MICROWAVE Diodes = Schottky Transistor = BJTs, FETs

In the microwave domain, diodes and transistor performance is affected by various effect: Diodes = Schottky Transistor = BJTs, FETs Discuss these effects and explain the special treatment required for these active devices due to their high frequency behavior which markedly departs from that low frequency components.

Special type of diode with a very low forward-voltage drop When current flow through a diode, it has some internal resistance to that current flow, which causes a small voltage across the diode terminal. Voltage drop is between approximately 0.15-0.45v (lower voltage drop translates into high system efficiency) compared to a normal diode has between 0.17-1.7v and the leakage current of a Schottky diode is higher than that of a pn-junction diode. The recovered charge of a Schottky diode is much less than that of an equivalent pn-junction diode. Since it is due only to the junction capacitance, it is largely independent of the reverse di/dt.

A Schottky diode with relatively low conduction voltage has relatively high leakage current, and vice versa. As a result, its maximum allowable voltage is limited to 100 v. The current ratings of Schottky diodes varies from 1 to 300 A. Schottky diodes are ideal for high-current and low-voltage dc power supplies. The Schottky diode is also used in power applications as a rectifier , again because of its lower forward voltage drop leading to lower level of power loss compared to ordinary pn-junction diodes.

CHARACTERISTIC The Schottky diode is what is called majority carrier device. This gives it tremendous advantages in term of speed because it does not rely on holes or electrons recombining when they enter the opposite type of region as in the case of a conventional diode . By making the device small the normal RC type time constant can be reduced, making these diode and order of magnitude faster than the conventional diode . This factor is the prime reason why they are so popular in radio frequency application. The diode has also a much higher current density than an ordinary pn-junction. This means that forward voltage drops are lower making the diode ideal for use in power rectification applications

I-V characteristic of a P-N junction diode (not to scale) I-V, characteristic curve is ascribed to the behavior of the so-called depletion layer or depletion zone which exists at the p-n junction between the differing semiconductors A diode’s I-V characteristic can be approximately by two regions of operation. Below a certain different in potential between the two leads, the depletion layer has significant width, and the diode can be though of as an open (non conductive) circuit. As the potential differences is creased, at some stage the diode will become conductive with zero ( or at least very low) resistance. More precisely, the transfer function is logarithmic, but so sharp that it looks like a corner on a zoomed-out-graph.

Referring to the I-V characteristic image, in the reverse bias region for a normal P-N rectifier diode, the current through the device is very low ( in the μ A range ) for all reverse voltages up to a point called the peak-inverse-voltage (PIV). Beyond this point a process called reverse breakdown occurs which causes the device to be damaged along with a large increase in current

APPLICATION /REQUIREMENT The Schottky barrier diode (formed by deposition of metal on a semiconductor surface) is the primary device used for mixer and detector applications at microwave and millimeterwave frequencies Below is Mixer function block:-

2FLO - FRF is called the image Mixing is defined as the conversion of a low-power level signal from one frequency to another by combining it with a higher-power (local oscillator) signal in a nonlinear device. In general, mixing produces a large number of sum and difference frequencies of the signal, local oscillator and their harmonics The local oscillator biases the diode to its nonlinear region and does not take part in the actual frequency conversion process, for example only RF signal power is converted to the IF and image frequencies. 2FLO - FRF is called the image

Equivalent Circuit or mixer diode The Schottky mixer or detector diode may be regarded as a nonlinear conductance, g, shunted by a capacitance (CJ), in series with a resistance (RS), as shown above At low frequencies the capacitance does not affect rectification, but at microwave frequencies its shunting action reduces the RF voltage across the barrier. Since it is impossible to tune out C at microwave frequencies with an external inductance due to the presence of RS, CJ must be kept small so as to minimize reduction in rectification efficiency.

In addition to this their low turn on voltage and high frequency capability and low capacitance make them ideal as RF detectors. Detector are essentially low sensitivity receivers which function on the basis of direct rectification of the RF signal through the used of a non-linear resistive element; a diode Detector are essentially low sensitivity receivers which function on the basis of direct rectification of the FR signal through the used of a non-linear resistive element; a diode

Typical detector output voltage verses input power The small detectors operation is dependant on the slope and curvature of the VI characteristic of the diode in the neighborhood of the bias point. The output of the detectors is proportional to the power input to the diode that is the output voltage( or current) is proportional to the square of the input voltage (or current), hence the term ‘square law

Dynamic range improvement with bias The large signal detector operation is dependent on the slope of the VI characteristic in the linear portion, consequently the diode functions essentially as a switch. In large detection the diode conducts over a portion of the input cycle and the output current of the diode follows the peaks of the input signal waveform with a linear relationship between the output current and the input voltage. Dynamic range improvement with bias Example of the illustrares is the effect of bias current on Agilent Technologies detector, measured at 10GH0z. T diode impedance was matched to the 50ohm system at each bias level. The tunnel was adjust at an input level of 30dBm

The square law dynamic range may be defined as the difference between the power input for 1dB deviation from the ideal square law response (compression point) and the power input corresponding to the tangential signal sensitivity (TSS). Normal operating conditions for the Schottky detector call for a large load resistance (100kΩ) and a small bias current (20μA). These normal conditions assure the minimum value of TTS input level but not the maximum value of compression level . The compression level can be raised by reducing the value of RL, the load resistance. However the sensitivity degrades by the factor =RL/(RL+RV) Where RV is the diode’s video resistance. This degradation in TSS exceeds the improvement in the compression, so there is no improvement in square law dynamic range. Another technique to raising the compression level is to increase the bias current. This also degrades the sensitivity. But the improvement in compression exceeds this degradation so square a law dynamic range is increase.

The Scottky diode is widely used in the electronics industry finding many uses as a general purpose rectifier However it has come into its own for radio frequency applications because of it high switching speed and high frequency capability. Scottky diode are also use high power application , as rectifiers. Their high current density and low forward voltage drop mean that less power is wasted than if ordinary PN-junction diodes were used.

An NPN transistor with Schottky diode clamp Figure above shown Scottky diode may also be used as a clamp diode in a transistor circuit to speed the operation when used as a switch. The diodes is inserted between the collector and base of the driver transistor to acts a clamp. When the Scottky diode is present this takes most of the current and allows the turn off time of the transistor to be greatly reduced, thereby improving the speed of the circuit.

Current vs. Voltage Relation The current versus voltage relationship for a Schottky diode is given by the following equation known as the Richardson equation. where: I = I S (e qV/η kT -1) Saturation Current, I S = AA* T 2 e -q B /kT A* = modified Richardson constant A = area (cm 2 ) k = Boltzmann’s Constant T = absolute temperature B = (Vm-x) barrier height in volts q = electronic charge η = ideality factor (forward slope factor, depends upon metal — semiconductor interface)

The barrier height (ФB), of a Schottky diode can be determined experimentally by fitting the forward V versus I characteristic to the given equation. I = I S (e qV/η kT -1) Saturation Current, I S = AA* T 2 e -q B /kT Notice that ФB, the potential barrier for electrons in the metal moving towards the semiconductor, influences the forward current. The barrier height (Ф B), is important because it determines the local oscillator power necessary to bias the diode into its non-linear region. In many radar receiver systems, available local oscillator power is less than 1 mW for mixer diodes, so low-barrier silicon Schottky diodes are desired. Schottky diodes have been fabricated with several metals and alloys using P and N type silicon with barriers ranging from 0.27–0.90 V. However, this ideal behavior is not observed for a metal/GaAs system. The barrier height is found to be virtually independent of metal work function. This is thought to be due to “Fermi level pinning” at the metal-semiconductor interface.

metal semiconductor ohmic contacts The Schottky metal contact is sunk via a "recess" channel into the depletion zone layer, which is located buffed under a highly doped contact layer. Published limit frequencies of fco<1.5 THz and cutoff voltages between 0.7 V and 0.8 V . This process does not make it possible to prepare diodes with reduced cutoff voltages and to reach very low Rs values with simultaneously low CjO values, because the conductivity of the ohmic lead layer prepared by silicon implantation is limited, the current must flow through the low-doped depletion zone layer (PET arrangement), and the Schottky metal contact resistance already increases markedly in these small structure geometries (<0.3 .mu.m) and it dominantly determines Rs. If the semiconductor is very highly doped, the Schottky barrier depletion region becomes very thin.  At very high doping levels, a thin depletion layer becomes quite transparent for electron tunneling.  This suggests that a practical way to make a good ohmic contact is to make a very highly doped semiconductor region between the contact metal and the semiconductor. A good ohmic contact should have a linear current-voltage characteristic and a very small resistance that is negligible compared to the resistance of the active region of the semiconductor device.

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