146 CHAPTER 5 Nanomorphic cell communication unit resistance. Also, all pn-junctions have junction capacitance, due to the finite size of the barrier length (depletion length) W, which acts as an effective insulating layer separating two `plates' formed by n- and p-regions. The junction capacitance of a nanomorphic LED can be estimated as C pn w 3 0 Kd 2 : W (5.37) A numerical estimate for a GaAs LED (K GaAs ¼ 13.1) with dimensions d ¼ 2 m m and W ¼ 1 m m gives C pn w5 Â 10 ­16 F. When the switch in Figure 5.13 is in the ON state, the LED is activated after the junction capacitor C pn is charged to a voltage larger than the turn-on voltage V th (see (5.33) and Table 5.2). Let V th w1 V; then the energy of charging the junction capacitor needed to activate the LED will be E LED ¼ 2 2 C pn V th 2 w 5x10 À16 ,1 2 ¼ 2:5,10 À16 J 2 (5.38) Equation (5.38) can be used as an optimistic estimate of the energy needed to send a bit of information using directional optical communication at l w1 m m. It should be noted that in practical LED there are additional factors which severely limit efficiency. In fact a considerable portion of light generated in a LED is trapped inside by internal reflection due to a high refractive index of light-emitting semiconductor materials. This internal reflection limits the photon extraction efficiency. For example, a conventional planar InGaN light-emitting diode has the photon extraction efficiency of less than 5% [27]. Different approaches to enhance light extraction include the use of shaped die, rough surfaces or textured semiconductor surfaces, micro-lens arrays, and photonic crystals [27]. However all these techniques require an increase in the total size of the structure compared to the active region, and thus are difficult to implement in the nanomorphic cell. 5.10 STATUS OF m -SCALED LEDs AND PDs Currently, there is considerable interest in development of LEDs and PDs of several micrometers in size ( m LED and m PD). Example application drivers for m LED are microdisplays, as well as different bioelectronic applications, such as lab-on-chip systems, neural stimulation, etc. [27­29]. There remain open questions regarding the performance of optoelectronic devices with device size scaling [28]. A recent example of the small `conventional' m LEDs are arrays of InGaN disk-shaped light-emitting diodes with a diameter of 12 m m and emission wavelength of l ¼ 408 nm, i.e. d/ l z 30 [27]. A new class of light emitters, the light-emitting transistor, has been recently proposed [30,31] targeting high-speed optical data communication. Scaling properties of this light-emitting transistor have been investigated and devices with emitter aperture size as small as 5 m m have been demonstrated [31]. The peak emission wavelength of the device wasw1 m m, i.e. d/ l ¼ 5. `Unconventional' subwavelength LEDs based on quantum dots have also been reported, which target special applications, such as single-photon sources [32]. In [32], quantum dot (InAs/GaAs) light-emitting diodes were reported with an active area of 0.6 m m, emission wavelength of 1.3 m m (d/ l z 0.5), and efficiency ofw0.01% [32]. Developments in microphotodiodes are supported by several emerging bioelectronic applications, such as retinal implants for artificial vision [32,33], `video pills' for visual inspection inside the human