Rf and microwave semiconductor device handbook

At very high frequencies the series resistance can be frequency dependent, due to current crowding in the semiconductor associated with the skin effect. This frequency dependence loss can also be included in simulations. Computer programs with these features are widely used to design high performance frequency multipliers. An alternative solution technique is the fixed point method [10]. This technique uses a circuit similar to Fig. However, this approach uses a fixed point iteration to arrive at a converged solution.

The solution starts Schottky Diode Frequency Multipliers with arbitrary voltages at the local oscillator frequency at the harmonics. These two equations provide an iterative solution of the nonlinear problem. They are particularly useful when a simple equivalent circuit for the nonlinear device is not available.

We now have the numerical tools to investigate the nonlinear operation of multipliers. However, there are some operating conditions where this equivalent circuit approach breaks down. Some of these limitations will be discussed in the next section. However, many multiplier applications require very high frequency output signals with reasonable output power levels.

Under these conditions, the output powers and efficiencies predicted are always higher than the experimental results.

There are several possible reasons. Circuit loss increases with frequency, so the loss between the diode and the external connection should be higher. Measurements are less accurate at these frequencies, so the differences between the desired designed circuit embedding impedances and the actual values may be different. Parasitic effects are also more important, degrading the performance.

However, even when all these effects are taken into account, the experimental powers and efficiencies are still low. The problem is with the equivalent circuit of the diode in Fig.

It does not correctly represent the high frequency device physics [11]. The difficulty can be explained by referring back to Fig. The device is a series connection of the depletion layer capacitance and a bulk series resistance. The displacement current flowing through the capacitor must be equal to the conduction current flowing through the undepleted resistive region. At modest drive levels and frequencies the undepleted region can support this current and the equivalent resistance remains constant.

However at higher electric fields, the velocity begins to saturate at a constant value. Additional increases in the electric field do not increase the conduction current through the varactor. This current saturation is a fundamental limit on the multiplier performance. A more physical explanation also uses Fig. A nonlinear capacitor implies a change in the depletion layer width with voltage.

However, changing the depletion layer width involves moving electrons from the depletion layer edge. These electrons are limited by their saturated velocity, so the time rate of change of the capacitance is also unlimited. This simple theory allows a modified device design. Equations 2. Our goal is usually to optimize the power or efficiency available from a multiplier circuit.

Clearly one option is to increase the doping Nd in Eq. However, increasing the doping decreases the breakdown voltage and thus the maximum RF voltage that can be present across the reverse biased depletion layer. The device doping design becomes a parameter in the overall multiplier design.

The optimum efficiency and optimum power operating points for the same input and output frequency are usually different. Although this simple description provides useful information, a detailed physical model is usually needed for best results [12]. The discussion so far has been based on a uniformly doped abrupt junction varactor. However, other doping or material layer combinations are possible [13].

One option is to tailor the doping or material profile to obtain a capacitance vs. Two options are the hyperabrupt varactor and the BNN structure. These devices are shown in Fig. The doping profile of a hyperabrupt varactor is shown in Fig. Instead of a uniform doping, this structure has a much smaller doping over most of the structure with a high doping or doping spike near the metal semiconductor junction. The corresponding capacitance vs. At modest reverse biases the depletion layer extends from the metal contact to the doping spike.

The resulting narrow depletion layer produces a high capacitance. Higher applied voltages begin to deplete the charge in the doping spike. When the spike charge is depleted, there is a rapid increase in the depletion layer width through the second lightly doped region and a corresponding decrease in the capacitance. This structure can produce a more nonlinear capacitance variation than a uniformly doped device. This structure uses combinations of Barriers Intrinsic and N doped regions formed with combinations of different epitaxial materials and doping to produce optimized capacitance structures.

This structure can have either ohmic or Schottky contacts. A typical structure is shown in Fig. Notice that conduction band energy rather than doping is being plotted. This structure can have a highly nonlinear capacitance characteristics as shown in Fig.

These BNN structures have potential advantages in monolithic integration and can be fabricated in a stacked or series configuration for higher output powers. Since the major application of frequency multipliers is for high frequency local oscillator sources, it would be reasonable to try to fabricate higher order multipliers, triplers for example, instead of doublers.

Although based on Eq. Efficient higher order multiplication requires currents and voltages at intermediate frequencies, at the second harmonic in a tripler for example. However, in order to avoid loss, this frequency must have the correct reactive termination. This adds to the complexity of the circuit design. Some details of doublers and triplers are given in Reference An alternative that avoids the idlers is an even or symmetrical capacitance voltage characteristic.

One possibility is the single barrier or quantum barrier varactor [15]. The structure and associated capacitance voltage characteristic are shown in Fig. This structure, shown in Fig.

This will be a series connection of a depletion layer, a barrier and a depletion RF and Microwave Semiconductor Device Handbook region, with ohmic contacts on each end.

The capacitance is maximum at zero applied bias, with a builtin depletion layer on each side of the barrier. When a voltage is applied, one to the depletion layers will become forward biased and shrink and the other one will be reverse biased and expand. The series combination capacitance will become smaller. Reversing the applied voltage will produce the same capacitance. The resulting symmetrical capacitance is shown in Fig. This capacitance characteristic is a useful starting point for odd order multipliers.

Although a variety of other solid state sources are available for lower frequency sources, these devices are a critical component of future space-based applications. Held and A. Siegel, A. Kerr and W. East, E. Kollberg and M. Frerking and J. Trew U. S Dept. These devices can be fabricated from a variety of semiconductor materials, but Si, GaAs, and InP are generally used.

The ability to fabricate devices with layer thicknesses on this scale permits these devices to operate at frequencies well into the millimeter-wave region. These devices have been in practical use since the s and their availability enabled a wide variety of solid-state system components to be designed and fabricated.

The charge transport properties describe the ease with which free charge can flow through the material. This is described by the charge velocity-electric field characteristic, as shown in Fig. At low values of electric field, the charge transport is ohmic and the charge velocity is directly proportional to the magnitude of the electric field.

Both of these behaviors have implications for device fabrication, especially for devices intended for high frequency operation. Generally, for transit time devices, a high velocity is desired since current is directly proportional to velocity. The greatest saturated velocity is demonstrated for electrons in the wide bandgap semiconductors, SiC and GaN.

This is one of the main reasons these materials are being developed for high frequency electronic devices. Also, a low value for the magnitude of the electric field at which velocity saturation occurs is desirable since this implies high charge mobility.

High mobility produces low resistivity, and therefore low values for parasitic and access resistances for semiconductor devices. The negative slope of the velocity versus electric field characteristic implies a decreasing current with increasing voltage. That is, the device has a negative resistance.

When a properly sized piece of these materials is biased in the region of decreasing current with voltage, and placed in a resonant cavity, the device will be unstable up to very high frequencies. By proper selection of embedding impedances, oscillators or amplifiers can be constructed.

Other semiconductor material parameters of interest include thermal conductivity, dielectric constant, energy bandgap, electric breakdown critical field, and minority carrier lifetime. The thermal conductivity of the material is important because it describes how easily heat can be extracted from the device. Compound semiconductors, such as GaAs and InP, have relatively poor thermal conductivity compared to elemental semiconductors such as Si.

Materials such as SiC have excellent thermal conductivity and are used in high power electronic devices. The dielectric constant is important since it represents capacitive loading and, therefore, affects the size of the semiconductor device.

Low values of dielectric constant are desirable since this permits larger device area, which in turn results in increased RF current and increased RF power that can be developed. Electric breakdown characteristics are important since electronic breakdown limits the magnitudes of the DC and RF voltages that can be applied to the device.

A low magnitude for electric field breakdown limits the DC bias that can be applied to a device, and thereby limits the RF power that can be handled or generated by the device. The electric breakdown for the material is generally described by the critical value of electric field that produces avalanche ionization. Minority carrier lifetime is important for bipolar devices, such as pn junction diodes, rectifiers, and bipolar junction transistors BJTs.

A long minority carrier lifetime is desirable for devices such as bipolar transistors. For materials such as Si and SiC the minority carrier lifetime can be varied by controlled impurity doping. A comparison of some of the important material parameters for several common semiconductors is presented in Table 3. The large variation for minority lifetime shown in Table 3.

These devices can operate from the low microwave through high mm-wave frequencies, extending to several hundred GHz. They were the first semiconductor devices that could provide useful RF power levels at microwave and mm-wave frequencies and were extensively used in early systems as solid-state replacements for vacuum tubes.

The three devices are similar in that they are fabricated from diode or diode-like semiconductor structures. DC bias is applied through two metal contacts that form the anode and cathode electrodes. The same electrodes are used for both the DC and RF ports and since only two electrodes are available, the devices must be operated as a one-port RF network, as shown in Fig. This causes little difficulty for oscillator circuits, but is problematic for amplifiers since a means of separating the input RF signal from the output RF signal must be devised.

The use of a nonreciprocal device, such as a circulator can be used to accomplish the task. Circulators, however, are large, bulky, and their performance is sensitive to thermal variations. In general, circulators are difficult to use for integrated circuit applications. The one-port character of diodes has limited their use in modern microwave systems, particularly for amplifiers, since transistors, which have three terminals and are two-port networks, can be designed to operate with comparable RF performance, and are much easier to integrate.

Diodes, however, are often used in oscillator circuits since these components are by nature one-port networks. Which description to use is determined by the physical operating principles of the particular device, and the two descriptions are, in general, not interchangeable. Bias and RF circuits for the two active characteristics must satisfy different stability and impedance matching criteria.

Transit time effects alone cannot generate active characteristics. This is illustrated in Fig. All passive circuits, no matter how complex or how many circuit elements are included, when arranged into a one-port network as shown in Fig. The network resistance will be positive and real, and the reactance will be inductive or capacitive. This type of network is not capable of active performance and cannot add energy to a signal. Transit time effects can only produce terminal impedances with inductive or capacitive reactive effects, depending upon the magnitude of the delay relative to the RF period of the signal.

The additional delay can be generated by feedback that can be developed by physical phenomena internal to the device structure, or created by circuit design external to the device. When a bias is applied, charge carriers can tunnel through the electrostatic barrier separating the p-type and n-type regions, rather than be thermionically emitted over the barrier, as generally occurs in most diodes.

When the diode is biased either forward or reverse bias current immediately flows and ohmic conduction characteristics are obtained. The tunnel current then decreases and normal, thermionic junction conduction occurs.

In the forward bias region where the tunnel current is decreasing with increasing bias voltage an N-type negative immittance characteristic is generated, as shown in Fig. This type of active element is current driven and is short-circuit stable. It is described by a negative conductance in shunt with a capacitance, as shown in Fig.

Tunnel diodes are limited in operation frequency by the time it takes for charge carriers to tunnel through the junction.

Since this time is very short on the order of 10—12 s operation frequency can be very high, approaching GHz. Tunnel diodes have been operated at s of GHz, and are primarily limited in frequency response by practical packaging and parasitic impedance considerations.

Increased RF power can only be obtained by increasing device area to increase RF current. However, increases in diode area will limit operation frequency due to increased diode capacitance. Tunnel diodes have moderate DC-to-RF conversion efficiency 50, thanks to matching of co-integrated devices. Figure 4. As shown in Fig. Now if Ib is considered a parasitic nuisance rather than a fundamental aspect, it becomes even more appropriate to view the BJT as a voltage-controlled device that behaves as a transconductor, albeit with exponential characteristics.

Instead, the designer takes advantage of device matching in an IC environment and translinearity to provide the appropriate voltage drive. This approach can be shown to be robust against device, process, temperature, and supply voltage variations. Superimposed on the basic model are parasitic ohmic resistances in series with each active terminal Rb, Re, Rc and parasitic capacitances associated with all pn junction depletion regions Cjc, Cje , Cjs , including the collector-substrate junction present to some extent in all technologies.

Their values and bias dependencies can be estimated from the physical device structure and layout. In particular, the base and emitter resistance, Rb and Re , soften the elegant exponential characteristics in Eq. This departure from ideal translinearity can introduce unwelcome distortion into many otherwise linear ICs. Furthermore, these resistances add unwelcome noise and degeneration voltage drops as will be discussed later.

The idealized formulation for ft given in Eq. To achieve peak ft, high currents are required to overcome the capacitances. It should also be kept in mind that ft only captures a snapshot of the device high frequency performance. In circuits, transistors are rarely current driven and short circuited at the output. The base resistance and substrate capacitance that do not appear in Eq. While various other figures of merit such as fmax have been proposed, none can capture the complex effects of device interactions with source and load impedances in a compact form.

The moral of the story is that ideal BJTs should have low inertia all around, i. At the high currents required for high ft, second order phenomena known in general as high level injection begin to corrupt the DC and RF characteristics.

Essentially, the electron concentration responsible for carrying the current becomes comparable to the background doping levels in the device causing deviations from the basic theory that assumes low level injection. The dominant phenomenon known as the Kirk effect or base-pushout manifests itself as a sudden widening of the base width at the expense of the collector.

High level injection sets a practical maximum current at which peak ft can be realized. To counteract high level injection, doping levels throughout the device have increased at a cost of higher depletion capacitances and lower breakdown voltages.

Since modeling of these effects is very complex and not necessarily included in many models, it is dangerous to design in this regime. So far, the transistor output has been considered a perfect current source with infinite output resistance. In reality, as the output voltage swings, the base width is modulated, causing Is and thus Ic to vary for a fixed Vbe.

Vce , shown in Fig. The net effect illustrated in Fig. This is often termed soft breakdown or weak avalanche in contrast to actual breakdown which will be discussed shortly. The effect of a varying Va is to introduce another form of distortion since the gain will vary according to bias point.

It resembles a simplified version of the popular Gummel-Poon model found in many simulators. Another form of pseudo-breakdown occurs in ultra-narrow base BJTs operating at high voltages.

If the Early effect or base-width modulation is taken to an extreme, the base eventually becomes completely depleted. After this point, a further change in collector voltage directly modulates the emitter-base junction leading to an exponential increase in current flow.

This phenomenon known as punchthrough fortunately has been mitigated by the fact that as base widths have narrowed, the base doping has been forced to increase so as to maintain a reasonable base resistance. Furthermore, since higher collector doping levels have been necessary to fight high level injection, true breakdown has become the voltage limiting mechanism rather than punchthrough. Just as high level injection limits the maximum operating current, junction breakdown restricts the maximum operating voltage.

When the collector voltage is raised, the collector base junction is reverse biased. The resulting electric field reaches a critical point where valence electrons are literally ripped out of their energy band and promoted to the conduction band while leaving holes in the valence band. The observed effect known as avalanche breakdown is a dramatic increase in current. The breakdown of the collector-base junction in isolation, i. Another limiting case of interest is when the base is open while a collectoremitter voltage is applied.

In this case, an initial avalanche current acts as base current that induces more current flow, which further drives the avalanche process.

The resulting positive feedback process causes the BVceo to be significantly lower than BVcbo. A third limit occurs when the base is AC shorted to ground via a low impedance. Therefore, BVces represents an absolute maximum value for Vce while BVceo represents a pessimistic limit since the base is rarely open. Operating in the intermediate region requires care in setting the base impedance to ground and knowing its effect on breakdown.

The base-emitter junction is also sensitive to reverse bias. In this case, the breakdown is usually related to Zener tunneling and is represented Bipolar Junction Transistors BVceo 0.

The crystalline structure in an ordered GaInP layer is such that sheets of pure Ga, P, In, and P atoms alternate on the planes of the basic unit cell, without the intermixing of the Ga and In atoms on the same lattice plane. Both wet and dry etching are used in production environments, and ion implantation is an effective technique to isolate the active devices from the rest.

Because of the narrow energy-gap of the InGaAs layer, achieving an effective device isolation often requires a complete removal of the inactive area surrounding the device, literally digging out trenches to form islands of devices. The turnon characteristics of various HBTs are shown in Fig. A calculation illustrates the advantage of an HBT.

For a graded Al0. The ratio for the graded HBT, according to Eq. After Ref. Typical HBT epitaxial structure designed for power amplifier applications. Therefore, Eq. Using Eq. This means the device is useless. In contrast, both the graded and the abrupt HBTs remain functional, despite the higher base doping in comparison to the emitter.

Once IB,back-inject is made small in a HBT through the use of a heterojunction, the remaining four components become noteworthy. All of these components are recombination currents; they differ only in the locations where the recombinations take place, as shown in Fig.

They are: 1 extrinsic base surface recombination current, IB,surf ; 2 base contact surface recombination current, IB,cont ; 3 bulk recombination current in the base layer, IB,bulk ; and 4 space-charge recombination current in the base-emitter junction depletion region, IB,scr. In the following, the characteristics of each of the five base components are described, so that we can better interpret the current gain from measurement and establish some insight about the measured device.

Figure 5. Without special consideration, a conventional fabrication process results in an exposed base surface at the extrinsic base regions. Intrinsic region is that underneath the emitter, and extrinsic region is outside the emitter mesa, as shown in Fig.

Various surface passivation techniques have been tested. The most effective method is ledge passivation,11,12 formed with, for example, an AlGaAs layer on top of the GaAs base. The AlGaAs ledge must be thin enough so that it is fully depleted by a combination of the free surface Fermi level pinning above and the base-emitter junction below. If the passivation ledge is not fully depleted, the active device area would be much larger than the designed emitter.

The fifth base current component, not a recombination current, is IB,back-inject shown in Figure 5. After Liu, W. Although the ledge passivation was originally designed to minimize IB,surf , it is also crucial to long-term reliability. For high frequency devices whose emitter is in a strip form thus the perimeter-to-area ratio is large , IB,surf is a major component to the overall base current. The current gain is substantially reduced from that of a large squarish device whose perimeter-to-area ratio is small.

The emitter area is small enough to demonstrate the benefit of the surface passivation. A large device has negligible surface recombination current and the current gain does not depend on whether the surface is passivated or not.

The second base recombination current, IB,cont, is in principle the same as IB,surf. Both are surface recombination currents, except IB,cont takes place on the base contacts whereas IB,surf , on the extrinsic base surfaces. Because the contacts are located further away from the intrinsic emitter than the extrinsic base surface, IB,cont is generally smaller than IB,surf when the surface is unpassivated. However, it may replace IB,surf in significance in passivated devices or in Si BJTs whose silicon dioxide is famous in passivating silicon.

There is a characteristic distance for the minority carrier concentration to decrease exponentially from the emitter edge toward the extrinsic base region. The base contact cannot be placed too far from the emitter, however.

The above two recombination currents occur in the extrinsic base region. Developing analytical expressions for them requires a solution of the two-dimensional carrier profile. Although this is possible without a full-blown numerical analysis,3 the resulting analytical equations are quite complicated. The base bulk recombination current, in contrast, can be accurately determined from a one-dimensional analysis since most of the base minority carriers reside in the intrinsic region.

Electron Devices, 39, —, , with permission. However, the transit time through the GaAs is also much shorter than in Si due to the higher carrier mobility in GaAs. Equation 5. The recombination lifetime of a semiconductor, a material property, is found to be inversely proportional to the doping level.

As the base doping increases to cm—3, IB,bulk dominates all other base current components, and the current gain decreases to only about 10, independent of whether the extrinsic surface is passivated or not. It is a compromise between achieving a reasonable current gain between 40 and and minimizing the intrinsic base resistance to boost the high frequency performance.

More and more organizations are investing and researching on this topic with huge potential in academic and commercial areas.

This is the first book on the market to. Chapter 2: Schottky Diode Frequency Multipliers. Chapter 3: Transit Time Microwave Devices. Chapter 4: Bipolar Junction Transistors. Chapter 5: Heterostructure Bipolar Transistors. Chapter 8: High Electron Mobility Transistors. Chapter Semiconductors. Chapter Metals. Chapter Technology Computer Aided Design. Click here to download Wait You will be directed to the download link after the count has ended.

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