SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

5.3. METAL SEMICONDUCTOR JUNCTION: SCHOTTKY BARRIER 227

saturation current(pn=2. 2 × 104 cm−^3 )(note that the saturation current is essentially

due to hole injection into then-side for ap+-ndiode)

Io =(10−^3 cm^2 )×(1. 6 × 10 −^19 C)×

(10.5cm^2 /s−^1 )

(3. 24 × 10 −^3 cm)

×(2. 25 × 104 cm−^3 )

=1. 17 × 10 −^14 A

This is an extremely small value of the current. At 0.3 V, the diode current becomes

I=Isexp

(

eV

kBT

)

=1. 2 × 10 −^9 A

a value which is almost six orders of magnitude smaller than the value in the Schottky

diode. For thep-ndiode to have the same current that the Schottky diode has at 0.3 V, the

voltage required is 0.71 V.

This example highlights the important differences between Schottky and junction diodes.

The Schottky diode turns on (i.e., the current is∼1 mA) at 0.3 V while thep-ndiode turns

on at closer to 0.7 V.

5.3.4 Comparison of Schottky andp-ndiodes .................

Both thep−n diode and the Schottky diode can be used for rectification and non-linear
I−Vresponse. One may ask which provides superior performance. The answer depends upon
specific applications. The questions of turn on voltage, speed needed, reverse leakage, etc. are
important in deciding whether ap−n diode or Schottky diode should be used. The Schottky
diodes have a number of important advantages overp−n diodes. Some of these are listed in
figure 5.6. The temperature dependence of the Schottky barrier current is quite weak compared
to that of ap-ndiode. This is because in ap-ndiode, the currents are controlled by the diffusion
current of minority carriers, which in turn depends on minority carrier concentration that has a
rather strong temperature dependence.
The fact that the Schottky barrier is a majority carrier device gives it a tremendous advantage
overp-ndiodes in terms of the device speed. Device speed is no longer dependent upon ex-
tracting minority charge via diffusion or recombination. By making small devices, theRCtime
constant of a Schottky barrier can approach a few picoseconds, which is orders of magnitude
faster than that ofp-ndiodes.
Another important advantage of the Schottky diode is the fact that there is essentially no
recombination in the depletion region and the ideality factor is very close to unity. Inp-ndiodes,
there is significant recombination in the depletion region and ideality factors range from 1.2 to
2.0.
The main disadvantage of Schottky diodes is a higher reverse current density. The thermionic-
emission-controlled prefactor gives a current density in the range of∼ 10 −^7 Acm−^2 ,whichis
three to four orders of magnitude higher than that of thep−ndiode. Thus for a given applied
bias, the Schottky barrier has much higher current than thep−ndiode. As a result the Schottky
diode is preferred as a low-voltage high-current rectifier. Since, the reverse current in a Schottky