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Zener Diode

A Zener diode has a well-controlled breakdown voltage, called Zener voltage, and sharp breakdown characteristics in the reverse bias regime. In spite of the name, the breakdown can be due to either impact ionization or Zener tunneling. Zener breakdown is caused by quantum-mechanical tunneling of carriers between the conduction band and the valence band (see Appendix B7-Tunneling). It occurs in junctions with higher doping concentrations and the critical field required is approximately 1 MV/cm. A Zener diode is usually used to establish a fixed reference voltage.

Step-Recovery Diode

The step-recovery diode is sometimes called a fast-recovery diode, a snap-off diode, or snap-back diode. The response of a standard p-n junction is limited by the minority-carrier storage, with the reverse recovery represented by Fig. 1.5. A step-recovery diode has a special doping profile such that the field confines the injected carriers much closer to the vicinity of the junction. This results in a much shorter transition time tlr (but with the same delay time tj). The sharp turn-off of current approaches a square waveform which contains rich harmonics, and is often used in applications of harmonic generation and pulse shaping.

Anisotype Heterojunction

An anisotype heterojunction is a junction not only of opposite types, but also of different semiconductor materials. The structure requires good lattice match between the two materials, and Ge-GaAs can be used as an example.8 The distinct features are the discontinuity in the conduction band AEC and the valence band AEy as shown in Fig. 1.8. These values can be determined graphically to be

AЈc = q(z{ ~X2) O-19)

AEV= (Eg2-Eg{)-AEC. (1.20)

The static characteristic described by Eqs. (1.1)-(1.7) have to be modified by the two dielectric constants Ki and K2 in the two materials. Specifically,

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(a) (b)

FIGURE 1.8

(a) Energy-band diagram of two isolated different semiconductor materials with opposite types, (b) Example of an anisotype heterojunction between «-type Ge and />-type GaAs. (After Ref. 8)

= K2S2 has to be satisfied at the interface. The potential variation across the «-type and p-type materials are given by

k2na

Vt ^^D +

kxnd

YT knd + K2Na (1.21)

According to Fig. 1.8(b), equilibrium is the difference between the two work functions, q<f>sl-q<l>sy

The current conduction, however, can be either diffusion limited or thermionic-emission limited. In the example shown in Fig. 1.8, the barrier for holes is similar to a standard p-n junction, and hole transport from GaAs to Ge is diffusion limited. Under forward bias, this component is similar to a homojunction

(iVt

qn, D

• r,

J =

p

exp

L Nn

P D

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(1.22)

 

The barrier for electrons is increased by AEc and the current is greatly reduced. Unlike a homostructure p-n junction in which current is dominated by diffusion current in the lightly doped side, an anisotype heterojunction usually favors injection of carriers from the material of larger energy gap. Other current

components are due to tunneling and recombination arising from a non-ideal interface. The suppression of one type of carriers improves the injection efficiency, which makes it beneficial for the emitter-base junction of a bipolar transistor.9 Other applications include photodetectors in which a local absorption coefficient can be optimized.

Varactor

The word varactor comes from variable reactor. A varactor, also called a varactor diode or varicap (variable capacitance) diode, is in principle any two-terminal device whose capacitance varies with the DC bias. In practice, a p-n junction is the most common structure. A Schottky-barrier diode can also perform the same function, and is used especially in ultra-high-speed operations.

When a p-n junction is under a reverse bias, the depletion layer widens, and its capacitance changes according to Eq. (1.17). Forward bias is to be avoided from excessive current which is undesirable for any capacitor. The dependence of capacitance on the DC reverse bias is determined by the doping profile near the junction. It can be described by the form

C = C, (Ybi+vr)~S • (1.23)

For a one-sided junction, if the profile of the lighter doping is approximated by

N(x) = C2xm, (1.24)

it can be shown that6

1

(1.25)

m + 2

For a one-sided step profile, m = 0 and s = 1/2. For a linearly graded junction, m = 1 and s = 1/3. If m < 0, the junction is said to be hyper-abrupt. Specific cases of interest are m = -1, -3/2, -5/3 and s = 1, 2, 3, respectively.

The applications of a varactor are in filters, oscillators, tuning circuits of radio and TV receivers, parametric amplifiers, and automatic frequency control circuits.

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