pin diode

Scheme of a pin diode

The pin diode ( English positive intrinsic negative diode ) is an electrical component . The structure is similar to a pn diode , with the crucial difference that there is an additional weakly or undoped layer between the p- and n- doped layer. This layer is therefore only intrinsically conductive ( intrinsically conductive ) and is therefore called the i-layer . The p- and n-layers are therefore not in direct contact, and when a reverse voltage is applied, a larger space charge zone is formed than with the classic pn diode. Since the i-layer contains only a few free charge carriers , it has a high resistance.

The pin diode is also called psn diode (s for weakly doped) or power diode ( due to its use in power electronics ).

construction

A pin diode essentially consists of a weakly n-conductive silicon base material ( substrate ), which is provided with a strong p-doping on one side and a strong n-doping on the other (weak use is also possible p-doped substrates, but n-material is usually available in higher purity).

The doping can optionally be achieved by diffusion processes , epitaxy or ion implantation . For contacting, metal layers are applied to both highly doped areas , creating an ohmic contact . Aluminum is often used as the metallization material .

function

If the pin diode is positively biased, then holes are injected into the i-layer from the p-layer and electrons from the n-layer . The service life of the charge carriers is particularly long in the undoped i-layer ( ≈ 0.05… 5 µs for silicon). The pin diode therefore remains conductive even if only short voltage pulses with a pulse duration of are applied. If the pin diode is operated in reverse direction, a space charge zone depleted of charge carriers results between the p and i zones. With a given reverse voltage, the depth of this zone is given by the following equation (see also pn junction ): ${\displaystyle \tau }$ ${\displaystyle \tau }$ ${\displaystyle t_{\rm {P}}\ll \tau }$ ${\displaystyle z_{\mathrm {d} }}$ ${\displaystyle U_{\mathrm {bias} }}$

${\displaystyle z_{\mathrm {d} }={\sqrt {{\frac {2\varepsilon _{0}\varepsilon _{\mathrm {r} }U_{\mathrm {bias} }}{e}}\left({\frac {N_{\mathrm {A} }+N_{\mathrm {D} }}{N_{\mathrm {A} }N_{\mathrm {D} }}}\right)}}\approx {\sqrt {\frac {2\varepsilon _{0}\varepsilon _{\mathrm {r} }U_{\mathrm {bias} }}{e\,N_{\mathrm {D} }}}}}$

Here ε ₀ = 8.85 × 10 ⁻¹²  F / m is the electric field constant , ε _{r is} the dielectric constant and e is the elementary charge . The approximation on the right-hand side applies to the case of the pin diode, since the acceptor concentration in the p-doping is very much greater than the donor concentration in the n-doping of the i-layer (of the substrate is typically 10 ¹² -10 ¹⁴  cm ⁻³ and the p-doping at 10 ¹⁸ –10 ²⁰  cm ⁻³ ). ${\displaystyle N_{\mathrm {A} }}$ ${\displaystyle N_{\mathrm {D} }}$ ${\displaystyle N_{\mathrm {D} }}$ ${\displaystyle N_{\mathrm {A} }}$

In direct current operation, the pin diode works in a similar way to a normal semiconductor diode; the high number of charge carriers stored in the i-layer is only noticeable during switching processes. For alternating currents , the pin diode has rectifier properties up to approx. 10 MHz (depending on the thickness of the i-layer). Above 10 MHz, it behaves like an ohmic resistance which is inversely proportional to the average current through the D is iode. ${\displaystyle {\overline {I}}_{\rm {D,{pin}}}}$

${\displaystyle r_{\rm {D,{pin}}}\approx {\frac {n\cdot U_{T}}{{\overline {I}}_{\rm {D,{pin}}}}}}$, with the temperature stress : and n ≈ 1… 2. (Limited by the contact resistance , a minimum of approx. 0.8–8 Ω can be achieved.) ${\displaystyle U_{T}={\frac {k\,T}{e}}}$

Space charge distribution in a negatively biased pin diode (above) and the associated electrical field strength distribution (below).

The capacitance of negatively biased pin diodes has a functional dependence on volume comparable to that of plate capacitors , which depends on the area and the plate spacing . ${\displaystyle A}$ ${\displaystyle d}$

${\displaystyle C=\varepsilon _{\mathrm {r} }\varepsilon _{0}{\frac {A}{d}}}$

In the case of pin photodiodes, the area corresponds to the active area of ​​the detector, and the plate spacing corresponds to the depth of the space charge zone . A completely depleted pin photodiode corresponds approximately to the chip thickness and with a detector area of ​​5 mm² and a chip thickness of 0.5 mm, a capacitance of 1 pF is obtained. ${\displaystyle z_{\mathrm {d} }}$ ${\displaystyle z_{\mathrm {d} }}$

The voltage change that can be measured by incident radiation in pin photodiodes is , where is the charge of the electrons or holes and the detector capacitance. The electron-hole pairs generated by the radiation are separated by the electric field, with the electrons drifting to the most positive and the holes to the most negative potential. The voltage difference should be as large as possible so that the signal-to-noise ratio is high. For this purpose, the capacitance should be as small as possible, for which purpose one either minimizes the sensitive area or increases the sensitive thickness with conventionally constructed pin photodiodes. On the other hand, the largest possible area is often desired. However, this should not be too large so that an extremely high reverse voltage does not have to be applied. ${\displaystyle \Delta U={\frac {\Delta q}{C}}}$ ${\displaystyle \Delta q}$ ${\displaystyle C}$ ${\displaystyle \Delta U}$ ${\displaystyle C}$

application

pin diodes are mainly used in high-frequency technology as DC-controlled resistors (attenuators or amplitude regulators) or DC voltage-controlled HF switches. Due to the existing i-layer, a better transmission behavior is achieved in power electronics at voltages above 1 kV and, thanks to the wide space charge zone, dielectric strength is 5 times higher than with pn diodes, which is why they are used as rectifier and freewheeling diodes for high voltages and currents can be used. As photodiodes , they are used to measure radiation and as receivers in fiber optic (LWL) transmission technology.

DC controlled resistor

Because it behaves as an ohmic resistor at high frequencies, a pin diode can be used as a DC-controlled AC voltage resistor . The high-frequency alternating current is superimposed with a direct current, which allows the resistance of the i-zone to be controlled. ${\displaystyle f\gg {\tau }^{-1}}$

In high-frequency circuits often are - attenuators used with three pin diodes. This enables a signal attenuation with constant adaptation to the wave resistance (usually 50  ). ${\displaystyle f>{2\dots 100\,{\rm {MHz}}}\gg {\tau }^{-1}}$ ${\displaystyle \pi }$ ${\displaystyle \Omega }$

In addition, due to the relatively thick i-zone, pin diodes have a low junction capacitance. As a result, with the circuit of the π-attenuator in short-circuit-series-short-circuit operation, this can also be used as a high-frequency switch , with strong blocking attenuation. ${\displaystyle {\overline {I}}_{\rm {D,{pin}}}=0}$

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  Voltage divider for alternating voltages with pin diode.

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  Circuit of a π attenuator.

Photodiode

Absorption of photons in the intrinsic layer ( space charge zone ) and generation of charge carrier pairs . The material becomes transparent for photons with less energy than the band gap ( E <E _g ).

BPW-34 photodiode

The pin photodiode and the avalanche photodiode are mainly used in optoelectronics for optical signal transmission in communications technology. The pin photodiode is the most important detector for fiber optic applications. Pin photodiodes are more temperature-stable and cheaper due to their thick i-layer, but less sensitive than avalanche photodiodes due to the lack of internal amplification. Peak values ​​for the sensitivity for Si- pin diodes are at a maximum at 850 nm between −40  dBm (25  Mi b s ⁻¹ ) and −55 dBm (2 Mib s ⁻¹ ). For wavelengths above 1000 nm, materials such as germanium (Ge), indium gallium arsenide (InGaAs) and indium gallium arsenide phosphide (InGaAsP) are used, with InGaAs having the largest cut-off wavelength of 1600 nm.

In a position sensitive device , the lateral photo effect of a flat pin diode with several electrodes is used to localize a light spot on the diode.

literature

• Ulrich Tietze, Christoph Schenk, Eberhard Gamm: Semiconductor circuit technology. 12th edition, Springer 2002, ISBN 3-540-42849-6 .

Web links

• From the diode switch to the electronic FM antenna switch . Electronics compendium (diode and high frequency switch).
•  ( Page no longer available , search in web archives ) The PIN Diode Circuit Designers' Handbook (pdf, English; 1.26 MB)

Individual evidence

1. ↑ ^{a b} L. Stiny: Handbook of active electronic components . Franzis' Verlag GmbH, 2009, ISBN 978-3-7723-5116-7 , p. 186 f .
2. ↑ J. Specovius: Basic course in power electronics: components, circuits and systems . Vieweg + Teubner, 2010, ISBN 978-3-8348-1307-7 , pp. 18–29 ( limited preview in Google Book Search).
3. ^ ^{A b} D. Gustedt, W. Wiesner: Fiber optic transmission technology . Franzis' Verlag GmbH, 1998, ISBN 978-3-7723-5634-6 , pp. 105 f .