PLANAR-DOPED-BARRIER DIODE

HISTORY

The planar-doped-barrier (PDB) diode is often called the cS-doped-barrier diode or triangular-barrier diode, due to the shape of the potential barrier. It belongs to a class of bulk-barrier diodes which are different from the most common majority-carrier device, the Schottky-barrier diode whose barrier is formed at the semiconductor surface. The development that led to this device started from the idea of enhancing the Schottky-barrier height by a thin, depleted region of high concentration of the opposite type at the surface, proposed by Shannon in 1974.1 Shannon, later in 1979, developed a new majority-carrier device called a camel diode in which the metal-semiconductor barrier was eliminated and the barrier was created by an n++-p+-n structure (see Section 4.5.1).2 The planar-doped barrier was first reported by Malik et al. in 1980.3 The structure differs from the camel diode by having layers of intrinsic regions inserted between the oppositely doped layers, i. e., n+-i-p+-i-n+ structure. More detailed discussions of the device can be found in Refs. 4-7.

STRUCTURE

A typical planar-doped-barrier diode in GaAs is shown in Fig. 4.1. Because the middle p+-layer has to be fully depleted, it is very thin and it lies in the range of 2-10 nm. Such a thin, heavily doped layer can only be controlled by MBE or MOCVD growth. Besides, thermal cycles during subsequent processing have to be constrained to avoid excessive diffusion. The doping level is typically in the 1018 cm-3 range. The intrinsic regions range from tens of nm to several hundred

FIGURE 4.1

PLANAR-DOPED-BARRIER DIODECross-section of a planar-doped-barrier diode in GaAs 3

nm, and have concentrations in the 1014 cm 3 range. The mesa structure can be obtained by etch-back of the planar epitaxial layers.

CHARACTERISTICS

The charge density per-unit-area in the middle p+-layer, when fully depleted, has a value equal to

(4.1)

Подпись: (4.1)qN.8

where S is the planar-doped layer thickness. This charge sheet induces space-charge regions in the two w+-layers (Fig. 4.2), giving rise to electric fields

(4.2)

(4.3)

Подпись: (4.2) (4.3) ?rh = №2

The barrier height (from Fermi level) at zero bias can be given by

x2Qa

(4.4)

Подпись: (4.4)

’’bo

Подпись: ’'bo+ V

Equation (4.4) ignores fields set up at the junctions between the w+-layer and the intrinsic layer. It underestimates <j>bo by an amount which becomes non-negligible for small Qyj.5,6 These equations show that the barrier height can be tailored by parameters NA, S, X] and x2, and is much more flexible than the Schottky-barrier diode whose barrier is determined mainly by the metal work function. The capacitance of the PDB diode is given by

PLANAR-DOPED-BARRIER DIODE(a)

(b)

 

(c)

 

PLANAR-DOPED-BARRIER DIODE

PLANAR-DOPED-BARRIER DIODE(d)

(e)

 

FIGURE 4.2

Properties of a planar-doped-barner diode (a) Doping profile (b) Space-charge density and (c) electric-field profile in equilibrium Energy-band diagram (d) in equilibrium and (e) under forward bias, with positive voltage applied to the top (left) layer

 

PLANAR-DOPED-BARRIER DIODE

and is independent of bias.

It is known that, in the injecting side of the barrier where carriers are going against the potential, if the potential variation exceeds kTlq within one mean free path, the current conduction is limited by thermionic emission. A PDB diode typically falls into this category. Under bias, the barriers faced by the two H+-layers are no longer symmetrical, as shown in Fig. 4.2(e). With a positive voltage V applied to the surface (left) layer, the barriers are modified to

x, F

Подпись: x,F

bo

Подпись: bo(4.6)

(4.8)

where A* is the effective Richardson constant (see Chapter 3 and Appendix B5). The applied voltage V can be either positive or negative, and both polarities are valid in Eq. (4.8). Typical characteristics are shown in Fig. 4.3. The factors, also known as barrier leverage factors, xi/(xi+x2) and x2/(x]+x2) determine the portion of voltage applied to 0and 0b2, respectively. These factors control the effective

J= A* I2 = /T2

(jC j + X^)

bo

The thermionic-emission current under this condition is given by

 

exp

I kT )

— exp

I kT ;

 

exp

-q(j>

bo

exp

(Xj +x2)kT,

■ exp

FIGURE 4.3

Typical I-V characteristics of a planar-doped-barrier diode in (a) semi-log scale and (b) linear scale Asymmetry is due to x, < jc2.

(a) (b)

x qV

-x. qV

kT

(.x + x-) kT

x2V

 

(4.7)

 

PLANAR-DOPED-BARRIER DIODE
PLANAR-DOPED-BARRIER DIODE

PLANAR-DOPED-BARRIER DIODE PLANAR-DOPED-BARRIER DIODE

ideality factor (n value) of the I-V characteristics. Also, in an asymmetrical structure (x, * x2), the voltage polarity that forces higher current is considered the forward bias. In the above example, x2 > *i, and positive V is the forward direction.

APPLICATIONS

The PDB diode is a majority-carrier device. It has no minority-carrier storage and is capable of high-frequency operations. It has certain advantages over the Schottky-barrier diode. First, the barrier can be varied between zero to approximately the energy-gap value. The degree of symmetry between the forward and reverse directions can also be adjusted. Second, the barrier is not at the metal-semiconductor interface so that it is more stable in response to electrical stress. Third, since all the layers are semiconductors, the PDB structure is more flexible as a device building block. Applications of the PDB diode are listed below.

1. Referring to the energy-band diagram of Fig. 4.2(e), if extra minority carriers (holes) are supplied by external means, they would accumulate at the peak of the valence band. These positive charges set up a field that reduces the barrier heights <j>bi and <j>b2, resulting in a larger thermionic-emission (majority-carrier) current. This property of current gain is used in a photodetector (see Section 54.5.4) and switch (see Chapter 17).

2. Two PDB diodes back-to-back are used to form a hot-electron transistor (see Chapter 35). The planar-doped barrier has also been incorporated as the channel or the gate of various FETs.

3. As a microwave mixer and detector, it has performance similar to that of a Schottky-barrier diode.8 It can also be used as a special subharmonic mixer that requires symmetrical I-V characteristics.9 In this case it replaces two Schottky-barrier diodes in anti-parallel.

4. It can replace the Schottky barrier as the injecting junction in a BARITT diode10 or a TED.

RELATED DEVICE

Camel Diode

A camel diode can be viewed as the extreme case of asymmetry in a PDB diode with x = 0. It has a three-layer structure shown in Fig. 4.4. The center layer is again fully depleted. The doping concentrations of the three layers increase toward the surface. Since the heavily doped layers are very near the surface, and the sharpness of the doping profiles is less critical, ion implantation and standard

n++

p*

n

(a)

T~

qfao

* *

Подпись: n++ p* n (a) T~ qfao * *

&

Подпись: &FIGURE 4.4

(a) Doping profile and (b) energy-band diagram of a camel diode.

chemical vapor deposition, instead of MBE, can be used for fabrication. The barrier height, in this case, is given by2,4

<t>

+ V

bo

qNf

2 E

PLANAR-DOPED-BARRIER DIODE

(4.9)

 

(Vn is negative for degenerate semiconductor.) Under bias, it behaves like a Schottky-barrier diode except with a small dependence of barrier height on bias, giving an ideality factor slightly larger than unity (typically « 1.2). Both the camel diode and the PDB diode belong to what is called bulk unipolar diode.7,11

1.

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