P-i-n DIODE

HISTORY

The p-i-n diode is a refinement of the p-n junction for special applications. After the p-n junction was developed in the late 1940s, the p-i-n diode was first used as a low-frequency, high-power rectifier in 1952 by Hall,1 and in 1956 by Prince.2 The presence of an intrinsic layer can substantially increase the breakdown voltage for high-voltage application. This intrinsic layer also provides interesting properties when the device is operated at high frequencies in the microwave and radio-wave range. It was not until 1958 that the device started to be used in microwave applications by Uhlir.3 More details on this device can be found in Refs. 4-9.

STRUCTURE

A p-i-n diode consists of an intrinsic layer sandwiched between the opposite types of a p-n junction. The intrinsic layer has a very low concentration of either «-type or p-type in the order of 1013 cm-3, and a resistivity in the order of kfi-cm. The intrinsic-layer thickness (xj) ranges between 10 |_im to 200 |im. The outsidep — and «-layers are usually heavily doped. As shown in Fig. 2.1, the p-i-n diode can be realized as a planar structure or a mesa structure, both fabricated on degenerate substrate material. In the planar structure, an intrinsic epitaxial film is grown and the p+-region is introduced by either diffusion or ion implantation. A mesa structure has epitaxially grown layers with dopants incorporated, and is capable of higher-frequency operation because the intrinsic layer can be made thinner with better control. Isolation of the device is achieved by mesa etching and

+

n

+

p

P-i-n DIODE

(b)

 

I

 

P-i-n DIODE
P-i-n DIODE

FIGURE 2.1

A p-i-n diode with (a) planar structure and (b) mesa structure.

surface passivation such as oxidation. The advantages of a mesa structure are reduced fringing capacitance and inductance, and improved surface breakdown voltage. The substrate material for the p-i-n diode has been almost exclusively silicon until the early 1980s when GaAs was also studied.

CHARACTERISTICS

The special feature of a p-i-n diode is a wide intrinsic layer that provides unique properties such as low capacitance, high breakdown voltage with reverse bias, and most interestingly, carrier storage for microwave applications with forward bias. Near zero or at low reverse bias, the lightly doped intrinsic layer starts to be fully depleted (Fig. 2.2(c)), and the capacitance is given by

Once fully depleted, its capacitance is independent of reverse bias. Since there is little net charge within the intrinsic layer, the electric field is constant (Fig. 2.2(d)) and the reverse breakdown voltage is given by

(2.2)

For silicon, the breakdown field %bd >s approximately 2xl05 V/cm. These two equations show that the parameter xj controls the trade-off between frequency response (from capacitance) and power (from maximum voltage).

When the p-i-n diode is under forward bias, both types of carriers are injected into the intrinsic layer, and the carrier profiles are shown in Fig. 2.2(f). It is usually assumed that within the intrinsic layer, the electron and hole concentrations are the same (Pj~ns), and that they are uniform within the intrinsic layer. The current conduction is through recombination,

P-i-n DIODE

P-i-n DIODE

P-i-n DIODE

(0

 

FIGURE 2.2

p-i-n diode shown in (a) structure cross-section, (b) impurity profile, (c) space charge distribution, (d) electric field, (e) energy-band diagram at equilibrium, and (f) carrier concentrations under forward bias.

 

P-i-n DIODE

P-i-n DIODE

FIGURE 2.4

Typical RF resistance as a function of DC forward current (After Ref 5)

FIGURE 2.3

DC J-Vcharacteristics of ap—n diode

Подпись:

-r

Jo

Подпись: -r Jo

jr

Подпись: jrqUdx

0

qnjX,

T (2.3)

The carrier lifetime r is a critical parameter in designing a p-i-n diode. The relationship of n/ to applied voltage is complicated, and the final I-Vrelationship is given here without proof 9-11

Jf

2 kTJ

*qnPaFL

exp

P-i-n DIODE

(2.4)

 

The parameter FL is a further function of xj and r, and it has a value between 0.01 and 0.3. Da is called the ambipolar diffusion coefficient and is given by

D =

Подпись: D =n! + Pj

D + D

P «

2D D ~ p ”

~ D +D

P " (2.5)

The forward current is shown in Fig. 2.3. The ideality factor of 2 is a characteristic of recombination current.

A similar equation can be obtained from standard recombination/generation consideration which can provide some physical insight. The recombination current within a depletion region is given by

qh

2kT.

qxIni

J = ———- exp

re 2r

P-i-n DIODE

(2.6)

 

(see Appendix B2). Assuming that x} is comparable to the ambipolar diffusion length,

D

Подпись: D(2.7)

‘*V’

j

.2 kTj

Substitution of r into Eq. (2.6) gives

qn D

^ i a

-exp

2x,

P-i-n DIODE

(2.8)

 

This result is similar to Eq. (2.4).

It should be noted that since the p-i-n diode is similar to a p-n junction diode, the diffusion current component should also be considered. In practice, this component is small since both regions surrounding the intrinsic layers are heavily doped. High Na and ND result in small diffusion current, due to the relationship

f Tt 1 7

 

exp

— 1

 

J

_

^ kT,

_

 

 

D

 

D

 

J = qn

 

(2.9)

 

L N

 

The most interesting phenomenon for a p-i-n diode, however, is for small signals at high frequencies (> l/(2wr)) at which the stored carriers within the intrinsic layer are not completely swept by the RF signal or by recombination. At these frequencies there is no rectification or distortion and the p-i-n diode behaves like a pure resistor whose value is solely determined by the superimposed DC bias or current. This dynamic RF resistance is simply given by

Rrf Pa

^ni^n + Hp)A

(2.10)

Подпись: (2.10)Jfr^„ + Hp)A

Here Eq. (2.3) has been assumed for Jj. The RF resistance is controlled by the DC current. Typical characteristics are shown in Fig. 2.4.

For modulation and switching applications, even the mean bias point can vary with time. The upper limit of this modulation frequency is determined by the reverse recovery characteristics. When a p-i-n diode is switched from forward bias to reverse bias abruptly, the stored charges continue to contribute to a large reverse current until they are fully drained away (Fig. 2.5). The reverse current is determined by the series resistance Rs (Ir = I,/r/Rs), and the delay time td is given by

( JA

(2.11)

Подпись: (2.11)t. = r In 1 + — d I

The transition time tlr is a complicated function of the doping profile and diode geometry. This reverse recovery time is the sum of td and tlr and it puts an upper limit on the rate at which the quiescent bias point can be switched.

APPLICATIONS

1. RF switching: The RF resistance of a p-i-n diode is controlled by the quiescent bias. This feature makes it practical as a circuit, shown in Fig. 2.6, called a series switch. When the diode is forward biased it is considered a short. At zero or reverse bias, it is considered a capacitor or an open circuit.

2. Attenuation and modulation: Because the RF resistance is a continuous function of quiescent bias, it can be varied to attenuate and modulate the RF signal. The modulation frequency is limited by the reverse recovery given in Eq. (2.11). RF switching is an extreme case of attenuation and modulation.

3. Phase shifting: Phase shifting of an RF signal can be achieved by using transmission lines of different lengths. The p-i-n diodes can be used as switches for selecting these transmission lines.

4. Limiter: As mentioned before, at RF frequencies, a p-i-n diode behaves like a pure resistor. However, this is valid only when the RF signal is below a critical level. Above this level, the RF resistance drops, similar to that of the DC resistance. This property enables it to be used for protection of radar receivers, when it is connected in parallel, against excessive transmitter power.

5. Power rectifier: Due to the thick intrinsic layer, a p-i-n diode has a high breakdown voltage and can be used as high power rectifier.

6. Photodetector: For a photodetector, a wide region with a built-in field is advantageous such that light can be completely absorbed within this region. A p-i-n structure is used as a nuclear-radiation detector and is one of the most common photodetectors (see Chapter 50).

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *