Anthony E. Parker

Effect of baseband impedance on FET intermodulation

The intermodulation performance of an FET in the common-source configuration is dependent on the impedance presented to its gate and drain terminals, not only at fundamental, but also at harmonic and baseband frequencies. At baseband frequencies, these terminating impedances are usually determined by the bias networks, which may have varying impedance over the frequencies involved. This can give rise to asymmetry in two-tone intermodulation levels, and changing intermodulation levels with tone spacing, as previous studies have shown. In this paper, an FET is analyzed to gain an understanding, useful to the circuit designer, of the contributing mechanisms, and to enable the prediction of bias points and the design of networks that can minimize or maximize these effects. Compact formulas are given to facilitate this. An amplifier was tested, showing good agreement between the theoretical and measured results.

Pulsed device measurements and applications

A pulsed measurement system can provide more than just isothermal characteristic data. An off-the-shelf system can determine rapidly the timing necessary for both pulsed-I-V and pulsed-S-parameter measurements to be isothermal and isodynamic. Instantaneous channel temperature may be determined. Thermal and charge-trapping effects can be separated and time constants measured. Full gain-derivative surfaces can be obtained far more efficiently than by spectral sweep measurements. Characteristics and transient effects following excursions beyond the safe-operating-area and into breakdown may be observed nondestructively.

Broad-band characterization of FET self-heating

The temperature response of field-effect transistors to instantaneous power dissipation has been shown to be significant at high frequencies, even though the self-heating process has a very low time constant. This affects intermodulation at high frequencies, which is examined with the aid of a signal-flow description of the self-heating process. The impact on broad-band intermodulation is confirmed with measurements over a range of biases. Intermodulation measurements are then used to obtain parameters that describe the heating response in the frequency domain. This description is then implemented in a time-domain model suitable for transient analysis and compared with measured heating and cooling step responses.

Bias and frequency dependence of FET characteristics

A novel measurement of the dynamics of high electron-mobility transistor (HEMT) and MESFET behavior permits classification of rate-dependence mechanisms and identification of operating regions that they affect. This reveals a simple structure to the otherwise complicated behavior that has concerned circuit designers. Heating, impact ionization, and trapping contribute to transient behavior through rate-dependence mechanisms. These are illustrated by a simple description. Each has an effect on specific regions of bias and operating frequency. With this insight, it is possible to determine true isodyamic characteristics of HEMTs and MESFETs and to predict operating conditions that will or will not be affected by rate dependence. It is interesting to note that, for some devices, rate dependence can be seen to exist at microwave frequencies and may, therefore, contribute to intermodulation distortion.

Circuit Implementation of a Theoretical Model of Trap Centres in GaAs and GaN Devices

A novel and simple circuit implementation of trap centres in GaAs and GaN HEMTs, MESFETs and HFETs is presented. When included in transistor models it explains the potential-dependent time constants seen in the circuit manifestations of charge trapping, being gate lag and drain overshoot. The implementation is suitable for both time- and harmonic-domain simulations. The trap-centre model is based on Shockley-Read-Hall (SRH)1 statistics of the trapping process. It also accommodates carrier injection from other important device effects, such as impact ionization and light sensitivity. In the model, the ionization charge of the trap centre is represented by the charge in a capacitor. The potential across the capacitor is proportional to the potential across the region of the trap centre in the semiconductor. It is positive or negative depending on the polarity of the ionization charge - electrons or holes. When included in a transistor model, this potential is added to the gate potential that controls the drain-current description. The capacitor is charged or discharged by two opposing currents that are functions of the ionization potential and temperature: one models charge emission; and the other, which is also controlled by an external potential and injected current, models charge capture. The external potential is typically a linear function of a transistors terminal potentials. The injection current can model charge generated by light or by holes from impact ionization. The four parameters for the model are simply the signed potential of the trap centre when fully ionized, the time constant for charge emission at a specific temperature, the injection-current sensitivity, and the activation energy of the emission process. The latter is used to predict the temperature dependence of the emission rate. The capture rate is determined within the model by an exponential function of the external potential that controls capture. Thus the model elegantly predicts asymmetry between trap charging and discharging rates. The model accounts for variation in emission and capture rates with temperature, which is shown to vary significantly over typical transistor operating ranges.

Pulsed-bias/Pulsed-RF Device Measurement System Requirements

We describe a pulsed-bias, pulsed-RF device measurement system with high bias power (6A/40V), high RF power capability (50W at 2GHz and lOW at 50GHz), and high resolution (16-bit). This system is intended to support both RF characterisation outside device continuous safe operating area (SOA), and data-acquisition for device modelling. The system is modular and flexible, offers very small duty cycles (<0.001%) simultaneous wide dynamic range (75dB at 50GHz), and employs instruments which are already available. We present novel measurements on several GaAs devices and draw conclusions important for future device characterisation efforts.

Baseband impedance and linearization of FET circuits

Baseband impedance has been identified as having a positive or negative effect on the intermodulation distortion of microwave circuits. The effect can be assessed or utilized with the aid of previously proposed figures-of-merit. Under certain situations, intermodulation cancellation can be achieved simply by adding resistance to the bias network. The impact of baseband impedance on the performance of derivative superposition amplifiers is analyzed. A bias region was studied that exhibits a good second- and third-order intermodulation null with minimal intermodulation dependence on baseband impedance. This allows the effective use of the derivative superposition technique in baseband amplifiers, as well as giving wide-band linearization performance in RF amplifiers.

New model extraction for predicting distortion in HEMT and MESFET circuits

A new method is presented for extracting Taylor series coefficients directly from IM measurements for modeling IM distortion in HEMT and MESFET circuits. It is based on an improved model that uses better simplifying assumptions. The method gives a substantially more accurate characterization, especially in the saturation region, required for amplifier designs.

60GHz Radios: Enabling Next-Generation Wireless Applications

Up to 7 GHz of continuous bandwidth centred around 60 GHz has been allocated worldwide for license free wireless communications. Highly attenuated due to oxygen absorption and small in wavelength, this band is ideal for extremely high data rate wireless data applications. These include numerous WPAN/WLAN applications such as home multimedia streaming. Traditional RF circuits used in this band are based on expensive compound semiconductor technologies. However for viable consumer applications, alternatives must be found. SiGe and CMOS based circuits are showing promise for enabling this technology at a price within reach of the consumer. This paper summarises a joint project aimed at developing high rate consumer level mm-wave wireless data systems. In particular, results to date in our RF design efforts are summarised.

Robust Extraction of Access Elements for Broadband Small-signal FET Models

A small-signal transistor model extraction technique is proposed. It partitions access and intrinsic elements with a more accurate network for the intrinsic section. This resolves problems of nonphysical parameters and inconsistencies across bias. The technique uses low gate and zero drain bias measurements to directly determine an access network. There is no need to apply electrical stress to the device during measurement. The procedure is deterministic.