Photodetectors and Dark Current

A photodetector simply is a device that converts light energy to an electrical current. These devices are very much similar to lasers, although they are designed to operate in reverse bias. “Dark current” is a term that originates from this reverse bias condition. When you reverse bias any diode, there is some leakage current which is appropriately named reverse bias leakage current. For photsensitive devices, it is called dark current because there is no light absorption involved. The main cause of this current is random generation of electrons and holes in the depletion region. Ideally, this dark current is minimal (<< 1).

1

The basic structure of the photodiode is the “PIN” structure, similar to a semiconductor laser diode. An intrinsic (undoped) region occurs between the P-doped and N-doped region.  Although PIN diodes are poor rectifiers, they are much better suited for high speed, high frequency applications due to the high level injection process. The wide intrinsic region provides a lowered capacitance at high frequencies. For photodetectors, the process is photon energy being absorbed into the depletion region, causing an electron hole pair to be created when the electron moves to a higher energy level (from valence to conduction band). This is what causes an electrical current to be created from light.

Photodetectors are “photoconductive”. That is, conductivity changes with applied light. Like amplifiers and other devices, photodetectors have “Figures of Merit” which signify characteristics of the device. These will be briefly examined

Quantum Efficiency

Quantum efficiency refers to the number of carriers generated per photon. It is normally denoted by η. It can also be stated as carrier flux/incident photon flux. Sometimes anti-reflection coatings are applied to photodetectors to increase QE.

Responsivity

Responsivity is closely related to the QE (quantum efficiency). The units are amperes/watt. It can also be known as “input-out gain” of any photosensitive or detective device. For amplifiers this is known as “gain”. Responsivity can be increased by maximizing the quantum efficiency.

Response Time

This is the time required for the photodiode to increase its output from 10% to 90% of final output level.

Noise Equivalent power

This value corresponds to units of Watts/sqrt(Hz). It is another measure of sensitivity of the device in terms of power that gives a signal to noise ratio of one hertz per output bandwidth, Small NEP is due to increased sensitivity of the device.

The Electronic Oscillator

The semiconductor laser is a device that can be compared to an electronic oscillator. An oscillator can be thought of as a resonator (a circuit that resonates or produces a strong output at a specific frequency) with gain. Resonators naturally decay over time by some factor, so adding in gain (so long as the gain is greater than or equal to the loss) can allow the resonator to become an oscillator that does not decay or dampen.

The stimulation of the oscillations of an oscillator is caused by electronic noise. A block diagram can demonstrate an oscillator in an abstract, easier to understand way.

block

The oscillator is built using an amplifier (transistor that is biased into active/saturation region) or op amp with positive and negative feedback. Noise in the circuit begins the oscillation, and this output is fed back into the input and is filtered along the way. This becomes an oscillation at a single frequency.

Oscillators can be built from RC circuits, LC circuits or can be crystal oscillators. RC circuit oscillators tend to be lower frequency oscillators in the audio range. The LC oscillator is often compared to the laser in terms of functionality. The negative reactance of the capacitor and positive inductive reactance cancel at a specific frequency, leaving the circuit with only resistance and a strong current is achieved. LC oscillators are much more important for RF/microwave purposes. A crystal oscillator produces its frequency through mechanical vibrations and has a much higher Q factor than the other resonator types, which provides greater temperature and frequency stability.

Two very important oscillator types for RF/microwave/mmWave circuits are dielectric resonators and SAW (surface acoustic wave) resonators. Dielectric resonators are mainly used as mmWave oscillators to drive antennas. They are generally made of a “puck” of ceramic which oscillates at a certain frequency dependent on its dimensions. Waves are confined inside the material due to an abrupt change in the permittivity. When the waves inside interfere and produce a standing wave, this increase of amplitude creates the resonance effect. SAW resonators are often used in cell phones and have distinct advantages over the LC oscillator or other types due to cost and size.

In a semiconductor laser (laser diode), the source of oscillations is the noise generated by spontaneous emission. Spontaneous emission is the result of recombination of electron and hole pairs within the material which produces photons. This spontaneous emission is how lasers begin their operation, and this is continued by stimulated emission. Stimulated emission is electron hole recombination due to photon energy which also produces a photon. The light emitted by this type of emission is coherent, a characteristic of a laser.

Miller Effect

The Miller Effect is a generally negative consequence of broadband circuitry due to the fact that bandwidth is reduced when capacitance increases. The Miller effect is common to inverting amplifiers with negative gain. Miller capacitance can also limit the gain of a transistor due to transistors’ parasitic capacitance. A common way to mitigate the Miller Effect, which causes an increase in equivalent input capacitance, is to use cascode configuration. The cascode configuration features a two stage amplifier circuit consisting of a common emitter circuit feeding into a common base. Configuring transistors in a particular way to mitigate the Miller Effect can lead to much wider bandwidth. For FET devices, capacitance exists between the electrodes (conductors) which in turn leads to Miller Effect. The Miller capacitance is typically calculated at the input, but for high output impedance applications it is important to note the output capacitance as well.

cascode

Interesting note: the Miller effect can be used to create a larger capacitor from a smaller one. So in this way, it can be used for something productive. This can be important for designing integrated circuits, where having large bulky capacitors is not ideal as “real estate” must be conserved.

Transistor IV curves and Modes of Operation/Biasing

In the field of electronics, the most important active device is without a doubt the transistor. A transistor acts as a ON/OFF switch or as an amplifier. It is important to understand the modes of operation for these devices, both voltage controlled (FET) and current controlled (BJT).

For the MOSFET, the cutoff region is where no current flows through the inversion channel and functions as an open switch. The “Ohmic” or linear region, the drain-source current increases linearly with the drain-source voltage. In this region, the FET is acting as a closed switch or “ON” state. The “Saturation” region is where the drain-source current stays roughly constant despite the drain source voltage increasing. This region has the FET functioning as an amplifier.

ivcurve

The image above illustrates that for an enhancement mode FET, the gate-source voltage must be higher than a certain threshold voltage for the device to conduct. Before that happens, there is no channel for charge to flow. From there, the device enters the linear region until the drain-source voltage is high enough to be in saturation.

DC biasing is an extremely important topic in electronics. For example, if a designer wishes for the transistor to operate as an amplifier, the FET must stay within the saturation region. To achieve this, a biasing circuit is implemented. Another condition which effects the operating point of the transistor is temperature, but this can be mitigated with a DC bias circuit as well (this is known as stabilization). “Stability factor” is a measure of how well the biasing circuit achieves this effect. Biasing a MOSFET changes its DC operating point or Q point and is usually implemented with a simple voltage divider circuit. This can be done with a single DC voltage supply.  The following voltage transfer curve shows that the MOSFET amplifies best in the saturation region with less distortion than the triode/ohmic region.

output

Advanced Electronics and Optoelectronics: The MESFET

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.

MESFET

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)

Common Emitter Amplifier

cea1

The common emitter amplifier accepts AC signal inputs and amplifies the entire AC input signal. For this circuit to work however, the common emitter amplifier requires biasing to operate between the minimum and maximum peak values on the input signal. It is also necessary to keep the transistor operating in active mode.

In amplifier design, minimizing distortion is a major issue. The Q-point or quiescent operating point of an amplifier is the DC operating current or voltage at the transistor with no input signal supplied. The Q-point for a transistor is typically half of the supply.

 

Voltage Divider Biasing

To achieve correct biasing, R1 and R2 must be chosen to maintain the base voltage at the transistor at a constant level. The base voltage VB is a function of the supply voltage and the two resistors at the base of the transistor.

cea2

Once the amplifier is properly biased, the voltage gain calculation is shown below. An important note is that the gain of the circuit is different for low frequencies than it is for high frequencies and the gain is then a function of the load resistance and the internal resistance of the transistor. Coupling capacitors C1 and C2 are used to separate the AC input signal from the DC biasing voltage.

cea3

Diode Voltage Clipping Circuits

We’re already discussed the PN junction in a previous post. Let’s explore some of the applications of the PN diode.

diode2

It was already discussed that due to the nature of the PN junction, current is only allowed to flow in one direction. This results in two possible scenarios using a diode, depending on the direction it is facing with respect to the source.

diode1

Diode Clipping Circuits

Diodes in a forward bias allow current to pass, but thereby reducing the voltage level. In a reverse bias, current is stopped but the voltage remains unaffected.

Diode clipping can be done for either the positive or the negative voltage of a sinusoidal (or analog) input voltage. In order to clip both the positive and negative sides of the input voltage, two diodes are needed.

Positive voltage clipping:

d1

Negative voltage clipping:

d2

Two Diodes:

d3

If 0.7 Volts is not the desired output clipping voltage, a bias voltage can be added in each situation above.

d4d5

Review of OP-AMPs

Operational Amplifiers perform a number of different tasks such as amplification, filtering and performing mathematical operations. This is a review of a number of concepts related to OP-AMPS.

Open Loop and Closed Loop Gain

An op-amp’s amplification (open loop gain) without any feedback in the system is usually very large (~100,000 x). The system without any feedback is referred to as an open loop system. A closed loop system provides feedback to the system.

Capture1

The feedback in a system is determined by the impedances of the feedback loop and the input of the system. Furthermore, the op-amp can function as a non-inverting op-amp or an inverting op-amp, producing a system of positive or negative feedback respectively.

Capture2Capture3

It must be said however that the input and feedback impedances must not be only real-values resistances. The following system may easily make use of a complex impedance, and other RC combinations to produce a filter modeled by the transfer function.

Capture4

 

Mathematical operations using OP-Amps

Below are the following Op-Amps:

  • Summing Amplifier
  • Comparator Amplifier
  • Differentiator
  • Integrator
  • Low-Pass Filter example
  • Subtractor
  • Voltage Follower/Buffer

sumop

compopdiffopintegratorlowpsubopvfol