![]() ![]() There are generally two techniques to measure current under pulsed condition. The device under test (DUT) draws the output current from the driver FET, which should be selected carefully on the basis of maximum current capability and transient response. The current driving capability is greatly enhanced here without sacrificing rapid transitions. The resultant signals are amplified by current feedback type operational amplifiers and driver FET. Two multiplexers are used to switch to the pulsed bias and they are synchronized with the pulse generator. A digital oscilloscope may be configured to monitor voltage waveforms for possible oscillations, ringing or malfunctioning, but is not a necessity. Both DC and pulsed I-V measurements can be performed by enabling or disabling the pulse generator. The external equipment required to perform pulsed measurements is only a pulse generator. The measurement setup implemented to characterize trap and thermal effects is shown in Figure 1. Moreover, pulsed measurements with various pulse timing configurations will reveal temperature effect on the observed transient response. ![]() When the device is measured starting from different DC quiescent points with the same dissipated power, the data will contain the same thermal information illustrating how the trap effects are related to the bias point. One of the features that the pulsed setup can provide is that the bias point, from which the I-V characteristics are measured by pulsing both the input and output terminal stimuli, can be set anywhere in the I-V plane. They provide valuable insight into various aspects of the device as to which predominantly affects the observed characteristics. Pulsed measurements allow more degrees of freedom in characterizing high frequency devices. In HBTs, there is a systematic decrease in collector current under steady-state when internal power dissipation (P diss ) causes a rise in junction temperature. Pseudomorphic HEMTs fabricated in some materials systems are fairly robust against this phenomenon, but it does exist to some extent. For example, GaAs FETs are known to suffer from low frequency dispersion, which means that I-V characteristics at high frequency are markedly different from their DC counterparts. However, even basic I-V measurements pose some challenges. They have included I-V and S-parameter measurements at different biases when the most frequently used active devices for high frequency circuits are FETs, heterojunction FETs (HJFET) and HBTs. Accurate characterizations of active devices are the basic requirements for a successful circuit design. ![]()
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