Next: 6.2.1 DC Characteristics Up: 6 Applications Previous: 6.1.2.3 Reduction of the Recess Length

6.2 Power HEMT for 0.9/1.9 GHz and 40 GHz Applications

In contrast to small signal applications a large voltage swing is applied on HEMTs for power applications. To obtain high output power the devices have to sustain high voltages and high current densities. A principle amplifier circuit is shown in  Figure 6.19. If the transistor is biased with a certain DC bias voltage VDS, DC and VGS is altered with low frequency a straight load line in the output characteristics occurs. The slope of the load line is defined by the load resistance.

Figure 6.19 Amplifier circuit with a HEMT biased at VDS, DC and VGS, DC. Additionally an RF input signal is applied. The HEMT is driving a 50 W load resistor through a matching network with the impedance Zoutput.

However, as the frequency of the input signal increases the capacitive and inductive elements of the transistor and the matching networks eventually become significant. The voltages occurring on the transistor can be determined by circuit simulation. In  Figure 6.20 the output characteristics of HEMTref is shown along with ID/VDS characteristics in the plane AA' indicated in  Figure 6.19 for a sinusoidal input signal with 38 GHz. For each cycle of the input signal one corresponding ID/VDS characteristics is covered. For the shown example HEMTref is DC biased at VDS = 3.0V and VGS = 0.35 V. The largest trajectory was obtained for an input voltage swing of , the smallest for .

Figure 6.20 Measured DC output characteristics of HEMTref. With a DC bias of VDS, DC = 3 V and VGS DC = 0.35 V and a sinusoidal input signal with 38 GHz ID/VDS characteristics over time in plane AA' ( Figure6.19) can be obtained by circuit simulation with the HP software MDS. The largest trajectory is obtained for an input signal amplitude of 0.3 V the smallest for 0.01 V.

The trajectory is widened not only with the applied input voltage swing but also with increasing frequency, thus a much larger area of the output characteristics is covered than in the DC case. This increases the requirements for the devices used for high frequency applications substantially. Not only the source drain breakdown voltage Vbr for VGS near VT is important but also Vbr for high VGS is becoming increasingly crucial with increasing operation frequency.

As discussed in Section 5.3.2.2 different breakdown regimes can be identified in the output characteristics of a HEMT. In any case these are attributed to high electric fields in some regions of the device. In pseudomorphic HEMTs the most common breakdown occurs in the channel due to its small bandgap. Thus, investigations will focus on the dependences of various geometry parameters on the maximum electric field in the channel for different bias points. Besides breakdown the RF performance of the devices is equally important. The DC and RF properties will be investigated by means of simulation and measurements. The trade-off between RF performance and power capability will be addressed.

In  Figure 6.21 a schematic cross section of the investigated power HEMT is shown. The most significant changes from HEMTref to power HEMTs investigated in this work are a smaller dGC, larger lateral spacing LR, and an additionally recessed cap layer to improve the breakdown voltage. To investigate the dependence of the RF performance on the passivation thickness layer with two different dielectric constants er1 = 1 and er2 = 7 are considered in the simulation. In addition the influence of the T­gate shape on CG will be investigated. To analyze this effect quantitatively the gate geometry will be characterized by LT, LG, dT, and dG as shown in  Figure 6.1.

Figure 6.21 Schematic cross section of the simulated power DH­HEMTs with double recess. The shape of the T­gate is characterized by LG, LT, dT and dG. er 1 and er 2 account for a certain passivation dielectric.

The low noise HEMTs investigated in the previous section and HEMTref have self aligned contacts, i. e., the gate metal serves as a mask to define source and drain contacts, thus LT is the source to drain contact separation. This is different for the power HEMTs were LT is smaller than the source to drain separation as depicted in the cross section shown in  Figure 6.21.

Next: 6.2.1 DC Characteristics Up: 6 Applications Previous: 6.1.2.3 Reduction of the Recess Length

Helmut Brech
1998-03-11