The following is a TCAD simulation of a high speed UTC photodetector. An I-V curve is simulated for the photodetector, forward and reverse. A light beam is simulated to enter the photodetector. The photo-current response to a light impulse is simulated, followed by a frequency response in TCAD.
Beam Simulation Entering Photodetector:
Frequency Response in ATLAS:
The full project (pdf) is here: ece530_final_mbenker
When light reaches a semiconductor, the light is absorbed if the photon energy is greater than or equal to the band gap, creating electron-hole pairs. In a direct semiconductor, the minimum of the conduction band is aligned with the maximum of the valence band.
One example of a direct semiconductor is GaAs. The band diagram for GaAs is shown to
the right. As the gap between the valence band and conduction band is 1.42eV, if a
photon of same or greater energy is applied to the semiconductor, a hole-electron pair is created for each photon. This is termed the photo-excitation of semiconductors. The photon is thereby absorbed into the semiconductor.
Indirect Semiconductors and Phonons
For an indirect semiconductor to absorb a photon, the process must be mediated by phonons, which are quanta of sound and in this case refer to the acoustic vibration of crystal lattice. A phonon is also used to provide energy for radiative recombination. When understanding the essence of a phonon, one should recall that sound is not necessarily within hearing range (20 – 20kHz). In fact, the sound vibrations in a semiconductor may well be in the Terrahertz range. The diagram to the right shows how an indirect semiconductor band would appear and also the use of phonon energy to mediate the process of allowing the indirect semiconductor to behave as a semiconductor.
Excitons are bound electron-hole pairs that are created in pure semiconductors when a photon with bandgap energy or larger is absorbed. In bulk semiconductors, these excitons will dissipate rapidly. In quantum wells however, the excitons may remain, even at room temperature. The effect of the quantum well is to force an electron and hole to be very close to each other. This allows for a strong bonding effect to take place and allows the quantum well the ability to generate light as a semiconductor laser.
The band structure of a semiconductor is given by:
Where mc = 0.2 * m0 and mv = 0.8 * m0 and Eg = 1.6 eV. Sketch the E-k Diagram.
One of the more common FET transistor typologies is the MESFET (Metal Semiconductor field effect transistor). This active device is the oldest FET device concept. The MESFET is similar in structure to a JFET (Junction Field effect transistor) but includes a Schottky junction instead of a P-N junction.
The MESFET’s channel depends on three parameters: the velocity of the charge carriers, the density of these charge carriers, and the geometric cross section the carriers flow through. The gate electrode is connected directly to the semiconductor material, creating a Schottky diode. The MESFET is generally constructed from the compound semiconductor GaAs (Gallium Arsenide) to provide higher electron mobility. As shown, the substrate is semi-insulating to decrease parasitic capacitance.
The device works by limiting the electron flow from source to drain, similar to a JFET. The Schottky diode controls the resistance of the channel (size of depletion region). Varying the voltage across the Schottky gate changes the channel size. Similar to other FETs, there is a certain pinch off voltage that causes the current to be very small, making the MESFET a switch or variable resistor. MESFETs can be depletion mode or enhancement mode. The MESFET is often used in high frequency wireless communication devices such as cell phones or military radars.
(All information and photos obtained from “High Speed Electronics and Optoelectronics Devices and Circuits” by Sheila Prasad)