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 V_{DS, DC} and V_GS 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.

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 HEMT_ref is shown along with I_D/V_DS 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 I_D/V_DS characteristics is covered. For the shown example HEMT_ref is DC biased at V_DS = 3.0V and V_GS = 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 HEMT_ref. With a DC bias of V_{DS, DC} = 3 V and V_{GS DC} = 0.35 V and a sinusoidal input signal with 38 GHz I_D/V_DS 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 V_br for V_GS near V_T is important but also V_br for high V_GS 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 HEMT_ref to power HEMTs investigated in this work are a smaller d_GC, larger lateral spacing L_R, 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 e_r1 = 1 and e_r2 = 7 are considered in the simulation. In addition the influence of the T­gate shape on C_G will be investigated. To analyze this effect quantitatively the gate geometry will be characterized by L_T, L_G, d_T, and d_G as shown in  Figure 6.1.
 

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

• 6.2.1 DC Characteristics
• 6.2.2 Dependence of Recess Geometry on the E-Field Distribution
• 6.2.3 RF Characteristics
• 6.2.3.1 Reduction of the Gate Length
• 6.2.3.2 Contributions to the Gate Capacitance
• 6.2.3.3 Dependence of C_G on Passivation Thickness
• 6.2.3.4 Dependence of C_G on T-gate Cross Section
• 6.3 Millimeter Wave HEMTs
• 6.3.1 DC Characteristics
• 6.3.1.1 Dependence on the Gate-to-Channel Separation
• 6.3.1.2 Dependence on the Gate Length
• 6.3.2 RF Performance
• 6.3.2.1 Dependence on the Gate Length
• 6.3.2.2 Contributions to the Gate Capacitance
• 6.3.2.3 Dependence of f_T on d_GC
• 6.3.2.4 Dependence of f_T and f_max on L_R
• 6.3.2.5 Dependence on the Passivation Thickness
• 6.3.3 Optimization of Millimeter Wave HEMTs
• 6.4 Comparison between Low Noise, Power, and Millimeter Wave HEMTs
• 6.5 Influence of Backside Doping on the C_G(V_GS) Characteristics