S CHOTTKY-B ARRIER DIODE

HISTORY

The metal-semiconductor (MS) junction is more commonly known as the Schottky-barrier diode. It is sometimes called the surface-barrier diode. Due to the energy-band discontinuity at the interface, injected carriers possess excess energy and the structure is also referred to as a hot-carrier diode or a hot-electron diode. An MS junction is also a useful building block for many other devices. A special type of MS junction is the ohmic contact where the semiconductor is heavily doped. Obviously ohmic contacts are required for every semiconductor device because the final conductor at the chip level is always a metal.

The metal-semiconductor system is among the oldest semiconductor devices. Application of the device can be traced to before 1900. The realization of a potential barrier resulting from space charge in the semiconductor surface was initiated in 1938 by Schottky,1 and by Mott2 independently. The formulation of the thermionic-emission theory was established by Bethe in 1942.3 This theory was later refined by Crowell and Sze in 1966.4 The theory of surface states developed by Bardeen in 1947 was instrumental for better understanding of experimental results.5 The use of silicide in place of metal on silicon substrates was pioneered by Lepselter and coworkers in 1968.6,7 The epitaxial silicide process developed by Tung in 1984 provides new insight into intrinsic metal-semiconductor properties.8 In-depth treatment of the Schottky-barrier structure can be found in Refs. 9-14.

S CHOTTKY-B ARRIER DIODE

METAL

OXIDE

S CHOTTKY-B ARRIER DIODEMETAL

(b)

FIGURE 3.1

Schottky-barrier diode in the form of (a) point contact, (b) deposited metal, and (c) deposited metal with oxide isolation and diffusion guard ring.

STRUCTURE

The early version of the Schottky diode was in the form of a point contact (Fig. 3.1) where a metal wire, called a cat’s whisker, is pressed against a clean semiconductor surface. (A point contact has the characteristics of either a Schottky barrier or a p-n junction, depending on the forming process.) Such a structure was unreliable and not reproducible, and was subsequently replaced by vacuum-deposited metal. A diffused guard ring shown in Fig. 3.1(c) is often used to avoid leakage and breakdown effects caused by the high electric field at the perimeter of the diode. For silicon substrates, metallic silicides can also be used in place of the metals.

A critical step in fabricating a Schottky-barrier diode is to prepare a clean surface for an intimate contact of the metal. In manufacturing, the surface is cleaned chemically. Experimentalists have also explored cleaved surfaces, as well as cleaning by back-sputtering in vacuum. The metal is usually deposited in vacuum, either by evaporation or sputtering. Chemical deposition is gaining popularity, especially for refractory metals. Plating can also be used but contamination from the solution is not controllable. Silicides on silicon substrates are usually made by metal deposition, followed by heat treatment to form the silicides. Such a system can be potentially more ideal because the reaction consumes silicon and the silicide-semiconductor interface propagates below the original surface. One advantage of a Schottky structure is the low temperature processing. The need for high temperature steps in impurity diffusion or impurity activation after ion implantation can be avoided.

CHARACTERISTICS

The formation of a Schottky-barrier junction is shown by the energy-band diagrams in Fig. 3.2. Assuming an ideal interface, the barrier height should be given by

S CHOTTKY-B ARRIER DIODE

(a) (b)

dVb, + K)

Ec

ef

n

Ey

 

(C) (d)

 

FIGURE 3.2

Energy-band diagram for metal-semiconductor structure (a) Isolated metal and «-semiconductor (b) Connected and under equilibrium (c) Under forward bias (positive voltage applied to the metal for n-type substrate) (d) Under reverse bias Also shown are rounded, reduced barrier heights due to image-force lowering

 

S CHOTTKY-B ARRIER DIODE

h = K~Zs ‘ (3-J)

Experimentally, such a relationship is seldom realized exactly. The discrepancy is not yet fully understood. Presently, a few proposals exist, considering (1) an insulating layer at the interface (similar to that of Fig. 3.9), (2) surface traps, (3) metal-induced gap states, and (4) structure-dependent interface dipoles. As a result, the barrier formation has a weaker dependence than Eq. (3.1) on the metal work function. Typical barrier heights of metals and silicides on w-type semiconductors are listed in Table 3.1. Their corresponding barrier heights on p-type substrates, even with the above imperfections, are given by

4&bn + hi) = Eg ‘ (3.2)

This relationship is experimentally observed. The depletion width is given by

qN

and the corresponding capacitance is

SCHOTTKY-BARRDER DIODE

Подпись: SCHOTTKY-BARRDER DIODEBarrier heights for metals and silicides on n-type semiconductors (q<fi/,„ in eV) at 300 K (* at 77 K).u (*)15 Barrier heights on p-type materials « Eg — qtj>i, n.

Si

Ge

SiC

GaP

GaAs

GaSb

InP InAs

InSb

ZnS

ZnSe

CdS

CdSe

CdTe

AI

Ag

Au

Au/Ti

B)

0 68-0 74 0 56-0 79 0 81-0 83

048 0 45

20 1 95

I 05 1 2 1 18

0 73-0 80 0 88 0 90

0 89-0 92

0 61

Ohmic 0 54 Ohmic 0 40-0 49 Ohmic 0 53

0 18* 0 17*

08 1 65 20

0 74

1 22

1 35-151

1 14

Ohm )c 0 35-0 56 0 68-0 78 0 84

0 43 0 70

0 76 0 66-0 78 0 86

078

Ca

Co «

CoSi2#

Cr

Cu

0 40 0 64 0 64 0 57-0 59 0 66-0 79

05 0 48

1 18 1 20

0 56 0 82

1 75

1 10

0 36-0 50

0 33

0 82

Fe

In

lr

Mg

Mo

0 65

0 77 04 0 56-0 68

042 0 42

1 04

0 83 0 65

0 82

1 11 0 86

049

0 78 0 69

MoSо2*

Na

Mi M

NiSii

Os

0 63-0 69 0 43 0 66-0 70 0 66 07

04

078-0 83

045 0 53

0 83

Pb Pd « Pd2Si * * PtSi*

06 0 71 0 71-0 75 0 90 0 81-0 86

145

0 86

1 87 1 84

1 14 1 4

0 62 0 85-1 1

0 37

0 68 0 86

0 89

Rh 0 72 RhSi 0 70 Ru 0 76 (SN)xPolymer Sb

0 40 038

1 0-1 2 0 86

0 8-0 9

2 7-3 0

1 7 1 34

1 1

0 6-0 7

076

Sn Tl «

TiSi2#

w

WSi2

0 50 0 58-0 60 0 66 86

048

0 75 0 75-0 84

0 66-0 71

Zn

ZrSi2

0 75 0 55

FIGURE 3.3

S CHOTTKY-B ARRIER DIODECapacitance characteristics vs. reverse bias.

Ј

Подпись: Ј

(3.4)

Подпись: (3.4)1 2^bi+vr) ^2

qe N

Capacitance measurement plotted in this format (Fig. 3.3) can yield the barrier height as well as the doping concentration (or area). This technique can be extended to profile non-uniform doping concentration (see Eq. (1.18)).

The current transport in a Schottky barrier is thermionic emission of majority carriers over the barrier (see derivation in Appendix B5), given by

r2

J = A

— 1

exp

exp

~kT )

qIi

nkT)

S CHOTTKY-B ARRIER DIODE

(3.5)

 

with

(3.6)

Подпись: (3.6)* 4nqm k2

A = —————

Common effective Richardson constants A used for Ge, Si and GaAs are shown in Table. 3.2. The n value (> 1) in Eq. (3.5) is called the ideality factor. The

TABLE 3.2

Commonly used values of Richardson constants (A ) for Si, GaAs and Ge. (A/cm2-K2)

Si

GaAs

Ge

«-type

110

4.4

143

p-type

30

74

41

r

Подпись: r

/

Подпись: //(log)’

S CHOTTKY-B ARRIER DIODE

"V

(b)

FIGURE 3.4

Typical 1-V characteristics of a Schottky barrier in (a) linear current plot and (b) logarithmic current plot.

underlying cause for n > 1 can be an interfacial layer, barrier height inhomogeneity, or image-force lowering which is voltage dependent.

The minority-carrier hole current, on the other hand, is limited by diffusion as in the case of a p-n junction and is therefore given by

S CHOTTKY-B ARRIER DIODE(3.7)

This diffusion current is usually 4-6 orders of magnitude smaller than the thermionic-emission current. It is the dominance of the majority-carrier current with minimum minority-carrier storage that enables the Schottky barrier to operate at much higher frequencies (« 100 GHz) compared to a p-n junction (« 1 GHz).

Another current component in addition is the depletion-region generation/recombination current (see Appendix B2), given in the form

I — f rr

(3.8)

Подпись: (3.8)r qVf

J = J exp ———- — 1

The magnitude of Jre depends on the quality of the semiconductor material. If Jre is larger than the corresponding component in Eq. (3.5), the current for small forward bias, as well as the reverse current, will be increased.

Typical 1-V characteristics are shown in Fig. 3.4. The «value can be measured from the exponential rise of current with voltage. At high forward bias, current is leveled off by series resistance or high current injection. At high reverse bias, breakdown occurs whose mechanism is similar to the impact ionization breakdown in p-n junction.

After fabrication of a Schottky-barrier diode, it is often required to measure its barrier height. There are altogether five methods to do so and they are listed below:

1. 1-V characteristics: A current level is measured when Vj is extrapolated to zero. With a known effective Richardson constant, Eq. (3.5) is used to deduce the barrier height. Due to the exponential dependence of current on

the accuracy of A is not critical.

2. Temperature dependence: The dependence of forward current on temperature can yield the barrier height (Eq. (3.5)). For a fixed Vj, a plot of log(J/T2) vs. 1 IT gives the activation energy of q(<pi, — Vj).

3. C-Vcharacteristics: Equation (3.4) andFig. 3.3 are used to obtain the built-in potential and doping concentration. Barrier height is then the sum of qy//,, and qV„ (or qVp).

4. Photoresponse: The quantum efficiency for carriers excited from the metal over the barrier is known to be a function of photon energy hv. If the square root of the photoresponse is plotted against the photon energy, the barrier height can be obtained as shown in Fig. 3.5.

5. Photovoltaic effect: When a Schottky barrier is exposed to light, a short-circuit current Jsc or an open-circuit voltage can be obtained. The relationship is given by16

*c + l

J

0

S CHOTTKY-B ARRIER DIODE

v

oc q

 

(3.9)

 

S CHOTTKY-B ARRIER DIODEPlotting Voc vs. Jsc at different illumination levels provides J0 and n. The dark current J0 obtained can be used to calculate the barrier height.

FIGURE 3.5

Photoresponse vs photon energy to determine Schottky-barner height

In the presence of image-force lowering (see Appendix B6), the tip of the barrier is rounded off as shown in Fig. 3.2. The amount of barrier lowering is given by

S CHOTTKY-B ARRIER DIODE(3.10)

S CHOTTKY-B ARRIER DIODEwhere is the maximum field near the interface and is given by

(3.11)

Among the above measurement methods, all yield the final effective barrier height with the exception of the C-V method where the built-in potential y/bt obtained would be overestimated by A<f>i, and must be corrected to obtain the exact barrier height.

S CHOTTKY-B ARRIER DIODEThe barrier height for a metal on a semiconductor is known to be independent of the semiconductor doping level, if the doping is uniform. However, it can be modified by a thin sheet of heavily doped layer at the surface, as first suggested by Shannon.1718 As shown in Fig. 3.6, a layer of n+-region reduces the effective barrier height (on n-type substrate) due to increased image-force lowering and increased tunneling. A layer of //-region increases the barrier due to band bending in the bulk of the semiconductor.

S CHOTTKY-B ARRIER DIODE

FIGURE 3.6

Modification of barrier height from (a) original value, (b) with n+-layer to reduce <j>b, (c) with //’-layer to increase <j>b.

APPLICATIONS

The main features of a Schottky-barrier diode are high-frequency capability and low forward-voltage drop. These features plus the ease of fabrication make the device useful in a wide range of applications.19

1. As a general purpose rectifier, it can be used in many circuit applications outlined in Appendix C 1-Applications of Rectifiers.

2. Due to its high-frequency capability, among all rectifiers the Schottky barrier is the most widely used diode as microwave mixer and detector.

3. Due to its low loss (low voltage drop) in forward bias, it is used quite extensively in power electronics. In particular, it is used in low-voltage, high-current power supplies.

4. Due to the non-linear I-V characteristics, it can be used as a varistor (see Section 12.5.1).

5. It can be used as a varactor based on the variation of depletion-layer capacitance under reverse bias (see Section 1.5.4).

6. It is a fundamental building block for many other devices such as the solar cell, photodetector, metal-base transistor, MESFET, etc.

7. A special form of Schottky junction is the ohmic contact which is required to connect every semiconductor device to other devices or to the external environment.

8. In a clamped bipolar transistor, a Schottky diode is connected between the base and the collector as shown in Fig. 3.7. In the saturation regime of the transistor operation, the base-collector junction is under forward bias. When a Schottky diode is connected in parallel, most of the current passes through the Schottky device, and minority-carrier storage is eliminated in the base-collector p-n junction. As a result the turn-off time of the bipolar transistor is greatly reduced.

FIGURE 3.8

Energy-band diagram for Mott diode

Подпись:COLLECTOR?

-X-

BASE

EMITTER

FIGURE 3.7

An n-p-n bipolar transistor with Schottky-diode clamp

b

•i h

S CHOTTKY-B ARRIER DIODEN

S CHOTTKY-B ARRIER DIODE

n

S CHOTTKY-B ARRIER DIODE

FIGURE 3.9

Подпись: Energy-band diagram for MIS tunnel diode

9. It is also used as a clamping diode in integrated injection logic circuits and transistor-transistor logic circuits.

10. Due to the low temperature processing, a Schottky barrier is used as a tool for characterization of the semiconductor material, especially on surface properties.

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