Device Modeling of a PNP HBT Using ATLAS


The development of PNP heterojunction bipolar transistors (HBTs) is of interest for integration with NPN HBTs for high frequency applications in microwave and optoelectronic integrated circuits [1]. While PNP devices have received relatively little attention compared with their NPN counterpart, analysis of PNP HBTs has shown that these devices have considerable potential for microwave and millimeter-wave high frequency performance [2]. So monolithic integration of PNP HBTs with NPN HBTs is very attractive for a number of applications where the PNP transistor can serve as an active load or as a matched component in push-pull amplifiers [3].

While hole mobilities are much lower than electron mobilities in GaAs material, the HBT is a minority carrier device whose performance depends not only on the minority carrier transport across the narrow base region, but also on the base resistance which is dependent on the majority carrier electron mobility. In this article, the high frequency performance of the PNP HBT has been simulated by using ATLAS. The DC and AC characteristics of an AlGaAs/GaAs PNP HBT are simulated. All the parasitic effects are inherently taken into account in ATLAS, including drift-diffusion, Fermi-Dirac, bandgap marrowing, thermionic emission at the heterojunction.


Device Structure

The device structure used for simulation is a single heterojunction PNP HBT of AlGaAs/GaAs material system with the large bandgap material of AlGaAs as emitter and the narrow bandgap material of GaAs as base and collector. The composition of Al is 30% and is gradually reduced to 0% to form a graded heterojunction in order to improve the high frequency performance. The emitter thickness is 1000Å with p-type doping of 2.0e18 cm. The base thickness is 500Å with n-type doping of 8.0e18 cm. The collector thickness is 5000 with lightly p-doped at 5.0e16 cm to form large depletion width in the collector side and reduce the base-collector junction capacitance. The highly doped (5.0e18 cm) sub-collector is used to form ohmic contact with the metal electrode. The non self-aligned structure is used with base metal contact and emitter mesa width equal to 1.0 µm.



Figure 1 gives equilibrium energy band diagram of this PNP device. It can be seen that the heterojunction is formed at 0.15µm from the left. In Figure 2, the Gummel plot of this device at Vce=-2.0V is given. From the figure, we can see that the device's turn-on voltage is about -0.7V. It shows also that the device has large current amplifying capability over several orders of magnitude of collector current and has a peak current gain of 32.


Figure 1. The Equilibrium Energy Band
Diagram of PNP AIGaAs/GaAs HBT.



Figure 2. The Gummel Plot of PNP
AlGaAs/GaAs HBT at Vce=-2.0V.



Figure 3 shows the device's output characteristics by using current boundary condition at the base and voltage boundary condition at the collector for base current at 1µA, 2µA, 3µA, 4µA and 5µA, respectively. We can see that this device has a small offset voltage and a large early voltage. In Figure 4, it gives the small signal current gain as a function of collector current at Vce=-2.0V and a operating frequency=1MHz. It shows that the ac current gain remains high and stable over about four orders of magnitude of collector current and falls down at high collector current due to the high collector series resistance.


Figure 3. The Output characteristics of the


Figure 4. The Small Signal Current Gain
Characteristics of the PNP AlGaAs HBT.



Figure 5 presents the high frequency characteristics. It plots the current gain and power gain as a function of frequency at DC bias of Vce=-2.0V and Vbe=-1.45V. From the figure, we can see that the cutoff frequency(frequency at unit current gain) is 5.70 GHz and the maximum frequency of oscillation (fa mx, frequency at unit unilateral power gain) is 22.91GHz. We can see here that the high frequency operation of PNP HBT has been demonstrated, which can provide current gain and power gain comparable to the NPN HBT at microwave frequency.


Figure 5. The Frequency Response Characteristics
of the PND AlGaAs/GaAs HBT.


The DC and AC characteristics of the AlGaAs/GaAs PNP HBT have been simulated by using ATLAS. The high frequency operation has been shown which can be used with NPN HBTs for application in monolithic microwave and optoelectronic integrated circuits (MMICs and OEICs). The software shows the possibility and feasibility of obtaining the characteristics of the device. This is a good way to investigate the more advanced devices.



[1] J. A. Hutch by, "High-performance PNP AlGaAs/GaAs heterojunction bipolar transistors: A theoretical study," IEEE Electro. Dev. Lett., vol.17, P.108-111, (1986).

[2] G.B. Gao, D.J. Roulston and H. Morkoc, "Is NPN or PNP the better choice for millimeter-wave AlGaAs/GaAs heterojunction bipolar transistors," Solid State Electronics, vol.33, P.1209-1210,(1990).

[3] H. Q. Tserng, D. G. Hill and T. S. Kim, "A 0.5W complementary AlGaAs-GaAs HBT push-pull amplifier at 10GHz," IEEE Microwave & Guided Wave Lett., vol.3, P.45-47, (1993).