technology photodiodes
high-power photodiode performance). In an ideal world the electrical output of the photodiode follows its optical input in a linear fashion. But in practice this is never the case – nearly every physical mechanism has some degree of nonlinearity. This is present in optical transmission, carrier generation and transport, and compounding this issue, the non-linear mechanism that dominates varies, depending on the power level, frequency range and bias voltage of the photodiode. So to produce a diode with good performance, an engineer has to weave a well-chosen path that trades conflicting requirements for high-linearity with the need for certain levels of performance in key areas.
While addressing these issues, the photodiode designer must not neglect thermal management. Multiplying the photocurrent by the bias voltage reveals that a high- power photodiode needs to dissipate nearly 1 Watt from an active area of less than 10-4
cm2 . The junction
temperature of the diode – which is governed by factors such as heat conductance of semiconductor layers, the photodiode geometry and heat sink design – can exceed 200 °C (see Figure 4). Even higher diode temperatures are possible when the bias voltage is cranked up to improve photodiode saturation. This further increases the importance of good thermal management.
New variants
During the last decade engineers have increased the performance of the photodiode by modifying its design. Variants introduced include the uni-traveling carrier (UTC) photodiode, the partially-depleted-absorber photodiode, and the dual-depletion region photodiode.
Our group at the Department of Electrical and Computer Engineering at the University of Virginia believes that the UTC photodiode merits special attention: It delivers superb performance and has potential for further improvements. This class of photodiode features a quasi-neutral absorber and a transparent depletion region. When incident photons create electron-hole pairs in the un-depleted absorption layer, electrons behave as minority carriers and holes as majority carriers. Via a combination of diffusion and drift, electrons are transported to the depleted, high-field collection layer, where they then drift toward the n- contact at their high saturation velocity. In contrast, holes, thanks to the quasi-neutrality of the absorption layer, respond very fast – within the dielectric relaxation time that is determined by their collective motion. This means that there is a fundamental difference between a UTC photodiode and its PIN counterpart: In the former structure, electrons and holes contribute to the response current, with the low-velocity hole transport dictating the device’s speed. An additional strength of the UTC design is that it alleviates the space-charge effect, thanks to a more balanced electron and hole distribution profile. Electrons are able to maintain their high velocity at relatively low electric fields, enabling the
UTC photodiode to achieve high speed and high saturation output photocurrent, even at a low bias.
Through modifications to the UTC structure, we have taken the device to a new level of performance in certain areas, such as output power. Our modified UTC (MUTC) features several elements that contribute to the device’s outstanding performance (see Figure 5). This includes an intrinsic InGaAs layer inserted between the p-type InGaAs absorber and the InP drift layer. Additional design flexibility follows from this – the device can then be optimized for both high responsivity and high speed.
Our quasi-neutral InGaAs absorber consists of four step-graded p-type layers. Grading creates a quasi- electric field that drives electron transport in the absorber region. To increase the saturation current, we use charge compensation. To this end, we slightly dope the depletion region to pre-distort the electric field in such a way that it is initially higher where the space- charge effect is most severe at high current.
Another feature that pre-distorts the electric field is the cliff layer – a very thin layer of n-type InP sandwiched between the InGaAs absorber and the InP drift layer. The cliff layer increases the electric field in the intrinsic InGaAs absorber and speeds the passage of electrons through the InGaAs/InP hetero-junction interface.
Figure 3. Factors that limit the performance of high-power photodiodes
Figure 4 (a).The layout image and (b) thermal image of 34-µm backside- illuminated MUTC
photodiode obtained by thermal reflectance imaging
April / May 2012
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