can also form an ohmic contact with a metal-like semiconductor,
Ohmic where Φ > Φ M for an n-type semiconductor, see Figure 7.27b. This
S
Contact
is achieved by adjusting the doping level to moderate levels so that
the built-in potential is negative. In this case the electrons in the semi-
conductor experience no barrier towards the metal and can flow
freely. In turn the correct choice of metal can significantly reduce the
barrier height and hence permit metal electrons to easily enter the
semiconductor states. In this case the built-in voltage is negative
qV = χ + ( E – E ) Φ < 0 (7.185)
–
bi c FM M
Schottky The Schottky contact can be considered to be a diode, and is often used
Barrier as a rectifier. Its switching speed is much higher than that of a semicon-
ductor PN-junction. This is because:

• the Schottky barrier has very little minority carrier storage, i.e.,
• the main conduction mechanism is via majority carriers, the minority
carriers do not contribute appreciably to the current;
• the metal has a very large supply of majority carriers ready to enter
unoccupied semiconductor states;
• the metal Fermi band edge does not appreciably change when elec-
trons are drained into the semiconductor, due to the extremely short
dielectric relaxation time of the metal;
• the Schottky barrier has almost no diffusion capacitance, so that the
depletion capacitance is almost independent of frequency at normal

320 Semiconductors for Micro and Nanosystem Technology