Technical Overview Of Products

I. Electro-Optical Detectors

A: Operating Principle

The Electro-Optic Detectors manufactured by Emerge Semiconductor are solid state photo-detectors. The photo-detectors are made from ultra-high purity, (nearly Intrinsic) Silicon using a double diffusion technique where a heavily doped P-layer is diffused into the rear face of the Silicon and a lightly doped N-layer is diffused into the entrance face obtaining a so-called PIN diode structure. By controlling the thickness of the intrinsic layer, the speed and responsivity of the photodiode can be controlled. Photodiodes, when biased, must be operated in the reverse bias mode.

1: Photon Interaction in Silicon

Photons interact in the intrinsic layer by means of the Photoelectric and Compton Effects. Silicon is a semiconductor with a band gap energy of 1.12 eV at room temperature. This is the gap between the valence band and the conduction band. At absolute zero temperature the valence band is completely filled and the conduction band is vacant. As the temperature increases, the electrons become excited and escalate from the valence band to the conduction band by thermal energy. The Photoelectric and Compton Effects describe two cases of electrons being escalated to the conduction band by particles or photons with energies greater than 1.12 eV, which corresponds to wavelengths shorter than 1100 nm. The resulting electrons in the conduction band are free to conduct current.

Due to concentration gradient, the diffusion of electrons from the N-type region to the P-type region and the diffusion of holes from the P-type region to the N-type region, develops a built-in voltage across the junction. The inter-diffusion of electrons and holes between N and P regions across the junction results in a region with no free carriers. This is the depletion region. The built-in voltage across the depletion region results in an electric field with a maximum at the junction and no field outside the depletion region. Any applied reverse bias adds to the built in voltage and results in a wider depletion region. The electron-hole pairs generated by light are swept away by drift in the depletion region and are collected by diffusion from the un-depleted region. The current generated is proportional to the incident light or radiation power. The light is absorbed exponentially with distance and is proportional to the absorption coefficient. The absorption coefficient is very high for shorter wavelengths in the UV region and is small for longer wavelengths. Hence, short wavelength photons such as UV, are absorbed in a thin top surface layer while Silicon becomes transparent to light wavelengths longer than 1200 nm. Moreover, photons with energies smaller than the band gap are not absorbed at all.

a: Photoelectric Effect

In the case of the Photoelectric Effect the photon is totally absorbed by a Silicon atom electron cloud and an electron (usually from the K or L shell) is ejected from the atom with energy equal to the incident photon energy minus the binding energy of the ejected electron. The binding energy represents the excitation energy in the residual atom after ejection of the photoelectron. The excitation energy is given up by the atom as characteristic X-rays or as Auger electrons as the vacancy is filled. For Photoelectric detectors the Photoelectric Effect is dominant and it is assumed that all the recoil secondary electron energy as well as the Auger electron energy is absorbed in the detector.

b: Compton Effect

In the case of the Compton Effect the incident photon is absorbed by a "free" electron, which emits a lower energy photon and recoils with the energy difference between the incident and final photon energy.

B: Performance Parameters

1: Mechanical Parameters

a: Area

Opto-Electric Detectors are available from the Emerge product line with areas between 10 mm2 and 800 mm2. The detectors are essentially 100% efficient for photon detection and have negligible background. Since the price increases with detector area, while the performance decreases with area, then the smallest detector area consistent with the required sensitivity should be selected.

b: Temperature

All photodiode characteristics are affected by the change in temperature. They include shunt resistance, dark current, breakdown voltage, responsivity and to a lesser extent other parameters such as junction capacitance.

2: Electrical Parameters

a: Bias Voltage

A photodiode signal can be measured as a voltage or a current. Current measurement demonstrates far better linearity, offset, and bandwidth performance. The generated photocurrent requires a conversion to voltage using a trans-impedance configuration.

The photodiode can be operated with or without an applied reverse bias depending on the application specific requirements. They are referred to as "Photoconductive" (biased) and "Photovoltaic" (unbiased) modes.

For small active area devices, by definition breakdown voltage is defined as the voltage at which the dark current becomes 10µA. Since dark current increases with temperature, therefore, breakdown voltage decreases similarly with increase in temperature.

i: Photoconductive Mode

Application of a reverse bias can greatly improve the speed of response and linearity of the devices. This is due to the increase in the depletion region width and consequent decrease in junction capacitance. Applying a reverse bias however, will increase the dark and noise currents.

ii: Photovoltaic Mode

The photovoltaic mode of operation (unbiased) is preferred when a photodiode is used in low frequency applications (<350KHz) as well as ultra low light level applications. The photocurrents in this mode of operation have less variation in responsivity with temperature.

b: Reverse Leakage Current

When a reverse bias is applied to a silicon diode, a reverse leakage current is generated. The reverse leakage current is made up of two components. These components are the bulk and surface leakage currents.

The bulk leakage current is due to the collection of thermally generated electron-hole pairs in the sensitive intrinsic layer and produces a "white noise" spectrum.

The surface leakage current, as the name implies, is a result of surface leakage and is voltage and ambient sensitive producing a "1/f" noise spectrum.

c: Responsivity R

The responsivity of a silicon photodiode is a measure of the sensitivity to light, and it is defined as the ratio of the photocurrent I p to the incident light power Pwr at a given wavelength:

R = I p / Pwr

In another words, it is a measure of the effectiveness of the conversion of the light power into electrical current. It varies with the wavelength of the incident light as well as applied reverse bias and temperature.

Responsivity increases slightly with applied reverse bias due to improved charge collection efficiency in photodiode. Also there are responsivity variations due to change in temperature. This is due to decrease or increase of the band gap, because of increase or decrease in the temperature respectively. Spectral responsivity may vary from lot to lot and it is dependent on wavelength. However, the relative variations in responsivity can be reduced to les than 1% on a selected basis.

d: Resolution

For the Photo-Detectors the resolution is the equivalent of Quantum Efficiency, Q.E. Quantum efficiency is defined as the percentage of the incident photons that contribute to photocurrent. It is related to responsivity by:

Q.E. =( R? observed / R ? Ideal) x 100%

= R? ( h c / ? q )

= 1.24 x 103 ( R? / ? )

Where h=6.63 x 10-34 Joule-Sec, is the Planck constant, c=3 x 108 m/s, is the speed of light, q=1.6 x 10-19 C is the electron charge, R? is the responsivity in Amp/Watt and ? is the wavelength in nm.

i: Non-Uniformity

Non-Uniformity of response is defined a variations of responsivity observed over the surface of the photodiode active area with a small spot of light. Non-uniformity is inversely proportional to spot size, i.e. larger non-uniformity for smaller spot size.

ii: Non-Linearity

A silicon photodiode is considered linear if the generated photocurrent increases linearly with the incident light power. Photocurrent linearity is determined by measuring the small change in photocurrent as a result as a result of a small change in the incident light power as a function of total photocurrent of incident light power. Non-Linearity is the variation of the ratio of the change in photocurrent to the same change in light power, i.e. ?I p / ? P ?. In another words, linearity exhibits the consistency of responsivity over a range of light power. Non-linearity of less than +1% is specified over 6-9 decades for double diffused photodiodes. The lower limit of the photocurrent linearity is determined by the noise current and the upper limit by the series resistance and the load resistance.

As the photocurrent increases, first the non-linearity sets in, gradually increasing with increasing photocurrent, and finally at saturation level, the photocurrent remains constant with increasing incident light power. In general, the change in photocurrent generated for the same change in incident light power, is smaller at higher current levels, when the photo-detector exhibits non-linearity. The linearity range can be slightly extended by applying a reverse bias to the photodiode.

e: Response Time

The rise time ( t rise ) is defined as the time for the signal of a photodiode to rise from 10% to 90% and fall time ( t fall ) is defined as the time for the signal of a photodiode to fall from 90% to 10% of the final value respectively. This parameter can be also expressed as frequency response, which is the frequency ( f 3dB )at which the photodiode output decreases by 3dB. It is roughly approximated by:

t rise = .35 / f 3dB

There are three factors defining the response time of a photodiode:

1. t Drift the charge collection time of the carriers in the depleted region of thephotodiode.
2. t Diffused the charge collection time of the carriers in the un-depleted region of the photodiode.
3. t RC the RC time constant of the diode-circuit combination.

t RC is determined by t RC = 2.2 RC, where R, is the sum of the diode series resistance and the load resistance ( R series + R load ), and C, is the sum of the photodiode junction and the stray capacitances (C junc +C stray ). Since the junction capacitance (C junc) is dependent on the diffused area of the photodiode and the applied reverse bias, faster rise times are obtained with smaller diffused area photodiodes, and larger applied reverse biases. In addition, stray capacitance can be minimized by using short leads, and careful lay out of the electronic components. The total rise time is determined by:

t rise = ((t Drift )2 + ( t Diffused )2 + (t RC )2)1/2

Generally, in photovoltaic mode of operation (no bias), rise time is dominated by the diffusion time for diffused areas less than 5 mm2 and by RC time constant for larger diffused areas for all wavelengths. When operated in photoconductive mode (applied reverse bias), if the photodiode is fully depleted, the dominant factor is the drift time. In non-fully depleted photodiodes, however, all three factors contribute to the response time.

f: Equivalent Circuit

A silicon photodiode can be represented by a current source (I photon) in parallel with an ideal diode. The current source represents the current generated by the incident radiation, and the diode represents the p-n junction. In addition, a junction capacitance (C junc) and a shunt resistance (R shunt) are in parallel with the other components. Series resistance (R series) is connected in series with all components in this model.

 

i: Junction Capacitance (C junc)

The boundaries of the depletion region act as the plates of a parallel plate capacitor. The junction capacitance is directly proportional to the diffused area and inversely proportional to the width of the depletion region. In addition, higher resistivity substrates have lower junction capacitance. Furthermore, the capacitance is dependent on the reverse bias as follows:

C junc = ( e Si e 0 A diffused ) / (2 e Si e 0 µ ? (VA + V bi ))1/2

Where e 0 = 8.854 x 10-14 F/cm, is the permittivity of free space, e Si = 11.7 is the silicon dielectric constant, µ = 1400 cm2 / VS is the mobility of the electrons at 300?K, ? is the resistivity of the silicon, V bi is the built in voltage of Silicon and VA is the applied bias. The capacitance is dependent on the applied reverse bias voltage. Junction capacitance is used to determine the speed of the response of the photodiode.

ii: Shunt Resistance( R shunt )

Shunt resistance is the slope of the current-voltage curve of the photodiode at the origin, i.e. V bias = 0. Although an ideal photodiode should have an infinite shunt resistance , actual values range from 10 MO to 1000 MO. Empirically the shunt resistance is calculated by applying +10 mV, and measuring the current. Shunt resistance is used to determine the noise current in the photodiode with no bias (photovoltaic mode). For best photodiode performance the highest shunt resistance is desired.

There are two major currents in a photodiode contributing to shunt resistance. Diffusion current is the dominating factor in a photovoltaic (unbiased) mode of operation, which determines the shunt resistance. It varies as square of the temperature. In photoconductive mode (reverse biased), however, the drift current becomes the dominant current and varies directly with temperature. Thus, change in temperature affects the photo-detector more in photovoltaic mode than in photoconductive mode of operation.

In photoconductive mode the drift current may approximately double for every 10°C increase change in temperature. The exact change is dependent on additional parameters such as the applied reverse bias, resistivity of the substrate as well as the thickness of the substrate.

iii: Series Resistance (R series)

Series resistance of a photodiode arises from the resistance of the contacts and the resistance of the un-depleted silicon. It is given by:

R series = ((W substrate - W depletion) ? / A diffused ) + R contact

Where W substrate is the thickness of the substrate, W depletion is the width of the depleted region, A diffused is the diffused area of the junction, ? is the resistivity of the substrate and R contact is the contact resistance. Series resistance is used to determine the linearity of the photodiode in photovoltaic mode (V bias = 0). Although an ideal photodiode should have no series resistance, typical values ranging from 10O to 100O are measured.