James G. Rathmell

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.

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.

Contribution of self heating to intermodulation in FETs

Self heating gives microwave FETs a temperature response to power dissipation. This is shown to be significant at high-frequency signal rates, even though the apparent time constant of heating is a few kilohertz. Self heating causes variation in intermodulation with frequency spacing and the extent of this variation is linked to bias. The frequency response of the heating process can be characterized from third-order intermodulation versus frequency spacing.

Measurement and characterization of HEMT dynamics

Large-signal dynamic behaviour of HEMTs is characterized for all bias points by bias-dependent pinch-off, gain, and drain feedback parameters. A novel drain current model uses these parameters to describe the dispersion effects. The model and parameters, presented here, give an insight into the operation of HEMTs.

Determining timing for isothermal pulsed-bias S-parameter measurements

S-parameters measured under pulsed conditions are shown to vary from their steady-state values with pulse measurement width and pulse repetition rate. A method is presented for determining suitable timing for isothermal, pulsed-bias, pulsed-RF, S-parameter measurement of GaAs devices. Variation of S-parameters with wafer temperature and with measurement duration and duty cycle are correlated.

Novel technique for determining bias, temperature and frequency dependence of FET characteristics

A novel measurement of the dynamics of HEMT and MESFET behavior permits classification of dispersion effects and identifies operating regions that they affect. This reveals a simple structure to the otherwise complicated dynamic behavior that has concerned circuit designers. With this insight, it is possible to predict biases, temperatures and frequencies that dispersion will or will not affect. It is interesting to note that, for some devices, dispersion effects can be seen to exist at microwave frequencies and may therefore contribute to intermodulation distortion.

Electrothermal gate and channel breakdown model for prediction of power and efficiency in FET amplifiers

A model of gate-junction leakage and impact ionization is used to predict catastrophic junction- and avalanche-breakdown mechanisms in a FET. It is shown that low-power dc measurements can be used to characterize breakdown and that the model correctly extrapolates to regions outside the safe-operating-area. When included in a large-signal FET model with dynamic calculation of junction temperature, the output power, power-added efficiency (PAE) and peak PAE of a common-source amplifier are well predicted.